09-09-2021, 03:55 PM
(09-09-2021, 01:55 PM)ppppenguin Wrote:(09-09-2021, 12:43 PM)Mark Hennessy Wrote: There should be about 30V across each of the main PSU caps, give or take the usual variables.
The main PSU caps are rated at 25V according to the circuit. That doesn't really square with the 21V AC quoted at the transformer.
I agree, but I wouldn't be surprised. I've lost count of the number of times I've seen over-volted caps in audio gear. Perhaps the most extreme example I've come across is the Musical Fidelity B200. This uses 63V caps, and the PSU rails idle at 75V! But in percentage terms, this could be about the same.
Perhaps surprisingly, the caps I've found in the B200 are always in pretty good health - certainly no worse than you'd expect for the age.
Of course, in this case, we don't know the idle current, and we don't know the conditions that 21V is measured at. It's likely the transformer is more like 18V at full load, perhaps. It's quite a small one (20VA, maybe?).
Either way, these things are very much built down to a price.
(09-09-2021, 01:48 PM)Murphyv310 Wrote: Well spotted Mark on the cooked resistor and I didn't see it on the phone. It looks as if its a feed from the PSU, long now since I've repaired this model so forgotten.
Looking again, I'm still pretty sure the cooked resistor is R40 (in the headphone circuit). A better photo would help, but the two resistors at that point - next to the headphone socket - are the only two that are 1/2W.
The 220R pre-amp supply resistors are elsewhere - between IC3 and TR5.
I do agree that these 220R resistors will be leading an unhappy life. Assuming 25V rails, they're dissipating practically 0.5W, rising to 1W if the rails do indeed reach 30V. They are standard 1/4W carbon film. The zeners are running warm, if C21 is any indication. In round numbers, the op-amps are only taking around 12mA, leaving around 55mA for the zeners (0.75W) at 30V rails. That falls to 35mA/0.5W if the rails are 25V. A bit marginal, but in keeping with the rest of the design...
The FR4 board doesn't look too discoloured around these hot-spots, suggesting it's not had a lot of use. But I think I'd be upgrading some of the components and spacing them away from the PCB with some ceramic spacers or similar.
