Cheap GC2020 resurrection attempt

[nevergiveup]

Hello first time poster. I have a habit of picking up these hopeless cases and this GC2020 is my latest. Inside it looks nearly brand new, the guy I got it from had no idea what the problem is.

I thought maybe the power supply could be faulty but I can't see any strange caps or burns other than a blue marker on one of the main caps.

Voltage after the rectifier is about 48v between the pos and negative, and checking any of the bare wires inside the unit some are as high as 42v, most around 30v. Seems high to me but I don't know what was normal for a compressor like this.

Controls all seem to work fine though switching on the comp doesn't engage the relays so I thought it might be a stuck relay as it seems to be stuck in bypass mode (or "protection" mode due to error condition). There is voltage in front of the 12v relays (27v) with the compressor switch is on however, so I am thinking that is not right.

I don't really like the idea of replacing a bunch of PSU caps on a unit like this if I can really help it. I will continue to trace lines with my multimeter to see if I can find any components producing bad output, but rn everything looks fine.

Thanks for any advice. -G

Service manual is here btw: http://bee.mif.pg.gda.pl/ciasteczkowypotwor/#pro_audio/Yamaha/Yamaha GC2020 Compressor Service Manual.pdf

Along with an almost identical model with a slightly better PDF scan: http://bee.mif.pg.gda.pl/ciasteczkowypotwor/#pro_audio/Yamaha/Yamaha GC2020B Compressor Service Manual.pdf

 

[avionic]

Welcome aboard. Just what appears to be the major malfunction with this piece?

 

[nevergiveup]

Welcome aboard. Just what appears to be the major malfunction with this piece?

Thank you! It appears to be stuck in bypass mode, so audio passes through whether it is on or off, but the compressor has no effect, no clicks or anything.

I'm beginning to think that the PSU transistors are fried, as the first two transistors should only be passing 15v and -15v to the rails.

 

[avionic]

Service manual is here btw:

Along with an almost identical model with a slightly better PDF scan

Both are real piss poor scans.

first two transistors should only be passing 15v and -15v to the rails.

I believe they are 15v voltage regulator integrated circuits. Many, many transistors in each TO-220. If there passing more than 15v, yeah there probably toast.

Looks like the LM7815, LM7915 and LM7812 should fit the bill for all three. All are currently available from Mouser.

 

[nevergiveup]

Thank you for that! Yes I usually struggle with 80s schematics and these ones are really annoying.

I will look up those regulators. One thing that worries me is all 3 of them appear to be faulty, which I suppose they would tend to cascade if one of them burned out. Is this probably the result of a power surge or bad quality power input? I don't see evidence of more damage past the regulators but if it's possible that a bunch of transistors have burned out together I don't think it's really worth the repair.

I believe they are 15v voltage regulator integrated circuits. Many, many transistors in each TO-220.

You know... I've been calling those "transistors" for decades and it never crossed my mind how silly that sounds

 

[avionic]

Thank you for that! Yes I usually struggle with 80s schematics and these ones are really annoying.

I will look up those regulators. One thing that worries me is all 3 of them appear to be faulty, which I suppose they would tend to cascade if one of them burned out. Is this probably the result of a power surge or bad quality power input? I don't see evidence of more damage past the regulators but if it's possible that a bunch of transistors have burned out together I don't think it's really worth the repair.

Throw the regulators at it. They are inexpensive. Then re-evaluate.

 

[nevergiveup]

Throw the regulators at it. They are inexpensive. Then re-evaluate.

That seemed to do the trick! Thanks for your help, it's certainly debatable the quality of compression of these units, but for what it's worth I needed the experience of getting it working.

 

[avionic]

Sometimes you get lucky.