Repair of an HP8904A signal generator - Page 1
Forum section: electronics / repair

author: douardda
author_rank: Regular Contributor
timestamp: March 04, 2016, 12:36:46 am
content: Hi,I have a couple of HP 3904A signal generator. One of them had a failing PSU and I recently took the time to work on it.I've made a description of the problem and the repair on a small blog I'm beginning therehttps://whatever.sdfa3.org/hp8904a-multifunction-synthetizer-restoration.htmlLong story short, the culprit was a resistor on the small "secondary" board of the PSU (labeled R7, a 20k 1% resistor involved in the current limitation portion of the voltage regulatorI've made a (very) partial schematic of the +16.3V rail of the PSU, if it may help someone:Now, I have another (small) problem? I've discovered that the 10MHz ref output is flat on the other HP8904A unit. I've begun to probe a bit around the 10MHz output (which is near the 10MHz xtal), and it looks to me the transistor Q106 is dead.But I cannot find this reference on the net. It looks it's a Motorola, but the numbers (30659) does help so much to find same specs. Having a couple of other same transistors on the board, I think it's a simple PNP bipolar transistor. I wonder if I can use a basic ref like a 2N3905 instead there.Anyone have an idea where I can find some specs of this transistor?BTW, does anyone have the schematic of the A2 (logic) board? It's not in the service manual...DavidLast edit: PNP, not NPN

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 04, 2016, 01:13:06 am
content: Ins't 2n3905 a PNP?

author: rf+tech
author_rank: Frequent Contributor
timestamp: March 04, 2016, 04:47:53 am
content: douardda,The complete HP part number is 1853-0659, one has to know the proper prefix to prepend to these mysterious part numbers.The HP-JEDEC cross reference lists this as a 2N3906(SEL), which indicates it is selected for a specific parameter. We have no way of knowing what the selection criteria may have been but I suspect the single digit 3 on the top line might be a clue. Seeing how this transistor sits in a schematic might provide another hint.For now, pop in whatever 2N3906 happens to be lying about.RF+ Tech

author: brob
author_rank: Newbie
timestamp: March 04, 2016, 04:58:01 am
content: It looks like the transistor has a broken wire on the one leg. might be able to patch it back together...

author: douardda
author_rank: Regular Contributor
timestamp: March 05, 2016, 01:31:56 pm
content: Quote from: SAUL BRITTO on March 04, 2016, 01:13:06 amIndeed. I've corrected my post. Thanks

author: douardda
author_rank: Regular Contributor
timestamp: March 05, 2016, 01:33:28 pm
content: Quote from: rf+tech on March 04, 2016, 04:47:53 amThanks a lot for pointing me to the exact reference number.These custom part references are really a pain in the ...David

author: douardda
author_rank: Regular Contributor
timestamp: March 05, 2016, 01:38:14 pm
content: Quote from: brob on March 04, 2016, 04:58:01 amAmazingly you are right! I did not notice this (impossible to see it by direct sight, it's obvious on the macro picture.) I've made a quick fix by putting a solder drop on the wire and my 10MHz signal is back on the BNC socket.ThanksDavid