PA System Problem

[dm99]

I run a small restaurant and we have a PA system, mostly for piped in music. The PA system seems quite old; it was here when I bought the place a few years ago.

The system started seeming a little off a few months ago. I noticed needing to turn up the master volume and that the music sounded increasingly mushy. Then last week it started making a strange popping sound periodically. Today the thing let out this horrific shriek, then a loud bang, and is totally dead.

Where do I start?

 

[dm99]

Does anyone have any idea how I'd start troubleshooting this? There's a pre-amp and an amplifier. I assume that the problem is in either the pre-amp or the amp. Can I plug an the output from the tuner right into the amplifier bypassing the pre-amp?

 

[davenn]

unless you have circuit info its probably a waste of time
would end up cheaper just to replace it

but if you insist
check for blown fuses, if more that one fuse in the unit ( often there's a DC and an AC fuse) seeing which one has blown will give an indication as to where the problem lies

look for blow resistors, capacitors, transistors
take some sharp well lit pix and post them on here .... keep the size to 800x600 and less than 100kb

Dave

 

[dm99]

It's an older one and it uses vacuum tubes. If one of those has blown would that be something I could troubleshoot and repair myself or do those only blow at the end of life of the entire unit?

 

[davenn]

so did you find anything that had visibly blown ?
the loud bang you said happened should be an indication that there would be obvious damage

Dave

 

[dm99]

I don't know a lot about electronics, but I took both the amp and the pre-amp apart. Nothing looks burned or anything in either of them and all the vacuum tubes still light up the way they used to. Also, the indicator lights come on like before and the fuses appear intact.

Would I do damage to the amplifier if I disconnected the pre-amp and tried connecting the tuner or CD player directly into the main input on the amplifier?

 

[davenn]

you need to find out what went BANG before you even consider putting power back into the unit
You have admitted to not knowing too much about electronics
BE CAREFUL old valve gear has HIGH voltages you could easily zap yourself if not
careful. The most likely thing that would have gone bang would be one of the smoothing capacitors with a value of probably ~ 8uF and 250V give or take a bit

Dave

 

[davenn]

Seriously tho....
with your admitted lack of skills in electronics, you really shouldnt be playing with it,
junk it and buy something more modern

I say this for your safety !!

Dave

 

[dm99]

The loud shriek and bang came out of the speakers not out of the equipment.

Would there be any value in an old public address amplifier or should I just junk it?

There is an old repair shop sticker on it. If they are in business still I will see if they can fix this or if they'd help me with the new install.

 

[davenn]

The loud shriek and bang came out of the speakers not out of the equipment.

info like that is helpful at the right time

Would there be any value in an old public address amplifier or should I just junk it?

none whatsoever

There is an old repair shop sticker on it. If they are in business still I will see if they can fix this or if they'd help me with the new install.

worth a try, get a quote first ... I suspect you will find it may be cheaper to buy a decent newer second hand one that resurecting the old one ( unless its a really basic fault)

Dave

 

[duke37]

I would think that there may be some value in the amp. The valves could be worth something and if the output valves are SP25s you could go on holiday!

I would try an auction website, you have nothing to lose.

I have an amp here with a couple of EL34s in the output. You could have this at a reasonable cost if you popped in to collect it.

 

[dm99]

I had someone come out and take a look at it for me. The repair shop was still in business and the tech told me he'd fully rebuilt this amplifier in the mid 90s. He replaced all the tubes and told me that if I ever start noticing changes in the performance of the amplifier I should call him in right away.