I am working on 49-1401 that has no audio output. All tubes test good. Capacitors replaced. Resistors test good. The schematic has a very nice troubleshooting chart which I have attached. Using a signal generator, I get no audio at test points A and E. I do get audio a point C, but not point D. A tried another 6AQ6 with no difference. The 10meg resistor is shown to go between pins 1-2. On my unit the 10meg go to pin 1 and pin 6 with a wire to pin3. Pins 2 and 3 tied together. The schematic is difficult to read clearly. Should pins 2-3 be tied together? Should the 10meg go to pin 2 via pins 6 and 3? Any help is appreciated. A photo of the wiring on tube 6AQ6 might help as well. Thanks
The 6AQ6 tube is wired correctly. Pins 2,3 and 6 are all connected together and connect to circuit negative common. So it does not matter if one end of the 10 meg connects to pin 2 or pin 6 as they are the same circuit wise.
Check and see if you have voltage on pin 7 of the 6AQ6. Should be around 50 to 75 VDC
Thanks for the reply. I’m not sure which test point to use. If I use test point B, which is the B-, the voltage at pin 7 is 190 volts DC. If I use the chassis as ground, the pin 7 voltage is 100 millivolts. I tested the 220k and it tests within 10% on the high side.
I know you said you tried another 6AQ6 but there still seems to be a problem with the operation of the tube itself.
If you measure 190V from pin 7 to B- point B, that indicates the tube is not conducting plate current at all. Since the 220K R202 is within tolerance, the voltage at pin 7 should be much lower than 190V because of the plate current drawn by the 6AQ6. The place current should produce a voltage drop across R202 of at least 100V. You can confirm by measuring across the terminals of R202. I would guess that you will see no voltage across R202.
Does the 6AQ6 filament light up? If there is a short to B- on the wire from pin 4 of the 6AQ6 to pin 3 (or 4) of the 12BE6, all the tubes except the 6AQ6 will light normally.
If the 6AQ6 does light up, I would still try some other tubes. You can substitute a 12AT6 or 12AV6 from another radio for the 6AQ6.Those are very common AA5 tubes, so you can take a known good one from a working radio to try out.
I was just looking at the schematic and it appears that the plate voltage at pin 7 of the 6AQ6 to B- should be around 110 volts if the tube is not conducting. If it's 190 there may be a problem in the power supply also.
Why don't we back up a stage and check the voltage between B- (the negative side of c102) and pins 3 and then 4 of the 35L6. There is no connection from the lower b+ to the higher b+ once it's it's passed r 101 and r102. 6AQ6 should be at 55volts and it's 190v. Seems like it would have to haul down a fair amount of current to get to 55v. I'm thinking the opt is open and the 35L6 isn't drawing much just the the screen grid. Lets have a look.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry, something is wrong that the supply voltage after the B+ dropping resistors is so high, but I don't think its the output transformer. Owner said he had speaker audio if he applied signal to the grid of the 35L6.
Even with 190V supply to the 6AQ6 plate, there appears to be no voltage drop across the 220K plate resistor? less than half a milliamp would give over a 100V drop. Something definitely wrong there.
If you have audio at point C but not point D, the 6AQ6 is working. Whether it's working WELL or not is another question. If it were not, you would have no audio anywhere because it could not detect a signal. Point C according to the schematic I have, is the control grid of the 6AQ5 output tube (actually, there must have been more than one run of this model, because the Riders shows a series string set.) The lack of audio at point D could just mean the sensitivity of whatever you are using to test is not good enough. Check for audio at pin 3 of the output tube. Meanwhile, I'm going to see if I can find the version of the model that you have.
Hi, Cu879,
To clarify, you state that you got Audio at Point C. Do you mean that you injected a signal at Point C and got audio out of the speaker? If that is the case, that proves the 35L6 is good. One does have to put quite a bit into the 35L6, like about 3V to get good volume.
One of the real puzzlers in this mystery is getting 190V on the 6AQ6 (Detector, 1st AF Amp) plate, when the plate is fed through R202 from the 110V B+ supply. (This set employs a half wave voltage doubler power supply; if you fix this it will be aa nice performer.
What is the voltage on the 35L6 screen if there is no signal (volume control set max low)? if it is also 190V, then it is likely that nothing is drawing current and you have a break in the B- circuit. Remember, this is not technically a hot chassis. Chassis is connected to ground through a 0.1 uF Cap (C305) and a 120K resistor (R103). If either shorts, then you will have a hot chassis! However, the 6AQ6 is a class A Amp; it should draw current, even with no signal. If injecting a signal into the grid of the 35L6 produced audio (do this through a 0.01 uF Cap) produces sound, check the following, using the B- side of the 35L6 Cathode Resistor (R204, 150 Ohm) as the B- Reference:
Confirm the plate voltage of the 6AQ6 as being 180V.
Check the Plate voltage of the 12BA6 IF Amp. It should be 110V. If it is also 190V, look for a break in the B- wiring. "Mecca" for B- centers around the 6AQ6, Pins 2, 3 and 6. Check for 0V at the Cathodes of the 12BA6 and the 6AQ6 These cathodes are tied to ground (B-); there is no cathode resistor. Bias is through grid leak. If they are at 190V, check for a cut wire or cold joint. reheat the joints, there seems to be a lot of old 1049 flux on those pins. Don't discount that the cathode of the 6AQ6 is floating.
Hope you get this working. I had the end table version of this set. With an8" Speaker and 190V on the plate, a 35L6 can kick a little @$$
Hope this helps.
Best Regards,
John "MrFixr55"
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
(This post was last modified: 05-09-2022, 09:15 PM by MrFixr55.)
Hello all. Sorry for the delay. I finally got back to it. I’m not any closer to getting it working. It was mentioned that maybe there is power supply issue. I schematic has a fairly good troubleshooting guide, so I went back to the power section. Both test points A and C have voltages close to 190 volts. C should be 200 volts, but the A point should be 110. Unfortunately, there is no troubleshooting for high voltage. The 2 resistors test in spec. The 50y6 and 35l6 both get very hot quickly, due to the over voltage. The 6AQ6 is cold. The B- between the 6AQ6 and 12BE6 is good.
I’m being to think this is above my pay grade and it is time to move onto the next project.
Another thing to keep in mind is that over time these radios have been worked on and not always by someone who knew what they were doing. When you see a voltage that isn't close to what it should be then it very well could be wired wrong. So, here's some logic. If point A is 110 volts and 6AQ6 pin 7 (plate) is connected to point A through a 220K ohm resistor then that plate should have considerably less voltage, around 50 volts. There's one of your dilemmas. If you measure 190 on the plate then it is connected wrong or the power supply is wired wrong or not working right.
I’m new to fixing radios. I’ve seen a lot of good ideas (a bit over my head). I’m watching details of the thought process to learn. This problem maybe the extra wires and pin connections and I know that was explained away by someone that knows more than I ever will. But the schismatics don’t show those extra connections. I say this in my ignorance but maybe try matching the circuit to the the schismatic? I don’t want to offend anyone.
