Hot Rod Deville low output Power amp out is strong

[lo-watt]

I got this one from a customer who had brought it to a previous repair shop. They could not fix it. I have repaired many of these amps and am familiar with most of the issues(16v Zener,470/330 5 watt resistors,plate load resistors at the PI,filter caps) But this one has me stumped.

The amp is working and making sound,but very faint. All the switching functions are working and plugging into the Power Amp jack reveals a loud signal,so im thinking preamp.

Checked most components at the beginning of the schematic and changed most of the coupling caps for good measure but everything seems to be normal.

Thoughts? This is my first post,but i have used the forum in the past to expand my knowledge.

Thanks to all!

[The Dude]

It seems like maybe you have, but check all of the preamp plate resistors and/or voltage on the plates. Also relatively common is bad filter caps, particularly on the last node- not sure why mostly there, but C36 I believe. Those crappy Illinois caps. It usually results in more hum than signal.

P.S. Welcome to the place!

[nickb]

It seems almost too obvious but ... have you connected a signal generator and gone through stage by stage comparing the signal levels with the reference values in the service manual?

[lo-watt]

I have already changed all the filter caps with F&T's and changed all the plate load resistors in the preamp to Dale CMF60's. Pin 1 of V1 plate voltage is rather high at over 400VDC the others were more in line with what i were expecting. NickB,I do not have a signal generator at this time,my scope is broken as well. Gotta do it with a DVM.

[52 Bill]

Pin 1 of V1 plate voltage is rather high at over 400VDC the others were more in line with what i were expecting.

That is an indication that the tube is not conducting. What is the dc voltage at the cathode pin 3?

[The Dude]

FYI, there are test tone files all over the web (many on YouTube). You can use your tablet, phone, PC, etc. to generate a test tone. Then, use your DVM measuring AC volts. It should be plenty good enough to see where your signal level is dropping.

[Enzo]

NO one has said it yet, did you try the old patch cord from send to return in the loop jacks?

And I agree with Bill, if the plate of a triode goes way high, always check the cathode to se if it is at zero, meaning that triode is not conducting. Do both heaters glow on that tube?

[lo-watt]

That is an indication that the tube is not conducting. What is the dc voltage at the cathode pin 3?

Im home from work now. I had considered the tubes and swapped all 5 with known good examples and no change. I'll gather voltages at all the pins tommorrow and report then. In the meantime,Im gathering theories to test tomm.

[lo-watt]

NO one has said it yet, did you try the old patch cord from send to return in the loop jacks?

And I agree with Bill, if the plate of a triode goes way high, always check the cathode to se if it is at zero, meaning that triode is not conducting. Do both heaters glow on that tube?

I'll try that tommorrow. V1 in this amp has a separate board to mount the tubes from the other 4. Two green wires fly from the main tube board to the V1 board(i think heaters). It's possible there could be a break in the wires. I have already replaced all the ribbon cables with PTFE stranded wired as the originals fell apart from the bending back and forth. Also,a previous repair shop broke quite a few traces and i repaired all of those and tested them for continuity.

[lo-watt]

That is an indication that the tube is not conducting. What is the dc voltage at the cathode pin 3?

Pin 3 is tied to the 47uf and the 1.5k resistor. I had not changed that cap,i'll check it tomm along with the voltage.

[The Dude]

Also check to be sure you haven't lost the ground for the cathode circuit.

[lo-watt]

Also check to be sure you haven't lost the ground for the cathode circuit.

Will do. Im frustrated as i know its something simple,but ive been swamped and its hard to concentrate with some many other repairs and task. Glued on a headstock plate on an old Harmony semi hollow today. Only one shot to get it right. So much stuff!

Thanks for all the replies,feel free to chime in!~

[lo-watt]

That is an indication that the tube is not conducting. What is the dc voltage at the cathode pin 3?

The voltage at pin 3 is 2vdc. The ground at the cathode resistor and cap is good to chassis.

[lo-watt]

Pin 6 has the high voltage over 400VDC. Looks like pin 8 has over 20vdc and has a .068 and 1.5k and a 100k tied to it. Im pulling the board and checking traces in the back.

[lo-watt]

Broken trace(microscopic) from pin 8 to W4(jumper tied to .068/150v cap. bypassed the jumper from ribbon cable,amp works great!! Ugh,now i look like a dummy. Thanks all for advice and places to look. I look forward to contributing often now.And asking a lot of questions!!