I have an RCA Radiola model 76ZX11 the has a few problems. It comes in with weak volume and distorted audio. When powered up, the rectifier tube will glow very bright and blow after awhile. I have recapped the radio and changed a few of the resistors. What could be the problem?

Steve

:I have an RCA Radiola model 76ZX11 the has a few problems. It comes in with weak volume and distorted audio. When powered up, the rectifier tube will glow very bright and blow after awhile. I have recapped the radio and changed a few of the resistors. What could be the problem?
:
:Steve

Looking at the schematic i would think the dial lamp would have blown before the rectifier. Double check your capacitors for polarity, voltage rating and capacitence. Is the Dial lamp been bypassed? Usualy they use this as a saftey ( like a fuse) to protect the rectifier from shorts. Some one correct me if i'm wrong But i think that it why the used them.

Sorry, I forgot, it also blows the dial lamp. Thanks.

::I have an RCA Radiola model 76ZX11 the has a few problems. It comes in with weak volume and distorted audio. When powered up, the rectifier tube will glow very bright and blow after awhile. I have recapped the radio and changed a few of the resistors. What could be the problem?
::
::Steve
:
:
:Looking at the schematic i would think the dial lamp would have blown before the rectifier. Double check your capacitors for polarity, voltage rating and capacitence. Is the Dial lamp been bypassed? Usualy they use this as a saftey ( like a fuse) to protect the rectifier from shorts. Some one correct me if i'm wrong But i think that it why the used them.

OK, that helps. Usually, that's caused by half of your rectifier heater open. But you've replaced the tube?

If both sections of the rectifier heater are OK, then something putting too much voltage across the rectifier, which is hard for me to understand. Just for grins, double check C23.

This is a post-war AC-DC set with a 35Z5 rectifier tube, which has a tapped heater for the pilot lamp.

Steve, when you say the recifier is glowing brightly, are you talking about the heater or the plate? The plate shouldn't glow at all. The heater would glow brightly if the voltage across it is, somehow, too high. Measure the voltage.

If it's the plate that is glowing, then something is shorting your B+. The usual suspects would be the filter caps, which you say that you replaced?

If the pilot lamp is open, the rectifier filament will glow a little brighter than usual, but that wouldn't produce all the symptoms you describe.

The plate glows brightly. I will double check c23 like Doug suggested.

:This is a post-war AC-DC set with a 35Z5 rectifier tube, which has a tapped heater for the pilot lamp.
:
:Steve, when you say the recifier is glowing brightly, are you talking about the heater or the plate? The plate shouldn't glow at all. The heater would glow brightly if the voltage across it is, somehow, too high. Measure the voltage.
:
:If it's the plate that is glowing, then something is shorting your B+. The usual suspects would be the filter caps, which you say that you replaced?
:
:If the pilot lamp is open, the rectifier filament will glow a little brighter than usual, but that wouldn't produce all the symptoms you describe.

Hi Steve

Be sure C20, 30 mfd and C21, 50 mfd have been replaced with 150 volts or greater rating. Polarity must be correct. Plus toward 35Z5 cathode, pin #8.

http://www.nostalgiaair.org/PagesByModel/130/M0015130.pdf

C23 as Doug mentioned could be leaky?

This radio uses a 12J5GT for oscillator. There is no protection for this tube. Plate is connected to B+ and cathode through a low resistance coil to ground. If something goes wrong with this circuit B+ can be shorted.

Your 35L6GT may be drawing too much current? Pin #5 must be around 5 volts negative in relation to pin #8.

Operate you radio off a variac or 40 watt series lamp. This can save you from burning out 35Z5GT's.

Norm

:The plate glows brightly. I will double check c23 like Doug suggested.
:
::This is a post-war AC-DC set with a 35Z5 rectifier tube, which has a tapped heater for the pilot lamp.
::
::Steve, when you say the recifier is glowing brightly, are you talking about the heater or the plate? The plate shouldn't glow at all. The heater would glow brightly if the voltage across it is, somehow, too high. Measure the voltage.
::
::If it's the plate that is glowing, then something is shorting your B+. The usual suspects would be the filter caps, which you say that you replaced?
::
::If the pilot lamp is open, the rectifier filament will glow a little brighter than usual, but that wouldn't produce all the symptoms you describe.

The plate glows brightly?
Very odd. Try disconnecting the entire B+ and filter circuit load from the cathode just to make sure that that's it.
If it still "blows"... then..
To my mind... without think'n on it too hard..that doesn't seem to agree with pilot lamp failure.
so...It's my wild guess then..that you may have a short in the filament string somewhere.

If the plate indeed glows, usually it'll glow a dim red. The pilot lamp and its associated shunt (part of the 35Z5 filament) should blow first as everyone on here seems to agree. If you have a short in the B circuit of any type (usually electrolytics or some other condenser), first the pilot lamp will blow because it is delicate. Then its associated shunt, the small section of the 35Z5 filament, will glow brightly and burn out. This section is in series with both the rest of the filaments and the plate of the rectifier, which is part of the B circuit. If the filament is what is glowing brightly before it burns out, then what I have just described to you is happening. This may also cause the cathode to glow brightly (small cylinder around the filament). Do not confuse this with the plate, which has a cylindrical part in the middle which surrounds the cathode, but is otherwise usually a large flat piece of metal, which is also usually black or gray in color.

Thomas

Like Thomas says, we really need to figure out whether it's the plate or the heater that is glowing brightly. Two horses of a different color.

Thanks for all the advice. I am looking at C23. Could someone please tell me if I am reading the schematic right. It appears to me that the C23 should have one end directly connected to pin 8 and pin 5.

:Like Thomas says, we really need to figure out whether it's the plate or the heater that is glowing brightly. Two horses of a different color.

Yes, Steve: One side of C23 goes to Pin 8. The other side is connected to Pins 3, 5, and the pilot lamp.

Please understand that the actual physical wiring may not be the same as shown on the schematic. If you're uncertain, use you ohmmeter to verify what is connected to where.

Have you replaced the electrolytic filter caps, C20 and C21, and verified their correct polarity? This needs to be done.

Did this set work before you started working on it?

More: if the set worked before you started working on it, then I would tend to suspect that you introduced a wiring error. In which case, you'd need to verify the wiring versus the schematic by tracing with colored pencils.

Not to add more info to the pile, but if you're wondering what C23 is, it is a sort of RF filter which filters out interference that would otherwise get into the radio from the line cord. I do not want to confuse matters worse, so I'll leave explanations out for now. If this condenser did short, it would blow the small portion of the rectifier filament.

Remember that when you test a condenser of any kind, you must disconnect it from the circuit which it is in. Take a reading with the most sensitive resistance scale on your meter. Condensers should have ABSOLUTELY NO leakage, though a small amount of leakage is probably not the cause of your rectifier failure.

Thomas

Steve:I'd like to know if it ever worked too...
But (try this)...to narrow things down very quickly and easily. Just temporarily unsolder ANYTHING connected to pin 8 of the 35z5 tube.
This will completely and easily remove the entire B+ and associated filter section completely.
Then turn it on, and if anything blows... it cannot be the plate-cathode of the 35z5.... if anything blows it can only be in the filament string!

If nothing blows then you can begin to troubleshoot and/or replace the suspicious electrolytic caps in the filter or at least start localizing the suspect component or mis-wire.

Steve, this comment is nothing for you to concern yourself with - ignore me!

Thomas, etc: another thing about the connection between the 35Z5 cathode and the lamp tap on the heater: it adds plate current to the lamp circuit. This increases the lamp's brilliance and is what causes the pilot lamp's brilliance to flicker with strong signals - as the rectifier plate current increases, so does the lamp's.

Exactly! It's like a 4 way benefit program or something....not necessarily 4, but multi.

It seems to me that it's rather difficult for some to grasp all that goes on here at first.

Thomas

Yeah, you're correct. A lot of things going on around that rectifier tube.

I understood from a recent text ( I think the AA5 book)that the tap conects to the B+ to add that additional 30-50ma to balance the current through that section of filament

here's an excerpt from the text at "funwithtubes.com"
which details the current balancing issue in the 35z5 tap.
---
"The tube manufacturer had provided a tap on the heater of the 35Z5/35W4to operate a 6 volt light for illumination of the dial. But the lamp filament also needed .15 amps so it just couldn't be connected to the six volt tap The voltage would drop way down and part of the heater would be under heated while the rest would be over heated. The currents were equalized by having the B+ current flowing through the parallel combination of the lamp and portion of the heater. So the plate of the 35Z5/35W4 connects to the tap as shown in the figure. All of the tubes had been designed so the B+ current would be right for the lamp. So...no transformer was needed to operate a dial light.

But why didn't they just connect the lamp filament in series with the tube heater string? The overall error in voltage would have only been a little more than 5 percent.

Good question. If the dial light burned out the radio would go dead. But the main reason was the warm up time of the tubes as compared to the warm up time of the lamp. The resistance of the tube heaters and the lamp filament are very low at room temperature. The initial current is very large and falls off as things heat up. The lamp heats up much faster than the tube heaters so right after turn on the lamp would have the entire line voltage across it. It would go off like a flash bulb and then be dead. This warm up time accounts for the way a dial lamp behaves. At turn on the current through the heater string is very large maybe as high as 2 amps. This will supply the normal 6 volts to the dial lamp and it is bright at first but quickly dims down as the tube heaters come up to operating temperature and draw less current making less current available to the dial lamp. Then the tube cathodes begin to emit electrons and the plates begin to draw current from the B+ supply. As this current comes up to normal the lamp comes back to normal brightness. If the dial lamp burns out the voltage on part of the rectifier's heater will be too high but the radio will operate for many months before the tube finally burns out. Older models often have resistors across the lamp and the other part of the heater to minimize the effect of a burned out dial lamp. Later ones eliminated these resistors to save money figuring the owner would have to pay the cost of replacing the tube and it was no skin off the nose of the manufacturer.

Moral; if you are using an AA5 and the dial lamp burns out, turn it off and replace the dial lamp at once. No one is making any new 35Z5s or 35W4s so don't waste a scarce 6 dollar tube for a 50 cent light bulb.

Shame that noone is making 35Z5 tubes. My saying this may drive the eBay prices for these tubes sky high. They're already getting a little more expensive.

What's interesting, too, about the lamp tap system, is that the lamp shunt portion of the 35Z5 is also very low in resistance when the tubes are all cool (it, along with all the other tube filaments, dramatically decreases in resistance when it's cold). This bridges extra current around the bulb during turn-on, protecting the bulb even more. As the shunt warms up it increases in resistance and allows more current to flow through the bulb (since less current will flow through the shunt due to its increased resistance). Of course when B current starts conducting, this also increases bulb brilliance.

Take note of the resistance change, though, in the lamp-shunt portion of the 35Z5 filament. If you were to use a resistor equal to the hot resistance of the shunt, it would stay this resistance at all times, and would not offer as much protection to the pilot lamp. If you've ever owned a radio where a 47 or 50 ohm resistor provides the shunt for the pilot lamp, you know that the pilot lamp glows much more brightly during cold turn-on. If this radio also uses a solid state rectifier, this dramaticizes the problem even more because the rectifier will start drawing B current for the electrolytics instantly (when they charge up).

Thomas

Thomas

:Shame that noone is making 35Z5 tubes. My saying this may drive the eBay prices for these tubes sky high. They're already getting a little more expensive.
:
:What's interesting, too, about the lamp tap system, is that the lamp shunt portion of the 35Z5 is also very low in resistance when the tubes are all cool (it, along with all the other tube filaments, dramatically decreases in resistance when it's cold). This bridges extra current around the bulb during turn-on, protecting the bulb even more. As the shunt warms up it increases in resistance and allows more current to flow through the bulb (since less current will flow through the shunt due to its increased resistance). Of course when B current starts conducting, this also increases bulb brilliance.
:
:Take note of the resistance change, though, in the lamp-shunt portion of the 35Z5 filament. If you were to use a resistor equal to the hot resistance of the shunt, it would stay this resistance at all times, and would not offer as much protection to the pilot lamp. If you've ever owned a radio where a 47 or 50 ohm resistor provides the shunt for the pilot lamp, you know that the pilot lamp glows much more brightly during cold turn-on. If this radio also uses a solid state rectifier, this dramaticizes the problem even more because the rectifier will start drawing B current for the electrolytics instantly (when they charge up).
:
:Thomas
:
:Thomas

Gentlemen,

Wonder why a #51 lamp is called for in the parts list for this radio instead of a #47 lamp for the dial lamp.

Radiodoc

I don't really know... the #47 and the #51 are both able to handle from 6 to 8 volts.. but the #47 is .15a and the #51 is .2a
So essentialy the 51 draws an additional 50ma to achieve the same briteness... however not studying the schematic...I can't imgine that the circuit is that critical.

:Gentlemen,
:
:Wonder why a #51 lamp is called for in the parts list for this radio instead of a #47 lamp for the dial lamp.
:
:Radiodoc
:

There is only so much wattage available across the 35Z5 lamp shunt, though. This may be consumed in any way that will equal this wattage. A lower voltage bulb with slightly higher amperage will work, or a higher voltage (to a point) lower amperage (lower than .15 amperes) bulb will work. Using a 6 volt bulb that draws more than .15 amperes will cause the bulb to glow dimly or not at all. .5 amperes is not likely to light brightly.

Thomas

The radio did not work before I started working on it. I checked the electrolytics and the polarities are correct. I did splice them into old wires, as the leads would not reach otherwise. I wonder if this could be one of the problems?

Yes, Steve: One side of C23 goes to Pin 8. The other side is connected to Pins 3, 5, and the pilot lamp.
:
:Please understand that the actual physical wiring may not be the same as shown on the schematic. If you're uncertain, use you ohmmeter to verify what is connected to where.
:
:Have you replaced the electrolytic filter caps, C20 and C21, and verified their correct polarity? This needs to be done.
:
:Did this set work before you started working on it?

Well, I really think that Peter's idea is a great one. You should strongly consider disconnecting the B wiring from the cathode of the rectifier. Then power up the radio and see if anything blows.

If the radio uses rubber wiring, look for places where the wiring melted through. It should never rest against tube socket terminals. These get hot and will melt through the rubber. The 117Z6 tube in my Zenith 5-G-500, luckily not the original Zenith tube, exploded quite dramatically when one of the black rubber wires on its socket decided to melt and contact one of the socket terminals. The radio worked fine the night before, and I played it for an hour or so.

It is wise to disconnect each and every condenser in the radio and check it for shorts and leakage. Replace all condensers with any leakage, even if it is in the millions of ohms and keeps the needle lingering above infinity by only the thickness of a hair. Condensers hold charges. When you hook a condenser to your meter, the meter will swing up and then fall back down. The up-swing distance depends upon the size (capacity) of the condenser. Condensers smaller than .001 MFD will create no noticeable needle movement in your meter. Leaky condensers are not likely the cause of your rectifier explosion, but they cause other trouble like poor AVC performance and distortion in the audio. If you find a shorted condenser, though, this is probably the trouble.

If you search the radio over until you're blue in the face, disconnect all of the IF transformer wires. Make a careful note as to how they are connected before you disconnect them. Study your schematic and see how they are wired internally. Check for continuity on each coil. There should be continuity here. Check for continuity between the primary and secondary coils of each transformer. There SHOULDN"T be continuity here. There also SHOULDN"T be continuity between any of the wires and the transformer housing. If there is, something is touching the housing from the inside. Open and correct the trouble.

Do not be afraid to get into the radio and test every part.

Also, if you disconnect the B supply and the rectifier still blows up--the pilot lamp shunt, as well as the pilot lamp itself, consider looking for a short between the pilot lamp and its associated wiring, and the chassis or some other part. If the chassis is isolated from the line through a condenser, a short between the pilot lamp and the chassis shouldn't cause trouble. If the chassis is connected directly to the line, things will explode if the pilot lamp or its wiring touches the chassis.

Remember that when you work on an AC-DC radio, you should either be on an insulated surface, or you should use an isolation transformer. Carpeting in the living room works great, but others will object to little balls of solder stuck to the carpeting. Hot tubes of an inverted chassis also make marks in the carpeting. I know from first hand experience, and I know the reaction I get from my mother.

Thomas

Thanks all you guys for your great advice. I will probably tear into the radio later this week, as work has me busy. I will let you know if I am successful.

Thanks
Steve

:Well, I really think that Peter's idea is a great one. You should strongly consider disconnecting the B wiring from the cathode of the rectifier. Then power up the radio and see if anything blows.
:
:If the radio uses rubber wiring, look for places where the wiring melted through. It should never rest against tube socket terminals. These get hot and will melt through the rubber. The 117Z6 tube in my Zenith 5-G-500, luckily not the original Zenith tube, exploded quite dramatically when one of the black rubber wires on its socket decided to melt and contact one of the socket terminals. The radio worked fine the night before, and I played it for an hour or so.
:
:It is wise to disconnect each and every condenser in the radio and check it for shorts and leakage. Replace all condensers with any leakage, even if it is in the millions of ohms and keeps the needle lingering above infinity by only the thickness of a hair. Condensers hold charges. When you hook a condenser to your meter, the meter will swing up and then fall back down. The up-swing distance depends upon the size (capacity) of the condenser. Condensers smaller than .001 MFD will create no noticeable needle movement in your meter. Leaky condensers are not likely the cause of your rectifier explosion, but they cause other trouble like poor AVC performance and distortion in the audio. If you find a shorted condenser, though, this is probably the trouble.
:
:If you search the radio over until you're blue in the face, disconnect all of the IF transformer wires. Make a careful note as to how they are connected before you disconnect them. Study your schematic and see how they are wired internally. Check for continuity on each coil. There should be continuity here. Check for continuity between the primary and secondary coils of each transformer. There SHOULDN"T be continuity here. There also SHOULDN"T be continuity between any of the wires and the transformer housing. If there is, something is touching the housing from the inside. Open and correct the trouble.
:
:Do not be afraid to get into the radio and test every part.
:
:Also, if you disconnect the B supply and the rectifier still blows up--the pilot lamp shunt, as well as the pilot lamp itself, consider looking for a short between the pilot lamp and its associated wiring, and the chassis or some other part. If the chassis is isolated from the line through a condenser, a short between the pilot lamp and the chassis shouldn't cause trouble. If the chassis is connected directly to the line, things will explode if the pilot lamp or its wiring touches the chassis.
:
:Remember that when you work on an AC-DC radio, you should either be on an insulated surface, or you should use an isolation transformer. Carpeting in the living room works great, but others will object to little balls of solder stuck to the carpeting. Hot tubes of an inverted chassis also make marks in the carpeting. I know from first hand experience, and I know the reaction I get from my mother.
:
:Thomas