Arcam Alpha 2 - channel out

[Bushbaby]

Hi All,
In email conversation with Mark Hennessy, I mentioned that I had an old Arcam Alpha 2 where a channel just failed one day. I actually put it one side in the 'round to it' pile but thought it could be another good project to get underway. 2 questions though:

• any thoughts why a channel should just 'fail' when in use?
  
• does anyone have a schematic for this amp?
  

Best Regards

Rob

[Mark Hennessy]

Well, I can help with the easy question: https://golbornevintageradio.co.uk/forum...p?tid=7912

As to the more tricky one...

Can you remember how it failed? Was there a bang? Any smoke? Blown fuses? Were the speakers OK or did a woofer die?

If the output stage failed, then it could have been dramatic. On the other hand, it might just need switch cleaner, or perhaps some dry joints repairing. Anything in-between those extremes is possible, but a bit more info is required

[Bushbaby]

Hi Mark
Thanks for the link to the document.

I can remember that I was using it and one channel just completely died, no sound, blow fuses or speaker issues. It just wasnt there anymore - really strange indeed.

Best Regards

[Mark Hennessy]

Hopefully nothing too drastic then.

First step: visual inspection. Check fuses, look for overheated components, check for dry joints, etc, etc.

Next step: basic checks. Does it draw roughly the right current (power up using a lamp limiter)? Is the speaker output close to 0V DC?

After that, run a signal through both channels and follow it with your 'scope until you find the bit where the signal is no longer equally present on both channels. Then the fun begins!

BTW, some faults on audio power amplifiers can be extremely difficult. Hopefully this is not one such example. But don't be disheartened if it's not a quick fix, and do be prepared to do a bit of book-work with this - it will be a good learning experience. Luckily, the circuitry is very conventional, so knowledge and experience gained here will be useful in many other scenarios.