Power amp hum

[handh]

I have a pair of maplins GA28F power amp modules which were working fine until I shorted out the speakers and blew the fuses between the power supply and the amp boards on both channels :steam:
After I replaced the fuses the amp now has a mains hum on both channels without any source connected and independent of the passive volume control on the signal input. Both channels still work when connected to a source but there is this very loud low frequency hum. I put my meter on the speaker connections and read 2.5v DC on each channel. I am pretty new to this so I would appreciate any advice and pointers as to what is wrong. I am I correct in assuming that the power transisors are Ok as the amp still basically works?:scratch:

Thanks
John

[StanleyB]

The power transistors are breaking down under load. I myself wouldn't use the amp until I had it fixed. Have you tried going back to Maplin and asking for a replacemant?

[handh]

Thanks for the reply StanleyB. I am sure I caused the problem due to my carelessness & these things are ancient. I got them second hand. Each module uses a 2SK133 & 2SJ48 I have checked some prices for replacements but they are far too expensive, rather than just bin the amplifier are there any cheap drop in modern replacements I could use?

[awkwardbydesign]

Thanks for the reply StanleyB. I am sure I caused the problem due to my carelessness & these things are ancient. I got them second hand. Each module uses a 2SK133 & 2SJ48 I have checked some prices for replacements but they are far too expensive, rather than just bin the amplifier are there any cheap drop in modern replacements I could use?

I think I have a pair of these modules I'm not using. PM me if you are interested.

[handh]

PM sent

[Reffc]

Don't connect to speakers whatever else you do. The fact that you have measured 2.5v DC shows a breakdown of some of the output components, possibly the transistors, and DC should read "0v". DC on voice coils will heat them up and fry them, sooner or later (sooner with increased DC).

[Radford Revival]

This isn't a particularly complicated amplifier design, it should be fixable.

[handh]

Thanks again for the replies, Richard has offered me 2 replacement modules so I'll go down that route. If I could find some cheap alternative MOSFETS I'd have a go at a repair but the prices for them are ridiculous.