Acoustic 370 troubleshooting help needed

[mkreutz]

This is one of those ... "wish I had half a brain left?" ... R132 (330K) biasing resistor at the base of Q107 is wide open. All the other resistors around Q107 tested within tolerance except R131 100K which is reading just a hair over 105.8K ... ordered up the lot anyway along with new electrolytics. Haven't pulled Q107 to test yet (it tested good in circuit). Want to correct the biasing issue first and then see what we have.

I did jump around Q107 last night for a few seconds and everything worked well enough to drive the 301 cab way louder than the neighbors wanted to hear ... stress test ... and then a stress relief!

If everything works out I'll order up caps for a complete recap job and check everything else along the way.

Thanks for all the input from everyone. I am hopeful that correcting the biasing issue will fix the problem. This has been educational for a newbie ... or one who just forgot a lot.

[Enzo]

105k on a 100k resistor is only 5% off, probably the tolerance of the part anyway.

[mkreutz]

105k on a 100k resistor is only 5% off, probably the tolerance of the part anyway.

Yup ... I don't know what I was thinking on that one (bad math late at night). Replaced open R132 (330K) at Q107 and the lytics C102, 127 and 128 just because I was in there. She is back! I've read that it is advisable to not just replace all the 1uF tants but to do so with lytics as well .... actually all the tants. I expect most everything on the PA board is ... 47 years in age at least except the lytics.

There is a little oscillation (clicking) on power up which settles quickly (5 seconds or less). Noticed R144 and R145 fairly hot to the touch but the voltages check good there. All in all I played it for a while and then just ran tunes through it for about an hour with no issues. If I can get all the critters out of the house for a while tonight I will push it a bit harder (like above 2 maybe .

I'm waiting on a loaner scope to see if I can track down the oscillation or clicking ... whatever it is. It lasts for a whole 5 seconds at most so I'm expecting a challenge there.

Thanks again for all your input Enzo ... and encouragement. I am so tempted to call the music shop I took it to initially and ask if the tech even knew what he was looking at. Like ... Did he have a schematic and did he really trace the signal or just sit texting for a couple hours ... or something ... and charge me $75 bucks to say "it can't be fixed ... can't find the replacement parts." I've found nothing but options and information on this forum and the unofficial ACC forum ... and others! Supposedly the tech was "on two forums" per the owner ... sure ... not so sure I would recommend them to anyone but our options are few here in Jacksonville Florida. I might have suspected something was up when the shop owner began to bad mouth the other repair shops and brag about all the experience his techs had combined. Hmmmmm.

I may have just gotten lucky on this .. who knows ... I did a lot of research (and reeducating), stared at the schematic for a few hours before touching the board. I'm not a professional ... just determined.

So the (re)education continues ... here I am about to retire ... and relearning things long forgotten. I see some new toys coming for testing! Anyway, my plan is to breadboard the first few stages of the preamp (Q101 - Q102 and maybe through the tone controls) to the volume control and experiment with sub xters just for fun .... and then continue on.

[mkreutz]

Was suspicious of how I had things setup and that I may just have a grounding issue ... the PA board was upside down so I could trace the input, pots snugged up to the mounting bracket which was connected to chasis ground with a jumper wire ... so I decided to clean things up and assemble it properly before giving it another test. Quiet as a mouse ... perfect ... ecept for that hiss at full volume which says it is going to hurt ... cranked it wide open and had some fun. This house has never been eposed to that!

[Enzo]

And as to contacting the old shop, what is to be gained by doing that? Just don't go there for service again, and tell anyone interested what your experience was there. The market will take care of itself.

[ToneX]

Good afternoon fellas, I’m new to this Web site. I’m not sure how to start a new post but I have an acoustic 370. And have been all through it several times. I’m glad you found it in the preamp section. My problems have been in the output section using new were power transistors. I’m having trouble with the positive side portion getting too hot. Any thoughts out there?

[ToneX]

Thanks

Good afternoon fellas, I’m new to this Web site. I’m not sure how to start a new post but I have an acoustic 370. And have been all through it several times. I’m glad you found it in the preamp section. My problems have been in the output section using new were power transistors. I’m having trouble with the positive side portion getting too hot. Any thoughts out there?

[J M Fahey]

I’m not sure how to start a new post

1) welcome to the Forum

2) go to Maintenance, Troubleshooting & Repair and on the top left click "+ Post New Thread"

3) search for Acoustic 370 schematic (might even be in another MEF thread) and post it in your question, so we all refer to same schematic.

[J M Fahey]

I’m not sure how to start a new post

1) welcome to the Forum

2) go to Maintenance, Troubleshooting & Repair and on the top left click "+ Post New Thread"

3) search for Acoustic 370 schematic (might even be in another MEF thread) and post it in your question, so we all refer to same schematic.

EDIT: just to begin with something, I think (might be wrong) that 370 has a single supply capacitor coupled power amp.

If so, your symptoms are weird because all current going through the upper half, must also pass through the lower one, so in theory both should dissipate the same.