HP 6227b pots - Page 1
Forum section: electronics / general

author: Prime73
author_rank: Regular Contributor
timestamp: February 28, 2014, 06:09:29 am
content: I'm trying to locate double potentiometers for my HP 6227b power supply. There are two 5K - 50 and two 1.1K - 10. Does anyone know where the buy these?

author: Prime73
author_rank: Regular Contributor
timestamp: February 28, 2014, 03:58:25 pm
content: I've opened one of them:What's all of this grease inside? just a dirt, etc? Is it safe to clean it up completely with alcohol or something?

author: katzohki
author_rank: Frequent Contributor
timestamp: February 28, 2014, 05:03:20 pm
content: Probably electrical contact grease on there. Measure the full range of the pot and the number of "winds" to find the resolution. Then go shopping for an equivalent.

author: TerraHertz
author_rank: Super Contributor
timestamp: March 02, 2014, 01:42:04 am
content: Quote from: Prime73 on February 28, 2014, 06:09:29 amI'm not sure what you mean by 'double pot' if that picture you posted is an example. That is just a standard wirewound single turn, single gang pot.The grease is important; unless it's gone hard with age, leave it there. It stops the metal contacts oxidizing, and also lubricates. It will also be a grade designed for electrical contacts, not just any mechanical grease.Also I don't know where your values came from: "5K - 50 and two 1.1K - 10".Those wirewound pots are always 'zero to value' range. Like 0-10, 0-50, 0-1K, etc. Often in the HP circuits there'll be a resistor in series with the pot, on the PCB. Do you have a manual with schematic?A while ago I had the same problem - a few HP power supplies needing pots (they were configured as slaves, and had no pots fitted.) These:HP 6263B 0-20V, 0-10A.HP 6267B 0-40V, 0-10A.HP 6268B 0-40V, 0-30A.The pots needed were Clarostat 'Series 43' 5% 2W Wirewound pots.I'm in Australia, and I couldn't find any local supplier with the multiple wirewound pot values I needed. Eventually I found an ebay seller in China, who had some values in stock and very helpfully was able to source all the other values from other stockists. Good prices too. That seller:http://myworld.ebay.com/easywayelc/One more thing - don't be tempted to substitute non-wirewound pots. Reason is, in the HP circuits there is considerable current through the wiper to resistor contact point. Carbon film and even ceramic film pots don't like that at all - the resistive film gets damaged at the contact point, developing an open circuit at that spot. When the pot is controlling the output voltage of a beefy power supply, you don't want intermittent open circuits in the pots.

author: Rigby
author_rank: Super Contributor
timestamp: March 02, 2014, 05:13:14 am
content: I replace those on all my supplies. the coarse/fine control they use is terrible.I bought some resistors to put in parallel with 5k 10-turn pots I have for this reason. find two 2W 10-turn 5k pots, and just use one straight as it is for the voltage replacing the fine & course there with the single pot. for the current, find a resistor that parallels with 5k to equal 1.1k, and use it in parallel with the 5k.I can take a picture if you want to see how it is done.

author: Rigby
author_rank: Super Contributor
timestamp: March 02, 2014, 05:15:59 am
content: Quote from: TerraHertz on March 02, 2014, 01:42:04 amthey are ganged. two 1-turn pots, a 5k and a 50 ohm in series for the voltage. the current control is usually a 1.8k, 1.2k, or 1.1k and a 20 ohm in series. the picture only shows one, yes.

author: TerraHertz
author_rank: Super Contributor
timestamp: March 02, 2014, 07:35:06 am
content: Quote from: Rigby on March 02, 2014, 05:15:59 amOh right. I have some of these supplies and like the two-knob coarse-fine system. Mostly I just leave the fine one set at center, so it's there when needed. Usually there's no need for more accuracy than the coarse one provides, and multi-turn pots are a pain, like when you just want to go from 5V to 20V quickly.

author: Prime73
author_rank: Regular Contributor
timestamp: March 02, 2014, 07:47:07 am
content: ok. thanks for the clarifications. One of the pots I have was loosing contact at one position and that's why I took at apart to see if there is any damage to it. Unfortunately I didn't find anything suspicions. Cleaned it up from all the grease (yes, is was hard) and it seems to be ok for now. Question is - what grease to use to lubricate these pots and where to get it from?On a side note - I also had to open up the "voltage/current" switches and clean them up with alcohol - they were all greasy, dirty and didn't make any contact - that seems the main reason this power supply was on ebay as "for parts, not working". I've tried to find replacement parts for these as well with no luck and decided to clean them up - wasn't that difficult and they seem to work fine now. Hope this info would be useful for someone.

author: Napalm2002
author_rank: Regular Contributor
timestamp: March 02, 2014, 11:27:45 pm
content: I would like a pic Rigby. I have two b& k 1601 power supplies that could use a tweak!

author: Rigby
author_rank: Super Contributor
timestamp: March 03, 2014, 12:03:56 am
content: http://imgur.com/a/awKxZ5k pot in parallel with a 2k8 resistor to create a 0-1k8 range that the power supply expects.

author: robrenz
author_rank: Super Contributor
timestamp: March 03, 2014, 12:47:29 am
content: Quote from: Prime73 on March 02, 2014, 07:47:07 amCaig Laboratories DeoxIT G100L on both situations. Excellent stuff

author: Prime73
author_rank: Regular Contributor
timestamp: March 03, 2014, 01:51:36 am
content: I don't get how this grease thing works. If it improves a connection between moving parts then it must conductive. If I put a conductive grease on a wirewound potentiometer this must basically create a short between the wires shorting out the potentiometer... Doesn't seem logical to me.

author: robrenz
author_rank: Super Contributor
timestamp: March 03, 2014, 03:18:04 am
content: Grease or lube designed for a pot wiper is typically not conductive in itself. It may decrease contact resistance by working on oxides but also needs to minimize mechanical wear of the wiper/element interface. Also different lubes are necessary depending on the element/wiper types, cermet, plastic, carbon, etc.

author: TerraHertz
author_rank: Super Contributor
timestamp: March 06, 2014, 09:29:03 am
content: Incidentally, if that 6227B has the same kind of panel meter mounting system as many other HP supplies in the 62xx series, you may be interested to know - there's a design flaw in them that results in the meter mounts breaking. But it's easy to rectify.The front bezel has two side flaps that snap into slots on the sides of the meter body. The assembly is kept under tension by four metal springs that sit in recesses of the meter body, pushing it back from inside of the metal face plate, thus tight against the bezel clips.But the springs are way too strong, and put the bezel clips under too much stress. The bezel plastic is pretty tough (hard nylon?) but nonetheless they still tend to snap off eventually. This is why you see many of these on ebay with loose meters.The fix is easy:- Unclip the meter, by sliding a thin metal shim like a windscreen wiper blade spring down between the meter body and the bezel clip to pop the retainer hook out. It's under a lot of tension, so hold the meter body as you do it.- Lift the meter body back inside the case, and remove the four springs.- With a pair of big tough sidecutters, cut about 1.5 turns off one end of each spring, making it a bit shorter.The springs are hard, thick spring steel, do I need to tell you not to use your nice cutters?- Replace the springs, so the cut end (not a flat face any more) will bear against the aluminium panel, not the meter plastic.- Clip it all back together.Now the plastic of the bezel and meter body is under much less stress, but still enough to keep it in place.Pic is of a HP 6205B in which I repaired a broken bezel clip, by making a small aluminium clip then riveting it (using tiny brass nails hammered) to the remnant of the plastic bezel. Painful.Around the center of the pic you can see one of the springs sitting in its hole in the clear plastic of the meter. It has had some of its end turns cut off.

author: robrenz
author_rank: Super Contributor
timestamp: March 06, 2014, 12:31:37 pm
content: Nice repair there!

author: Rigby
author_rank: Super Contributor
timestamp: March 06, 2014, 12:44:22 pm
content: I fixed the exact same issue by 3D printing a new bezel clip, and using some self-adhesive foam as replacement for the springs.http://www.thingiverse.com/thing:225449