Acoustic 370 repair

[sverige_cruz]

Hey all,

I have an Acoustic 370 on my bench, trying to figure out if it is acting like it should. I got the schematic from Enzo, and have read others' posts regarding their problems with this model. Don't know where I might be able to put this, for reference...(somebody know of a safe, free file-hosting site?)

The amp works, but I'm wondering if the output I'm getting at clipping is what it should be. I replaced a couple output transistors that were shorted. Everything else seems to be checking out ok. Main DC (off PSU cap) is 7-10V down from what the schematic shows (87V), 120V on the primary of the pwr xfmr.

At clipping, I get ~ 22Vrms into 2 ohms = ~240W. The DC voltage at the output cap at this point sits at ~ 34Vdc (with no signal, it's ~ 42Vdc).

I've disconnected the preamp stage and am feeding the power amp from a signal generator.

Can someone save me from a bit of grief and either verify that this sounds right, maybe from personal experience with this amp, or from their expertise of SS circuits, or say otherwise? I'm more of a tube guy myself and am reaching the limits of my knowledge of SS amps.

Many thanks!

- Jesus

[Enzo]

I find it odd the output bus drops 8v under load. Does the power rail drop 16v under load? I am not happy with the power railk being that low either.

HAve you scoped the output looking for any clues? Is the power rail excessivley rippled?

What is the crossover distortion situation? Does the unit run hot? Possible overbias dragging the power down.

[gbono]

Here's my $.02 -

ACC rated the 370 (per service manual) at 205W @ 3.2 ohms 5% distortion and 325W @ 1.6 ohms at 5% distortion. That would be 25 and 23 Vrms respectively. I believe they referenced 5% distortion as just clipping of the output waveform.

Can't speak to the supply voltage/rail sag issue. Though if your input source is a sine-wave then the amplifier is really working hard when driven to the rated output. Power supply rejection wasn't a major design criteria on the 370??

Somewhat unrelated factoid:
I have also seen notes in a 370 service manual concerning the use of only using RCA transistors in the output stages (2N3055 or 2N3773). I'm not sure if they are refering to osc issues with the higher gain bandwidth transistor designs that came along after the RCA homotaxial process?? I have used output devices from On Semi (formerly Motorola) and STmicro without issue but I replaced all the output transistors.

The unofficial Acoustic web site has had some discussion on the above issue but I could never understand why someone wanted to use 25 year old transistors to repair a 370.

[Enzo]

My take on the RCA only thing is the same thing Peavey warns about in their power semis: mixing. RCA and Moto might make the same transistor, but it is made to spec, not molecule by molecule identical. SO the process for building the parts differ. The parts if mixed may not thermal track very well, and they may not share current very well. If you use all RCA xstrs there is no problem. I wouldn't hesitate to install a bunch of MJ15003 or something in place of the old items. And yes, I would change them all so they would all be the same.

I would also suspect they had characterized their circuit on the RCA parts and were not going to warrant that other brand parts would perform similarly - even if most likely they would

[sverige_cruz]

Hey guys - thanks for the replies.

So, main DC sags ~12V at full power.

I had previously swapped out all 3 big caps to see if there was any difference, but noticed no change.

Got ~500mV AC on the large cap.

Does this help at all? The DC values noted on the schematic I got from Enzo match up fairly well with the real life value.

Thanks again,
Jesus

[acousticcontrolcorporation]

Acoustic Schematic Diagrams Available!

www.acousticcontrolcorporation.com

[reno88]

hello! this is my first post, and i hope i'm not hijacking this thread. i have an acoutstic 370 bass head that doesn't work. it powers up (light come on) but no sound. can anyone recommend a repair technician in the atlanta, ga area? thanks!

[Jam]

reno88 start new

Hi if you want to get more help I would start a new thread

[Enzo]

Hi Reno, welcome. Jam is right. Start a new thread, you'll get better response.