Acoustic Model 125 With Low Output

[Johnrcurry]

I am working on a 1977 Acoustic Model 125 guitar amp. All controls work as they are supposed to, except the bright switch does not seem to make any difference. The problem I'm having is that the output volume is not very high. At full volume, both volume and master volume, the amp is quiet enough that one can stand right next it while playing. It is not whisper quiet, but nothing approaching 100 watts.

To try to figure out whether the problem was in the power amp or the preamp I did the following: Took a cable from the preamp out jack of the Acoustic into the power amp in jack of a Peavey Bandit 65. The output was very low. I then took a cable from the preamp out of the Bandit 65 into the power amp in jack of the Acoustic. It was ear-splitting loud. That suggests to me that the problem with the Acoustic 125 is in the preamp section. Does that sound right?

If it is in the preamp, is there some fault in particular that could cause the low output problem I’m having? Is there any point in the circuit I should be looking at in particular? The preamp board is 170078A; the power amp board is 170080, the EQ board is 17006B. I have attached the 170078 preamp board schematic (the closest I could find to 170078A).

Thanks for your help.

[DrGonz78]

Confirm the 32v and the 15v DC throughout the preamp circuit. Assuming voltages are good then move towards signal tracing. You have already cut the problem in half so work your way from back to front. Start tracing the signal from preamp out backwards towards the input jack. There are even test points for both VAC and VDC on the schematic.

Edit: Plus note that there are vDC test points in the circuit that are 4v, 9v and 12v. Not to mention additional measurements in the reverb cicuit too.

[DrGonz78]

I forgot to mention that as you go backwards tracing the signal up to the input jack that you are wanting to hear if it get louder at some point. If the signal gets louder at some point going back then stop there and concentrate on that part of the circuit forwards. It could be as simple of a problem as a weak solder joint. Also, try injecting signal and tapping around the board to see if it triggers anything that makes the signal kick in stronger.

[Johnrcurry]

DrGonz,

Thanks for your reply. I will work my way through the circuit as you suggest, and report back.

John

[Johnrcurry]

I've found and fixed my low volume problem. I followed the signal path, as DrGonz suggested, only I started at the input. The signal got progressively louder until it passed through C10 on the schematic, where it got considerably quieter. I replaced C10 and tested. The amp is now ridiculously loud. The bright switch is now working as well. Problem solved. Thanks for your help.

John

[R.G.]

I replaced C10 and tested. The amp is now ridiculously loud. The bright switch is now working as well. Problem solved.

Good work.

Let me insert a comment in favor of preventive maintenance. Since you're already tinkering inside, consider replacing all the electrolytic caps. These do go bad over time, and you just had one failure. If the others are originals, they will start failing too, at random intervals. Prophylactically replacing them will give the amp another three or four decades of freedom from that problem again.

[52 Bill]

The bright switch is now working as well.

The bright switch was always working, but due to the low volume you had the volume control turned up so high that the bypass cap was not having any effect on the signal.

[Johnrcurry]

R.G.: That is very good advice. As you suggest, I should just do it now while I've got the amp apart. . . . even though it seems there's about a million of them. . . .

Bill: I hadn't thought of that; interesting observation.

Thanks for your responses.

[Johnrcurry]

I am looking to follow R.G.'s advice and replace the e-caps in this amp. The huge one pictured below is a type I've never encountered before. Does anyone have a suggestion for a replacement for this part and a source? Is this a type with a limited useful life that ought to be replaced prophylactically after 30+ years?

Thanks

[Enzo]

There are a million caps, but only some of them are electrolytics.


I don't disagree with RG really, but I seldom see that main cap failing. The amp is now working, right? But for a few dollars, what the heck. On the other hand, all those little signal path electrolytics, get rid of them.

Look at the print on your big cap. Says 7800uf -10 +75% it is a 7800uf cap, but the rest of that means it can be as much as 10% lower and up to 75% higher than that and be in spec. That means it isn't rocket surgery. 7800uf is not a standard value, and you won;t likely find one... and you don't need to. 8200uf or even 10,000uf would be standard values, easy to find, and just fine to use. Next, 100 working volts, means it is rated for use in a 100v circuit. 125v surge means it is happy with going up to 125v, like maybe when it is first switched on. Most caps have surge ratings, so a 100v cap ought to work for you. 85C max ambient refers to temperature. This is made to have a reasonable life at 85 degrees centigrade. This is a common rating, the other common one is 105 degrees. One could make a case that 105 degree caps will "last longer" than 85 degree ones. Go ahead, make the case, then look at the last line on the part - 188-7741 That is a date code meaning the cap was made in the 41st week of 1977. That is 37 years ago. And we are not changing this part because it has failed, we are being preemptive. So really, 85 degrees ought to work fine. And I skipped one line: "Made in USA" Well, you are a few years too late for that.

Rightly or wrongly we usually call the screw terminal caps "computer grade" caps. They were so large because that is how big caps had to be for a large value that far back in history. Caps are much smaller today for the same ratings. The only reason we might look for this type today is for your convenience, just screw it in. But electrically, nothing wrong with any 8200uf 100v cap you can find. The main issue will be making up a way to mount it and solder the wires to it.

Mouser has 8200uf 100v screw terminal caps. $55 each. Mouser has regular old wire lead ones but only up to 50v. They do have a selection of 8200uf 100v "snap-in" types and a couple are under $10. Starting to sound like maybe the old one is OK.

I did check 10,000uf 100v as probably more common, and they have screw terminals on them for $25-30 each and up. Snap-in 10,000uf cost about like the 8200uf.

[Johnrcurry]

Thanks, Enzo. As usual, a wealth of information. Just out of curiosity, why the need for such a large value capacitor in this circuit?


John

[Johnrcurry]

Still Working on Recapping this Amp

I am trying to identify all the small e-caps in this amp and have come up with a question. There are almost two dozen 1uf e-caps on the three boards in this amp (per-amp, power amp and eq). Most of them are labeled 1uF 50V. Some examples are circled in green in the attached picture. There are a handful, however, that are labelled 1uf 35DC. A couple of examples are circled in red in the attached picture. The schematic does not note any difference between the two types.

My questions are. . . .

1) Is there any significance to the “DC” on the ones labeled 35DC, as opposed to the “V” on the ones labeled 50V? If so, what should I be looking for in a replacement part?

2) If there is no significance between the “DC” and “V, ” can I just replace them all with the same type of 1uF/50V caps? (i.e. Mouser part #75-516D105M050JL6AE). In other words, am I right that I can replace a cap rated at 35 volts with a cap rated at 50V?

Thanks for your help.

[Enzo]

The caps are all rated at DC voltage, so there is no difference between V and DC or DCV or whatever.

The smaller ones might be tantalum or something, I'd have to see the schematic. Got a link? But it is just a guitar amp, so I can;t imagine it making much difference. yes, all 1uf 50v ought to be fine.

[The Dude]

As long as the caps are rated at least the required voltage, you are fine. It's more than safe to use a cap rated at a higher voltage.

[gbono]

Where to begin? The capacitors you are replacing are aluminum electrolytic technology. These parts have a voltage (the V) rating that should be greater than what is actually available in the circuit. The "DC" stands for direct current and is related to the voltage rating of the part. Yes you can use a higher voltage rating but don't go to far over since the size of the part increases and so does the cost.