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8116A Code 42

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Mick B:
Hay Mark, just a quick note: I went through the 8116A again and have results and pictures. I have been writing it up and rewriting it to get the explanations right, comparing both units. I live in Asheville, North Carolina and that has delayed my response I will get back to you as soon as the power is restored and stable. Thanks for your understanding.

MarkL:
Wow - I hope you and family are ok.

Take your time.  I imagine there are other much more important things to deal with right now.

Mick B:
Hello Mark, I'm back. I have been busy helping with the clean-up in outlying towns, things are back to simi- normal in Asheville still have to boil water to drink it, but that should end next week HOPEFULLY!
As for the HP 8116A I have traced the trigger overshoot and +offset to a bad transistor that would be Q262 on Pg. 10.4 schematic 1G lower right corner, (Trigger Output Amp). it tests as a diode. I will need to replace both Q262 & Q263 as it's a diff amp. The HP part number is 1853-0357 the part number on the transistor is 53-0357. I just found one maybe 2 on eBay, better than nothing at least I can move forward with the output. I believe this will clear up the Trigger problem.
Also, I'm aware of the function of the ground spring. I was going through the schematic starting at test point 2 and working my way back to R265. Note: R265 is a little over an inch from terminal to terminal, I found a ground nearby. doing this lead me to the dead Q262. I just over complicated it with the different scope triggers.

MarkL:
Glad to hear things are on the way back to normal!  From what we saw here of the flooding, I'm sure there's still much work to be done.

Q262 and Q263 form a current mirror differential amplifier.  Unfortunately the specs for the 1853-0357 can't be found either, so it's hard to look for a substitute.  I found a couple of references on-line for 2N2894.  I don't think the characteristics in this circuit should be too critical since this section is just amplifying the incoming "TRIGGER SIGNAL" with two stages of current mirror differential amp.

Q260 and Q261 are the first stage using MPSH10 NPN VHF/UHF transistors.  You might try looking for a PNP complement.


Edit: Not a current mirror, rather a differential amp.

Mick B:
Hi Mark, they lifted the boil water warning, that was the last thing for us, the flooding was so bad some of the places are just gone and not coming back, others it will take decades to complete the repairs and bring the population back to its former numbers. Living through this experience REALLY SUCKED, but I feel VERY LUCKY. Enough about the hurricane.

Yes, current mirrors, when I saw the HP part number was the same number on the transistor minus the 18 I knew I was in for a long search, but I wasn't ready for NO INFORMATION other than back in the day they were used in transmitters. Not helpful. I have a pair of used 1853-0357 coming maybe as soon as 22nd. I looked at the MPSH10 there is no complementary transistor. I looked for substitutes for the MPSH10 there are substitutes, but I could find no complementary transistor for these either. they must be special for high frequency 50MHz why VHF/UHF?  Hopefully the ones I'm getting will be ok. then back to the main output problem massive overshoot. I will get back to you after the 22nd.
 Thanks     

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