[REPAIRED] HP3478A repair - help needed - Page 1
Forum section: electronics / repair

author: octal
author_rank: Contributor
timestamp: July 27, 2025, 01:20:31 pm
content: Hello,I got an HP3478A in quite nice cosmetic state. I tried it and it's extremely well calibrated, and I tried measuring almost all my refs at home (voltage, resistors, ...) and I compared it to a new Brymen BM869S and they differs almost only on last digit.What's annoying is that the HP3478A smells burning parts despite the fact it works correctly (apparently). I opened it to check and I found a resistor being changing color (and maybe the capacitor near it). I wanted to know if some of you guys specialist in electronics can help in diagnosing what's happening (I can change any part, repair pcb ...., I miss electronic knowledge).The problematic part seems to be R769 and maybe C760. Below the pictures. I checked the scematic and I don't really want to just change those parts as this may be only the visible "result" of a deeper problem.When I checked R769 with multimeter I got a value (don't remember) but it doesn't seem to be burnt completely.R769 seems to be 316 Ohm 1% 0.125W (from documentation) and it has been completely removed from Revision D of the multimeter (I have revision A)C760 is 0.1µF +80-20% 50VDC CERAlso I checked the battery, it looks like an original battery (there is no trace of soldering rework), it looks like those original Panasonic 3V batteries, but when I measure it (with the Brymen) it shows about 3V6. Is that normal?If anyone can help, I would be very thankful.

author: MarkL
author_rank: Unknown
timestamp: July 27, 2025, 10:27:45 pm
content: C760 may be shorted or have a partial failure. It's the only path where current could cause R769 to heat up like that.I would remove C760 and check its resistance with a multimeter. Normally it would be infinite. Or, if you have a capacitor tester that can test for breakdown voltage, that would be ideal. It's unusual for these types of capacitors to partially fail, but even if it shows not shorted I would probably replace it anyway due to the discoloration (presumably from heat). My guess is that it's probably shorted.While C760 is out, you can measure the resistance of R769 to make sure it's ok and verify it's around 316 ohms.Also, these units can be wired for multiple mains voltages. If your mains voltage is 240V in France and the 3478A is wired for 120V (for example), it might run ok but would put the power supply under quite a bit of stress. The linear regulators could be running hot and contributing to a burning smell.

author: bdunham7
author_rank: Super Contributor
timestamp: July 27, 2025, 10:42:16 pm
content: First, simply remove R769 and C760. These appear to be a snubber circuit for the rectifiers and their absence won't cause any real issues. Clean up the circuit board and then evaluate both components. C760 appears to be an MLCC of some sort and you'll likely find it shorted--although just removing it or handling it could change that. There's really no other realistic explanation for this since at 60Hz, C760 should have an impedance of 20-30k. There's no reason you need that particular type of capacitor so a 0.1µF mylar or PP version that fits ought to be fine. A replacement resistor can be a 0.25-0.40W or so (whatever fits) carbon film or metal film of 330R or so. IDK why they used those particular components in the first place, 316R is a weird value and unless they hand-tuned the snubbers for each unit the tolerances of the capacitors would seem to rule out any need for that exact value.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 28, 2025, 09:28:04 am
content: A candidate for burned smell are EMI capacitors. There may be a filter included in the IEC power input connector.

author: octal
author_rank: Contributor
timestamp: July 29, 2025, 09:35:07 am
content: Thank you all for your help.@MarkLAt France we are (like almost all Europe) at 230V and my 3478A is correctly wired to the 220V input (this is fine 220+/-10%), I can switch it to 240V (maybe I'll do it).I didn't got time to check what's happening, I ordered all parts from Mouser and will work on it hopefully this weekend. One of the reasons I can see for this resistor to burn is if the diode CR760 died and become open the only remaining path for current will be through R769.I ordered replacement for those diodes (both 1N4004), ordered a replacement for U550 (LM339 if I remember) and U760 (5V regulator), with their corresponding resistors and capacitors. I found the 316Ohm resistor (unusual value).I'll work on it this weekend and keep you informed.Thanks for all.Regards

author: Kleinstein
author_rank: Super Contributor
timestamp: July 29, 2025, 10:55:57 am
content: The resistor in the snubber could also have been stressed from a noisy main, like operating near a poor PV inverter or with a low qualitity UPS.

author: octal
author_rank: Contributor
timestamp: July 29, 2025, 12:35:50 pm
content: @KleinsteinThank you for your input.I’m not entirely sure how the unit was used before, but I’ve noticed something interesting: I have another HP3478A with a later PCB revision (Rev-D), and in that version, HP completely removed this resistor—this change is documented in the Service Manual.Some have suggested replacing the original 1/8W resistor with a 1W or 2W version to improve its ability to handle power. Personally, I believe that’s not the right approach. Increasing the power rating might only mask the real problem. To me, this resistor burning out is likely a symptom of a deeper issue—possibly excessive current draw caused by a failing regulator, leaky capacitor, or damaged comparator.So yes, I’ll replace the resistor, but I definitely plan to investigate and identify the root cause before powering the unit again. I’ll share any findings here in case it helps others facing the same issue.

author: bdunham7
author_rank: Super Contributor
timestamp: July 29, 2025, 02:03:05 pm
content: Quote from: octal on July 29, 2025, 12:35:50 pmIMO the only tenable explanations are the failure of C760 or exposure to much more HF noise than expected. IDK why these parts were revised out but perhaps this was a common issue. If you verify that the transformer part number is the same for Rev A and Rev B-D, then perhaps you could replace the resistor with a jumper or much lower value and then make sure C760 is OK.

author: m k
author_rank: Super Contributor
timestamp: July 29, 2025, 07:18:32 pm
content: So the resistor needs over 6V before it starts cooking.How much is the raw AC voltage?

author: octal
author_rank: Contributor
timestamp: July 29, 2025, 07:22:12 pm
content: I didn't start exploration yet. I ordered replacement parts and also the (RIFA and electrolytic capacitors) and the Mouser package will arrive on Friday, hopefully I'll be able to work on it this weekend. I'll also change the backup battery this weekend (should arrive tomorrow from Germany).

author: m k
author_rank: Super Contributor
timestamp: July 29, 2025, 07:26:21 pm
content: Check that GPIB backup method before you start fiddling with the battery.

author: octal
author_rank: Contributor
timestamp: July 29, 2025, 08:40:18 pm
content: Already done. I used an Agilent GPIB tag and I backed up calibration parameters and checked that the CRC is ok.

author: octal
author_rank: Contributor
timestamp: August 01, 2025, 01:07:04 pm
content: Well, I started my repair journey with the multimeter.Well C760 broke completely when I just touched it to remove it, and R769 measure give 245 Ohm instead of 316 Ohm. Diodes measurements shows they are both ok.I soldered new parts (received them from Mouser yesturday). So new resistor, new capacitor and I changed also both the two diodes 1N4004 CR760 and CR761.Now the multimeter starts OK and I have no burn smell, BUT:I measured the voltage AFTER the U760 regulator and it shows 5V, this is perfectBut when I measured the voltage at the output of the diodes so before the U760 regulator, I found a little bit beyond 13V, while from the schematic it's supposed to be unregulated 5V.Am I reading badly the schematics, or is there a problem (maybe C761 the big 3000µF has problem)?Any suggestion would be welcome

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2025, 01:15:08 pm
content: The voltage at the input of the regulator has to be quite a bit higher than 5 V. A 7805 would want at least 8 V to be on the safe side. There chould be alowance for maybe 1 V of ripple and the mains voltage can drop by 10%. The old design may also consider a nominal 220 V mains. So some 10-11 V would be the minimum for a sound design and a bit more to avoid an early brown out is good practice.A problem with C761 would more cause a reduction of the voltage.So the 13 V look OK and it would be an odd failure to cause the voltage to go higher.

author: octal
author_rank: Contributor
timestamp: August 01, 2025, 01:20:34 pm
content: Yes, you are right (we have 230V at France as in most European countries). I think this value is OK, as the voltage is about 14V ACV at the output of the transformer.Now I'll replace the RIFA capacitors and the battery for calibration data backup. I'll also change the electrolytic capacitors.

author: nfmax
author_rank: Super Contributor
timestamp: August 01, 2025, 01:41:38 pm
content: The 3478A has an internal line voltage selector that has positions for both 220V and 240V. Depending on your local voltage, you might do better setting it to the higher voltage. The service manual covers this. You just solder a flying lead to one of 4 test points (!)

author: bdunham7
author_rank: Super Contributor
timestamp: August 01, 2025, 03:15:08 pm
content: Quote from: octal on August 01, 2025, 01:07:04 pmThat's probably the whole story right there--C760 shorted and caused too much current in R769. It should be fixed now.I second the notion of changing your mains transformer to the 240V tap, things will run cooler that way and these have plenty of overhead for the power supplies. The 13V you are seeing pre-regulator is not unusual for designs like this. It works if the currents are small enough.

author: octal
author_rank: Contributor
timestamp: August 01, 2025, 06:44:25 pm
content: Quote from: nfmax on August 01, 2025, 01:41:38 pmThe voltage selector was on 220V (the multimeter was used in a lab at Denmark), most European countries are at 230V (like here at France), so it should work normally because we are at V +/-10%, but I prefer to have it under powered than overpowered. With the selector in the 240 V position, the allowable range becomes 216–264 V (240 V ±10%) which covers almost all kind of fluctuations.

author: octal
author_rank: Contributor
timestamp: August 02, 2025, 08:57:52 pm
content: Well,- I checked all voltages at various test points suggested by the service manual and all of them are at correct value and very stable.So I consider the multimeter as completely repaired.Since I ordered other parts for this multimeter from Mouser, I have gone in a long maintenance journey:- I took opportunity to replace all electrolyctics despite the fact they were in good shape. I prefered to use new ones with a little bit higher voltage for some.- I replaced the battery backing the SRAM with calibration data. I used an external battery as backup (soldered in parallel with the needed diode at the end), removed the old battery and installed the new one. I have done that with peace in mind because I used a python script fromhttps://t.co/GeoBgOxVNp(thanks to its author and to EEvBlog community) to backup all calibration parameters via GPIB before I start the procedure. Replacement battery was an FDK (Fujitsu Dielectrics), brand of battery already used by HP in some series of the 3478A.- I also replaced the RIFA. Replacing RIFA today was the best decision I took, old one were really preparing to spread magic smoke. They were already cracked (check images next post).I tested the multimeter and it works perfectly, and since it has been calibrated recently, I think with all those refurbishments it's ready to serve for at least upcoming 20 years.Thanks to all of you for your help.Kind regards

author: octal
author_rank: Contributor
timestamp: August 02, 2025, 09:03:49 pm
content: As I said, changing the RIFA was the best decision of the day, here is the state of mine.Hopefull I changed them.Accessibilty was a problem. I had to detach the power switch and the regulator from the casing side, and for the small ones at the rear I had to disassemble the GPIB connector to be able to access them and replace them.