30-05-2013, 04:23 PM
(This post was last modified: 01-06-2013, 06:40 AM by Old Sparky.)
I have now run the main amplifier board from a +25/0/-25v DC supply. Admittedly this is still only a little more than half the voltage it's designed to run from, but, on the basis of short-term tests with this supply and signals from an Audio Generator, the amplifier is working perfectly from this. I have, as a precaution, replaced all 8 2200uf 50v Electrolytics with new ones, but I'm rapidly coming to the conclusion that the only fault was the o/c primary of the original mains transformer. There are quite a few threads about the A500 on 'diyaudio.com', where the consensus of opinion seems to be that it's a decent design let down by indifferent build quality and inadequate heat-sinking, but generally OK if 8 ohm speakers are used, and it's not played for many hours at high volume, or into too low a load impedance. The speaker terminals on these are simply wired in parallel, which means that, if 4 speakers are connected, ideally they should each be 15 or 16 ohm types. In addition no cooling fan is fitted. Since posting, I have run the amplifier from a 25-0-25v AC supply, giving +35/0-35vDC, and it is working OK, at least from a test signal at this voltage. I don't know the history of this particular A500. It was brought to me with no screws in the outer cover, and, when I came to put the fuses back, I noticed that the two from the secondary windings of the transformer to the rectifier were 6.3A (T) each, not the specified 4A(T). Enough said?!!
(Naturally I shall replace them with the correct types!)
(Naturally I shall replace them with the correct types!)
