Asking for help on a Philco 37-89. All capacitors except mica and out of tolerance resistors have been replaced. This radio worked well for awhile but now has no RF. The speaker makes hissing sounds as would be normal with no RF station.
So a voltage check reveals that the plate voltage on the 6Q7G/2nd detector/1st audio, is very low at 25v - should be 150v. The tube tests “good”. All other voltages are OK. Based on the circuit (schematic below), what might be the probable cause? Hope this isn’t too much of a general question but I don’t understand how the plate voltage is being produced.
Thanks for any info.

http://www.nostalgiaair.org/PagesByModel/226/M0013226.pdf

:Asking for help on a Philco 37-89. All capacitors except mica and out of tolerance resistors have been replaced. This radio worked well for awhile but now has no RF. The speaker makes hissing sounds as would be normal with no RF station.
:So a voltage check reveals that the plate voltage on the 6Q7G/2nd detector/1st audio, is very low at 25v - should be 150v. The tube tests “good”. All other voltages are OK. Based on the circuit (schematic below), what might be the probable cause? Hope this isn’t too much of a general question but I don’t understand how the plate voltage is being produced.
:Thanks for any info.
:
:http://www.nostalgiaair.org/PagesByModel/226/M0013226.pdf
:
Check R36,R40 for changed value, also C38 & C41 for leakage. Plate voltage comes off load side of field coil thru R36/R40 voltage divider. A bad C38 or
C41 would definitely load down the D.C. John

Plate voltage on a 6Q7 can read low depending on type of meter. This voltage is supplied through a fairly high value resistor. Some meters load down circuits more than others. A Digital voltage will be accurate. 1000 ohm/volt meter may read around 25.

Has cap 41 , 015 mf, been replaced? Voltage will also be low if this cap is leaky. Wouldn't expect a problem in this area to completely stop reception.

Check grid #1 voltage on 6A8 tube. It should be several volts negative if the oscillator is operating. Not oscillating is a common cause of receiving nothing.

Norm

::Asking for help on a Philco 37-89. All capacitors except mica and out of tolerance resistors have been replaced. This radio worked well for awhile but now has no RF. The speaker makes hissing sounds as would be normal with no RF station.
::So a voltage check reveals that the plate voltage on the 6Q7G/2nd detector/1st audio, is very low at 25v - should be 150v. The tube tests �good�. All other voltages are OK. Based on the circuit (schematic below), what might be the probable cause? Hope this isn�t too much of a general question but I don�t understand how the plate voltage is being produced.
::Thanks for any info.
::
::http://www.nostalgiaair.org/PagesByModel/226/M0013226.pdf
::
:Check R36,R40 for changed value, also C38 & C41 for leakage. Plate voltage comes off load side of field coil thru R36/R40 voltage divider. A bad C38 or
:C41 would definitely load down the D.C. John
:

Thanks John/Norm. Will check this out. Measured the voltage with both simpson and digital with similar results. thanks for the tip on grid voltage on 6a8.
Appreciate the info.

:Asking for help on a Philco 37-89. All capacitors except mica and out of tolerance resistors have been replaced. This radio worked well for awhile but now has no RF. The speaker makes hissing sounds as would be normal with no RF station.
:So a voltage check reveals that the plate voltage on the 6Q7G/2nd detector/1st audio, is very low at 25v - should be 150v. The tube tests “good”. All other voltages are OK. Based on the circuit (schematic below), what might be the probable cause? Hope this isn’t too much of a general question but I don’t understand how the plate voltage is being produced.
:Thanks for any info.
:

Try pulling the 6Q7 tube and powering up the set without it. Then check the voltage at the plate pin. If it is near normal, the tube has lost the negative voltage bias on its grid. This causes the tube to pull high current and load down the plate. The resistor divider string (between the power transformer B+ winding center tap and chassis ground) is a common source of trouble.

If the plate voltage is still low even with the tube out, something is loading down the path between the plate pin and the B+ bus. Since you said that you recapped the set, my thinking is that you have a bad solder connection or an errant blob of solder causing the problem in that case.

Another thing to check: you said that the 6Q7 tube tests "good". Did you test all three parts of it (two diode sections and a triode)? I've known techs (lazy or pressed for time, take your pick of explanations) who will test just the triode and call the tube "good", assuming that the diodes will be OK if the triode tests OK. That's not the case; and a bad detector diode will kill the audio dead, dead, dead.

OK - Verified C38 and C41.
R36 and R40 were within tolerance but changed them out and also replaced R39.
Poked around 6Q7 and found that the grid cap wire was pinched between the shield and base and wire was frayed so this could have been causing an issue.
Checked Grid #1/6A8 and have zero negative voltage.
CV asked if all 3 sections of 6Q7 were tested and the answer is yes, although the first diode was considerably stronger than the second.
I pulled 6Q7 and checked plate voltage and it was well over spec at 320v.
Also touched up a few solder joints.
So after all this I fired up and the voltage read 160v on plate of 6Q7.…for about 2 minutes and then started drifting down slowly until it went to zero and smoke was detected on the lead section of the 3 section bias resistor. After cooling the ohms reading on the bias resistor was close to called for 202 at 210 , and the other 2 section were close as well. So I once again turned on the radio but still had zero plate volts on 6Q7.
So my thinking is:
3 section bias resistor is possibly toast, even though the ohms values are within reason after cool down.
Based on the Grid check on 6A8, the lack of RF is a separate troubleshooting issue not associated with plate voltage issue.
Does this seem reasonable? Feedback appreciated.

:OK - Verified C38 and C41.
:R36 and R40 were within tolerance but changed them out and also replaced R39.
:Poked around 6Q7 and found that the grid cap wire was pinched between the shield and base and wire was frayed so this could have been causing an issue.
:Checked Grid #1/6A8 and have zero negative voltage.
:CV asked if all 3 sections of 6Q7 were tested and the answer is yes, although the first diode was considerably stronger than the second.
:I pulled 6Q7 and checked plate voltage and it was well over spec at 320v.
:Also touched up a few solder joints.
:So after all this I fired up and the voltage read 160v on plate of 6Q7.…for about 2 minutes and then started drifting down slowly until it went to zero and smoke was detected on the lead section of the 3 section bias resistor. After cooling the ohms reading on the bias resistor was close to called for 202 at 210 , and the other 2 section were close as well. So I once again turned on the radio but still had zero plate volts on 6Q7.
:So my thinking is:
: 3 section bias resistor is possibly toast, even though the ohms values are within reason after cool down.
:Based on the Grid check on 6A8, the lack of RF is a separate troubleshooting issue not associated with plate voltage issue.
:Does this seem reasonable? Feedback appreciated.
:

What are the voltages at the 3 taps on bias resistor R50, with respect to chassis ground? At the end tap opposite the grounded lug, it should be around -15 V.
The other two taps will be higher, with the one next to ground only a couple of volts negative.

All of the B+ current in the set flows through R50, so a short or overload on the B+ bus will heat it up as you described.

Here is the info on the (Part 50) Bias Resistor- voltage readings:
When I first turned on radio today the readings were,
Tap 1.….minus116v
Tap 2.…..minus 20v
Tap 3.…..minus 17v

Now because the first tap is very high I couldn’t leave the set on very long because the bias resistor heats up quickly. When the above readings were taken there was zero volts at the plate of 6Q7 and should be 160v. So there is a definite problem.

So, I poked around the solder connections again and took some continuity checks but really without much direction cause I haven’t come up with any smart ideas.

After doing not very much and the set was cool again I turned it on and the bias resistor readings were minus 19v, minus 3.3v, and minus 2.7v and there was 170v on 6q7 plate.
After a couple minutes there was a “single pop sound” that could be heard in the speaker. Then the voltage begins to slowly go down on 6Q7 plate. The bias resistor remained as steady proper voltage.
I thought maybe the 6Q7 tube was shorting internally, but I tested it on my Hickok immediately after this popping incident and it tested good on all 3 elements and even after leaving it in the tester for 5 minutes the values remained unchanged.
So I have some trouble shooting to do. Could be a lot of head scratching going on with this one.
Thanks for the help.

:Here is the info on the (Part 50) Bias Resistor- voltage readings:
:When I first turned on radio today the readings were,
:Tap 1.….minus116v
:Tap 2.…..minus 20v
:Tap 3.…..minus 17v
:
:Now because the first tap is very high I couldn’t leave the set on very long because the bias resistor heats up quickly. When the above readings were taken there was zero volts at the plate of 6Q7 and should be 160v. So there is a definite problem.
:
:So, I poked around the solder connections again and took some continuity checks but really without much direction cause I haven’t come up with any smart ideas.
:
:After doing not very much and the set was cool again I turned it on and the bias resistor readings were minus 19v, minus 3.3v, and minus 2.7v and there was 170v on 6q7 plate.
:After a couple minutes there was a “single pop sound” that could be heard in the speaker. Then the voltage begins to slowly go down on 6Q7 plate. The bias resistor remained as steady proper voltage.
:I thought maybe the 6Q7 tube was shorting internally, but I tested it on my Hickok immediately after this popping incident and it tested good on all 3 elements and even after leaving it in the tester for 5 minutes the values remained unchanged.
:So I have some trouble shooting to do. Could be a lot of head scratching going on with this one.
:Thanks for the help.

:
Something is warming up and putting a near dead short on B+. Possibilities: filter cap breaking down under voltage; speaker field coil heating up, expanding, and shorting to core; audio output transformer shorting to core. Has to be something that is sitting directly on the B+ bus and either thermal or voltage-level sensitive.