The output amplifier board also showed significant heat damage.
It's a little easier to spot looking at the back of the board.
Both transistors in the push-pull output stage were shorts on all terminals. I decided to simply replace all the transistors in the output stage and any passives that were obviously damaged or located over the heat stressed area of the board.
The 2N3866 is still available from Mouser, but the 2N3866A (a higher bandwith spec) is not. Also, I had to track down the PNP transistor. The markings on the part itself 3-440 402 were not helpful to me. The service manual gives an HP part number 1853-0440 wich I was able to cross-reference to an MM4018 with Google's help. I was able to get NOS 2N3866A and MM4018 transistors from US suppliers on eBay
Again, the damaged components were cut out, leads desoldered and the board cleaned with IPA. This time one trace had lifted and a repair bodge would be necessary.
After reinstalling the board and powering up the 3325B didn't throw a "Fail 023" which it had been
However, hooking up the scope showed no output
At this point I powered it off and was going to put it aside for a while, but I decided to track down the part I had heard rattling around inside the case first. After tilting the rather large instrument this way and that and gently shaking, out popped a loose scorched resistor. WTF? The markings were unreadable, but a quick check with the DMM showed it to be 50.0
. No, I thought to myself, it couldn't be the output resistor. A quick check showed that sure enough it was. I had overlooked the filled solder pads assuming they were filled vias
After replacing with a new 50
resistor the generator has an output signal
It's clearly a little out of spec, but I can confirm that at least all 3 attenuator relays are working!
Next I need to tackle the odd HPIB problem...