Acoustic 370 troubleshooting help needed

[mkreutz]

I own an Acoustic 370 (with original 301 cabinet) which failed a few weeks back.. The output to any speaker cab and from the monitor outputs to a power amp produces weak and broken up (crackling) output. I've already taken it to one repair shop which evaluated it as not repairable due to availability (or the lack thereof) of old transistors and diodes. I've asked for a list of what they think is the failed components so I can begin my own search.

I have a little electronics knowledge but no test equipment other than a digital and analog dvm. I'm not willing to make this a "for display only" amp as it has been my main amp since originally purchased back in 1970 while playing in high school bands (yes I just aged myself).

I would appreciate any help, advice, direction, information that will steer towards restoring my 370 to full operation.

[Enzo]

"Can't get transistors" is just silly.

The line outs are from the preamp. So it would be the problem so far. Now the LAST thing I am is a replace all the caps all the time guy, but in an amp this age (over 45 years) all those little 1uf caps in the signal path are instantly suspect. But before changing them all, the schematic has DC voltages all over it, so go through the preamp and compare yours.

I would also apply a steady strong input to the amp and follow it through teh preamp to see where it goes away.

[Dave H]

Schematic

[g1]

As far as the transistors go, yes some of the exact replacements are obsolete. But there are perfectly usable substitutes available.

[Enzo]

Exactly, most any good small signal transistor of the right polarity should work just fine in the preamp so long as voltage spec is met.

[The Dude]

And, since this is a single supply output section with an output coupling cap, don't forget to check C404.

[m1989jmp]

Does the sound get loud during that crackling or stays weak?

I had a similar problem with my Acoustic 230 a couple of months ago. The crackling would stop if I pushed the volume knob at a certain angle. After reflowing all the pots and cleaning them the amp came back to life.

I'm sure the guys at the repair shop would check for something like that, but given their response, anything is possible.

[Sowhat]

Got one of those as well sitting in my shop collecting dust, the customer abandoned it when I told him it was beyond economical repair. That's a pretty harsh assessment but that's probably what the tech you took it to was trying to say. It's not that the parts are unavailable or even expensive, it's the time involved in making things right that eat right into the economics of this repair. You might get lucky with some De-oxit but Enzo is probably right as usual when he suspects the 50 year old caps, the silicon stuff is probably just fine.

[nsubulysses]

Check all those heavy ass inductors and orange drops in the preamp board to see if one of the legs has broken the solder connection.

[mkreutz]

Schematic

[ATTACH=CONFIG]45932[/ATTACH]

Dave, Thanks for the schematic ... I have a couple other copies of this but yours is a bit clearer .... I also have a parts list for the power amp section but do not have that for the pre-amp section .... if anyone has that I would greatly appreciate it as well. Not giving up on this just yet!

[mkreutz]

I do understand the response in a way .... and the economics of it. I can easily pick up a PF500 and sell my 301 bottom ... or perhaps a used Carvin which will run down to 2 ohm loads, but it isn't my 370. .... I spent a good part of Friday sourcing parts for the power amp section ... mostly the transistors ... and found pretty much all of them ... not having a parts list for the pre-amp ... I haven't gotten to that yet and am hunting that parts list (if anyone has one would appreciate it).

It was cleaned and recapped about 15 years ago and a few transistors were replaced ... not sure which ones ... the entire job cost me a whopping $80 but it was done by a tech I was in the Navy with ... he moved to California or I would take it back to him.

Right now I'm just sourcing parts ... and then will decide if it is economical ... and then decide if I can part with it. I've not used any other amp for any performance in all of my over 50 years of playing ... except recently here where I've been playing lead and acoustic ... but that is a different story ... it is just hard to part with something that has performed so well and has been the basis of my bass sound .....

[mkreutz]

That is my thought as well ... nice thump still from the power amp section on power up ... once I locate a parts list for the preamp section I'll start sourcing parts ... based on the schematic it looks like some of the pre-amp transistors are the same as some used in the power amp section.

[mkreutz]

Yup .. that was my thought as well ... and I never really dug into the electronics in all my years in the service ... had all the training ... not that it means anything ... took a lot of college classes ... not that it means anything either ... don't have the test equipment other than a few good dvm's ... ... that slows me down a bit. I know I can generate a steady tone and feed that to the input but tracing it may be fun without a scope.... not exactly what I would consider ideal trouble shooting.

I don't think I can make it any worse unless I do something really stupid ... short short zap ... but the preamp section doesn't really rely on matched output transistors ... more of the power amp section where that comes into play. If I had to work there I'd definitely be at a disadvantage without the proper test equipment to find matching power trans.

[Enzo]

I am a little concerned over this parts list thing. Are you planning wholesale parts replacement?

The parts are mostly identified on the schematic. The 49-23 transistors are house numbered, also called 480023, the lower left notes tell you about component sorts.

What parts do you need to identify that a list would help?

[mkreutz]

Brought my 370 home from the shop last night ... I'll definitely check on all the recommendations everyone has offered so far ... and continue searching for parts. My wife's stepfather was an engineer at the cape ... had a very nice scope and signal generator ... he passed away a few years back and I'm not sure my mother in law kept all that or sold it but going to check that out ... having a decent scope will help trace the signal path ...

Thanks for all the input ...

Needs ... which I haven't found yet ...: Pre-Amp parts list if anyone can point me to that
Common sense?

I'm not playing bass right now and it was one of our other base players that was playing when my 370 gave it up .... the guy actually feels bad that it happened while he was playing. I understand the age issue (not me the amp!) ... he though maybe he did something. I did my best to explain to him that it was a tank but stuff breaks ... especially old stuff.