| :::::::::::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 :::::::::: :::::::::Radiodoc, :::::::::I have continuity from the LARGE pins on the 80 tube rectifier to the speaker and have continuity of the speaker. ::::::::: :::::::::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 :::::: ::::::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 :::: ::::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, : :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 |
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