How much can a RIFA crack before it's considered dead? Mettler Toledo PG3001 - Page 1
Forum section: electronics / repair

author: Ionforbes
author_rank: Contributor
timestamp: October 29, 2020, 07:18:56 pm
content: I'm trying to repair a Mettler Toledo PG3001 top-pan-balance which has a blank display, though I think it's still alive on the inside as I can hear a high pitch hum coming from the front 'console' (where all the logic happens; I think the analog balancee stuff is OK). It runs off aan 80C52 uC and the 14.7MHz crystal is oscillating nicely. The logic chips all get 4.8V (good enough for a 5V rail?) and the SN75518 VFD driver is getting 28.5V which seems okay for a VFD.I checked the power supplky first. Annoyingly there is no silkscreen so I just have to guess when a voltage seems right, which they all did (though there was an LM317 outputting 23.6V, which seems a bit odd). Removing the output caps showed no leakage and they all had good ESR and capacitance rating so I put them back in (didn't have replacements on hand anyway). However I did notice 2 RIFA caps on the input, one 47nf and one 220nf. The case shows a bit of cracking but nothing catastrophic. According to my cheapy £5 LCR meter, the big one haas 2.2ohm ESR and 240nf and the small one shows 0 ESR and 56nf. Is it worth replacing these? Would they even have an effect on the working of the scales? Given that the outputsseemOK I don't see why I should spend the money on new ones (to be clear I will put the old ones back in so I don't compromise safety).Many thanks,Ion

author: elekorsi
author_rank: Regular Contributor
timestamp: October 29, 2020, 07:26:43 pm
content: I wouldn't even consider about that one, just replace them. It's cheap and really not worth the feeling that the device may fail weeks or even days after you worked on it...

author: Ionforbes
author_rank: Contributor
timestamp: October 29, 2020, 07:37:22 pm
content: OK so I'll replace them, especially the one with 2.2Ohm ESR, but in the meantime should they have any effect in the functionality of the device? That way I could still troubleshoot the rest of the scales until the caps arrive

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 29, 2020, 07:40:48 pm
content: Close to no effect on the functionality.If you're afraid of them going off in your face, remove them and continue troubleshooting.

author: tggzzz
author_rank: Super Contributor
timestamp: October 29, 2020, 07:43:17 pm
content: Quote from: Ionforbes on October 29, 2020, 07:37:22 pmIf they are performing their usual function, EMI suppression, you can remove them for debugging and replace them later.There will be a time delay between switching on and their making their presence known. There is a good reason they are known as "delayed action smoke generators".

author: Ionforbes
author_rank: Contributor
timestamp: October 29, 2020, 09:06:15 pm
content: Is 28V sufficient for a vacuum fluorescent display? Everything else on all the other boards seems to appear completely fine, and probing random pins on a scope shows data transfeerring and clocks running so I assumee the issue is with the VFD or SN75518. Are there any 'standard' measurements I should make that would show a VFD to be working fine? Also the 23.6V output on the LM317 in the power supply is a bit suspicious, but I don't see the voltage appear anywhere else. Any opinions are much appreciated!

author: srb1954
author_rank: Super Contributor
timestamp: October 29, 2020, 09:33:33 pm
content: Quote from: Ionforbes on October 29, 2020, 07:18:56 pmThose RIFA caps are not long for this world. Best to replace them ASAP. You can temporarily remove these caps from circuit if they are just used for RFI suppression on the mains input without affecting the operation of the circuit.However, I would make sure that one of them isn't being used as a snubber capacitor on power transistor in the switching convertor. The one that measures 2.2ESR might incorporate an internal resistor for use in a snubber circuit. You can't safely operate a switching convertor without the necessary snubber circuits in place.

author: james_s
author_rank: Super Contributor
timestamp: October 29, 2020, 09:44:48 pm
content: I don't proactively replace them, but you've already got it out and you live in a 240V region so I would certainly not consider putting the old one back in. Even if it showed no signs of cracking it's better to just replace, they are not expensive.

author: amyk
author_rank: Super Contributor
timestamp: October 30, 2020, 12:02:16 am
content: 28V is normal for VFD anode voltage. Check the filament and grid voltages too.