Here's more.
https://www.hifiengine.com/manual_library/gas/son-of-ampzilla.shtml
Wonder if the transistor leads could be corroded.
There may be DC offset after replacing the transistors.
Adjust R131 to compensate.
https://www.hifiengine.com/manual_library/gas/son-of-ampzilla.shtml
Wonder if the transistor leads could be corroded.
There may be DC offset after replacing the transistors.
Adjust R131 to compensate.
I don't think it's corrosion at all. First, there does not appear to be any visible corrosion. Second, usually a corroded contact point responds to mechanical disturbance. Wiggling these transistors in their sockets does not cause any change.
If you need to order the input diffs try 2N5210 and 2N5087 from Central Semi. I got 100 of each from Mouser and all are fairly high hfe and it doesn't take many to get matches.
I see my UNMODIFIED SoA schematic made it to HiFi Engine.
I see my UNMODIFIED SoA schematic made it to HiFi Engine.
Hmm.. didn't know about the matching. What am I matching - hfe? Do all four need to match? I ordered 20 each of the original numbers yesterday,
2N5551 and 2N5401.
2N5551 and 2N5401.
The originals will work fine, the higher the hfe the better though. I just use the hfe function on my DMM, match the NPNs and match the PNPs. VERY seldom will you get NPNs to match PNPs but sometimes they do.
Problem solved. I received the transistor assortment last night and matched hfe on the two pairs. I have a DCA55 semiconductor analyzer purchased years ago and hardly ever used. Just for kicks I tested the old transistors: the two NPN tested ok but with low hfe; one of the PNP was identified as a Darlington and the other was identified as "device not connected". Guess that one was the root problem, probably needed some initial current to even start working. The numbers pulled all started with "MPS".
Thanks for everyone's help!
Thanks for everyone's help!