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Ampeg B3158, smoked

N

nomoniker

Apr 16, 2012
2
0
#1
I picked this combo up free thanks to craigslist. The previous owner could only tell me that he gradually noticed that he wasn't getting enough volume, so he increased gain (using it as a keyboard speaker for gigging) at which point it began to smoke.

I pulled out the head and turned it on for about 30 seconds, I'm pretty sure it's the transformer. Where can I find parts for this? Sure, maybe I should take it to someone who knows what they're doing, thought I'd give it a shot though.

Jistc.jpg


Thanks.
 
#4
What makes you think it's the tranzformer? That's the least likely suspect. Look/sniff around for burned resistors and bulging/leaking caps. Use the sends/returns to try and isolate it to pre or power sections.
 
#5
RickenBoogie said:
Always best to let a qualified amp tech, with a shop full of the proper testing equipment and know-how to do the job. very rarely a good idea to "do it yourself".
Click to expand...

Good advise. Although, and especially when something's free, I'm even more inclined to tinker. Basically have nothing to lose equipment/moneywise. As long as you take some common sense safety precautions, you basically can't make it any more broke.

Although, when the OP points to the tranzformer without having done any tinkering, a tech may indeed be his best bet. Resistors/caps, etc. are for the most part cheap. If you go through a few and it still doesn't work, you're not out any more than going to a movie and might've learned something along the way.....at least learned you're not cut out for fixing amps.:D
 
#8
Usually with SS amps the output transistors are where the problem lies. If the power fuse doesn't blow, the transformer with be subject to excessive current flow and will complain. I too would suggest a trip to the tech. It could save you money over just guessing what the problem may be.
 
#9
BassmanPaul said:
Usually with SS amps the output transistors are where the problem lies. If the power fuse doesn't blow the transformer with be subject to excessive current flow and will complain. I too would suggest a trip to the tech. It could save you money over just guessing waht the problem may be.
Click to expand...

And then i'd imagine its a whole bank of Output transistors.... they like to travel in pairs, or quads... when one goes, usually more than one goes. :D
 
#10
Hi.

With a quick glance the schematic is not available (what a bloody surprise ;)).

Because of that, without a Ampeg tech, You're pretty much SOL. I doubt that they distribute the schematics freely, otherwise it'd pop up in a Google search.


So yet another vote for taking it to a Ampeg approved tech.

Regards
Sam
 
#11
I believe Ampeg will send you schematics if you call their tech support line. I just had a similar experience - got one of these amps for nothing because it was fried. The previous owner got the schematics from Ampeg and replaced all the output transistors. When I got it, it still wasn't working. Using the schematic and a volt-ohm meter, I found that the +16v supply was shorted to ground. I had to de-solder a couple of components to localize the short and was able to narrow it down to a resistor-cap or the quad op amp chip. I didn't see any sign of excessive heat on the PC board under the resistor or cap, so I replaced the op amp chip. I also had to replace a couple of the big 5 watt ceramic resistors in the output circuit because my ohmmeter showed they were orders of magnitude out of spec. It took a few days but the amp is now working.

One caveat - after I got the amp working, I found out that both speakers were also blown (read infinite resistance with ohmmeter). I don't know the history of the amp or what event caused all the destruction. Now I'm wondering - how much should I invest in speakers for this thing? Although I'm not a bass player, I know Ampeg is one of those iconic brands. Was this particular model one of those "oops" mistakes in an otherwise stellar historical lineup or are the dead ones that people like me pick up really the outlier? So anyway, I'm asking for suggestions on what speakers and dollar value appropriate to invest in this amp. Thanks.
 
#12
I have the schematic for it......

That amp has nice common parts..... TIP142/TIP147, pair of each in LF, one of each for HF. Couple of MPS transistors, an NE5532, some diodes..... not so bad. It's even self-biasing (although I am not especially fond of that particular design).

Tech is best, and can be the cheapest approach, too, overall.

It would surprise me if it were the transformer.
 
#13
nomoniker said:
I picked this combo up free thanks to craigslist. The previous owner could only tell me that he gradually noticed that he wasn't getting enough volume, so he increased gain (using it as a keyboard speaker for gigging) at which point it began to smoke.

I pulled out the head and turned it on for about 30 seconds, I'm pretty sure it's the transformer. Where can I find parts for this? Sure, maybe I should take it to someone who knows what they're doing, thought I'd give it a shot though.

View attachment 1937379

Thanks.
Click to expand...
@nomoniker Did you ever get your amp fixed? I'm looking at one on sale right now...
 

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