"Got this amplifier with a crackle... can you have a look?"
Yeah ... why not.
It's a Dual PA5030, a good quality hifi amp from around 1988.
Off with the lid, and a few issues are immediately apparent....
... leaking capacitors ...
... so they're evicted, the board cleaned up with some IPA, and the caps replaced with some nice new ones ...
Thankfully Dual provided us with a nice access panel on the bottom, making repair a breeze...
Switches and pots are duly clean ed up, the bias set, and the amp put on test. After a few minutes, the bias is checked, all is not well. The left channel's bias is moving about. It's set again, and something's obviously wrong. A low rumble is apparent in the left hand channel. Out with some freezer-spray-on- the-cheap. Spraying the Left hand output transistors restores operation. Damn. They're obsolete...
At this point I'm gifted another identical Dual amp with a fault! The output transistors are swapped out of this amp, and set up again. The fault remains....
Some head-scratching and a quick perusal of the manual is required...The voltage on the emittor of Q617, a 2SB631 is wobbling about as the amp warms up. Replacement provides a cure.
The amp is a really impressive performer. The moving-coil input is a touch noisy, but looking at it's topology, not surprising. It's better than my Rotel. MM is very quiet, as is everything else.
Another saved from landfill!
You said "The voltage on the collector of Q617, a 2SB817 is wobbling about [...]"
ReplyDeleteIs that what you meant to say? On the circuit Q617 is identified as 2SB631, and there is a 2SB817, but that is Q621. In any case, both of these transistors have their collectors on the negative supply rail.
Or is this another case of the circuit diagram not actually reflecting the build version on your bench? (Sadly, not uncommon).
Good job, anyway.
I really should check these things before pressing go.
ReplyDelete