Acoustic 125 with low output

[01redcobra]

I have to admit that solid state amps aren't my cup of tea, but I promised a friend I'd give this a shot. This Acoustic 125 has a very low output. When I opened it up, I found that it had been repaired by someone before. I don't know the history, but as you can see in the one photo, there have been diodes replaced and a resistor added for some reason. I did find a burnt resistor on the power board, but I'm not sure of the value because of it's burnt state. I don't have a schematic so I'm up in the air with what could cause this. I saw a past post on repairs to the same model amp but it didn't help me out. Wondering what could be causing the low output in the first place, what the resistor value is and what it's purpose is in this circuit, and what could be the reason someone added that resistor. Another thing that bothered me was why did they solder the new components to the clipped leads rather than just desolder and remove the old components and replace them? Seems like the same amount of work to me. Thanks in advance!

[Marina]

You might be able to get free service data at this website.

http://fileshare.eshop.bg/index.php?what=search2

Sounds like someone was in a hurry & did a bit of a hack job. You could try measuring voltage on both sides of the resistor. If it's in a supply line and has increased in value, it may be limitting the supply to the circuit it's supposed to provide voltage for. Just a thought.

Your thumbnails were kind of small & I couldn't figure out a way to make them larger, so I couldn't see what was being pointed out very well.

Good luck with it.

[gbono]

Acc 126

You will need the schematic to go any further. Sorry don't have the 126 maybe someone else does - there is always MusicParts. Couldn't read your images either.

Why do people do senseless things when "repairing" amplifiers (or anything for that matter) - just human nature and they didn't have a solder sucker

[01redcobra]

Thanks for reading my post. Sorry 'bout the photos, I guess I made them too small. I reupped them and these are the new links. I did see a thread that Enzo answered a few years ago on the 125 and wondered if he might have any ideas on this problem. Enzo, are you out there?

[Marina]

Those look like signal diodes in a power supply. Have you checked them? The resistor combination is probably just a parallel equivalent for whatever the value is supposed to be there. Someone was making do, no doubt. Thumbnails are legible now.

[Enzo]

I don't recall the thread, but in any case, without a schematic I can;t tell you what resistance the part should have. Oh wait, I do have them

I will look in my warehouse when I get the chance, because I just might have a few of those drive boards in storag, I can look on one.

What number of on that board? SInce it is just the power amp that is burnt up (right?) we don't need the whole schematic. Is it the 170074 or the 170080? Or possibly 17-74 or 17-80, they changed number styles.

The diodes look to be clamps across the driver bases. two in series for each side. The 6.8k with a parallel tacked across it is in the bias circuit. Th resultant lower resistance would turn the bias transistor on harder, pulling the output bases closer together. In other words they modded it to run a little cooler. WHy on top of the board? They were lazy I guess.

Here:

[01redcobra]

Thanks for getting back to me. The board number is170080. And many thanks for the schematic. I'll wait to hear back from you. I checked the diodes in circuit and they appear to be good.

[Enzo]

Don't wait, I don;t know how long it will take to find them. Meanwhile, this is not a complex circuit, what does that resistor connect to? If it brunt up, it is a pretty good bet some semi is shorted nearby. SHould be able to ident those from the schematic.

[acousticcontrolcorporation]

126 has many variations!

www.acousticcontrolcorporation.com

[Bruce / Mission Amps]

If you are going to take the time to type... why don't you participate instead of leaving your website URL?
How about just sharing some info like the rest of us.
Otherwise your just another annoying free commercial here and that is not in the true spirit of AMPAGE.