Vintage Armstrong 600 series

My Mum has the receiver, can't remember the exact number (621?) and one channel is dead. I had a look at Christmas hoping it was just a blown output fuse. The fuse was blown but after careful checking of speakers/wiring and replacement of fuse it blew again. Swapping speakers over confirmed the problem is with a specific channel of the amp. Any suggestions as to where to look next or if there are any common faults other than shorting speaker cables that cause the output fuses to blow? I know this is an ancient piece of kit but it still sounds fab with the KK3's it normally drives.

 

change the small electrolytic caps on the board and the driver transistors

 

Most likely to be the output transistors. The 40636 type fitted is equivalent to the 2N3055 which is easily available. The drivers may have gone as well, these are 2N5320 and 2N5322 and should still be available. If not try ZTX653 and ZTX753 which should do the trick.
After replacing any or all of these you will need to re set the bias to 20 mA.

These are good amps and better (IMHO) than the Sugdan or A+R amps of the time. Well worth fixing!
Good luck.

 

Thanks very much for this. The amp lives at the other end of the country unfortunately so I won't now get a chance to look at it until Easter, but I'll go up armed with the relevant transistors. This amp was my introduction to proper hifi back in the 1970s so I'd really like to bring it back to life and get the old KefKit3 speakers singing again.