The 3326A has been working for several days now, with no problem, but the current repairs were only temporary, with some parts that were at hand, but not quite up to the requirements.
Still, I didn’t have the service manual, and no luck on the internet, except for some companies that want to charge me 20 or even 50 USD, for a 30+ year old manual. Well, these manual dealers somehow have to make a living, but I have no dollars to give away, just to get some part numbers. After a few requests sent around to various people that might be proud owners of a 3326a service manual. Finally, some luck: a generous engineer out there provided me with a pdf copy today, of the service manual HP 03326-90010.

Let’s get started on the remaining items to call this repair complete:

(1) Replace the input cap with a 100n X7R, 100 Voltage rating – so far, only 50 V was in stock, but recently, a package of Kemet brand, mil-spec C430C104M1R5CA arrived, hope these will last!

(2) The over voltage SCR crow-bar circuit (currently with a 1884-0261 installed, 4 Amp rms). Looking at the parts list, this needs to be a 1884-0231, a very rare HP numbered SCR. Apparently, it is a TIP116A, 100 V, 8 Amp rms, 20 mA max. gate trigger current equivalent. So the 4 Amp part currently in there might be a bit too weak.

The only SCRs around are a 1200 V, 25 Amp, 40 mA max. gate trigger current.

Question – are there any issues to be expected, when replacing a 20 mA max. gate current, with a 40 mA gate current part? Well, there could be. After a more careful look at the TYN1225 datasheet, there is some hope that it could work. 4 to 40 mA, that’s a large range. Maybe the part will trigger already at much lower current, given that I don’t intend to operate the instrument at very low temperatures, below freezing, where the SCR gate trigger current typically increases.

After a quick test with a power supply, a resistor, and a current source – the particular TYN1225 which I randomly picked from the bag triggers at about 4.5 mA. All should be fine.

Note that the SCR is triggered via the Q800 transistor (which has been checked and found OK), and this circuit could supply well over 30 mA to the gate, if needed… also, there is connection via the mother board, so the over-voltage circuit will only work when the power supply assembly is mounted in the 3326A. Otherwise, the gate trigger will be open circuit.

… with the SCR soldered in, the 3326A repair is now complete!