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Tech: Gremlin in old Carlsboro PA amp - help?

Richard Bridge 30 Jun 10 - 06:29 PM
Mitch the Bass 01 Jul 10 - 06:27 AM
pavane 01 Jul 10 - 07:27 AM
pavane 01 Jul 10 - 07:29 AM
Richard Bridge 01 Jul 10 - 09:33 AM
John J 01 Jul 10 - 10:06 AM
Richard Bridge 01 Jul 10 - 10:42 AM
pavane 02 Jul 10 - 05:05 AM
John J 02 Jul 10 - 07:51 AM
The Fooles Troupe 02 Jul 10 - 10:55 PM
Richard Bridge 03 Jul 10 - 02:38 AM
Richard Bridge 03 Jul 10 - 12:47 PM
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Subject: Tech: Gremlin in old Carlsboro PA amp - help?
From: Richard Bridge
Date: 30 Jun 10 - 06:29 PM

It's a very very very old Carlsboro Marlin 150 box mixer amp. Belongs to friends. They said it didn't. I took it home, found one speaker out that seemed as if it might have usable contacts, attached speaker and crap mic and bingo it worked sort of.

So then I re-attached all the jack speaker outs, used lots of switch cleaner on all the pots sockets and switches and let it dry out. Hoorah - all inputs work with mic, no crackling on pots.

Notice one electrolytic is encrusted and looks crappy (4,700 muf 50VDC). Clean it up with solvent and sod me one terminal comes off in my hand. Make very large solder joint to re-attach terminal.

Hoorah - all inputs work with mic, no crackling on pots.

Attach one channel to left out of a CD player and one to right and turn down quite a lot. Forget to hit the "attenuate" buttons. Insert and play CD. Er - now I have 100mv up a pair of mic inputs.

One big distorted blast, then a big thump and a nasty mid-range oscillation, maybe about 500 hz but not a pure sine wave.

Oh, sez I (well, I did use another word as well) - I've put max volts up old electorlytics (one of which I have thermally stressed making a BIG solder joint on its terminal). So I bought two new electrolytics 4,700 muf and 63 VDC, 50s were twice the price), and carefully observing the polarity and adjusting themounts, and re-jigging wire lengths, inserted same into circuit.

Result - no freaping difference.

Do we think I have destroyed the output device - it appears to be a square plastic thing, quite small, about a quarter of an inch thick and less than an inch square, with no observable heatsink? I wouldnt expect to blow a plastic thingy with just one blast and no apparent heat excess.

Or is that something else and not the output device at all?

Or is there another more likely fault?


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Subject: RE: Tech: Gremlin in old Carlsboro PA amp - help?
From: Mitch the Bass
Date: 01 Jul 10 - 06:27 AM

Richard,

The output device you describe sounds like the driver transistor rather than the output devices themselves. I'd be suprised if they are not on heat sinks.

There's a schematic at http://music-electronics-forum.com/t18783/ which may help. I think you're seeing TR3 but need to look for TR6, 7, 8, 9.

Mitch


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Subject: RE: Tech: Gremlin in old Carlsboro PA amp - help?
From: pavane
Date: 01 Jul 10 - 07:27 AM

We have two marlins, and yes, OP transistors are on heatsinks.
Have blown a channel once, due to feedback. Probably the output transistors.

Did this blow BOTH channels or just one?

Overloading the input is very unlikely to blow the capacitors. But you would then get a very load mains hum - they are just there to smooth it out.

Much more likely to have blown the power transistors. But I am surprised that his would happen unless you had the volume turned up full. Otherwise you would just have heard distortion from the overload of the input stages.


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Subject: RE: Tech: Gremlin in old Carlsboro PA amp - help?
From: pavane
Date: 01 Jul 10 - 07:29 AM

PS if you don't want the amp, let me know. Might take it off your hands


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Subject: RE: Tech: Gremlin in old Carlsboro PA amp - help?
From: Richard Bridge
Date: 01 Jul 10 - 09:33 AM

It's only got one channel, it's mono.

I'm going to have another look inside while I eat a sandwich.


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Subject: RE: Tech: Gremlin in old Carlsboro PA amp - help?
From: John J
Date: 01 Jul 10 - 10:06 AM

As an electrolytic failed it may have taken out the rectifier, this could be a discrete bridge rectifier (4 separate diodes) or an encapsulated type. It may be that 1 or 2 of the diodes have failed.

Just looked at the schematics, it's a split-rail supply. Have you checked the supply rails?

JJ


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Subject: RE: Tech: Gremlin in old Carlsboro PA amp - help?
From: Richard Bridge
Date: 01 Jul 10 - 10:42 AM

Well, I don't know that an electrolytic failed. I assumed it was that and changed them.

Yes, on closer inspection what I thought was a plastic power device is almost certainly an encapsulated rectifier (four wires) so if I squint a lot two should bring in AC from the transformer and the other two push mostly DC with a huge ripple factor out to somewhere (probably the electrolytics for smoothing). What am I looking for, 50/0/50 or something?

I have also found two dinky little black heatsinks with fins, about two or three inches square, with one three legged object on each. To look more closely at the three legged devices will be interesting as the legs go through the wooden case, and then are soldered to short wires so I can't just hoik the heatsinks off with the trannies, and there is a circuit board the other side with (helpfully) wires at both ends so to angle that out of the way I will have to take a lot of jack sockets off the case. And I will have to take the wires off the trannies (careful with the heat, Richard) to perform a crude check with a meter.

Must do some work now.


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Subject: RE: Tech: Gremlin in old Carlsboro PA amp - help?
From: pavane
Date: 02 Jul 10 - 05:05 AM

Of course its mono, silly me.


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Subject: RE: Tech: Gremlin in old Carlsboro PA amp - help?
From: John J
Date: 02 Jul 10 - 07:51 AM

It's a complimentary output pair - I think it needs a split rail supply to work efficiently.

If an electrolytic failed it's possible that it damaged the bridge rectifier.

JJ


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Subject: RE: Tech: Gremlin in old Carlsboro PA amp - help?
From: The Fooles Troupe
Date: 02 Jul 10 - 10:55 PM

"Notice one electrolytic is encrusted and looks crappy (4,700 muf 50VDC). Clean it up with solvent and sod me one terminal comes off in my hand. Make very large solder joint to re-attach terminal."

If the electros are in this visible condition, it is extremely UNWISE to apply power! The surge as they short and dump their insides everywhere can (and usually will) take out almost anything and every thing... as well as leaving conductive goop all over bridges between conductor tracks that can take out the rest of the circuit...

Well now you know....

Step ONE - replace ALL old electros....

The rectifier thingie was probably taken down thru this - you can check this with a multimeter - they usually only have a a 1.5 V battery, so little chance of blowing much else... check lead to lead open circuit every where is dead - each leg should only conduct one way...

As for the rest of the circuit - if you have shorted other semiconductor devices later in the circuit, you may well blow another rectifier with the resultant short circuits...


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Subject: RE: Tech: Gremlin in old Carlsboro PA amp - help?
From: Richard Bridge
Date: 03 Jul 10 - 02:38 AM

No conductor tracks anywhere hear the electrolytics - this thing is old enough to be largely point to point wiring (beautifully loomed, apparently by hand) and replace the electrolytics was the first thing I did. When I'm more awake I think I'll have another look at the schematic and measure the rail voltages.


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Subject: RE: Tech: Gremlin in old Carlsboro PA amp - help?
From: Richard Bridge
Date: 03 Jul 10 - 12:47 PM

One blown MJ802 output trannie, readily available from Maplins for £4.55. So unless there is another fault that caused that, a cheap solution - even when you allow for changing two electrolytics, not a bad thing to do in any amp that old.

Stickers inside showing previous repair work by old man Pyne (Dartford) and he's been dead for over 10 years!


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