Old school Coustic Amp 160:turns on with no remote power

[spooney]

I have an oldschool Coustic Amp 160 here that likes to turn on when only power and ground are applied.No remote power necessary.It functions properly other than this issue.No problems with noise or other output related issues.This amp was working fine previously but I had a brain fart one day and reversed the power input polarity.I could smell some warm components but never noticed until recently that it turned on regardless of whether remote signal was applied or not.I'm not looking to spend a ton to repair this as I got the amp for free and I only use it in my basement to run a small setup but I would like to get it back in working order if at all possible.Is there a component that commonly fails when reverse polarity happens that would cause the issue I'm having?

 

[yesterdaysyouth]

i would start by tracing the path from the remote terminal, probally a small bjt that sends a signal to the pwm to turn the amp on is shorted....

 

[I Am An Idiot]

As stated above, I seem to remember there being a small transistor on the corner of the board, that when it shorts, it causes the symptoms you are experiencing. I think the remote wire connects directly to the transistor. I looked at ampguts but there is no picture of that amp.

 

[spooney]

thanks for the replies.I will check out that transistor when I get a chance

 

[spooney]

i traced the remote wire and found several small transistors connected to the remote lead but none appear to be shorted.i noticed a small ic in the power supply ,does anyone know perhaps if this ic has antyhing to do with turn on ?i really need to post some pics,it would probably greatly simplify this process

 

[ben04srt]

I had the same problem on mine , but mine was from the coating on the wires breaking inside the grommet and touching pos and remote wire.
You might want to check there to.

 

[Perry Babin]

What's the part number on the IC?

Yes. Photos would be helpful.

 

[spooney]

the IC is a njm4558d .I will get some pics asap

 

[spooney]

here is a link with pics of the top of the board ,I have a pic of the bottom but my putfile account is being screwy right now and will not let me upload anything : http://www.putfile.com/album/171983

 

[Perry Babin]

Some of these self oscillating amplifiers would start up without remote or fail to shut down if the power supply transistors were leaking (electrically). Pull them and check them carefully on both ohms and diode check to make sure that there is no reverse leakage.

 

[spooney]

i'll give that a shot . it made quite a stink when i hooked it up backwards although there was never any magic smoke.

 

[Perry Babin]

That was probably the reverse protection diode (connected across B+ and ground).

 

[spooney]

i pulled the power supply transistors and they appear to be in working order.I also pulled a couple other small transitors near the remote turn on input but found nothing unusual

 

[Perry Babin]

When testing the transistors on ohms, you should have only read something other than OL when you had the red probe on the base and the black probe on the collector and emitter. Did you read anything other than OL with any other connections?

 

[spooney]

no i didn't. the only time i got any reading was with the red lead on the base and the black lead on either the collector or the emitter.

 

[Perry Babin]

They would appear to be OK.

Does the TO-220 case transistor standing in the power supply feed the small wires on the transformer?

If so, check it. If it's OK, is the voltage from leg 1 to leg 3 more than 0.3v with no remote voltage?

 

[spooney]

I stumbled upon the problem just a few minutes ago.I was pulling the rest of the small transistors in the power supply when I found a leaky 2n3904 transistor.I threw in an mpsa06 in its place just for a test run and it luckily worked.the amp now only powered up with remote turn on applied.Is the mpsa 06 suitable to use in this application or should i just get the 2n3904?I have plenty of the mpsa06's on hand I just don't have any 2n3904's so if the 06's will work I can get the amp up and running much faster

 

[Perry Babin]

The A06 is a good sub. There's no need to replace it with a 3904.