Acoustic 270 problem

[Nutjob]

Got an Acoustic 270 with real low distorted output on my bench. Power amp voltages look good, and the reverb and trem come thru the speaker just fine, but nothing from the preamp unless its a real hot signal, then it sounds real low and distorted.

Any advice on what to check first? Any common issues in these with the preamp board?

Ive repaired a few Acoustics previously, but they were all power amp issues. I mostly deal with valve amps, but have had this one sitting around for a while.

Any advice or assistance would be much appreciated.

[olddawg]

Got an Acoustic 270 with real low distorted output on my bench. Power amp voltages look good, and the reverb and trem come thru the speaker just fine, but nothing from the preamp unless its a real hot signal, then it sounds real low and distorted.

Any advice on what to check first? Any common issues in these with the preamp board?

Ive repaired a few Acoustics previously, but they were all power amp issues. I mostly deal with valve amps, but have had this one sitting around for a while.

Any advice or assistance would be much appreciated.

It's been a while and I don't have a schematic, but the first thing I would check would be the power supply voltages to the preamp. Probably +\- 15v rails. Make sure you don't have a Zener or bad cap pulling down a supply. After that run a signal in the input and follow it. Do you have a scope or a signal injection probe?

[Nutjob]

It's been a while and I don't have a schematic, but the first thing I would check would be the power supply voltages to the preamp. Probably +\- 15v rails. Make sure you don't have a Zener or bad cap pulling down a supply. After that run a signal in the input and follow it. Do you have a scope or a signal injection probe?

Thanks for the reply. Looks like the preamp should be +/- 35v according to the schematic. I'll check that out. I do have a scope and signal generator. I'll trace the signal after I check the rails.

Appreciate the help, just thought I'd ask first and see if theres any common issues to look for.

Len

[J M Fahey]

AFAIK no Op Amps there but discrete single supply preamp.
Mostly transistor complementary pairs , the 2nd collector in every pair should be at about 1/2 preamp +V rail.
There may also be some single transistor stage, same considerations.
Inject signal at the input and follow it along the path, check that it drives the reverb tank (should feel the spring vibration with your fingertips), etc..

[Nutjob]

AFAIK no Op Amps there but discrete single supply preamp.
Mostly transistor complementary pairs , the 2nd collector in every pair should be at about 1/2 preamp +V rail.
There may also be some single transistor stage, same considerations.
Inject signal at the input and follow it along the path, check that it drives the reverb tank (should feel the spring vibration with your fingertips), etc..

Pretty sure signal is getting held up somewhere well before the reverb driver, possibly between the initial preamp and tone stack. I do hear the graphic EQ change tone clearly throught the speakers when I adjust the sliders. Reverb springs are clearly heard as well when I tap the tank.

[52 Bill]

One common problem with any vintage solid state amp is dead or dying electrolytic coupling caps. But a dead transistor will cause the symptoms that you have. Inject a signal and see where it stops.

[Nutjob]

One common problem with any vintage solid state amp is dead or dying electrolytic coupling caps. But a dead transistor will cause the symptoms that you have. Inject a signal and see where it stops.

On the scope, signal turned weird around the tone amplifier. Transistor before the tone stack Q106 appears to be dead reading no voltage on the emitter where there should be 25.5 volts. Scratched my head for days wondering wtf a 48-23 transistor was and where I could find one, before i looked at the parts list to see that it was a 2n2484. Doh! Hopefully this sorts it out. Thanks for the suggestions.

[xjbear]

Posting in here with a 270 Schematic request. Could some one please share a clear good copy? The ACC archives are gone

[Nutjob]

Posting in here with a 270 Schematic request. Could some one please share a clear good copy? The ACC archives are gone

here ya go.

[xjbear]

Thank you so much nutjob.

[xjbear]

Well it just opens a new tab and nothings there ill try on my laptop

Still not workin

[g1]

nutjob will need to zip the file and repost it as a zip.

[xjbear]

Well I got a schem finally and took the time to dig thru this thing. Ive got pretty solid out put when I play hard thru it but its got a nasty distortion and crackle/fades out quick rather than sustaining. Replaced all the transistors on the power board along with one bad 1ohm 1/2 watt resistor and still the exact same result. The row of transistors directly off the 85v rail get hot but the 4 on the other side do not. The line out signal into another amp is perfect. Clear and clean so the preamp board is fine. Anyone have some ideas to check? Im really tired of working on this and would love to put it back in my bass rig.

[g1]

its got a nasty distortion and crackle/fades out quick rather than sustaining. Replaced all the transistors on the power board along with one bad 1ohm 1/2 watt resistor and still the exact same result. The row of transistors directly off the 85v rail get hot but the 4 on the other side do not.

Sounds like you may be describing a crossover distortion/bias issue.

[xjbear]

Im hoping thats it and I can spin the bias pot and get it happy again.
Been reading this thread and im sure its the same power section
http://music-electronics-forum.com/t...um.com/t26639/