Used HP 6644A: Transformer hum and a questionable X2 capacitor - Page 1
Forum section: electronics / repair

author: pigrew
author_rank: Frequent Contributor
timestamp: August 24, 2016, 12:00:05 am
content: I recently bought a used HP 6644A power supply (from 1993 or so, a bit over $100 on eBay (parts only/not working)), and am trying to do a quick check-up on it. I have two small concerns that I'm unsure if I should ignore or not:1) Taking a look at this unit (and also my 6633A), the clear plastic surround of the Y2 capacitor has some small cracks. Should I expect it to explode soon? [see attached cell-phone photo]2) The transformer hums at 60 Hz. It gets louder when the output current increases. It never get very loud, but it bothers me that it makes a noise while the other units don't. Or... this unit's fan is off, so perhaps other units make a hum, but I can't hear it over the fan noise? Do I need to worry about the buzz? Is it indicative of some other component failing?Otherwise, it works, and its calibration of voltage and current outputs seem dead on (have not verified OVP or COP). With its output disabled, it outputs about -0.07 V, which I think is normal [I also have a 6633A which outputs a bit of voltage when disabled]. I did have to resolder the output connector (cracked solder joint because there is no mechanical support of the output jack).Thanks,NathanEDIT: X2, not Y2

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 24, 2016, 12:39:00 am
content: Yes, that cap is going to explode very soon! I know from experience!The transformer screws might just be loose making it vibrate.EDIT: my personal experience

author: pigrew
author_rank: Frequent Contributor
timestamp: August 24, 2016, 06:32:56 pm
content: Next up is the fan. It seems to be (nearly) an open (very low current with 12V supplied).I tried to check the PSU's output voltage to the fan (with no fan connected), and it is 27 V. But, after spending a while with the circuit diagram (and checking many testpoints), I noticed that there is a 270 ohm pull up resistor (R778) from the fan fanout to the 25V rail (which is actually about 27V). So, I guess this is the expected behavior, though I don't know WHY one would design the supply to output such a high voltage to a 12V fan while it's current is low.Time to buy a new fan.

author: wraper
author_rank: Unknown
timestamp: August 24, 2016, 06:36:44 pm
content: Those are infamous exploding Rifa caps. Replace Rifa Y capacitors too.

author: wraper
author_rank: Unknown
timestamp: August 24, 2016, 06:39:10 pm
content: As of vibrating, that's most likely not transformer itself but enclosure vibrating in it's magnetic field. You also likely hear some louder sound on turning on.

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 24, 2016, 11:56:26 pm
content: *GASP* That company is still around!Rifa is now called "Kemet". Apparently, audiophools buy them, I hope they like "party poppers"!It might be a 24V fan, notice Q770 is shunting that resistor, I assume to speed up the fan. So it may not draw much current at 12V (or it is actually bad, you'll have to test it).

author: pigrew
author_rank: Frequent Contributor
timestamp: August 25, 2016, 12:20:01 am
content: Quote from: Cyberdragon on August 24, 2016, 11:56:26 pmI ended up ordering identical Kemet/Rifa capacitors from Digikey. They were the only available capacitors with the same footprint. I hope that their experience over the last 25 years has improved their productThe part numbers I used for the 6644A are "2x (PME271Y410MR30) and PME271M660KR30". I also ordered the capacitors for my 6633A (which shows the same cracks): "2x (PME271Y422MR30) and PME271M622KR30". They both take the same line of caps... just different values.Quote from: Cyberdragon on August 24, 2016, 11:56:26 pmIt's a 12V fan (Papst 612), rated for up to 15V. It's supply bus is unregulated, nominally 25 V, but 27 V for me. My line voltage is 120.3V. Perhaps I should double-check the transformer connections. I did verify that the voltage-switches are set to 120V.I attached a slightly smaller load than the fan should be, and watched the voltage fall to about 12V. But, the voltage should really have been lower because this unit lowers the fan speed based on the heat-sink temperature (but wasn't due to my using a 1W fan instead of a 2.5W fan). I ordered the official replacement fan, which is a " NMB 2410ML-04W-B60" off of eBay (I hope it's a genuine product, and not some knock-off). The new fan's connector is wired differently, so I'll have to change its connector before using it.One I get the new fan and replace the EMI filter, I'll do a proper load test. I'll then try readjusting the transformer mounting to see if the hum goes away.

author: wraper
author_rank: Unknown
timestamp: August 25, 2016, 12:30:24 am
content: Quote from: Cyberdragon on August 24, 2016, 11:56:26 pmKemet bought quiet a lot of companies. Rifa capacitors are fine, except those transparent time bomb series.

author: VK5RC
author_rank: Unknown
timestamp: August 25, 2016, 03:20:40 am
content: I have a couple of the 66xx series and they all hum to some degree, fans are not quiet, but otherwise a tough PSU. The Rifa caps issue I recall is only from a selected series and also includes the IEC/EMC filters that had them inside. The manual is hard to read as the schematics cover several models on the same schematic (FFS!) so for some models they don't have parts shown on the schematic, you have to refer to the parts list and edit the schematic!!!

author: calmtron
author_rank: Regular Contributor
timestamp: August 25, 2016, 06:00:56 pm
content: I just had one of those lovely RIFA caps explode on me a couple of days ago. Took a while to clear the apartment of the magic smoke.

author: pigrew
author_rank: Frequent Contributor
timestamp: August 27, 2016, 05:55:36 am
content: I've received the replacement EMI filter caps, and popped them into both my 6644A and 6633. Note that the new capacitors have MANY fewer regulatory logos than the old ones, and also the new ones are rated for a higher voltage (see attached image).And then, I noticed that there were another few of the same Rifa Y2 caps on the output terminals (to ground), also with micro-cracks in their case. With these, there are also polypropylene film caps with the same dimensions that I could use.So, do I go with the paper capacitors (which may be less likely to catch on fire and maybe better at self-healing though may explode in another 20 years), or polypropylene (which is slightly cheaper, and has a higher DC voltage rating, and isn't yellow).I'm leaning towards the 300VAC rated Rifa caps. I should double-check to see what Keysight uses these days.

author: singapol
author_rank: Frequent Contributor
timestamp: August 27, 2016, 06:46:27 am
content: Since you bought the Rifas your choice to use or not (if they are of recent manufacture). Personally I would use plastic polypropylene caps similarly rated (you wil not have cracked clear polycarbonate casing in future).

author: tronde
author_rank: Frequent Contributor
timestamp: August 28, 2016, 04:17:22 pm
content: Quote from: pigrew on August 27, 2016, 05:55:36 amThe old one is from 1993 and pre CE-mark era. Today the ENEC mark replaces the old national logos.https://en.wikipedia.org/wiki/ENEC_MarkThe ENEC mark is a trade mark so the chinese can't just cheat as they do with the non-trade marked CE pretending their version to mean Chinese Export or whatever they call it when caught pants down.Edit: When I write trade mark, I mean registered trade mark.