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?

[old-wos-is-name]

Hi Tremesurus,

No, once it has completely shut down, it stays off until I switch the mains switch off and on again.

[Hymie]

It is common for guitar amplifiers to be fitted with a thermal cut out (tco) mounted on the output transistor heatsink.

The reason being that it is possible to drive the amp to the point where the transistors will die (due to getting too hot). To prevent this, manufacturers fit a tco, which cuts the power to the amp if it get too hot.

If you have had the amp chassis out of the case, the tco (mounted on the heatsink) usually has only two electrical connections and is cylindrical in shape, with a diameter of around 15mm and a length of around 10mm – with one of the circular faces mounted to the heatsink.

If for some reason, the amp is getting too hot – then this device is saving the amp. The device could be faulty, but I would advise against shorting it out. And it is possible that the device is switching the mains to the amp – so I would advise that you pull the mains plug before dismantling the amp.

[old-wos-is-name]

Hi,
thanks for all the input. I have odered the fan, thanks builder, and the heat sink compound. I will leave the amp on for a few hours with a keyboard plugged in playing a demo tune and see if it cuts out, then fit the fan and goo. will keep you all informed.

[Hymie]

To be effective, heatsink compound needs to be spread very thinly between the mating surfaces of the transistor(s) and the heatsink. I would not recommend displacing the transistors to achieve this – you are likely to damage the fixing, which may include an insulation sheet, and/or stress the electrical connections – leading to further problems.

Placing a dollop of the stuff around the transistor will act as an insulator, making things worse.

If a fan fixes the problem and you can live with the fan noise – I would go with that.

[old-wos-is-name]

Hi All,
Thanks for the input. situation is, the three metal rods are getting hot.
at the bottom of the rods, (they have a metal block/base ) that they are next to/ up against a black flat object with three legs..
as I said before, it seems strange that the rods have a threaded top, as though something should be screwed into them. all still working o.k. out of the box, but when back inside, 10 minutes and all goes off.