Acoustic 370 troubleshooting help

[poveglia99]

I recently inherited a vintage Acoustic 370 solid state bass amp that blew up. Smoke came out when the amp blew. I have a general idea on where to start but could use some help from someone who does this all the time so I don't do more damage than good. From what I have read the obvious things to check would be the driver and output transistors, associated resistors, traces and grounds. Is it a good idea to simply pull all the transisters and test one by one or is there a faster way to troubleshoot. How would a good technician approach this problem?

[jrfrond]

When something smokes, the first thing you need to do is to ascertain the source. Sometimes it's obvious, sometimes not. I might opt to bring the amp up SLOWLY on a Variac to get it to smoke again just to see where it is coming from.

Acoustic amps used a common DC-coupled predriver/driver/output stage. Chances are you have some resistors blown in the driver and bias areas, maybe a driver and/or an output transistor or two.

The issue with SS amps is that unless you have experience and are aware of the pitfalls and practices, you might power it up only to have it all blow up in your face again. Next time, you might not get another chance and get a bad PCB burn. Then you are sunk. Tube amps can be dangerous for inexperienced hands, but are inherently more forgiving. If it is out of your league, you might want to call in a pro. Troubleshooting SS amps by proxy (i.e. asking questions on a forum) is dicey at best.

[52 Bill]

How would a good technician approach this problem?

I don't know how a good technician would approach this, but here's what I would do.

First, isolate the problem area. If something burned causing the amp to smoke, that would be the first thing to look for. Usually, but not always, this will be a resistor in series with something that was drawing too much current, a shorted transistor maybe. Wherever you find the burned component, will be the starting location of your search. This could be anywhere in the amp, power amp, power supply or pre-amp. Try and locate the problem by doing the least amount of dis-assembly as possible.

The power transistors in your amp are parallel wired, so if one is shorted it may cause all of the transistors in the same circuit with it to test as shorted. So you might have to remove the transistor to get an accurate reading.

I start by checking them in circuit. If I find that there is a short let's say from collector to emitter on the first transistor, I move on to the next transistor to see if that one shows a short as well. If it does, then I pull the first transistor out of circuit and retest it. On most Acoustic amps, the output transistors are socketed. On a metal cased TO-3 transistor, the collector is connected to the case, so removing the two screws that mount it to the heatsink disconnects the collector from the circuit, essentially removing it from the circuit. If after removing the two mounting screws it still tests shorted, I pull it out of the socket, retest it completely out of the amp and set it aside. If it tests ok, I replace the mounting screws and repeat the same test for the next transistor and so on until I find all of the shorted transistors.

Hope this helps.

[poveglia99]

I popped the cover on the 370 last night and found the 10 ohm 2 Watt resistor thats in parallel with the choke on the output stage was burned. The choke had a bad solder connection where the solder had melted. I tested the driver transistors in circuit and they seemed fine. I went ahead and pulled the output transistors and they also tested fine. I went ahead and ordered the resistor replacement. I got 4 because the site had a minimum order. I figure if it burns up another one then I can get the other test equipment I need and do it right.



I think it should be fine though. I am going to redo the heat sink compound on the output transistors and where the heat sink attaches to the frame.



I should end up with a great amp in the end. I'm planning on fixing all of the cosmetic items like sliders, knobs, jack hardware, spare fuse holder and am even going to add a fan.



Thanks for your help. I will keep you posted.



Poveglia99

[Mars Amp Repair]

yep,
Sounds like with the choke not connected, all the power went to the speaker via the resistor, which is not designed to handle that much current.

Ya got lucky. I just worked on one of these & I would definitely re-silicone grease all the power transistors as well as the aluminum sink to chassis, too. that old silicone dries out & doesn't transfer heat like it used to. glen

[Enzo]

It must be Acoustic month. I have a smoking 370 to fix next to me, and out in the warehouse is a repaired 370 waiting to be picked up. it got all new EQ sliders. And a 360/361 just came through a few days ago.

[poveglia99]

It's fixed -)

I replaced the resistor on Saturday afternoon. Redid a couple of other solder connections and silicon greased the outputs and heatsink.

This amp is awesome. I have a Laney 300W amp I have to turn up to seven to get near the sound I get out of the Acoustic 370 at two. This amp has pure raw power and I now understand what all the hoopla is about. Thanks to everyone that answered the thread.

Poveglia99

[PRNDL]

It must be Acoustic month.

What a coincidence ... I'm waiting for someone to drop off their Acoustic 370!

This one has a strange noise on top of bass notes. It sounds very much like a parasitic oscillation, which I've become very familiar with.

So far it's been to three techs without resolving the issue ... the buck seems to be stopping with me on a lot of things ... last time it was Mutron Phasors and Auto-Wahs.

Here's a schematics page
http://acc.homeunix.net/schematics.html
Unofficial Acoustic Control Corporation Homepage -- Schematics

[poveglia99]

Replacement knobs for Acoustic 370 and slider control knobs

Cool, Thanks for the link to the prints and manual. Would you or anyone else know where I can find original replacement knobs for the Acoustic 370?

Thanks,

Poveglia99

[Mark Norwine]

Need a PT

Hey guys,

I just found this forum via a google search for "Acoustic 370", and I hope you can help. This is my first post here; I apologize if my post voids any board etiquette.

Anyway.....I'm a retired amp tech who was asked to take a look at a friend's neighbor's son's bandmate's bass amp.

(visions of Pacino in G-III: "They keep pulling me back in")

Anyway....It's an Acoustic 370. An earthmover, if ever there was one!

Anyway, this one has a PT with the pri winding shorted to ground.

The schematic shows this as Acoustic part # T-401. It's massive.

Since Acoustic Control Corporation is deader than this kid's amp, getting a replacement is not going to happen via the "usual suspects" (Mojo, New Sensor, etc.).

Now, I have no real issue in declaring this amp to be "dumpster food", but I told my friend I'd "reach out".

Anyone, by chance, have an Acoustic 370 "organ donor" with a good PT? Or a source for a NOS PT?

Anyway....thanks for reading!

Mark Norwine

[Enzo]

Hi Mark, welcome to the forum. May I suffest starting a new thread for your amp instead of tacking on to the end of a dormant one, you'll get better response.

SHorted to ground? Do you mean the primary winding is shorted to the frame of the transformer? Disconnect ALL primary wires from ANYTHING. Still shorted to something?

I don't know about finding a vintage transformer, I wish you well there, maybe they are around. Otherwise, the amp runs on a single 93v rail, which means the transformer puts out about 65VAC at sufficient current to make the 325 watt rating into 1.6 ohms. I am going to call that a 2 ohm load. If you stick to 4 ohms it can be smaller. You are still looking for a 300-400 watt transformer.

Not sure what the odds are finding a high current 65V transformer, but a 30-0-30 or 32-0-32 would work as well and be more likely. I might look at power transformers for 300-400 watt solid state bass heads from Peavey or someone. Or for that matter for plain old power amps that run on a signal split winding - we just ignore the split.

[acousticcontrolcorporation]

www.acousticcontrolcorporation.com

www.acousticcontrolcorporation.com

Transformers for sale 50% of all remaining inventory!

[Bruce / Mission Amps]

Transformers for sale 50% of all remaining inventory!

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.

[Mars Amp Repair]

I second that! g

[Panhead]

Hey Enzo, where did you get new 370 eq sliders? Thanks, Tom