I suspect the transformers that generate these voltages or a "common" feed to these transformers.
I would appreictae any information that anyone can share. I have .pdf files of the chassis showing graphics as well as a schematic.
Grigsby-Grunow Co.
Model: 70B
Power:
Chassis:
Circuit:
Year: Pre 1932
IF:
Tubes:
Bands:
Resources
Riders Volume 1 - MAJESTIC 1-5
Riders Volume 1 - MAJESTIC 1-10
Riders Volume 1 - MAJESTIC 1-24
Courtesy Nostalgia Air
Courtesy Nostalgia Air
Courtesy Nostalgia Air
Courtesy Nostalgia Air
I am persistent and not ready to give up on bringing this radio back to life.
Thanks in advance for all of your help.
Terry
These higher voltages come from the 80 rectifier rather than direct from your power transformer.
http://www.nostalgiaair.org/PagesByModel/002/M0040002.pdf
Check voltage on one of the large pins of your 80 (UX280) tube. From there voltage goes through a 400 ohm choke. Is it open?
Norm
:I have a Majestic radio model 70B with 7P6 power supply. The voltages are present for the 5Vpower, 1 1/2 V A.F, 2 1/2 det and 1 1/2 RF, but no +50, +96 or +260. All of the tubes light up and the radio is in very good shape (the old wiring has been replaced).
:
:I suspect the transformers that generate these voltages or a "common" feed to these transformers.
:
:I would appreictae any information that anyone can share. I have .pdf files of the chassis showing graphics as well as a schematic.
:
:Grigsby-Grunow Co.
:Model: 70B
:Power:
:Chassis:
:Circuit:
:Year: Pre 1932
:IF:
:Tubes:
:Bands:
:Resources
:Riders Volume 1 - MAJESTIC 1-5
:Riders Volume 1 - MAJESTIC 1-10
:Riders Volume 1 - MAJESTIC 1-24
:Courtesy Nostalgia Air
:Courtesy Nostalgia Air
:Courtesy Nostalgia Air
:Courtesy Nostalgia Air
:
:I am persistent and not ready to give up on bringing this radio back to life.
:
:Thanks in advance for all of your help.
:
:Terry
Hi Terry,
One of the first things I would check would be continuity of the speaker field winding. If it is open you will not get any +B voltages.
Radiodoc
I should have said in my previous post that you would not get the +96 or +50 volts if the field coil was open. Norm had the better idea in his post about checking the plus voltage on the rectifier and checking for continuity through the 400 ohm choke.
Radiodoc
When I measure the AC voltage at the large pins of the 80 tube I get 3.8V and 356V at the small pins. Is this 400 ohm resistor in the bottom part of the power supply?
Any help woul be appreciated.
TF
Hi Terry,
The AC voltage on the large (filament) pins of the 80 should be 5 volts. You said you had continuity from the large pins to the speaker (this goes through the filter choke). Was it around 400 ohms as the PS schematic shows? Have you changed the filter capacitors A,B,C & D? If the filters are original they are probably bad, shorted even. With the power off check for continuity between the small pins of the 80 tube and the black (ground) wire from the power supply. Measure for plus voltage between one of the big pins of the 80 to the black (ground) lead to see if you have voltage.
Radiodoc
Radiodoc,
I have 378 ohms from the speaker to the large pins of the 80 rect. No, I have not replaced the caps, but I ordered them today. I have continuity from the small pins to the ground ( I assume it is the top connection of the 'condensor bank" as shown on the 7P6 power supply (I traced the "neg" connection on the terminal strip back to this connection as reference. If this is not correct, please let me know.
I do not have any voltage from the big pins of the 80 to the negative (ground) lead.
I hope this helps you.
TF
:
Terry,
With filament voltage on the 80 filament pins and high AC on the 80 plates there should be a plus voltage on one of the big pins to ground. It is possible, since you have not changed them, A or B capacitor or both are shorted and possibly damaged the 80. Just for the heck of it you may want to check the resistance between either of the big pins to ground (black wire). The resistance should be fairly high. May want to do the same from the red wire to the black wire. If you can locate the wires from the A and B capacitors, may want to disconnect these and see if you have plus voltage on the big pins of the 80.
Radiodoc
No + voltage from the big pins of the 80 to ground. The 80 has continuity on the big pins. The small pin voltage is now 715V AC (I didn't do anything either) just remeasured the voltage on the small pins. The big pin voltage is now dropped to <1.0 volt.
The resistance of the big pins to ground is 230 ohms. I disconnected the A and B capacitors and still do not have + voltage on the big pins to ground.
Is the 80 the culprit? I am at a loss.....
As always, any help is grealy appreciated.
Terry
Hi Terry,
You may not have had a good connection with the meter probes. Between the black lead (center tap of the HV AC winding you should measure approximately half of the 715 volts or 357 VAC. The 80 could be bad. If you know someone who has a tester may check it. There is the possibility the socket is bad. Measure the voltage on the big pins of the 80 in the socket and measure the voltage on the socket wiring tabs. I have had radios with bad sockets sometimes. If the voltage is the same, try removing the 80 and measure the AC voltage on the big pins and see if it measures around 5 volts. I'm going to study the schematic for the power supply again. I am concerned about the 230 ohms from the big pins to ground as this is too low. Please be very careful testing as the voltages in tube type power supplies can be lethal! I use the one-hand-behind-my-back approach.
Radiodoc
I discovered a loose wire from the large pins to the plate trans. It is now reconnected.
I removed the 80 and measured the pins in the socket. 5V AC on the large pins - pin to pin and 716V AC on the small pins - pin to pin.
At one point, the resistance from the large pins to ground was 412 ohms, but now it is back to the previous reading. The shock of my life was last night BFORE you sent me the warning. I am still here but wiser.
As always, any help is greatly appreciated.
Terry
Hi again Terry,
The 230 ohms from the big pins of the 80 to ground still bugs me. With the radio unpluged try disconnecting the terminal strip coming from the radio from the power supply and with the 80 out of its socket and caps A & B disconnected, measure the resistance from the big pins of the 80 to ground. The resistance should be high. This pretty much isolates everything from ground.
Radiodoc
Radiodoc,
330 ohms is the best I can get.
Terry
:
Radiodoc,
Here is some news. After I checked the resistance as you suggested, I put the power strip back on. I powered up the radio and I got a distintive hum coming from the speaker. I attempted to tune in a station. Again, I got close but no cigar.
I checked for loose wires and I think I have one from the terminal strip up to the chassis. I will check out the wires and will advise tomorrow. I am encouraged by the progress.
Terry
:
Terry,
Getting some hum from the radio is encouring. What I was attempting to do was isolate everything from the plus voltage path that could load it down.. I was going to suggest a 10K about 10 watt resistor from the red wire to the black wire for a load on the plus supply with the radio completely disconnected. Getting the power supply working right and then worry about the possible problems with the radio.
Radiodoc
Radiodoc,
I was only able to get 2-6.8K resistors +/- 10% at 15 watt. I have them soldered in series (13.7K ohms) and before I solder them to the red and black wire, I WANT TO BE SURE that I am looking at the same two locations on the power supply wiring diagram that you are.
Since I rewired the device, I went from wire to wire location by location, so what you call a black may be a red to me. I wasn't worried about color as I should have been. Stupid move on my part.
On the plate transformer, I interpret these wires to be the top two terminals on the left. Please advise if this is correct before I solder the resistor bank to the power supply.
This may seem dumb, but .....I have to ask. You are considerably more knowledeable than I am.
As always, your help (and patience) is greatly appreciated for helping me step through this problem.
Terry
:
Hi Terry,
I hope I have not confused you too much. I had been looking at schematics for this power supply in two different books. It dawned on me you were probably using the schematic from this site so I decided to download it and print it out since is showed the physical wiring too. Might know it! My printer in the shop let me know the toner was out...the printout showed this. I will have to take the file home and print out. It also dawned on me that the wire colors may have faded out. The plus voltage point is the +220V and the ground is either the junction between the green (2170 Ohm) resistor and brown (1000 Ohm) resistor on the old wiring or the junction between the blue (4180 Ohm) resistor and the brown (1000 Ohm) resistor. I just wanted some sort of load on the +220 volt point while testing the power supply with the radio chassis disconnected. When I get a printout of the wiring diagram that I can read I will get back to you.
Radiodoc
Radiodoc,
The 4180 resistor measures only 348 ohms. I will try to get a functional replacement tomorrow and will advise. The 15000 ohm and the 1000 ohm resistors on the power supply top measure OK.
Terry
:
Hello Terry,
To get close to the 4180 you can use resistors in parallel or in series. In series the values of the resistors will add and in parallel the total will be somewhere less than the smallest value. Another reason for putting a load on the power supply when testing it disconnected from the radio chassis is that capacitors can retain a charge after the power is removed unless there is a load to drain the charge off them. You can get a nasty shock. Just be careful and most of all don't get discouraged.
Radiodoc
Thomas
Radiodoc,
I decided to retrace my steps since I found a loose connection.
From the big pin to ground is now 93VDC and the resistance from the big pin to ground is 390 ohms.
Just for your information.
Terry
:
Hi Terry,
Over the weekend is looked the schematic over and was trying to figure out where the 230 ohm or so could be coming from. Your latest post you stated you measured 390 ohms from the big pins to ground. This is almost the value of the choke coil. With the caps, receiver chassis disconnected and our 10K load resistor removed, measure the resistance at the 80 big pins to ground and the +220v position to ground and see what the resistance is. We may have a shorted winding in the choke to ground or a wire from the choke to the +220 volt point shorted to ground.
Radiodoc
Radiodoc,
From the big pins of the 80 to ground is 367 ohms. From the 220V position to ground is 2.1 ohms.
Terry
:
Terry,
I know the 2.1 Ohms is not right as is the 367 Ohms. If you can, disconnect the leads of the choke. This will isolate it from the circuit and then measure the resistance from either lead to ground. A low resistance reading will indicate the choke coil is shorted to the metal frame of the choke. If the reading is infinite on both choke wires, will have to look for a short in the +220 volt wiring from the choke.
Radiodoc
Radiodoc,
I disconnected both wires from the choke Each wire is infinite to ground. Choke wire to choke wire is 368 ohms.
Terry
OK Terry,
I bet the resistance reading (I hope) from the 80 big pins to ground should be high now. I also bet there is a short in the wiring from the choke to the +220 point. That would result in our measuring 300 plus Ohms before. Could be a pinched or cut wire there or insulation off the wire letting it get against ground.
Radiodoc
Radiodoc,
If I remember, the resistance from the big pins to ground was infinite when I took the choke wires off to verify the resistance of the choke. I'll let you know tonight.The choke wires are still disconnected. Let me know if I need to solder them back in place.
Terry
:
Hi Terry,
If I remember correctly you said you measured 2.1 Ohms on the +220 volt point. The resistance there should be high. You can resolder the wire to the input to the choke. I would leave the wire from the choke to the +220 volts disconnected until what is causing the 2.1 Ohms reading is found. The best I can tell from the wiring diagram is a wire goes from the choke to a capacitor (you did disconnect this cap) in the capacitor bank and then a wire from there to the +220 volt contact where the radio cable connects.
Radiodoc
Radiodoc,
I will reconnect the right hand tap of the choke to the condesnor bank tap (third from bottom) tonight. This will leave the center tap of the choke disconnected.
I want to make sure that when I say ground (it is the top connection on the condensor bank.) This is where I measured the 2.1 ohms from the 220V pin to.
I looked for pinched wires, contact with the outer case and wires last night. I couldn't find anything. Could a poor solder joint give me a resistance reading that was false? I am fishing here...
Terry
:
Terry,
We will get this hashed out sooner or later. Generally a poor solder joint will give a high reading or intermittent reading. With the cap bank disconnected from the right tap on the choke and the right choke tap hooked back where it belongs try connecting our 10K load resistor from the center tap of the choke to ground and try powering up the power supply and CAREFULLY measure for any DC voltage across the load resistor. I keep a handy supply or clip leads on hand just for the purpose of testing...cuts down on soldering when experimenting.
Radiodoc
Radiodoc,
Here goes:
Both caps are disconnected as per previous instructions. I hooked the right choke wire back to the condensor bank (3rd from the bottom). I connected the 10K resistor bank from the center tap of the choke to ground then I powered up the unit. and found 1.6 VDC from the load resistor to ground.
Ground to neg pin = 2.2 ohms
Ground to 96 V pin = 407 ohms
Ground to 50V pin = 1746 ohms
Ground to 220 V pin = 2.2 ohms
Big pins to ground = 9.3 ohms
That's about it.
Terry
:
Somehow we have backed up. I have printed out our postings and will study the events up to this point and try to figure out what is going on.
Radiodoc
With all condensers removed, you should be able to check all of the resistors without removing them, save those which connect from the resistor bank to B-. As I said before, it is very rare that resistors decrease in value. These decreases in value lead me to believe that condensers are shorted and are causing these low values. Resistance checks, when dealing with old condensers, which more likely than not are leaky, cannot be accurately made until all condensers are removed. It is very bad practice to turn on an old radio and make voltage checks without first checking the components. It is extremely bad practice to assume that all of the condensers in such an old radio are still good. All of the condensers and resistors should be checked individually, one at a time, by removing them from the circuit. Only after all bad components have been replaced should the radio be fired up. Until then you are only asking for trouble if you try to find the problem by using voltage checks. You may damage the power supply further. The only voltage checks that should ever be made on a radio prior to examining components as stated exactly above, are AC voltage checks. These are made with all tubes removed. Voltage is tested at the rectifier plate pins, filament pins, and at all other tube filament pins. Only after all components have been tested and bad ones replaced, should DC voltage checks be made after carefully powering up the radio for the first time with all tubes in place. As I said before, firing up a radio in unknown electrical condition puts terrible stress on parts when some are malfunctioning and you don't know which ones are malfunctioning.
Also, unless the wiring is all rubber and is falling apart, you don't usually need to replace all of it. From my experience, some Majestics use rubber wiring even at this early time (rubber wire is often dried up and terrible to work with), but otherwise cloth wire is often encountered. Cloth wire doesn't usually fall apart unless the radio has been baked in an attic, soaked in water, or invaded by mice.
Thomas
Thomas,
I removed all the wires and checked the condensors. I have readings, but how do iIknow what they should be?
I also checked the resistors and found the 4180 ohm to be 7.26K ohm. I replaced it with a series set up and now have 4.3K ohms. The other two were right 1.048K and 17.4K ohms
Terry
:With all condensers removed, you should be able to check all of the resistors without removing them, save those which connect from the resistor bank to B-. As I said before, it is very rare that resistors decrease in value. These decreases in value lead me to believe that condensers are shorted and are causing these low values. Resistance checks, when dealing with old condensers, which more likely than not are leaky, cannot be accurately made until all condensers are removed. It is very bad practice to turn on an old radio and make voltage checks without first checking the components. It is extremely bad practice to assume that all of the condensers in such an old radio are still good. All of the condensers and resistors should be checked individually, one at a time, by removing them from the circuit. Only after all bad components have been replaced should the radio be fired up. Until then you are only asking for trouble if you try to find the problem by using voltage checks. You may damage the power supply further. The only voltage checks that should ever be made on a radio prior to examining components as stated exactly above, are AC voltage checks. These are made with all tubes removed. Voltage is tested at the rectifier plate pins, filament pins, and at all other tube filament pins. Only after all components have been tested and bad ones replaced, should DC voltage checks be made after carefully powering up the radio for the first time with all tubes in place. As I said before, firing up a radio in unknown electrical condition puts terrible stress on parts when some are malfunctioning and you don't know which ones are malfunctioning.
:
:Also, unless the wiring is all rubber and is falling apart, you don't usually need to replace all of it. From my experience, some Majestics use rubber wiring even at this early time (rubber wire is often dried up and terrible to work with), but otherwise cloth wire is often encountered. Cloth wire doesn't usually fall apart unless the radio has been baked in an attic, soaked in water, or invaded by mice.
:
:Thomas
Another situation would be where a pump is supplying a circuit with water, but in surges. On the outlet side of the pump, you can place a narrow piece of pipe. Prior to this you can install regular sized pipe that runs from the outlet to the inlet. Install a rubber diaphram on this pipe so that no water can actually flow through. The smaller pipe will act like a resistor, limiting the maximum amount of current flow. This will allow the rubber diaphram to have a better chance at "charging up." The rubber diaphram will build up a "charge" each time the pump puts out a surge, and the diaphram will release this charge whenever the pump isn't surging. This will smooth out the surges so that you have more of a smooth flow of water. This is how the filter system in your power supply works. Actually, the explanation I gave prior to this one, how a rubber diaphram is used to pass impulses from one circuit to another, is kind of how your amplifier works--sending audio information from one stage to the next without allowing the direct current in one stage to interfere with the current in the next stage. Explaining this more thoroughly with water would require a paper drawing. I'm sick and kind of dizzy right now, so I'm not up to explaining things in too much detail.
Anyway, so now that you understand that condensers charge up and all that, you should have a better understanding about what you are looking at when you test them on your multi-meter. Whenever you are going to test a condenser, remove all of its leads from the circuit. If the unit only has two leads, then removal of one lead is all that is necessary. When you connect your multi-meter to a condenser, depending upon its capacity, the needle will swing up to a greater or lesser degree. Then it will fall back down. It rises up as current rushes into the condenser (from the meter) to charge it. Once the condenser becomes "full," the needle swings back down. Condensers of smaller capacity will swing the needle up less than those with greater capacity. Generally, any condenser rated at 4 MFD or higher should swing the needle all the way up, or almost all the way up to zero ohms when concerning values right at 4 MFD. Smaller values may not swing the needle up much at all (as you move on to smaller values, deflection of the needle becomes less and less). When dealing with condensers around .05 MFD or .01 MFD, the needle will swing up a very small amount--almost unnoticeable. With values smaller than .001 MFD, you may not see any up-swing at all. When testing electrolytic condensers (those with polarity), if the value is above 4 MFD and the needle doesn't swing all the way up to zero ohms, then you can assume that the condenser has dried up (electrolytics have liquid in them) and is kind of useless. Since most electrolytics are larger in value than 4 MFD, you can pretty much use this as a testing method for all electrolytics regarding whether they have dried up or not. With values right around 4 MFD, if you are unsure, compare with a new condenser of the same value. They should be very similar in how far the needle swings up and how long it takes to fall back down. Some electrolytics that have sat for a long time may make the needle take longer to fall back down. These are electrolytics that need to be "reformed." This is explained a bit later. The process is complicated when dealing with good condensers in new condition, but the explanation I give below is good for old ones that probably need to be replaced anyway.
Now, regarding leakage--that is, when a condenser has a partial or total short, you want to watch the needle as it swings back down. Watch it until it stops moving. When the needle takes a while to fall down, it may simply be that the condenser needs reforming. The number that the needle stops at is your leakage. For electrolytics, leakage with a number lower than 1 MEG ohm is unacceptable. If you have an electrolytic that has 1 MEG leakage, you can try it in the radio for a while and see if it gets better--"reforming the condenser" (with the radio in series with a 100 watt bulb). If it doesn't get better (go to 10 meg or higher) after an hour of usage, replace the electrolytic. For continual electrolytic use, any leakage with a value less than 10 MEG is unacceptable. This means that the condenser is breaking down (usually), and could soon fail, damaging your power supply.
Now, with all other condensers (non-electrolytic), you do not want ANY leakage at all. Regardless of whether or not you normally can see the needle swing up (with values lower than .001 MFD, for instance), you will see the needle swing up when there's leakage. If the needle swings up, it must swing back down. As I said before, some condensers may only swing the needle up the thickness of a hair. If the needle stays up even just the thickness of a hair, the condenser has unacceptable leakage. Tube circuits are high impedance circuits, and are EXTREMELY sensitive. Leakage in the 10s of millions of ohms is enough to throw off AVC voltage and is enough to cause horrible distortion in the audio.
When testing condensers, use the X10,000 scale on your meter (the most sensitive one). Do not touch both meter terminals or both condenser terminals with your fingers at the same time. The moisture in your hands is enough to give false readings--you detect leakage even though there isn't any, because current is actually flowing through your fingers.
Whenever you acquire any radio, you should always test every condenser in the radio. Many radio repairmen just replace all of the condensers to be on the safe side. Modern condensers are far more reliable than old condensers (save mica condensers and air insulated tuning condensers). You can do many things to preserve the appearance of the chassis of your radio. You can stash the new condensers within the housings of the old condensers, and then seal them with hot glue or some more appropriately colored substance. Metalized film condensers fit nicely within the cardboard tubes of old paper condensers. Metalized film or ceramic condensers fit well in the bakelite condenser blocks found in Philco radios. Improvize. With electrolytics, sometimes you can figure out how to open the old ones in an inconspicuous way and stash new units inside (with all leads of new units well dressed--tape or spaghetti, so that no shorts occur). In other cases, simply stashing new units under the chassis and leaving the old units on top of the chassis (if this is where they are mounted) unwired, is the only option, especially when you can't figure out how to open up the old units.
Thomas
With power supplies, it is wise to replace with same or slightly higher values. Never go lower. Actually you can go lower, but you must have a bit of experience with situations in which this is acceptable. With AC power supplies, it is wise to never go above 40 MFD unless higher values were originally used. Using values higher than 40 MFD can sometimes overload rectifier tubes during turn-on of the receiver, especially if the receiver has just been turned off (like a second ago). One rather sensitive rectifier tube is the 6X5. The 80 is less sensitive due to its filament cooling off almost as soon as power is turned off. With AC/DC type receivers (no power transformer, tube filaments strung in series), you can go higher, say as high as 60 MFD. I've gone as high as 80 MFD with the 35Z5 rectifier, but I don't recommend this. Such large condensers can overload rectifiers when the receiver is turned off and then back on when still warmed up.
Thomas
When you see bakelite units with colored dots, not all of these are condensers. Micamold made resistors in this form. The resistors will have three colored dots which are the resistance value. They may have a silver or gold dot in the corner, corresponding to the tolerance. If you find a unit with colored dots and the leakage tests out to be the same resistance as the numbers given by the colored dots, consult your schematic to see if it is in fact a resistor instead of a condenser. These resistors aren't too common, but you find them from time to time. Zenith used them a lot.
Thomas
All who have helped,
After building a new "condensor bank", replacing one resistor and much trouble shooting thanks to all of you, my radio started working tonight November 30, 2005. It received KMOX 1120AM here in St. Louis. I am very pleased that this fine piece is working. It is magnificent and a tribute the golden age of radio.
Again, thanks to all of you who helped me through the trouble shooting.
Terry Foushee
A long 70 foot wire should be used with the long wire antenna terminal on the rear of the set. A shorter, say 15 to 25 foot antenna will work for the short antenna terminal. Use which ever length is most practical for you. Run wire in a straight line. Never make turns or bends or have wire fold back on itself. Turns and bends cause inverse waves in the antenna, which cancel out signals and make the antenna inefficient. Suspend the wire outside. Fifteen feet is good. Higher is better, but not necessary. Too high and you attract lightening.
The variable coil you see all the way to the left of the schematic is your antenna coil. This coil has a big metal cup that moves over it or away from it, depending upon which way the antenna trimmer control knob is turned on the front of the radio. On my radio this is the right hand knob. You can use this to fine tune stations. You will have to adjust it as you tune over the dial, as it reacts differently for different portions of the dial. If you leave it in one position for one side of the dial, the other side of the dial may be completely dead. Play with this knob before ever suspecting radio frequency alignment issues within the radio--if you haven't alrady played around with it. Once you get acquainted with it, you should find that the radio performs quite well all over the dial.
The trimmer condensers on each of the tuning condensers can be tweaked (found in the big barn looking metal structure), but I don't recommend this if your radio functions fine. Your dial is most likely marked from 0 to 100 instead of in frequency markings. If you mess with the trimmers, you can throw things off since you don't really know where stations are supposed to fall. You could lose coverage at the high end of the dial if you tweak the trimmers in the wrong way. If you are having sensitivity issues, though, you can tune to a quiet station around 1400 KC. Align all of the trimmers for maximum loudness and selectivity (ability to differenciate between this and other stations).
Thomas