HP 33120A  repair.... I need help [EPILOG] - Page 1
Forum section: electronics / repair

author: Unknown
author_rank: Unknown
timestamp: May 08, 2013, 01:35:16 pm
content: HelloI bough my HP 33120A from ebay for about 130 Euros, it was sold "as-is", and i got it today.It is not working properly.Let me describe the problem:33120A is switching on without problemsSelf-Test: PASSThe menu and display are working properly.The frequency is adjustable as it should be.Sync-Ouput seems to work just fine and follows the output frequency pretty accurate.The main output is srewed. Instead of desired frequency i get permanent -17 V.If i try to andjust the output amplitude the FG crashes randomly: display blinking and doesnt react to anything.In menus no error is shown.I am complete newbie in repairing thing, but i have everything i need : oscillocope, multimeter, soldering station:)Does anyone has an idea what is wrong with this device, or where should i start my journey?=)I am also looking for HP 33120 schematic, but cant find anything. Can someone pls post a link?I am really thankful for any help:)

author: c4757p
author_rank: Super Contributor
timestamp: May 08, 2013, 01:48:54 pm
content: Sounds like the negative (PNP) side of the push-pull output amplifier is blown. Shorted, perhaps, and upsetting the power supply (hence the crashes). Look for large-ish transistors near the output and test them out of circuit.Disclaimer: I've never had this particular function generator, haven't seen inside it, and haven't looked for a schematic. They're all reasonably similar, though.

author: c4757p
author_rank: Super Contributor
timestamp: May 08, 2013, 02:09:29 pm
content: Schematics are in the service manualhere. Second result on Google for "hp 33120a service manual"! How did you search for that - reference section at your local library??

author: Unknown
author_rank: Unknown
timestamp: May 08, 2013, 02:12:00 pm
content: Ok, i have done optical examination...Some things look strange.1: is it a fuse? And if it is one, why the F..k is it shorted??2: I found some crusty resistors(?) the doesnt look pretty healthy at all.Or is it normal for an old circuit?I have to go now, i will come back in few hours, and measure those bastards.

author: c4757p
author_rank: Super Contributor
timestamp: May 08, 2013, 02:17:45 pm
content: Now that you have the service manual, don't just speculate. Find anything that is questionable in the schematic and figure out what you should expect from it and what could cause failure. Even better, if there are troubleshooting procedures, follow them (I'm on my phone so I can't be bothered to check).

author: benemorius
author_rank: Regular Contributor
timestamp: May 08, 2013, 03:48:00 pm
content: Give the service manual a good long read. An HP manual from that era should still have some useful troubleshooting intuition to glean.

author: Paul Price
author_rank: Super Contributor
timestamp: May 08, 2013, 03:58:02 pm
content: The easiest way to kill a signal generator is to damage the output circuit, check out this area of the output amplifier.

author: Unknown
author_rank: Unknown
timestamp: May 08, 2013, 05:42:10 pm
content: Ok, i measured the R's around the damaged output area, here are the results.Most roasted resistances shows 35 kOhm instead of 100 Ohm.As shown in attahced PDF-File, all measured values differs from the datasheet. I think it is mainly because they are all somehow parallel to each other. So now i will calculate according to he schematics, if my measurments are correct.I just dont understand how can you burn a FG like tah, Why the hell you should apply high voltage on the OUTPUT??Moreover i think i will replace the fuse, now it seems to act like jumper:)Anyone knows, if i can use simple "very fast fuse" instead of original agilent one?

author: free_electron
author_rank: Super Contributor
timestamp: May 08, 2013, 06:47:37 pm
content: allright. looks like some dillweed really f-ed up that machine.that fuse is 0.5 ampere fast blow signal fuse from littlefuse. if you inject DC into the output the fuse would pop , protecting the output.but some dodo clearly soldered a couple of copper strands across it ( most likely because he blew it a few times and did not have the right fuse )so now, he fried the output attenuator.things you will have to replace :- all resistors in the output attenuator. these need to be KOA resistors or you will never get amplitude flat in the high range.- the aromat relays in the attenuatorsbut , before you even start with that you need to fix the output amplifier.most likely the two big thransisotrs with the circular heatsink on em are fried....

author: Unknown
author_rank: Unknown
timestamp: May 08, 2013, 08:53:31 pm
content: Ok, seems like 16 Resistors are dead...+ I think those two transistor are also dead. At least one of them gives short with my diodtester.So i will buy 2 of them from ebay, and theni will look for resistors.

author: Unknown
author_rank: Unknown
timestamp: May 28, 2013, 10:45:40 pm
content: So, if anyone wonders what happened to this FG, here are my results.I had to replace both Motorolla transistors Q713 and Q178. Then i really tried to desolder those fried resistors. But the problem was, that most of them were burned in the PCB, so it was almost impossible to remove the chip, and not to damage the trace.But somehow i managed to rescue some pads. On places where pads where removed completely, i just soldered resistors sandwichwise,they are parallel anyway.For one resistor i had to use jumper...instead of 69 Ohm i used 69.8 Ohm.One more thing: i had to remove the protection diodes CR801 and CR802, because one was dead. The device seems to work without problems. Do i need to replace them, or is it not necessary?All in all the result is satisfying. Even if it looks ugly, the frequency is very accurate, all functions are working great. The only thing is, that the amplitude is about 10% off (except Sinewave). Maybe some calibration will help there.I will attach some pics, but pls dont shoot, it was my first repair attempt:)

author: SeanB
author_rank: Super Contributor
timestamp: May 29, 2013, 03:51:49 pm
content: Put the diodes in, and get a pack of spare fuses as well, and keep them in a sealed package with a desiccant sachet inside the meter. Hard to not peel tracks with having charred remains there, just clean up the board with a fine brush and alcohol, and trim where you cooked the relays with the soldering iron.

author: sorbik
author_rank: Newbie
timestamp: May 30, 2013, 05:17:14 am
content: Quote from: Citizen on May 08, 2013, 05:42:10 pmI've seen techs put the output of a 5700 meter calibrator into another 5700 thinking it was a 5790 AC measurement standard (they look pretty identical). Maybe they thought it was a fancy... function meter.. with bnc inputs?

author: Unknown
author_rank: Unknown
timestamp: May 30, 2013, 12:49:02 pm
content: I somehow cant find a replacement diodes. The datasheet says they areDIODE SI PN SOT23 100V 750MABut i cant find anything i googleCan anyone link me the manufacturer part number?

author: arekm
author_rank: Unknown
timestamp: December 13, 2013, 09:33:27 am
content: Quote from: Citizen on May 30, 2013, 12:49:02 pmList of parts says:CR801-CR802 1901-1386 2 DIODE SI PN SOT23 100V 750MA 28480 1901-1386where you google for agilent part number 1901-1386 and in dochttp://cp.literature.agilent.com/litweb/pdf/5989-5867EN.pdfyou can find that this is:1901-1386 DIODE-SWITCHING 100V 1A BAS78B OSRAM