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HP 33120A waveform is distorted. Video inside. What is going on?

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strawberry:
TTC015B

fran:
If the schematic is to be believed - and it "looks" like it matches the board, the transistors are:

AD711
PMBTH10
MMBTH81
MMBT3906
MMBT3904
PZT3906
PZT3904
2n5901
2n5583

Some things are in stock at mouser (AD711, MMBT3906, MMBT3904, PZT3904) but others are not, nor are they in RS or farnell. I must try digikey..... but this is looking increasingly like a dead duck.

As outlined above, I did swap in some BD139/140 as a quick check on the output transistors, but still had exactly the same waveform problem.

free_electron:

--- Quote from: fran on August 08, 2022, 09:20:51 pm ---If the schematic is to be believed - and it "looks" like it matches the board, the transistors are:

AD711
PMBTH10
MMBTH81
MMBT3906
MMBT3904
PZT3906
PZT3904
2n5901
2n5583

Some things are in stock at mouser (AD711, MMBT3906, MMBT3904, PZT3904) but others are not, nor are they in RS or farnell. I must try digikey..... but this is looking increasingly like a dead duck.

As outlined above, I did swap in some BD139/140 as a quick check on the output transistors, but still had exactly the same waveform problem.

--- End quote ---

let me check what i have laying around. i'm sitting on three of these generators with various faults. (one has a failed sram) . i know i have some o f the transistors and i want to buy the remaining ones as well.

-sent pm-

fran:
I'e continued looking at this now and again over the last week or so.

I found a faulty zener on the output transistor supply - CR710. I replaced that but it didn't fix the issue.

I also found that this board (which I cannot see any version marking on) might be a little newer than the one in the service manual. It uses motorola SO-8 transistors for the driver stage of the amp circuit:

Q715/716 were PZT3904 are MRF5943 (NPN)
Q711/714 were PZT3906 are MRF5583 (PNP)

Diode junctions measure fine on each of these.

When I measure DC on Q712 and Q717 - there is a difference - voltage on the emitters of both is spot on +/-13V or so, base again is equal at around 12.x volts on each. The collector though shows some difference - about 2V less on the 3904 compared to 3906

free_electron:
nope, that is version 1.0 with the so8 transistors. i have repaired two of that generation. i don't know if i still have spares for those , back then they were already extremely difficult to find ( i'm talking 10+ years ago ... )

The schematic for that machine is print only. none of the pdf's out there have the schematic. There are other differences too. the main DAC is a totally different animal. it uses a raytheon dac, where the new machines are an SPT ECL dac with associated level shifting.

check cr703 and cr704. cr710 should have a counterpart on the other rail as well

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