Acoustic 370 Voltage Intermittent

[ke7vhn]

Hey guys. I have an Acoustic 370 bass amp that the First and second stage voltages look good until I run a 1k sine wave through the input. The base of Q101 goes from 1.0v down to -2.3, the collector goes from 27v down to 7v. The signal pulls way down through the first stage. Obviously the base and emitter of the Q102 also pulls down. This amp experienced some kind volume drop, according to my buddy after playing one night for 5 hours. It still works but is definitly
lacking power. I have changed all of the coupling caps on this board a while ago and check all of the resistors to ground in the signal path for the first and second stages.
The voltage pulls from 27 volts down to 7v post R111 only when signal is run through the input. Voltage holds at 32 pre R111

[J M Fahey]

Please post a schematic or a link to it, otherwise "Qxxx" or "Rxxx" means nothing to us.
Thanks.
PS: or at least hand draw and post the schematic of the suspect area.

[ke7vhn]

Acoustic 370 Schematic

SchematicAcoustic 370-PL[1].pdfAcoustic 370-2[1].pdfhttp://music-electronics-forum.com/attachments/10474d1278947188-acoustic-370-1-1-.pdf

[J M Fahey]

OK, thanks.
What DC voltage do you measure om Q102 collector?
There is *no way* that Q101 base gets to -2.3V with any instrument and even line level signal. unless you are driving it with the speaker signal, full blast, from another amplifier.
What are you using to drive it?
I suggest an MP3 (or CD) player, with an audio level around 100 or 200 mV and a 1KHz (or 440 Hz) MP3.
On the schematic they suggest 15 to 35mV signals.
On Q102 collector you should measure around 20X the input signal.(AC) and the DC voltage should change little, if any.

[Enzo]

I agree, there is no way for the DC voltage at the base of the transistors to go negative, since the circuit is stretched between ground and +25v. If you are getting such a reading, I'd have to guess your meter was not really on ground. One way this can happen is of a ground trace is burnt open on the board somewhere. Then what should have been ground is now floating, and it could float to a more positive level than that base, so teh base APPEARS to be negative.

[jrfrond]

Acoustic amps were known for bad solder joints. I'd bet money that you've got at least one in there. Try a wholesale resoldering of the power amp PCB. Heck, resolder ALL of them. I've also seen many where components just BARELY made it through to the foil side of the board.

[ke7vhn]

Sorry guys. I had my function generator too hot. Around 2 volts. Oops! I turned it down and now voltages are holding true to schematic specs. That was when I was trying to drive 1k sine wave into the amp with a load resistor on the output. The amp still has a gain issue though. Now that I have signal cleanly passing through stages, I tracked the signal clamping down after C108. It’s after the R115,116, 117, 118 that it goes really low. I ran a jumper from out of C108 directly into the base of Q104 and plugged in the bass and speaker cab and this amp is loud! Tone circuit seems to be the issue. Now to find out why it’s pulling the signal low.
A

[J M Fahey]

There are many possibilities, but just to start with something:
1) Set all pots to "0" (full CCW) , and with no signal measure DC voltages in Q104/5/6 .
Do they reasonably match those in the schematic?
Unfortunately the most important one , the base of Q104 won't be accurate. 8.2 Megohms bias resistor: what were they thinking?
Very easy to drift because of a leaky C112 or even too much rosin or dirt deposited around the solder itself or condensation.
And your tester probe will modify it too. Oh well.
2) Still like before, check how much signal you have *after* R117/118. After R115 you won't have much, unless you inject , say, 50 Hz.
Good luck.
PS: as was suggested above, redo solders , follow traces with a loupe and good light to find cracked traces, clean the rosin residues with a small brush and some alcohol, etc.

[Enzo]

Is C112 bad? I don't care that it was changed a while ago, is it bad now?

That jumper you ran from C108 to the base of Q104? Take it off Q104 and connect it to the other end of C112.

Is C112 in correct orientation? Note which end the + is.

Note that this EQ is active. Q104,105 take the signal and send it on through C116 and the graphic and later stages, but the signal atop Q104 is ALSO sampled off through Q106, the emitter follower whose sole function is to drive it back through the EQ. If Q106 is not working or C115 is bad, then the whole active EQ stage won;t happen.

SO connect that jumper to the base of Q104 like you did to make it loud, but when you do, look at the emitter of Q106. Is there strong signal there too? And if so, back through C115?

That is my thinking, that you don;t have signal being "clamped down" after C108, but you have a gain stage not complete.

Of course I could be all wet too. But my vote is on either C112 or the Q106/C115 stage.

[ke7vhn]

I finally found the problem. R125 had drifted bad. I couldn't get an accurate reading so I un-soldered one leg and it measured 8k! Yikes! I rounded up the closest resistor value to the 8.2M spec. The closest that I found was 8M for the R125. Turned on the amp and sine wave was strong! Turned it off, plugged in speaker cab and Bass and Voila! What number 8 was on volume is now at 2! Awesome!
Thank you Che' JMFAHEY! Viva Boca Jrs. !!! Thank You JRFrond and or course Thank You Douglas! You guys are great!! Thank You! Thank You!