Problem with '79 Beta Lead Combo

[Andy]

Hi guys and gals - great site,

I've just been given a '79 Beta Lead Combo, and once I'd cleaned up the pots it's obvious it's an amazing piece of kit!

I have a problem with it though - ten minutes or so after it's turned on, it starts to emit a real LOUD hum, which can't be changed by any of the pots, or by unplugging the guitar. The only thing that stops it is turning it off.

From what I've read generally, I would think that:
a) It is heat related - it doesn't happen for the first ten minutes or so, so I'm assuming it's once a critical component has had time to warm up.

b) From what I've read elsewhere, it sounds as though it may be one of the output stage resistors being fried. But I've checked all the Rs I can see and there are no obvious burn signs. I've tested a large proportion of them with a multimeter, and though I've long forgotten the colour codes for resistance, they are all sensible values (i.e. no shorts or breaks).

Can anyone tell from my description what's wrong?
Can anyone recommend further trouble-shooting steps?

I'm handy with a soldering iron, and know my way around a pcb to some extent, and know enough to read good 'net guides on how not to blow yourself up when dealing with HV capacitors......

I'm also looking for a schematic, but haven't found one yet.

Thanks for listening,

Andy

[mckinnon audio]

  Hi Andy,check your filter caps in the power supply,look for a leaky output transistor and check your bias volts.Put a dummy load on the amp and see if there is any D.C. volts at the output when you hear the hum.Hope this is of some help,good luck Mel.

[Andy]

Hi Mel,

Thanks for the reply.

I shall do my best to try those. I know what all the sentences you said mean, but I'm not 100% sure how to do them
I'll report back...

thanks,

Andy

[Andy]

Well I know it's been a couple of months, but after trying, I realised that there was no way I was going to be able to do anything with this - given my limited abilities and knowledge.

So, I took it to a pro - he charged me £75 and fixed it. Apparently one of the ICs had blown.

I am now the happy owner of a beautiful sounding bit of kit.

Thanks for your help

Now, all I have to do is:
a) find a footswitch for it
and
b) learn how to play properly

Andy