Looking for a single 2SD555 . . .

TimD

New Around These Parts
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Jun 26, 2025
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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
 
Not seeing and 2 letter code like that on the part. Here is a photo:
 

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The R28A is the number. Ill check the stash when i get in.
 
We have a winner. PM your address lad..
 

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The first letter is gain structure, the numbers and letters after that are date related. there were IIRC 3 gain ranges, R< S< T......
 
Thanks! Interesting that they tagged for gain . . . the newer parts are coded only for date and lot, as I recall.

PM sent.
 
@laatsch55 Just confirming that you got my PM, what with all the noise and spam that we were barraged with recently . . .
 
Tim, US Postal Service tracking number---- 9500 1119 3947 5234 7165 17 Arrives 8-26
 
Tim, US Postal Service tracking number---- 9500 1119 3947 5234 7165 17 Arrives 8-26
Thanks much! I see it in tracking, and eagerly anticipate being able to get this back up (and be able to deny my screwup . . . . ). :-)

Hopefully, one day I can either return the favor, or pass it forward . . .
 
@laatsch55 FYI, got the part today, and have it installed. No time yet to power up and set bias . . . hopefully tomorrow.
 
OK, brought it up on the variac, and all looks good, and bias sets OK. Passed signal as expected, so back where I started . . .

Looked at right chan output on my 'scope, and has a nice 6 MHz oscillation (with possible overtones of something higher riding atop that). This amp as I got it already had a 180pF disc cap on just the pos side pre-driver on the "good" channel . . . did see a small asymmetric ringing there as well, but nothing like on the right.

As per the PL service bulletins, I added 250pF caps to all 4 pre-drivers base to collector (replaced the one existing disc . . .) I have 5% silver micas in my stock, and they measured dead-on, so went with the higher end of the recommemded range, and now both channels are pretty much silent on the scope. (a few mv of hash - likely just a tad of hiss . . .)

Load test and button up pending!

Also plan to first update LF356 to LF351 op amps, and also change the 1.8K 5W power resistors to the 7.5K part to keep from baking the PCB any more, and eventually still thinking about going full comp (but no urgency on that . . .).

Again, thanks to all who assisted!
 
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