It sounds as if a tube is being overdriven. So this weekend I decided to put it back on the bench and try and find the problem. A year ago, I completely recapped this set, rebuilt all of the capacitor packs, and replaced most of the old dogbone resistors as well. The tubes check OK in my Jackson 648S so I flipped the set over and started measuring tube voltages.
All tube voltages are normal, being off just a few volts either side of the specification. However, when I get in to the audio section, I have huge negative voltages on the control grids of the audio tubes. For example, the 47 output tube should have +1 volt on the control grid and I measured -14.42 In addition, the AVC-DET 27 tube should have +1.7 volts on the control grid, but I measured -1.8 And the real problem seems to be the control grid of the DET-AMP 27 tube with a measured -14.67 volts where I should find +1.8 volts.
Can anyone point me in the direction to look for these errant voltages? Thanks!
Found my notes when restoring this radio. Measured -15V on the 47 control grid. -9V on the 1st AF control grid, and 0V on the Det-Amp 27 tube control grid. Agree something is amiss on the Det-Amp stage. Not sure why the schematic gives the voltage readings you mentioned, especially for the 47 output tube. Can only recommend the volume be checked for correct resistance and its wiper is making good contact and the wiring is correct since many components got replaced.
Richard
Thanks, yes I'm currently rebuilding the volume control since it was giving me some serious trouble. After I get it back together I'll put it back in and see what happens. Thanks for the suggestions.
David
:Hi David,
:
:Found my notes when restoring this radio. Measured -15V on the 47 control grid. -9V on the 1st AF control grid, and 0V on the Det-Amp 27 tube control grid. Agree something is amiss on the Det-Amp stage. Not sure why the schematic gives the voltage readings you mentioned, especially for the 47 output tube. Can only recommend the volume be checked for correct resistance and its wiper is making good contact and the wiring is correct since many components got replaced.
:
:Richard
:
Hmmmm. . . .I wonder, what with the disparity in the time frame, would be the possibility of there having been an error in the re build of some of the unitized components, or possibly a miss wiring.
Initially, by your reference to :
I have huge negative voltages on the control grids of the audio tubes. For example, the 47 output tube should have +1 volt on the control grid and I measured -14.42.
And that ~ neg 15 is in the order of what should be expected, however, if being proportionatively MORE negative that, one would be getting choppy sound by virtue of excess bias having shifting the stages class of operation.
On the +1.8V bias value that you mentioned, one would be expecting the output stage to be pulling current like a hound dog.
One would probably only expecting to be seeing that altered value, if the prior stages .01 (item 28) was leaking plate voltage across and upsetting the A.F. output stages grid bias.
Firstly, how’s about consulting:
http://www.philcoradio.com/tech/90error.htm
Refer to that links info in order to confirm that you referenced the correct rebuild info on items 30 and 24 and have re- done them accordingly.
Consult my submitted markup schema and you will see that the items 24 and 30 provide filter / decoupling functions for the negative bias supply [GREEN BUSS] and the sub B+ supply [ORANGE BUSS] , as well as plate supply bypassing of two of the A.F. stages [The right two 27’s] .
Also look at the item 58 cluster of three resistors [RED-BLACK box] and confirm that no error has made been made there and confirm the values of the resistors currently present there.
The 3500 ohm unit is one leg of a voltage divider for acquiring the proper level of sub B+, the 60 ohm value of resistance is serving as a common cathode resistor for several stages, while the final 180 ohms is holding your center tap from the HV secondary of the power supply above ground and thereby acquiring that negative voltage that you are seeing present on the [GREEN BUSS] and feeding the final two AF stages 1st grids biasing.
On that aspect, notice the red box markup of item 56 and confirm that its negative is going to the 180 ohm resistor and not to chassis ground.
Lastly in referencing to the volume control. . . .in the square blue box markup. . . .that stages bias is derived by a cathode resistor, so
If you want to read its value just jab your DVM’s negative lead into the cathode connection and meter the negative voltage on the first grid of that tube.
Peeelco 90 Markup:
73's de Edd
Thanks for the reply. Yes, I replaced the caps with the correct value according the the corrected schematic from Ron Remeriez's website. This radio has been working well for over a year and I just noticed it wasn't sounding very well. I'm having a friend of mine with more radio parts than I, rebuild the volume control of this set. As you mentioned it could be a source of the problem. I checked all the tubes on my jackson 648S tube tester and they all seem to be at least in the marginal range and actually work OK in the set. However, I might change them around a bit to see if I can narrow down the problem to one tube. I don't know what this might be, however, I did discover I was using the wrong tube voltage chart when I made my initial assess ment of the problems with my voltages. According the the correct schematic and tube voltage chart, my tube voltages are all very close to what I should have in this set. I'll keep looking and thanks again for the suggestions and your work in marking up that Pheelco schematic!
David R
:
:Thanks for the reply. Yes, I replaced the caps with the correct value according the the corrected schematic from Ron Remeriez's website. This radio has been working well for over a year and I just noticed it wasn't sounding very well. I'm having a friend of mine with more radio parts than I, rebuild the volume control of this set. As you mentioned it could be a source of the problem. I checked all the tubes on my jackson 648S tube tester and they all seem to be at least in the marginal range and actually work OK in the set. However, I might change them around a bit to see if I can narrow down the problem to one tube. I don't know what this might be, however, I did discover I was using the wrong tube voltage chart when I made my initial assess ment of the problems with my voltages. According the the correct schematic and tube voltage chart, my tube voltages are all very close to what I should have in this set. I'll keep looking and thanks again for the suggestions and your work in marking up that Pheelco schematic!
:
:David R
David, you may want to try one of the NRI How to Fix series. The one for sound distortion can be downloade here:
http://retrotronicsource.com/cls/NRI/NRI_V25.pdf
Good Luck, Doug