HP 651B Help - Page 1
Forum section: electronics / repair

author: outlawpsd
author_rank: Contributor
timestamp: June 04, 2022, 09:10:09 pm
content: I've got a HP 651B that I've had for a few years now and went to use it a few days ago but it was not working properly. I did some quick checks and found the +30 rail was at -.88 and the voltage on the negative rail was fine at -25. Going through the troubleshooting in the manual it recommended pulling the two connectors going from the +30 rail on the A1 board that go to the A2 board and see if the voltage will return to normal. Upon doing that I get +25 on the +30 rail with no load, so I am pretty sure the problem lies on the A2 board but I'm still learning and not sure what to do next? The manual just says check the associated circuits on A2. Not much help.

author: Swainster
author_rank: Frequent Contributor
timestamp: June 05, 2022, 03:16:57 am
content: Recent enthusiast of old test equipment here with no direct experience of this particular model, however from the manual on the Web this board is fairly simple and normal troubleshooting techniques should apply.I'd start with visual/smell inspection to identify likely faults, then check the resistance from the +30 inputs (there are 2 on the Web schematic) to ground. If one of these shows unusually low resistance to ground (perhaps <100 ohms) then I'd concentrate on that particular part of the circuit.At this point I'd start checking the polarised caps in that part of the circuit (resistance mode on your multimeter would give an indication, or better still one of those cheap component testers). If all the caps test good then I'd use the component tester or diode test mode on the multimeter to check the transistors.I'd be pretty surprised if you hadn't found the culprit by this stage.

author: outlawpsd
author_rank: Contributor
timestamp: June 05, 2022, 01:09:52 pm
content: Well, found something last night that's probably not going to make this any easier to troubleshoot. I have some kind of cross between a 651b and a 652a. The A1 board does NOT resemble the one in the 651b manual. Guess I can use the 652a manual for that. The A2 board gets even more interesting. I was checking resistors last night and found some that were out of spec according to the 651b manual. After consulting the 652a manual there were correct, but my board does not have the adjustable A2R16 - its just a 315 OHM resistor that has to be changed out to set the value of TP1 like in the 651b manual.I am guessing this is a later model 651b and they must have tried to keep costs down by using some parts from the 652a. Does anyone have to appropriate manual for this version?

author: factory
author_rank: Super Contributor
timestamp: June 05, 2022, 01:23:13 pm
content: A1 provides the power supply, if the outputs are still incorrect with the load disconnected, then the problem is with the power supply section, or associated chassis mounted parts.You will need a manual that matches the serial number prefix of your unit. Some parts & assemblies will be shared between the 651x to 654x oscillators, you may notice this in the board numbering, the 651B (647 prefix) manual gives 00652-66502 for the A1 board for example.The output attenuator & front panel switching, will be where the most of the differences are between the models.Edit: Looking at the 652A (prefix 1226A) on the web, the late version has the PSU section redesigned to use all silicon transistors and a couple of Op-amp ICs, this has part number is 00652-66504 for A1.The older one (651B, 647 prefix) uses a mix of germanium & silicon transistors, hence why I mentioned looking for the correct manual.David

author: wn1fju
author_rank: Frequent Contributor
timestamp: June 05, 2022, 03:19:13 pm
content: Swainster's comment is exactly the right thing to do. Don't worry about slight differences in schematics or part numbers, and I wouldn't be checking resistors at this point. If the positive power supply is being dragged down by A2, you should suspect that you have a short (or near short) on the A2 board. Disconnect the A2 board and put an ohmmeter across the positive power rails to ground. Assuming you get a low reading, the most likely culprit is a shorted electrolytic cap (in series with a low ohm current limiting resistor on A2).I've fixed many boards like this, including my HP 651B, without even looking at the schematic. If there are a small number of electrolytics on the board, I just leave my ohmmeter hooked up (showing the near short) and start unsoldering one leg of each capacitor until the short goes away. Then replace that capacitor.Other possible shorts are transistors and diodes as Swainster indicates. Simple ohmmeter checks usually find those. But my guess is a capacitor.

author: factory
author_rank: Super Contributor
timestamp: June 05, 2022, 03:40:51 pm
content: If I read the first post correctly, the A1 board +30V PSU output was still low (at +25V)withthe A2 board disconnected, this can't be normal.And the early & late production power supply section design is quite different, they would have had to do the redesign when germanium parts started to become obsolete. The later design uses separate circuits for the +30V & -25V outputs and has current limiting, the early design uses a common reference for both supplies.I've had more transistors in fail than capacitors in oldhpTE, the little plastic package ones occasionally fail, the late design has these in the PSU section.David

author: free_electron
author_rank: Super Contributor
timestamp: June 05, 2022, 03:50:48 pm
content: Quote from: Swainster on June 05, 2022, 03:16:57 amdon't forget to lick the occasional thing , especially when it is powered up...

author: outlawpsd
author_rank: Contributor
timestamp: June 05, 2022, 09:35:09 pm
content: I went through A2 and everything looks OK as far as I can tell and have focused in on A1. Spent the day taking voltage readings on A1 and I hope I am on to something.Without the +30v rail from A1 plugged into A2 -Q3:B 44.3vC 54.2E 23With the +30v rail from A1 plugged into A2 -B 41.2C 52.0E -.3Now does anyone know the replacement part for Q3? The HP manual just has it listed as A1Q3 1854-0474 Tstr: 5i PNP

author: wn1fju
author_rank: Frequent Contributor
timestamp: June 05, 2022, 11:43:48 pm
content: 1854-0474 is a 2N5551.You measured the base at 44.3V and the emitter at 23V? Don't know too many NPN transistors that like a 21.3V drop from B to E!

author: outlawpsd
author_rank: Contributor
timestamp: June 06, 2022, 12:36:45 am
content: Thank you for that got a few on order and will replace and report back. What do you use to cross reference old HP part numbers if you dont mind sharing?

author: wn1fju
author_rank: Frequent Contributor
timestamp: June 06, 2022, 12:12:18 pm
content: There is a 22 MB HP Cross Reference List. One place you can find it is at sphere.bc.ca where you want to navigate to the "Used Electronic Test Equipment + Parts" page, then scroll down and click the "HP to Industry Number Cross-Ref Data" link on the left hand side and then select "Download the Master Consolidated 143 Page HP Cross-Reference"Alternatively, google "HP Cross Reference" where you will find multiple pdfs available for download.

author: outlawpsd
author_rank: Contributor
timestamp: June 07, 2022, 11:16:50 pm
content: Replacing Q3 on A1 fixed the power supply issue. Checked TP1 on A2 and instead of + voltage I have - voltage now for some reason. Didnt have time to go through it tonight but will start digging into it tomorrow evening.