On Apr 3, 2024, at 22:05 , Adam R. Maxwell via groups.io <amaxwell@...> wrote:
He suggested replacing Q309/Q317 in the trigger pulse generator with the pair from my working 7B70, and that did the trick: stable waveform with sine/triangle/square, and controls work as expected.
I had previously tried replacing them individually with a 2N3906 (one of the few PNP transistors I have on hand), with no luck. Replacing both Q309 and Q317 with 2N3906s actually shows a waveform instead of a blank screen, but it stutters so I've ordered some 151-0220-00 replacements.
Messing around with this a bit more tonight, I tested both of the 151-0220-00 transistors from the bad 7B70 in my cheapo tester (BSIDE ESR02 Pro). It recognizes both as PNP transistors, one with B=187, Uf=682 mV and the other with B=150, Uf=676 mV.
The 7B70 manual (S/N <205000) specifies 151-0220-00 for Q309/Q317, and the Common Design catalog says a 2N3906 is close (fT is 400 MHz for the 151-0220-00, 2N3906 is 250 MHz). I tried stealing a pair of 151-0220-00 transistors from elsewhere on the trigger board (assuming they were fine) for the Q309/Q317 sockets (replacing the donors with 2N3906s), and the bad 7B70 still did not work.
I finally realized my good 7B70 has a >205000 serial number, and the manual for that specifies a 151-0271-00 (which explains why the ones that work in both plugins are marked 151271). That transistor has a minimum fT of 2 GHz, and is an AST4261 or 2N4261 in a TO-92 case, so I might be SOL if the bad 7B70 needs those.
I assume the (much!) slower transistor worked at some point in the low serial number unit, but Tek must have had a reason for that change. I've grown a bit less optimistic that NOS 151-0220-00 transistors will be the silver bullet, though.
Further bulletins as events warrant, etc.
Adam
