zenith: High plate voltage 6Q7
Hello! Would appreciate help with the 5S119 I’m restoring.
Here’s where I am:
* All electrolytic and paper capacitors replaced
* All resistors beyond 15% replaced
* All tubes test OK, except 6Q7, awaiting replacement
So, although the 6Q7 is measuring weak in all sections (no shorts), I decided to fire the chassis up with it in place as the replacement hasn’t arrived. I brought it up slowly with the variac/isolation/dim-bulb, and the dial lamps lit and there were no problems with shorts. The only sound produced, though, is a buzz which is there regardless of volume setting. All controls seem to have no effect on sound.
I consulted the “socket voltage” guidelines in the schematic and found the following upon testing the relevant points:
1. 5Y3: 240V AC in, 240V DC out. Appears to be ok.
2. 6F6: Pins 3/4/7 measure correctly, pin 5 is supposed to be -5V, I’m getting -25V
3. 6Q7: Pin 3 is supposed to be 75V, I’m getting 240V. Pins 4/5 should be .1V, I’m getting 2.3V. Pin 7 measures OK. Pin 8 is supposed to be 1.5V, I’m getting 2.4V
4. 6K7: Pin 3 OK at 240V. Pin 4 is supposed to be 85V, I’m getting 112V. Pin 5 measures 0V. Pins 7/8 OK.
5. 6A8 Pin 3 OK at 240V. Pin 4 is supposed to be 85V, I’m getting 115V. Pin 5 supposed to be -1, I’m getting 114V. Pin 6 supposed to be 166, I’m getting 196. Pins 7/8 measure OK.
Measurements were done with the Variac dialed to produce 112V at AC in.
The 6Q7 measures as a bad tube, but even without it in the circuit, I’m still getting 240V at pin 3 where there is only supposed to be 75V.
All voltage tests were also done with the speaker/field coil plugged into circuit. The schematic states that field coil resistance is supposed to be 2125 ohms, but mine measures 852 ohms. The speaker itself states that it is a model 49-165-X rated for 1000 ohms. The schematic states that speaker 49-143-6 is supposed to be the correct speaker. So, perhaps the wrong speaker is included with my radio. Because of the concern of improper field coil resistance, I wired a 2.2K resistor in place of the field coil. With the 2.2K resistor in place, I still had an excess of 230 volts on pin 3 of the 6Q7.
Further, without the field coil/speaker plugged into the chassis, I measure 30V on pin 3 of 6Q7.
Does anyone have any thoughts/suggestions? Although the 6Q7 is measuring weak, I don’t think that’s causing all these voltage anomalies. I’ve rechecked all the values of the resistors and everything appears to be in order there. Could a bad resistor or wrong value resistor be causing the issue? If so, which would be the most likely suspect? Or, is the field coil resistance discrepancy to blame? And why just hum with no volume control? Because the 6Q7 is weak?
Thanks for your help in advance!
A follow-up on my post:
I discovered that I had replaced the two 490K ohm resistors with 490 ohm resistors. This has been fixed and the correct values installed. Voltage at pin 3/plate of 6Q7 is now about 150v vs. the 240 I was getting before. Still far above the 75 volts called for in the schematic, but better.
After replacing the resistors I no longer have any sound coming from the speaker. I still haven’t received the replacement 6Q7 which reads bad and is installed now.
Very poor emission in the triode section of the 6Q7 will cause the plate voltage to read higher than normal. The new tube should lower this back to the normal range. However I would still have expected some output from the radio on local stations, so there may still be another problem. While waiting for the new tube, I would recheck the other resistors that you replaced and solder connections etc. With the volume control at mid point, you should get a hum if you touch the wiper. If you do, then there is still a problem up stream in the RF sections, or both diodes of the 6Q7 are dead. If they tested OK look further up stream. If you don't get a hum when touching the wiper, the problem is in an audio stage. Its important to see some negative bias voltage (-10 to -20V) on the control grid of the output tube.
Thanks, Tom. Really appreciate your help!
I’ve replaced all the tubes at this point and the radio has awakened somewhat. I was able to do an alignment and the test tone is pure with no hum, but it is very weak coming from the speaker. Through the antenna, I am only able to tune into an AM transmitter I have running in the house. Sound is ok, but again, very low volume.
I’m encouraged at least, that I’m getting sound, but need to figure out whether the front end is dead or there’s a problem with the output stage, perhaps the output transformer. If I hook another speaker to the voice coil leads I get the same low volume, so I don’t think the issue is the speaker or field coil.
I purchased an old signal tracer from ebay that I’m hoping will help me diagnose the issue, but that hasn’t arrived yet.
Referencing the “socket voltages” chart on the schematic, I’m still getting some anomolies and I can’t seem to figure out why. I’ve double and triple checked all the resistors and capacitors. These are my measured voltages (in parentheses) compared the chart (all filaments measure 6.4V):
6A8: Pin 3: 240 (290), Pin 4: 85 (67), Pin 5: -1 (5) Pin 6: 166 (226), Pin 8: 4 (5.1)
6K7: Pin 3: 240 (292), Pin 4: 85 (64), Pin 5: 3 (0), Pin 8: 3 (0)
6Q7: Pin 3: 75 (152), Pin 4: .1 (2.3), Pin 5: .1 (2.3), Pin 8: 1.5 (2.7)
6F6: Pin 3: 230 (273), Pin 4: 240 (293), Pin 5: -5 (-18)
5Y3: Pin 3: 240 (293), Pin 8: 240 (293)
Any additional help greatly appreciated!
With the volume control at maximum, touch the wiper. On a normally functioning radio, you should get a loud buzz.
Thanks again for your assistance, Tom.
Yes, I get a buzz when touching the volume wiper at highest volume setting, but the volume level of the buzz is not high.
The volume variable resistor is supposed to be 400K ohms and I measure:
Center wiper to high: 37 ohms - 356K
Center wiper to low: 356K - 266 ohms
Your measured voltages will be somewhat higher if you are using a modern DVM. The schematic indicates that the voltages are measured with a 1000 ohms per Volt meter, which was common at that time, and tended to load the circuits, thus reading a bit lower.
I suspect that you have a problem in the front end with sensitivity. Even if the volume was low due to an audio issue, you should still probably hear some local stations. (I assume you have some local stations?). Does the AVC change with tuning at all? It should sit around 0 Volts between stations, and go toward -10 Volts on a strong station. Do you have at least 10-20 feet of antenna wire connected?