I've recapped it, replaced three missing tubes and repaired cracks in the speaker cone. It plays really well on all three bands. I did install the electrolytics correctly (not connected to chassis ground). However I can meter 256 volts DC on the chassis. If I ground the chassis it goes dead with the power plug in one way, and a very loud 60hz hum with it reversed. The 256 volts is present everywhere on the chassis, tuning capacitor, control shafts, etc. I've just started trouble shooting it. Does anyone have experience with this kind of problem?

Measure resistance from rectifier filament to chassis. Should be at least 5 or 6K according to the schematic. If it's low, the filament winding might be shorted to the frame or the speaker output transformer might be shorted to frame. It could be in the transformers or in the wiring. I am assuming good caps since you recapped it and the radio plays OK with no ground.

It's a sure bet that B+ is shorted to the chassis. All you have to do is find out where .

Tony

:I've recapped it, replaced three missing tubes and repaired cracks in the speaker cone. It plays really well on all three bands. I did install the electrolytics correctly (not connected to chassis ground). However I can meter 256 volts DC on the chassis. If I ground the chassis it goes dead with the power plug in one way, and a very loud 60hz hum with it reversed. The 256 volts is present everywhere on the chassis, tuning capacitor, control shafts, etc. I've just started trouble shooting it. Does anyone have experience with this kind of problem?

: Measure resistance from rectifier filament to chassis. Should be at least 5 or 6K according to the schematic. If it's low, the filament winding might be shorted to the frame or the speaker output transformer might be shorted to frame. It could be in the transformers or in the wiring. I am assuming good caps since you recapped it and the radio plays OK with no ground.
:
:It's a sure bet that B+ is shorted to the chassis. All you have to do is find out where .
:
:Tony
:
:
::I've recapped it, replaced three missing tubes and repaired cracks in the speaker cone. It plays really well on all three bands. I did install the electrolytics correctly (not connected to chassis ground). However I can meter 256 volts DC on the chassis. If I ground the chassis it goes dead with the power plug in one way, and a very loud 60hz hum with it reversed. The 256 volts is present everywhere on the chassis, tuning capacitor, control shafts, etc. I've just started trouble shooting it. Does anyone have experience with this kind of problem?

I must have smoked too much pumpkin pie yesterday. If B+ were shorted to chassis we would have a short across the power supply output.

If the orientation of the AC plug makes a difference then I think that the primary and secondary of the power transformer are not isolated from one another. If it were simply a case of one side of the AC line being shorted to the chassis then I don't see how we would get the DC voltage with respect to ground. So, I vote for a primary to secondary short in the power transformer. Measure from the primary to each of the secondary windings.

: Measure resistance from rectifier filament to chassis. Should be at least 5 or 6K according to the schematic. If it's low, the filament winding might be shorted to the frame or the speaker output transformer might be shorted to frame. It could be in the transformers or in the wiring. I am assuming good caps since you recapped it and the radio plays OK with no ground.
:
:It's a sure bet that B+ is shorted to the chassis. All you have to do is find out where .
:
:Tony
:
:
::I've recapped it, replaced three missing tubes and repaired cracks in the speaker cone. It plays really well on all three bands. I did install the electrolytics correctly (not connected to chassis ground). However I can meter 256 volts DC on the chassis. If I ground the chassis it goes dead with the power plug in one way, and a very loud 60hz hum with it reversed. The 256 volts is present everywhere on the chassis, tuning capacitor, control shafts, etc. I've just started trouble shooting it. Does anyone have experience with this kind of problem?

:I must have smoked too much pumpkin pie yesterday. If B+ were shorted to chassis we would have a short across the power supply output.
:
:If the orientation of the AC plug makes a difference then I think that the primary and secondary of the power transformer are not isolated from one another. If it were simply a case of one side of the AC line being shorted to the chassis then I don't see how we would get the DC voltage with respect to ground. So, I vote for a primary to secondary short in the power transformer. Measure from the primary to each of the secondary windings.
:
:
:
:
:: Measure resistance from rectifier filament to chassis. Should be at least 5 or 6K according to the schematic. If it's low, the filament winding might be shorted to the frame or the speaker output transformer might be shorted to frame. It could be in the transformers or in the wiring. I am assuming good caps since you recapped it and the radio plays OK with no ground.
::
::It's a sure bet that B+ is shorted to the chassis. All you have to do is find out where .
::
::Tony
::
::
:::I've recapped it, replaced three missing tubes and repaired cracks in the speaker cone. It plays really well on all three bands. I did install the electrolytics correctly (not connected to chassis ground). However I can meter 256 volts DC on the chassis. If I ground the chassis it goes dead with the power plug in one way, and a very loud 60hz hum with it reversed. The 256 volts is present everywhere on the chassis, tuning capacitor, control shafts, etc. I've just started trouble shooting it. Does anyone have experience with this kind of problem?

::I must have smoked too much pumpkin pie yesterday. If B+ were shorted to chassis we would have a short across the power supply output.
::
::If the orientation of the AC plug makes a difference then I think that the primary and secondary of the power transformer are not isolated from one another. If it were simply a case of one side of the AC line being shorted to the chassis then I don't see how we would get the DC voltage with respect to ground. So, I vote for a primary to secondary short in the power transformer. Measure from the primary to each of the secondary windings.
::
::
::
::
::: Measure resistance from rectifier filament to chassis. Should be at least 5 or 6K according to the schematic. If it's low, the filament winding might be shorted to the frame or the speaker output transformer might be shorted to frame. It could be in the transformers or in the wiring. I am assuming good caps since you recapped it and the radio plays OK with no ground.
:::
:::It's a sure bet that B+ is shorted to the chassis. All you have to do is find out where .
:::
:::Tony
:::
:::
::::I've recapped it, replaced three missing tubes and repaired cracks in the speaker cone. It plays really well on all three bands. I did install the electrolytics correctly (not connected to chassis ground). However I can meter 256 volts DC on the chassis. If I ground the chassis it goes dead with the power plug in one way, and a very loud 60hz hum with it reversed. The 256 volts is present everywhere on the chassis, tuning capacitor, control shafts, etc. I've just started trouble shooting it. Does anyone have experience with this kind of problem?

:I must have smoked too much pumpkin pie yesterday. If B+ were shorted to chassis we would have a short across the power supply output.
:
:If the orientation of the AC plug makes a difference then I think that the primary and secondary of the power transformer are not isolated from one another. If it were simply a case of one side of the AC line being shorted to the chassis then I don't see how we would get the DC voltage with respect to ground. So, I vote for a primary to secondary short in the power transformer. Measure from the primary to each of the secondary windings.
:
:
:
:
:: Measure resistance from rectifier filament to chassis. Should be at least 5 or 6K according to the schematic. If it's low, the filament winding might be shorted to the frame or the speaker output transformer might be shorted to frame. It could be in the transformers or in the wiring. I am assuming good caps since you recapped it and the radio plays OK with no ground.
::
::It's a sure bet that B+ is shorted to the chassis. All you have to do is find out where .
::
::Tony
::
::
:::I've recapped it, replaced three missing tubes and repaired cracks in the speaker cone. It plays really well on all three bands. I did install the electrolytics correctly (not connected to chassis ground). However I can meter 256 volts DC on the chassis. If I ground the chassis it goes dead with the power plug in one way, and a very loud 60hz hum with it reversed. The 256 volts is present everywhere on the chassis, tuning capacitor, control shafts, etc. I've just started trouble shooting it. Does anyone have experience with this kind of problem?

Tony, thanks for the help. I think that you have nailed the problem as an internal short in the power transformer. Measured at the rectifier tube socket pins 4 to 6 show 441 ohms through the secondary and only 1.4K to the chassis. Pins 1 to 8 show 17K to the chassis. I took the transformer covers off but didn't find any obvious problem. I spent the rest of the day looking for a B+ to chassis short but didn't find any. The strange thing about this is that I've never received even a tingle from handling the chassis. I'm using a Fluke digital multimeter with all the bells and whistles. It shows 256 VDC to the house ground at 413 MA and 121 VAC at 120 MA and a freq of 59.99 hz. I began to doubt my Fluke meter so I dug out an old RCA analog meter. It showed me the same voltages. I plan to press on tommorrow and start looking for a replacement transformer. Thanks again.

::I must have smoked too much pumpkin pie yesterday. If B+ were shorted to chassis we would have a short across the power supply output.
::
::If the orientation of the AC plug makes a difference then I think that the primary and secondary of the power transformer are not isolated from one another. If it were simply a case of one side of the AC line being shorted to the chassis then I don't see how we would get the DC voltage with respect to ground. So, I vote for a primary to secondary short in the power transformer. Measure from the primary to each of the secondary windings.
::
::
::
::
::: Measure resistance from rectifier filament to chassis. Should be at least 5 or 6K according to the schematic. If it's low, the filament winding might be shorted to the frame or the speaker output transformer might be shorted to frame. It could be in the transformers or in the wiring. I am assuming good caps since you recapped it and the radio plays OK with no ground.
:::
:::It's a sure bet that B+ is shorted to the chassis. All you have to do is find out where .
:::
:::Tony
:::
:::
::::I've recapped it, replaced three missing tubes and repaired cracks in the speaker cone. It plays really well on all three bands. I did install the electrolytics correctly (not connected to chassis ground). However I can meter 256 volts DC on the chassis. If I ground the chassis it goes dead with the power plug in one way, and a very loud 60hz hum with it reversed. The 256 volts is present everywhere on the chassis, tuning capacitor, control shafts, etc. I've just started trouble shooting it. Does anyone have experience with this kind of problem?

To find a short between the 110 volt primary and the high voltage secondary, simply connect your meter between any point on the two (one lead to one coil at any point, and the other lead to the other coil at any point). If you find any continuity what-so-ever, then this is your problem. You must disconnect any line cord condensers that go to the chassis, however, or any leakage present within them may give you the false notion that you have leakage within the transformer.

If the transformer functions normally, otherwise, you may simply treat the set as an AC-DC set, and try not to ground the chassis. Insert a .01 MFD condenser between the antenna and the radio. You can also disassemble the transformer and check for shorts in the wiring, though the trouble may actually be in the windings. If trouble is in the windings, then try not to leave the set on unattended. It is not likely to blow up, but if things are deteriorating, then it is best to be cautious. Still, if the transformer is functioning normally, otherwise, I don't see the need for replacing it yet. Transformers are expensive. If you do disassemble the transformer for a wiring examination, be careful, as the wiring may be fragile. If it is, either be ready with some heat shrink, or avoid disassemble. Heat shrink might not be such a bad idea, though, if the wiring is deteriorating. It may actually save trouble that might otherwise happen in the future (internal shorts).

Thomas

Also, since the chassis is not directly connected to the center tap of the transformer, it is slightly positive with respect to the transformer. You may in fact have a short between the center tap wire of the transformer and one of the 110 volt wires of the transformer. It could be shorted anywhere, though. Probing around on the various leads of the two coils will give you a clue as to which leads the short is closest to. For accurate results, make sure that all leads are connected to nothing that will throw off readings (the plates of the rectifier when cold will not throw off resistance readings....if no condensers on the line cord, no trouble here, either).

Thomas

Will, I am sorry I sent you looking for a B+ short to chassis. I was thinking of an AC/DC set with a "floating" ground. In your set the chassis is the ground reference for your power supply. If B+ came into contact with the chassis it would short out the supply, probably cooking something in the process.

The only thing that makes any sense to me is that the primary is shorted to the HV secondary. As Thomas said, the way to check it is to place one lead of the ohm meter on one of the AC primary leads and the other on one of the secondary leads. My bet is for the HV secondary so one of the rectifier plates. It should read open. The primary is supposed to be completely isolated. It should be open to the chassis and all other points in the circuit.

Instead of replacing the transformer, you could add an isolation transformer between the line and the primary of your transformer. I find transformers with dual primaries useful for this. One primary to the line and the other primary becomes a secondary. Or, you can try to find a transformer designed specifically for isolation. Of course, you have to find room to hide it in the set somewhere.

The isolation transformer must have a high enough power rating for the job. If the isolation transformer core is about the same thickness (or thicker) as the transformer on your set, then you should be good to go.

Tony

Don't just check the primary to HV secondary. Check the primary to the recifier filament winding, and to ground.

:Don't just check the primary to HV secondary. Check the primary to the recifier filament winding, and to ground.
:Tony and Thomas, Thanks a million for all your help. I know next to nothing about tube circuits. I'm a retired main frame computer tech ('63 to '97) so I've had lots of troubleshooting time, but nothing with tubes. My only exposure to radios was in the army in the '50's as a high speed morse operator. I checked the transformer as you suggested but didn't find any shorts to the primary. I discovered why I wasn't getting a shock from the chassis. I said that there was 451 ma to ground and that was wrong, It's only 4 ma. 451 at 260 volts would give you a real wake up call but 4 wont. the power plug orientation gives me 122 vac on the chassis one way, and 241 mv the other way with the swithc off. So, I think that my problem is with the circuitry and not the transformer. I have not had any luck finding it though. One more question: should the tuning capacitor be isolated from the chassis? Zenith mounted it with rubber washers which are really in sad shape after 69 years and it's not isolated now if it ever was. the isolation transformer sounds like a good idea. It's a console cabinet so I have plenty of room to hide it. I bought this radio at an auction in 1966 for $3.00. It's been sitting around not being molested since then. I've restored the cabinet to its' original ebony finish and now I'm ready to put it in service in our basement bar. Thanks again for all your help. If I ever find the real problem I'll let you know.
:

As far as I can see, I think that C1, next to the 6A8 tube (in the schematic) in the antenna circuit, is one of the two gangs of your tuning condenser, and C1, connected to the oscillator circuit under the 6A8 tube (in the schematic), are the two gangs of your tuning condenser. They are both connected to the chassis, which means that the frame of your tuning condenser is connected to the chassis. Tuning condensers, whether connected directly to the chassis or not, are commonly isolated on rubber bushings. This dampens out vibration that might otherwise cause frequency modulation feedback. This happens when the speaker plays a really loud passage and then vibrates the condenser plates. This changes the frequency that is tuned in, and causes a fluctuation in the radio circuits, which then passes on to the speaker again, and it howls like a microphone placed too close to the speaker. This problem isn't common, but manufacturers often isolated the condenser anyway. New rubber bushings can be found at your hardware store. Purchase many. This way some can be placed in the original holes. Then others can be cut in half and used as shims, if necessary, for putting the condenser back to its original height.

Have fun with tubes. They are a lot of fun, and are easy to understand. I have heard, though, that people who start off with solid state technology, have trouble with tubes sometimes. Tubes are amazing devices, capable of far more than most people to-day realize. They are more sensitive than any other device of similar nature (transistor), and are faster responding than any other similar device. Unfortunately they aren't extremely stable (emission dies off with age), and they take up a lot of space. Though they could make computers faster than they are, the computers would take up huge buildings. Noone knows how to put a tube in a chip. They make radios sound really good, though, and will cook your food really fast. Your radio, though it will likely sound great, won't have the best sound, since it doesn't have push-pull output. Push-pull output is used on all fine radios, both modern and old. Most car stereos and high quality home stereos use push-pull output. Push-pull output, by nature, cancels a lot of distortion in audio. These distortions may either be in the form of clipped off audio waves or in the form of non-uniform frequency response. Push-pull is deep and rich, and of full range. A properly designed singe phase output can sound really nice, though.

Thomas

:As far as I can see, I think that C1, next to the 6A8 tube (in the schematic) in the antenna circuit, is one of the two gangs of your tuning condenser, and C1, connected to the oscillator circuit under the 6A8 tube (in the schematic), are the two gangs of your tuning condenser. They are both connected to the chassis, which means that the frame of your tuning condenser is connected to the chassis. Tuning condensers, whether connected directly to the chassis or not, are commonly isolated on rubber bushings. This dampens out vibration that might otherwise cause frequency modulation feedback. This happens when the speaker plays a really loud passage and then vibrates the condenser plates. This changes the frequency that is tuned in, and causes a fluctuation in the radio circuits, which then passes on to the speaker again, and it howls like a microphone placed too close to the speaker. This problem isn't common, but manufacturers often isolated the condenser anyway. New rubber bushings can be found at your hardware store. Purchase many. This way some can be placed in the original holes. Then others can be cut in half and used as shims, if necessary, for putting the condenser back to its original height.
:
:Have fun with tubes. They are a lot of fun, and are easy to understand. I have heard, though, that people who start off with solid state technology, have trouble with tubes sometimes. Tubes are amazing devices, capable of far more than most people to-day realize. They are more sensitive than any other device of similar nature (transistor), and are faster responding than any other similar device. Unfortunately they aren't extremely stable (emission dies off with age), and they take up a lot of space. Though they could make computers faster than they are, the computers would take up huge buildings. Noone knows how to put a tube in a chip. They make radios sound really good, though, and will cook your food really fast. Your radio, though it will likely sound great, won't have the best sound, since it doesn't have push-pull output. Push-pull output is used on all fine radios, both modern and old. Most car stereos and high quality home stereos use push-pull output. Push-pull output, by nature, cancels a lot of distortion in audio. These distortions may either be in the form of clipped off audio waves or in the form of non-uniform frequency response. Push-pull is deep and rich, and of full range. A properly designed singe phase output can sound really nice, though.
:
:Thomas

Thomas,
Thanks for the help. As I mentioned, I have AC present on the chassis when the set is plugged in but not turned on. the amplitude depends on the orientation of the plug. 120 volts when the hot side is connected to the on/off/volume switch and millivolts when the neutral is connected. This afternoon I traced the 120 from the line input to the switch and then to the 6B6 2nd detector. I may have a leak in the switch (R7 on the map). I plan to remove the switch and all its' leads and see if it measures correctly. If it doesn't I'll take it apart and have a look see. this evening I put the speaker and chassis back in the cabinet and I'm listening to it as I type this message. One other thing that I failed to mention is that the 260 VDC on the chassis when the set is turned on is -260 and not B+. I guess you call that B-, right? This is one of those sets where the power supply reference floats and the first filter cap is not grounded to the chassis but uses the speaker field coil as part of the supply. By the way, I checked the speaker field and voice coils for shorts or longs and they seem to be perfect.
Thanks again.

Will

Will, according to your first post, you state that the electrolytics are not connected to the chassis. I take this to mean that neither of the 8mfd caps are connected to chassis. However, the schematic shows that one of the C13 caps installs to chassis ground, making it a conventional power supply. If the negative side of this cap is connected anywhere else, then things just ain’t gonna work like they should. Maybe this is the cause of your strange voltage readings.

Perhaps the speaker field may be confusing you somewhat. As with many radios of this time period, the speaker field also serves as a choke for the power supply filter. This choke does the same thing, regardless if it is installed between the caps in the B+ or in the negative center tap of the power transformer. Manufacturers did them both ways. In the case of your Zenith, this part of the circuit supplies the bias or negative voltage to the output tube grid. Therefore, this voltage should be negative when measured from chassis ground. You did not state where you are measuring your DC chassis voltages from, but all B+ should be measured from chassis ground.

There is an 85 ohm resistor from one end of R7 to the chassis so it is probably the reason that you see 120 vac there as well. A check with the ohm meter between each terminal of the switch and the three terminals of the potentiometer should tell you if there is a short. While you are at it, you might check from each terminal of the switch to the case of the potentiometer.

Earlier I think you said that you measured 4ma. If this was an AC current reading and if this was with the meter connected between the chassis and the wall outlet ground then we are talking about 30K ohms. If it was a DC current reading then we are talking about 65K. In either case the short is not "hard".

The fact that the chassis is negative means that the short is probably to some point that normally sits at +260 VDC. This is consistent with a short between the line and the 5Y3 filament circuit.

Tony is right on about the short. It is probably happening at some positive point in the power supply.

Regarding electrolytics, they can connect between any positive and negative point within the power supply. The negative does not have to connect to the chassis. It depends upon what you are trying to do. Connecting an electrolytic in some places will actually increase hum or may have no effect at all. At any rate, if you look in the schematic, you'll see that one is connected with the negative to the center tap of the transformer, and the other is connected with the negative to the chassis. Think of the more common circuit with the field coil on the positive side. One electrolytic goes before the field and one goes after it. With the field coil in the negative portion of the circuit, everything is the same, only this time you're dealing with the negative side of the electrolytics.

Thomas

:Tony is right on about the short. It is probably happening at some positive point in the power supply.
:
:Regarding electrolytics, they can connect between any positive and negative point within the power supply. The negative does not have to connect to the chassis. It depends upon what you are trying to do. Connecting an electrolytic in some places will actually increase hum or may have no effect at all. At any rate, if you look in the schematic, you'll see that one is connected with the negative to the center tap of the transformer, and the other is connected with the negative to the chassis. Think of the more common circuit with the field coil on the positive side. One electrolytic goes before the field and one goes after it. With the field coil in the negative portion of the circuit, everything is the same, only this time you're dealing with the negative side of the electrolytics.
:
:Thomas

When I replaced the electrolytics I just cut the leads off the old cans noting the polarity and soldered in the new caps observing the correct polarity. Then I replaced all the paper caps with orange drops. Then I replaced the missing tubes. I powered it up with a dim blub tester I made with 25, 40, 60, and 100 watt bulbs and it worked well. After that I found the voltage on the chassis and we started this saga. I've checket all my capacitor work and it agrees with the schematic.
This morning I checked the volume control switch and didn't find any shorts. The tone control has never worked because the lead at the low resistance end to the plate of the power tube had been removed sometime before I bought the set. I policed up the guts of the tone pot and replaced the lead and it works very well. On the tube section of this site I checked up on the rectifier tube. Zenith marked the socket 5Y3 and 5Z4 and the schematic calls out both types. The rectifier tube was one of the missing tubes that I replaced. I installed a 5Z4. The tube section of this site doesn't list them as interchangeable. Does this indicate something that Zenith didn't know. The schematic tube picture is a 5Y3. I really appreciate all the help I have received from you all.
Later, Will

The rectifier tube is OK. The 5Z4 has a cathode and is rated at 125ma. The 5Y3 is directly heated and is rated at 135ma. Both should be OK in this application.

I still want to go back to the transformer. It just sounds exactly like the correct symptom for a short from the primary to the rectifier filament winding. When you measured from the rectifier pins to the primary (or AC line cord with the switch closed) did you put your ohmmeter on it's higher ranges to check for high resistance shorts? You probably did, but I am at a loss to explain this any other way. The only other thing I can think of, and I've never seen it before, is a transformer short that is only there when power is applied.

:The rectifier tube is OK. The 5Z4 has a cathode and is rated at 125ma. The 5Y3 is directly heated and is rated at 135ma. Both should be OK in this application.
:
:I still want to go back to the transformer. It just sounds exactly like the correct symptom for a short from the primary to the rectifier filament winding. When you measured from the rectifier pins to the primary (or AC line cord with the switch closed) did you put your ohmmeter on it's higher ranges to check for high resistance shorts? You probably did, but I am at a loss to explain this any other way. The only other thing I can think of, and I've never seen it before, is a transformer short that is only there when power is applied.

Tony,
Just got your resopnse. I have an auto rangeing meter. I just measured it again and this is what I found. From pin 2 to one side of the line cord I get 429 ohms and from the other side I get infinity (OL).
From pin 8 I get 421 ohms and OL. I am counting the pins counter clockwise from the index as viewed from the top. Whatcha think?
Will

::The rectifier tube is OK. The 5Z4 has a cathode and is rated at 125ma. The 5Y3 is directly heated and is rated at 135ma. Both should be OK in this application.
::
::I still want to go back to the transformer. It just sounds exactly like the correct symptom for a short from the primary to the rectifier filament winding. When you measured from the rectifier pins to the primary (or AC line cord with the switch closed) did you put your ohmmeter on it's higher ranges to check for high resistance shorts? You probably did, but I am at a loss to explain this any other way. The only other thing I can think of, and I've never seen it before, is a transformer short that is only there when power is applied.
:
:Tony,
:Just got your resopnse. I have an auto rangeing meter. I just measured it again and this is what I found. From pin 2 to one side of the line cord I get 429 ohms and from the other side I get infinity (OL).
:From pin 8 I get 421 ohms and OL. I am counting the pins counter clockwise from the index as viewed from the top. Whatcha think?
:Will

Tony, you are right, there is a short. In my last test I forgot to turn the switch on. With the switch on I get 421 and 422 from pin 8 and 428/433 from pin 2. I think that this saga is over and I want to thank you, Thomas, and Billy for the help.
Will

Congratulations on the transformer. Regarding the rectifier tube, if you didn't already get the idea, they are pretty much interchangeable electrically, but are not physically. Each is wired to the base slightly differently.

Thomas

I’m glad you have it going too Will, even though I was not of any help. Your comment about a “floating” power supply was enough get me thinking in that direction.

About the rectifier. My old tube manual copyrighted in 1940 by RCA, shows all specifications for these two tubes as being exactly the same, including the pin out. My later manual copyrighted in 1975 shows that a 5Y3GT has a maximum current rating of 125mA at 380 volts, and 150mA at 245 volts. The 5Z4 was obsolete at this time and the RCA tube manual says to use a 5Y3GT for a replacement.

Sorry everybody. I was thinking 5Y4, not 5Z4. Not sure how I did that. The 5Z4 is interchangable with the 5Y3 like Billy said. The 5V4 is also an interesting tube that I have sometimes used as a swap for the 5Y3. If you are one of those who prefers the radios with the center tapped rectifier filament winding, both the 5Z4 and the 5V4 give you the next best thing. The problem with using a filament as a cathode is that filament voltage is added and subtracted from the cathode voltage unless the cathode is tapped from some central point. Since the cathode in the 5Z4 and 5V4 tubes is not part of the filament circuit other than being connected to one side (it handles no filament current), the cathode is pretty much neutral with respect to the filament voltage--filament current has no effect on cathode current as long as cathode current is drawn directly off of the cathode and not through the cathode heater. As most of us know, though, with proper filtering, center tapping the filament winding of a filamentary rectifier is not necessary.

Sorry for the mistake before.

Thomas

I'm glad you found the short! Have fun with the tubes. I only recently came back to tubes after a career in digital electronics. I learned tubes as a kid when I got my ham license. What I am really lacking is a knowledge of the ins and outs of the various manufacturers and what they did in their designs. Reading this forum and others is a great way to pick up on the details. There are some real old-timers on here and on other forums that have a great deal to offer and are willing to do so.

Hi All,

Have to put my two cents in here since I actually saw some of the pre-transistor/solid state computers. Yeah, I'm fairly old in the electronics realm. Some of the first WWII/1950 era computers (Eniac comes to mind as well as some of the original Lincoln Lab analog and digital experiments) had vacuum tubes as their switches. By the time they got a few K worth of registers and some level of memory (again only in the few K worth) you needed a cooling system of the gigantic kind.

Take a look at Eniac's web site and realize what a miracle the modern PC is in such a short time.
http://www.library.upenn.edu/exhibits/rbm/mauchly/jwmintro.html.

You'll be hard pressed to believe that's how it all started. (19,000 tubes). All basically started to compute shell trajectories for blasting the hell out of the enemy.

In addition, for the younger members, I remember when the ARPANET first got started. It was the predecessor to today's Internet started by the Dept. of Defense for communication and idea sharing between them and the various research facilities (mostly colleges) for weapons development.

Recommend seeing where all of our current revolution started. Truly an eyeopener.

Rick

:As far as I can see, I think that C1, next to the 6A8 tube (in the schematic) in the antenna circuit, is one of the two gangs of your tuning condenser, and C1, connected to the oscillator circuit under the 6A8 tube (in the schematic), are the two gangs of your tuning condenser. They are both connected to the chassis, which means that the frame of your tuning condenser is connected to the chassis. Tuning condensers, whether connected directly to the chassis or not, are commonly isolated on rubber bushings. This dampens out vibration that might otherwise cause frequency modulation feedback. This happens when the speaker plays a really loud passage and then vibrates the condenser plates. This changes the frequency that is tuned in, and causes a fluctuation in the radio circuits, which then passes on to the speaker again, and it howls like a microphone placed too close to the speaker. This problem isn't common, but manufacturers often isolated the condenser anyway. New rubber bushings can be found at your hardware store. Purchase many. This way some can be placed in the original holes. Then others can be cut in half and used as shims, if necessary, for putting the condenser back to its original height.
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:Have fun with tubes. They are a lot of fun, and are easy to understand. I have heard, though, that people who start off with solid state technology, have trouble with tubes sometimes. Tubes are amazing devices, capable of far more than most people to-day realize. They are more sensitive than any other device of similar nature (transistor), and are faster responding than any other similar device. Unfortunately they aren't extremely stable (emission dies off with age), and they take up a lot of space. Though they could make computers faster than they are, the computers would take up huge buildings. Noone knows how to put a tube in a chip. They make radios sound really good, though, and will cook your food really fast. Your radio, though it will likely sound great, won't have the best sound, since it doesn't have push-pull output. Push-pull output is used on all fine radios, both modern and old. Most car stereos and high quality home stereos use push-pull output. Push-pull output, by nature, cancels a lot of distortion in audio. These distortions may either be in the form of clipped off audio waves or in the form of non-uniform frequency response. Push-pull is deep and rich, and of full range. A properly designed singe phase output can sound really nice, though.
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:Thomas

Amazing. Love it. I remember reading an article back in 1997, when I was a senior in high school. It talked about the future potential in vacuum tubes. It talked about how a radio tube, due to its purely electrical operation (electrons flow in a vacuum instead of a solid), is capable of computing faster than the transistor (for those of you who don't know, microchips are simply devices which hold a bunch of transistorized circuits--transistors, condensers, resistors, and the like). The problem with the radio tube is its size and that it cannot be made as microscopic as modern transistors due to the need for a filament. Some thought was being given to cold cathode tubes. Studies were being done and some success was being made with microchip tube circuits. Still, even with modern innovations, the current tube designs would still make the desktop computer much larger than it is when transistors are used.

A problem I see with tubes is fading emission. Perhaps with the digital on and off sequences (0s and 1s), this wouldn't matter, as digital coding only sees on or off. It doesn't see variables. I don't see how tubes can be made as small as the components found within a microchip. It's an interesting idea, though, and it'd be totally cool if tubes were used in computers. Since 1997 I haven't seen any new tube type computers, so we'll see.

Thomas

I ran into a similar problem with another radio. I assume that you are seing the 256v volts on the chassis with respect to AC ground? What is the voltage when measuring AC? When I had this issue, it was a short in the power switch. Since the power switch is in only one line, (the other line goes directly to the power transformer) if the line cords polarity is such that that the neutral side falls on the side of the switch the unit would not come up, the other polarity would let the set play, but with a very loud hum.

To quickly check, measure continuity from each line cord prong, to chassis. Make a measurement with the power switch on and off. There should be virtually no leakage to the chassis on the 5S56 (1meg or better).

This should help troubleshoot a bit.
Good luck

donnie

:I've recapped it, replaced three missing tubes and repaired cracks in the speaker cone. It plays really well on all three bands. I did install the electrolytics correctly (not connected to chassis ground). However I can meter 256 volts DC on the chassis. If I ground the chassis it goes dead with the power plug in one way, and a very loud 60hz hum with it reversed. The 256 volts is present everywhere on the chassis, tuning capacitor, control shafts, etc. I've just started trouble shooting it. Does anyone have experience with this kind of problem?