Cracked rifa replacements HP 3478a - Page 1
Forum section: electronics / repair

author: ALearningLad
author_rank: Contributor
timestamp: January 19, 2022, 02:35:19 am
content: What should I replace them with? Let me know if I forgot to add any information. Thanks in advance!

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: January 19, 2022, 03:18:37 am
content: Whole list of replacement caps and battery. List may need updated if out of stock or backorder.Highly advise you to watchhttps://youtu.be/e-itiJSftzshttps://www.digikey.com/short/zndr39

author: ALearningLad
author_rank: Contributor
timestamp: January 19, 2022, 04:53:14 am
content: I actually just found that, do plan on recapping the whole damn thing. Should I take those dreaded carbon film resistors out while I'm at it?

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 19, 2022, 08:15:18 am
content: Any appropriately rated (X/Y class) capacitor of the same capacitance and physical size can be used to replace the Rifa caps.I tend to replace all the electrolytics as I showed in the linked video, there is only a few to replace and it can only help with stability and longevity. Just be sure to use good quality caps from known sources.As for the carbon composite resistors, I was just about to have a look at that myself. The ones int he power supply sections could be replaced with equal rated metal films with no trouble, but if they are in the signal path somewhere, or have an effect on the reading of the signal, a recalibration/adjustment may be required after replacement.Having said that, if they have drifted then an adjustment may be needed anyway, and replacing them with metal film would be a good idea (and maybe pull the meter back to being closer in spec, depending when the last adjustment was performed).So the tl;dr of it all is: sure replace the carbon composites if you'll recalibrate/adjust the meter, or double check what the resistors are actually doing if no adjustment will be performed to check if they'll have an effect on your readings.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 19, 2022, 08:55:55 am
content: If there are carbon capacitors in the input section, they may be choosen for high surge voltage. For same size metal film capacitors often have lower maximum voltage ratings. This would especially be the series of resistors in series to the inputs. These are important for the voltage rating and the actual value has very little effect on the calibration. So even if the values would drift by 20% one would likely no see this in the calibration (maybe a little in the offset, but even that effect is small).Some of the resistors (both at the input and in the power supply) may be fusible ones and these should not be replaced with normal ones - so check the service manual before replacing.I don't see a need to replace resistors.The cracked Rifa caps at the input should be relaced with same X/Y class rating new one. The new ones may be physical a bit smaller.

author: m3vuv
author_rank: Super Contributor
timestamp: January 19, 2022, 11:06:12 am
content: Quote from: Kleinstein on January 19, 2022, 08:55:55 am

author: m3vuv
author_rank: Super Contributor
timestamp: January 19, 2022, 11:06:53 am
content: What are carbon capacitors?

author: ALearningLad
author_rank: Contributor
timestamp: January 19, 2022, 02:24:01 pm
content: Indeed what are carbon caps

author: inse
author_rank: Super Contributor
timestamp: January 19, 2022, 02:49:43 pm
content: The analog to electrolytic resistors!

author: Kleinstein
author_rank: Super Contributor
timestamp: January 19, 2022, 02:57:40 pm
content: Sorry for the confusion - it is carbon resistors, not capacitors.At least the moder 34401 versions seem to not use any carbon resistors. So no need to look for the resistors.

author: ALearningLad
author_rank: Contributor
timestamp: January 19, 2022, 03:09:14 pm
content: Wish I was that lucky haha, got me a old 2301 edition none of that newer 26-- rubbish lmao. The good old 1:1 isolation transformers.

author: ALearningLad
author_rank: Contributor
timestamp: January 25, 2022, 05:46:54 pm
content: So I actually have some X2 classes caps, can I chuck these in instead? .22 mic 275/250vac (live in us anyways so it's sure as heck not going close to that), and a .33 mic 250/275vac. Those ok to replace those rifas with? Same size/form factor.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: January 25, 2022, 08:58:45 pm
content: HiClass X and class Y capacitors are different. Class X capacitors can be used between phase and neutral where an internal failure would NOT produce a hazard. Class Y capacitors are designed to be used between phase and neutral to earth where an internal failure could cause a safety hazard.Please use the correct onesG Edmonds

author: ALearningLad
author_rank: Contributor
timestamp: January 26, 2022, 05:12:39 am
content: Fascinating, will need to do a few hours of research on that. Thank you!