HP3312A low amplitude output, sanity check on solution - FIXED - Page 1
Forum section: electronics / repair

author: Grey_Dawn_Breaking
author_rank: Newbie
timestamp: July 10, 2025, 12:35:16 am
content: Hi all, I'm fairly new to all this so pardon my ignorance. I'm hoping some folks could look at the following process I undertook to solve an issue and tell me if there's something I haven't overlooked:I have an HP3312A which was only outputting around 1.5V when the attenuator position was at the "10" and the vernier dial was fully clockwise. If I turned the attenuator to 1, .1 or .01 all I got a noisy signal in the millivolts range. I tried running the signal both through a 50terminator and without, and this didn't help at all.I started working my way through the service manual, doing readings at each test point and comparing the readings. The first bunch of TPs were on par with the manual but then I came across a few discrepancies:1) voltage at TP7 was higher than it should be: manual shows that it should be around 1.2-1.5V, but mine was 5V.2) TP 8 should done with the o-scope at 0.2ms/div, but mine needed to be at 20microseconds to match the same waveform.3) TP 9 should be around 5V; mine was 1.5VNot surprisingly, I started poking around more carefully near the "amplitude mod. and output amp//attenuator section" on board 03312-66501. The voltages near some of the transitors were off so I removed all of these to test them out of circuit:Q501Q502Q503Q504Q506Q507Q508Q509Q511I found that Q507 and Q508 are bad - they each only pass the usual diode range of voltage of 0.4-0.8 volts in one direction from Base to Emitter or Base to Collector, instead of in both directions.So I am going to replace both Q507 and Q508. I also checked the components in-circuit nearby these two faulty transistors, which include: CR502, CR503, CR504, CR506, CR507, CR508, RS29, RS32, RS31, RS33, RS34, RS36, RS37. All components seem to be ok.While I order and wait for the new parts I was wondering if there's anything I might've overlooked and/or if anyone else has had a similar issue with 2 transistors on opposite sides of the circuit crapping out like this.I'm also wondering if I should've tested the components leading into the base of Q507.Has anyone had this kind of issue before? Am I on the right track in putting in new transistors and switching it on, or are there some other things I should test beforehand?Any insights/suggestions for a newbie would be really appreciated, thanks!

author: Grey_Dawn_Breaking
author_rank: Newbie
timestamp: July 10, 2025, 12:34:56 pm
content: I should add that I applied contact cleaner to the connectors between the boards a couple of times and slid the connectors on and off many times, as I've read on other threads that bad connectors are often the issue in these machines.

author: Grey_Dawn_Breaking
author_rank: Newbie
timestamp: August 02, 2025, 05:01:44 pm
content: For future reference for anyone who might have the same problem, here's what I did: I only needed to swap out 2 transistors that were bad. I also put in a few new resistors nearby because they had started to turn brown from a previous short. The challenge was that some of the transistors used in this unit are really hard to find, either because they're not made anymore or because the part # doesn't show up anywhere.These are the transistors I swapped:Q507 - listed as 1854-0351 in the manual --> used a 2N3904Q508 - listed as 2N4035 (which isn't manufactured anymore) --> used a 2N3906I also put in new resistors around those transistors: R531, R533, R534, R536, R537.