Hot Rod Deville - distortion & low volume - suspect preamp

[sound=waves]

Hi all,

I've got very low volume and nasty distortion when playing the clean or dirty channel.

Plugging into the Power Amp In yields clean, loud sound, so I assume preamp section is at fault.

Some time ago I had replaced R57, R58, R61 and 62 so hopefully those aren't the problem. Unfortunately I lost all my notes re: that repair.

I'm getting crazy readings on some test points:

TP2 measures 515VAC, should be 196mV.
TP5 measures 510VAC, should be 324mV.

Any ideas?

Many thanks,
Jesse

[soundguruman]

besides a bad preamp tube, TLO 72 op amps U1, U2, or failure of low voltage power supply, check for + and minus 16 volts DC on diodes CR 13, and CR 14.
feel the op amps and see if they are getting hot. don't burn your fingy.
Oh, well actually there are several versions of the amp, so your part numbers can vary.
Does the schematic match your amp?

[sound=waves]

soundguruman,

thanks for the quick reply. I've tried swapping out each preamp tube one-at-a-time with a new one, and problem remains. Should I swap them all out at once?

Op amps U1 & U2 are cool to the touch, so me fingy remains unburnt.

I read -17V on CR14 (anode side) and +16.7 on CR13 (cathode side), and no voltage on the other side.

Also, we appear to be reading the same schematic, and my amp matches so far.

[sound=waves]

Actually, I may be onto something here. When I measure R16, it pops and seems to restore full volume to the amp. I read voltage drop of 382VDC across it. I'm still kind of a noob about these things - this would indicate that it has come open, yes?

[Chuck H]

"fingy"... I thought I would piss myself.

A signal trace would be a quick way to locate the problem. If you can follow the signal path you only need to find where it drops out and move to the amplifier immediately prior. Most likely. Probably not a tube. Your tube swap should have been sufficient to rule that out. It could still be a board trace though... Signal trace.

[soundguruman]

Actually, I may be onto something here. When I measure R16, it pops and seems to restore full volume to the amp. I read voltage drop of 382VDC across it. I'm still kind of a noob about these things - this would indicate that it has come open, yes?

So you have a bad solder connection? Those things are a pain to take out and re-solder. Hurts my fingys.

[soundguruman]

Actually, I may be onto something here. When I measure R16, it pops and seems to restore full volume to the amp. I read voltage drop of 382VDC across it. I'm still kind of a noob about these things - this would indicate that it has come open, yes?

So you have a bad solder connection? Those things are a pain to take out and re-solder. Hurts my fingys.
Or a bad resistor? anyhow, what a pain to take the board out, my fingys hurt.
come to think of it, may be the solder connections on the power resistors which get hot. but don't burn your fingy.

[sound=waves]

Heh. I'm actually considering just soldering another 100k in parallel, onto the legs of the installed one – super hack but I'm guessing the installed 100k is fully open, not passing any current, so I'd basically be replacing it...without unscrewing all those damn nuts. Any reason I should not do this?

[soundguruman]

as long as you practice fingy safety.
I would not do that cause it looks unprofessional,
but if you do it and it works, and nobody knows, who cares?
there are lots of techs who do that, but I don't.
I would take out the whole board, and solder a nice resistor in there. Even though it hurts the fingys.

[sound=waves]

as long as you practice fingy safety.
I would not do that cause it looks unprofessional,
but if you do it and it works, and nobody knows, who cares?
there are lots of techs who do that, but I don't.
I would take out the whole board, and solder a nice resistor in there. Even though it hurts the fingys.

I hear ya. I'm not a professional tech, just repairing my own gear. I am planning to trade this in soon though, so I'm motivated to do it right & save the next guy some pain.

Thank you much for the assistance, and the chuckles! Now I gotta go watch Yellow Submarine again...

[soundguruman]

hey, two halves make a hole you know.

[g1]

As far as R16, check the voltage at TP9. If TP9 voltage is correct when you have the low volume condition, then R16 is ok. If you want to jumper the new part across it, rather than flipping the board over, then clip out the old part after the new one is installed.
The fact that a big pop makes the sound cut back in and work properly makes me think this is a problem with a bad solder joint or connection.

[Chuck H]

As far as R16, check the voltage at TP9. If TP9 voltage is correct when you have the low volume condition, then R16 is ok. If you want to jumper the new part across it, rather than flipping the board over, then clip out the old part after the new one is installed.
The fact that a big pop makes the sound cut back in and work properly makes me think this is a problem with a bad solder joint or connection.

Ah, yes. It looks like the OP was posting at the same time as I was. I would definitely re-flow R16. Pushing the probe onto it seems to correct the issue because it's making it contact on that end. Probably. Nothing to lose by trying. I know the board is a PITA to get on the other side of. I have an iron with a curved tip and sometimes I can just loosten the tabs on a board and lift it enough to get contact with my curved tip iron without the need to unsolder a bunch of stuff. A real time saver. A dental mirror helps.