Just finishing up a new PL400S2 acquisition, and it was looking perfect, other than bias not getting below .4 on one chan and .42 on the other. Made power into a load fine, yada yada . . . Did a full recap (mains has a 10,000uF on B- and 5900uF on B+ as received, 10K slopped in with glue and solder . . . Ugh! Some folks should not be allowed to work on gear . . .butnI digress ), added the diode pair to prevent lockup, and DC protect board.
But, needed to fix the bias. Found that the bias trimmers were measuring just 720 ohms (not 1k as they should), but all other parts in the bias circuit were just fine. Pots held settings well also, so, realizing that I was at max resistance through the pot and the 680 ohm (1400 ohms in this case - 720 + 680) instead of the 1720 that should be possible, I changed the 680 to a 1K to get it back where it should be. Brought up slow, and tested - both channels now biased perfectly at midrange, and life is good.
Left it to warm up to finalize, and noted that the right was getting warm . . . bias had climbed a bit, so reset . . . looks good, but still heating. Checked drop on emitter resistors on "normal" channel, not seeing much, but much higher on the warm channel, despite bias at .35 on both 10 ohm resistors on this channel . . . .
Thinking I may have a failing/leaky output, I decide to read across each emitter resistor to see if the current is balanced. Using micro grabbers (very little bare) and looking good on four of the six . . .
And then a probe slip . . . Pop/flash, vaporized the tip off the grabber, and popped the low rail fuse. Replaced fuse, and slow bringup on variac (in case I just blew the fuse . . . ) but nope, drawing hard at minimal input voltage.
Pulled everything off the heatsink on that channel, the 2SD555 I was probing is shorted hard (all pins), all else are good. Put back 2 of the 3 pairs and drivers, came up fine . . .
So (sorry for the long story) it looks like all that is needed is the transistor. Since all else was 100% prior (well, I think the heat might have been an oscillation, but didn't get to scoping it), so no sane/visible reason to make major changes
*IF* someone has a single 2SD555 that they could sell me, it would get this going in the most cost effective way I can come up with.
(Otherwise, this is gonna hurt! I have a spare channel of outputs, but with the 15024/5 parts (comp), and while I am still thinking about converting this amp, was going to run it for a bit prior . . . but if I have to buy transistors, it's getting the comp conversion (the newest parts), PL36 conversion to comp, and backplane rework . . .
So (at the risk of redundancy) might someone have a 2SD555 leftover that I could talk you out of?
Thanks,
Tim
But, needed to fix the bias. Found that the bias trimmers were measuring just 720 ohms (not 1k as they should), but all other parts in the bias circuit were just fine. Pots held settings well also, so, realizing that I was at max resistance through the pot and the 680 ohm (1400 ohms in this case - 720 + 680) instead of the 1720 that should be possible, I changed the 680 to a 1K to get it back where it should be. Brought up slow, and tested - both channels now biased perfectly at midrange, and life is good.
Left it to warm up to finalize, and noted that the right was getting warm . . . bias had climbed a bit, so reset . . . looks good, but still heating. Checked drop on emitter resistors on "normal" channel, not seeing much, but much higher on the warm channel, despite bias at .35 on both 10 ohm resistors on this channel . . . .
Thinking I may have a failing/leaky output, I decide to read across each emitter resistor to see if the current is balanced. Using micro grabbers (very little bare) and looking good on four of the six . . .
And then a probe slip . . . Pop/flash, vaporized the tip off the grabber, and popped the low rail fuse. Replaced fuse, and slow bringup on variac (in case I just blew the fuse . . . ) but nope, drawing hard at minimal input voltage.
Pulled everything off the heatsink on that channel, the 2SD555 I was probing is shorted hard (all pins), all else are good. Put back 2 of the 3 pairs and drivers, came up fine . . .
So (sorry for the long story) it looks like all that is needed is the transistor. Since all else was 100% prior (well, I think the heat might have been an oscillation, but didn't get to scoping it), so no sane/visible reason to make major changes
*IF* someone has a single 2SD555 that they could sell me, it would get this going in the most cost effective way I can come up with.
(Otherwise, this is gonna hurt! I have a spare channel of outputs, but with the 15024/5 parts (comp), and while I am still thinking about converting this amp, was going to run it for a bit prior . . . but if I have to buy transistors, it's getting the comp conversion (the newest parts), PL36 conversion to comp, and backplane rework . . .
So (at the risk of redundancy) might someone have a 2SD555 leftover that I could talk you out of?
Thanks,
Tim
