Need help diagnosing a 70's vintage PA

[gellfex]

I've got a MTI Italian made PA that I bought in the early 80's, used till about 97, and brought out of storage for my son to use, only to find it's not working, though I don't remember why.

I couldn't find much online about the unit, though one's up for auction GUITAR AMP /DJ AMP/MTI POLY-PATCH 4 CHANNEL AMP#P-175 - eBay (item 380244589711 end time Dec-17-10 17:25:40 PST)

It powers up, but doesn't put out any signal to the speakers. I unplugged the speaker and saw a spark that alarmed me. A meter detected 66v dc across the line. I presume that's not normal, does this mean a shorted output transistor or what?

I'm pretty handy with electrical work and basic electronics, but amps are beyond me. I've looked all through the boards for something obviously toasted, but have found nothing. Are there any diagnostic flow charts out there to follow? I'd like to be able to repair this thing, the veteran rocker who sold it to me said it was more powerful than the 175w it was specked, and my experience was that he was right.

Thanks for reading this far, and any help at all would be appreciated.

Jon

[olddawg]

I've got a MTI Italian made PA that I bought in the early 80's, used till about 97, and brought out of storage for my son to use, only to find it's not working, though I don't remember why.

I couldn't find much online about the unit, though one's up for auction GUITAR AMP /DJ AMP/MTI POLY-PATCH 4 CHANNEL AMP#P-175 - eBay (item 380244589711 end time Dec-17-10 17:25:40 PST)

It powers up, but doesn't put out any signal to the speakers. I unplugged the speaker and saw a spark that alarmed me. A meter detected 66v dc across the line. I presume that's not normal, does this mean a shorted output transistor or what?

I'm pretty handy with electrical work and basic electronics, but amps are beyond me. I've looked all through the boards for something obviously toasted, but have found nothing. Are there any diagnostic flow charts out there to follow? I'd like to be able to repair this thing, the veteran rocker who sold it to me said it was more powerful than the 175w it was specked, and my experience was that he was right.

Thanks for reading this far, and any help at all would be appreciated.

Jon

Your output stage has a DC offset. More than likely the output devices are not shorted because that would usually draw enough current to blow the main fuse but not always. It could be a lot of things. If it has a diffamp input stage I would check that first. More than likely though, you have some electrolytic coupling caps gone bad. Make sure your rail voltages are symetrical within .5v or so and are clean. Without a lot of experience or a working amp to use to compare to, it will be difficult to diagnose without a schematic.

[gellfex]

Thanks olddawg. It sounds like diagnosing and fixing this thing is out of my league. Do you think a good repair shop could do it, and any suggestions on how to find one? The repair guy at my local music shop suggested a nearby TV repair shop that someone had had an amp successfully repaired at, but that makes me kinda nervous. Should I be?

Is there any network of guys doing this work? I'm in Hudson County NJ, right across the river from Manhattan. If the damn thing didn't weigh 50lb I'd take it to the music shops on 48th street where I bought it!

[Marc]

I'd call and ask the TV repair guy if he'd be willing to work on it. He might be cheaper than taking it into the city and certainly less muscle strain.
I thought MTI was a Japanese outfit...I have an MTI tube preamp on my bench that was MIJ. A '70's era foot pedal witha half watt amp and 2" speaker for a load. kinda nifty now that it works.

[J M Fahey]

So far you have a burnt power stage, meaning at least some power transistors shorted and who knows what else.
Maybe it does not blow the mains fuse because *something else* blew inside, working as a de facto internal fuse (wire wound emitter resistors/thin tracks) .
That does not make it any healthier.
It looks like a powerful beast, those 66V per rail you are now seeing say so, pity you don't have more experience on this as to walk you through repair.
I think the best bet would be to let the TV guy (nothing wrong on that, skills are similar) see it and try to determine if he can repair it for, say, no more than $150/180.
If possible, go ahead; if not, and you need it , buy the EBay one or something similar.
Put it on EBay too, as-is , stating the truth, maybe somebody is interested and wants to give it a try.
Don't expect more than a few bucks , yet better than nothing.
Good luck.

[gellfex]

So far you have a burnt power stage, meaning at least some power transistors shorted and who knows what else.
Maybe it does not blow the mains fuse because *something else* blew inside, working as a de facto internal fuse (wire wound emitter resistors/thin tracks) .
That does not make it any healthier.
It looks like a powerful beast, those 66V per rail you are now seeing say so, pity you don't have more experience on this as to walk you through repair.
I think the best bet would be to let the TV guy (nothing wrong on that, skills are similar) see it and try to determine if he can repair it for, say, no more than $150/180.
If possible, go ahead; if not, and you need it , buy the EBay one or something similar.
Put it on EBay too, as-is , stating the truth, maybe somebody is interested and wants to give it a try.
Don't expect more than a few bucks , yet better than nothing.
Good luck.

I guess taking it to the TV guy and hoping for the best makes sense. This thing doesn't have much complicated electronics, the only chips in it on are on the individual channel boards, presumably for EQ. I don't really know what an equivalent new PA would cost.

Thanks for the advice, and sorry I'm a little slow in responding. If you follow theater news you'll understand when I say I'm working on Spiderman the Musical. It's currently something of a trainwreck and the previews started on Sunday. I do costume mechanics and it's been crazy. Hopefully I'll get a chance to drag in the PA next week.

Happy Holidays!