overheating amplifier

[old-wos-is-name]

Hi,
Put this here rather than music topic as more a technical problem.
I have a guitar amplifier, which overheats, goes crazy then cuts out. need to switch off, leave for a while and all o.k. again. I have removed the head from the cabionet and run a wire from it to the speaker. its fine like that, plenty of air around it, so no problem. there is only one small vent section on the amp. I was thinking of drilling some holes at the other end of the chassis and fitting a fan of some sort. trouble is, what voltage fan can I install that will run off the amp transformer. I doubt if I will find a 240v fan as small as a computer fan. any ideas?

[The Builder]

Is it a valve amp Oldie?

[old-wos-is-name]

Hi Builder,
No its a spider line 6 mk3 75w all transistor. I can't actually feel anything getting over hot when I touch all the bits after unplugging and removing the chassis from the box, but overheating must be the problem as it works for hours out the cabinet,( has never switched off) but stick it back in the cabinet and ten minutes later off it goes.

[The Builder]

ok......... I've never heard of a fan in an amp, but anyway ....... how about this ...... 80mm by 80mm by 25mm

http://www.maplin.co....leeve-thin-fans-30265

[The Builder]

The only thing that could overheat, I guess, is the power supply. Maybe something's breaking down at a lower temp than it should. Whatever it is should probably pack up completely at some point :o(

[The Builder]

I agree, Trimbo. Component breaking down somewhere.

[ChuckFickens]

First thing I'd do is get an airline in it and blow out the years of dust and fluff.

If that doesn't help then I'd go looking for anything with a heatsink attached to it and remake the contact with the heatsink using decent thermal paste.

[ChuckFickens]

And I'd say the main thing that's likely to be overheating will be the power amp transistors, rather than the PSU.

[The Builder]

Aren't power transistors usually mounted on a heat sink Chuck?

[ChuckFickens]

They'll normally have a metal back plate that will be bolted to either a heatsink, or just the metal of the case, or just a big metal plate, if it's several years old then oxidisation can occur between them and cause problems, a good clean and remaking the joint with decent thermal paste is a good thing to do every couple of years.

(I'm speaking from more of a RF amp point, but audio amps tend to be the same)

[rojash]

Has it crossed your mind that when you remove it from the cabinet, you are changing two things, not one?

Ok you're increasing ventilation to the amp (although you say it doesn't actually feel hot, anyway), but you're also changing the way in which the speaker is connected. It's possible that the fault is with the internal speaker connection.

[old-wos-is-name]

Hi All,
now I was told about this thermal paste, that may be the next thing to do. The amp has no dust or anything, it is brand new, but no warranty as the chap I bought it from got it from a music shop that had it back for repair as a new under guarantee non-worker and line 6 replaced it, so it looks, and is, brand new. line 6 want £90 to look at it, I only paid £40 for it so I will try a modification if need be. one of my friends has a similar problem, he takes a floor standing oscillating fan with him and points it into the back of the amp and it works all night in a hot pub/club. my amp has the large heat sink fins sticking out of the back of the chassis, so are visible when all is back together, but they do not appear to be hot at any time.

[old-wos-is-name]

Hi again, the "go crazy bit" is where it suddenly turns itself up, starts making a rhyhmic sound like gunfire with echo, and all the lights start flashing, one by one. really deafening.

[AlBags]

Power Output Transistors ... Check them carefully with a magnifying glass if necessary .. any look bubbled or bulged?
I wonder if the amp is a crowbar design .. if so another poss. is a shorted input .. or heavily over-loaded one.