Acoustic 125 guitar combo issues.. no output!

[crowguitar]

This is an old late 70's Acoustic 125. Great sounding amp but hasnt worked in quite some time and I wanna get it back together. Speakers are okay (tested w leads from another amp) but theres no output from the actual amp.

I ran a cable from the preamp out to my effect return in my classic 30 and got a strong clear signal, the EQ sliders were functioning and tone/volume controls worked (Master Volume was not engaged - problem w footswitch as well.). Slight hum but that could either be from 1. old amp plugged into same outlet as computer and other amp that signal was being sent to 2. whatever issues the output stage has could be makin the hum..

Any help greatly appreciated. This was a gift from my uncle when I first started playing. THanks in advance.

[Jazz P Bass]

You might want to try to locate a schematic.
Tha Acoustic amps are a wiring nightmare.

[crowguitar]

Well, opened it up.. I guess when my uncle had it something happened and he decided to cut the leads to the power section. Also looks like a component is missing, not sure what without a parts list.

I do have to say, I slightly disagree with the wire nitemare part. This amp looks quite linear and Acoustic's use of colored wires makes it easy to see whats goin where. Its like a rainbow inside this thing..lol

So now that I realized the Preamp's fine but the 170080 power section just needs a lil work, the main goal is getting the parts list and schem for this.. but havin a hell of a time finding one. HELP!!

Here's some inside pics.

[Jazz P Bass]

Here is a link.
You will have to buy it.
Link:Acoustic*Schematics

[52 Bill]

The left side of the board connects with jumpers to the 4 large output transistors on the center heat sink. The right side wires connect to the power supply and the preamp.

Having the schematic will help you in figuring this out.

I'd start by checking the 2 driver transistors on the board, the ones with the built in heats sinks, as they will be the hardest parts to source.

[crowguitar]

Its not the leads or transistors that are the problem, I tested those. Look at the pic of the board. There's something actually missing that was attached to the PCB. the leads and solder r still there but it had to have been some big capacitor or something that must've blown up. Thats the part I need to find. its the only thing keeping this amp from working.

[Jazz P Bass]

Can you do some voltage, continuity checks.
Where that missing part was, it looks like two identical circuits around those heat sinked transistors.
What is the purpose of these two circuits.
+ - 15 volts?
Flip the board over. Where do the "missing component" traces go?
Is one of them at ground?
Is there any voltage on the leads?

[52 Bill]

Can you do some voltage, continuity checks.
Where that missing part was, it looks like two identical circuits around those heat sinked transistors.
What is the purpose of these two circuits.
+ - 15 volts?
Flip the board over. Where do the "missing component" traces go?
Is one of them at ground?
Is there any voltage on the leads?

The two heat sinked transistors on the board are the drivers RCA 40409 and RCA 40410.

[Jazz P Bass]

Quote:Flip the board over. Where do the "missing component" traces go?
Is one of them at ground?
Is there any voltage on the leads?
Any chance that you could answer this.
You have the amp, we don't.
Without more info we are dancing in the dark.

[Jazz P Bass]

It might be for a feedback capacitor.

[gbono]

I just spent 3 days trouble shooting an Acoustic model 123 without a schematic - good luck. BTW, I broke down and bought a schematic for this model from MusicParts it was 3 revisions behind the board design I had - useless.

The main problem I had was keeping track of all the leads that were breaking when I had to move PCBs. I thought this was ironic since Acoustic used edge connectors on their earlier designs (150, 140, etc).

Your picture is pretty blurry.....

[Enzo]

Is your driver board the 170074 or the 170080?

It appears the black wire is ground for power supply and speaker, the red wire is the V+ from the power supply. The orange wire is the output to speaker. The input ground is black with white stripe, and the input signal is green. That work with your wires?

I suspect you have the 170074 (possibly written 17-74), since the circuits are about the same, but the 170080 has the edge connector terminations on the end of the board. Your board lacks that apparently.

Has the large cap vented? Looks like cap spooge out the top end of it.

[crowguitar]

its the 170080.

[crowguitar]

nothing like a really expensive Canon to give the best blurry pics for the $$...oy.

Anyway, theres definitely signs that something blew up on this board. Whatever that thing is thats missing with its two cut leads still attached to the board and remaining solder. Right next to the missing * is a small black component with lettering on it reading:

FPN
2484
EBC

It has 3 leads coming out of the bottom of it (well used to, the one side blew out and the 3rd leg is hangin freely) This is on the board to the left of the Heat sunk RCA transistors..

Im gonna post some more pics here (sorry if they are blurry, this cam stinks) & hopefully someone can identify whats there and whats not.

[Enzo]

The blown apart guy on the end is your bias transistor. And that agrees with the schematic.

Wow, my scan was really crappy, I'll try to find a better one.

The missing part is probably the 0.24 ohm 7w output ballast resistor R26.

[crowguitar]

Thanks for what you've done so far.. getting closer to getting this thing back up and runnin..

For now Ive been runnin out the preamp out into my Tube 100 power amp and back into the speakers.
Just stinks seeing the power section sitting on my workbench lookin lonely and undone..lol

[J M Fahey]

Hi.
Start by using a series lamp, don't plug straight into the wall.
With the amp off check for shorts in all transistors.
If in doubt, pull the suspect one to measure better, but as a general rule pamper the board, avoid pulling components "just in case" .
Measure also the power ones, that's easier because they are socketed, but don't connect them yet to the main board.
Replace what's missing.
Even if you lack a part, see what is still connected to those pads through track or wire, and identify what's missing.
It might help printing a couple schematic copies, and check it part-by-part against the actual board.
Pencil every part you identify on the schematic and, if you wish, mark a small white dot on the actual part on the board, with an office typist correction pen.
Soon you'll be certain of what is mising, and where.
If you can't source 40409/10 you can replace them with Tip31C/32C, *but* the pinout will differ, and you must join the pads where the heat sinks were soldered with a piece of wire, Acoustic designers used them as jumpers.
Your board should work properly, that's to say have around 1/2 Vcc/+B on the + pin of C14, you should have signal there, (without speaker load, output transistors , even without C14) and you should have around 400 to 500mV DC across R¿21? and R24, the ¿50 ohm? resistors. The voltage across those is the actual bias voltage for the power transistors. Acoustics were biased hot, but I preferred to lower that a bit, even risking some slight crossover distortion, because of their non-use of emitter resistors.
Only when you have the board working you can connect the power transistors.
If the wiring is somewhat corroded, (what happened to our friend above), unsolder them (one by one), cut 1/4 inch of the insulated part, peel, tin and re-solder it on the pad, to avoid it cutting when moving the board.
When the amp is fully working, add 0.33r/5W emitter resistors to each power transistor, you don't need a board for that.