HP 3325A Synthesizer/ Function Generator

Repair of excessive spurious signals seen on the output of an HP 3325A Synthesizer/ Function Generator.

[@mnoxman]

The equipment on display is no less "scientific" than the 20 year old X-ray machine in the local hospital is. It may not have that 'new car' smell to it but dollars to donuts you could send them out to be calibrated and they would still be in calibration.

[@CraigPetersen12f36b]

Nice, I also have a 3325A and just did this test myself. I used a HP 70K series SA and discovered using the same parameters of CF 1MHz, Span 1MHz, RBW 300Hz and 1kHz VBW my 3325A @ 1MHz, 0dB out measures -89 to -88.42dB @ 400kHz from the carrier. I have also measured the phase noise performance of the 3325A that I have and have discovered that it is well within the specifications listed in the service manual. These instruments will last seemingly forever if properly cared for. *edit* Guess I'm going to have to get a "lab coat" now. :)

[@Peter-W1]

Beck to the 70´s then when Lab coat was popular haha

[@somedutchguy7582]

The service manual says: "If the VCO locks but there are large spurious signals present at the output, diodes A21CR3, CR4, CR8, and CR9 should be checked." If you do not already have that manual, get it now! It is as much of a gem as the actual generator it describes.

[@garyramsey4275]

@@somedutchguy7582 I do have the manual. I’m sure I’ve checked those diodes, because I’ve checked every component on the A21 board. But check for what, exactly?

[@somedutchguy7582]

@@garyramsey4275 leakage is probably the culprit here. It is difficult to ascertain which diodes these are exactly, but I've read stories of people succesfully using 1N4448/1N4148 as substitute. Could at least be worth a shot, now you _have_ a working unit.

[@somedutchguy7582]

​@@garyramsey4275strange, the answer I just gave has disappeared 🤔. I suspect leakage is the culprit here. As these are Schottky diodes, that would make the most sense. Apparently it is not quite clear which diodes these are exactly, but I've read reports of people succesfully replacing them with 1N4448 or 1N4148. Given that you have a working unit now, it would be worth a shot to try on your defective board.

[@paolomonai9511]

❤❤❤ ...what a nice collection! Congratulations. Regarding the spurs in the original board, I suggest to check Phase Detector and OPAMP Loop Filter in the main loop. You are right: there is an undesidered modulation there... A good strategy could be to test first of all passive elements for aging issues and than exchange the semicinductors with new parts. Good luck and keep us informed. Regards from Italy.

[@TonyBarr99]

I also have a 3325A and it is located exactly as yours, just above my bench. I'll be checking the spurious noise level as well and will report back!

[@W1RMD]

I LOVE your collection! I think that the multiplexing effect looks really cool on camera. I see it a lot on some of the airplane videos I watch. I love how they have the circuit traces on the same side of the board as the ic's. Cool video and nice jacket too.

[@davidv1289]

A lab coat to wear in the lab, very cool! I was wondering if you tried the DAC Offset (para. 5-8) and Analog Phase Interpolation alignment procedure (para. 5-10) on your original A21 board. API alignment removes or minimizes spurious output from the VCO output. I ask because I recently repaired a 3325A and ran the entire alignment procedure. Regards, David

[@garyramsey4275]

I went through the entire adjustment procedure, hoping that it would help. It did not. I’m sure, with enough time and effort, I could repair the problem to component level, and I will probably do just that at some point. Then I’ll have two working 3325As!