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Power Designs PS-2005 (see ABSE for partial Schem)

I got two Power Designs PS-2005A Precision sources from two different sellers on Ebay, and they both work, somewhat. They put out up to

20VDC at a half amp, so they can serve as a power supply. They're real old, made back in the early '70s so they're an all discrete design. Essentially they're a linear power supply with a reverence (yeah right:-) diode in an oven. The circuit furnishes a 1 mA precision current to a series of switched resistances from 0 to 20k, giving 0 to 20V. I posted half of the schematic on the alt.binaries.schematis.electronic newsgroup.

The problem seems to be the switches. There are two rotary switches, each having dual concentric rotors. These are arranged to give 0 to

10k. There is a 10k .01% resistor with a switch shorting it, to add to the switches to give the 10 to 20V. Another 1 ohm pot called the vernier is in series to vary the voltage by a millivolt. I can replace the DPDT switch across the 10k .01% resistor, if I find that it's so intermittent that it doesn't make good contact. But the rotary switches are a whole 'nother story. All these switches can be bypassed by setting them to zero and putting a 0 to 20k resistance across the terminals on the back. I could put a 20k ten turn pot there and make it into a power supply, losing the discrete settings the switches give. But I'd like to fix the switches if I can, obviously.

I was thinking of giving them a bath in tuner cleaner or some kind of cleaner to get rid of the intermittent problems. But one switch doesn't go thru the voltage steps correctly, so I may have to find that problem. My main question is how to get the rotary switches back in shape and get rid of the intermittent switch problems. I ran the switches thru the full ranges quite a number of times to get the contacts to wipe a lot, hopefully to clean some of the tarnish off them.

I have some tuner lube, I thought it might be good to lube the switch contacts after I clean them. One of the units smells of smoke, so I guess I'll have to clean it up and get rid of the dust and grime in it. The units are built well, all aluminum with easy access to stuff. The circuit board is just drilled and eyelets put in for the contacts. Basically point to point wiring. I have one concern that if I mess with any of the precision resistors, they probably can't be replaced, or else I'd have to use lower tolerance parts. So I gotta be careful about those. Anywqay, thanks for any ideas.

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Watson A.Name - "Watt Sun, Dar
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Yep, you have a couple of very nice power supplies. I also have one of the model 2005 supplies, and a model 2005A supply. Basically the same unit with a more modern front panel. I've never had any problems with my supplies, but maybe I can offer a bit of help. When you said that one of the switches doesn't go through the steps correctly, do you mean that when you increment the switch, the voltage doesn't change or changes by a large amount? If that is the case, then you probably have intermittent contacts on the switch or one or more open resistors on the bad switch. The resistors are in a 1-2-2-5 sequence, meaning that the switch is making several contacts per wafer at the same time, placing a short across selected resistor(s) at each position. If one of the resistors is open, that could cause some very strange voltages at the output. Check each resistor on the bad switch to find if any are open. Replacement resistors should be available from distributors such as Mouser or Digikey. Mouser lists the Vishay/Dale resistor line in their online catalog, which seem to be available in 0.1% values at 1/2 watts. Those units should work nicely as replacements if you need them. Since the resistors are high precision units, cleaning the switches with tuner cleaner, or any cleaner that leaves a residue may change the calibration when you get it working. I recommend that you use long wooden cotton-tipped Q-tips, or similar to clean the contacts. Dip them into a good deoxidant cleaner such as De-Oxit and clean ONLY the metal contacts. Stay away from the wafer itself as much as possible while cleaning. Sometimes the pressure wipers get spread open and fail to provide enough pressure to make an effective connection. If this is the case, try gently bending them back into position so that positive contact is made. You should be able to see the contacts spread when the switch is rotated, and makes contact at each successive position. I have had to resort to Tarn-X in extreme tarnish situations, and it works very well to remove the tarnish. The caveat is that everything must be cleaned and neutralized after the tarnish is removed. Since Tarn-X is acidic, use a solution of baking soda to neutralize, followed by a liberal clean water rinse. Be sure to lubricate the contacts with DeOxit when you're finished cleaning. Again, only the metal contacts should get the lube. It may help to completely remove the switches from the chassis to make repairs and clean them. Good luck!!

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In article , snipped-for-privacy@att99.net mentioned...

Wow, great - I'm gonna try to take the front panel off and see what's up. I have some tuner lube, I don't know if it's similar to De-oxit. It's a grease that's used on the old mechanical rotary tuners, and comes in a tube. I believe it was made by RCA.

I've seen some tarnish cleaners in the shelves along with detergents, etc, at the market. I'm guessing that this is the same as Tarn-X. I'll have to check at the store. The one that smells like smoke needs a good cleaning, I'll have to try to do something about that, too.

I'm hoping that this doesn't turn up any bent or damaged contacts. I have the schematic so I'm somewhat prepared. Thank you for the help.

BTW, I found the DeoxIT locally, so I'm going to check it out.

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My email address is whitelisted.  *All* email sent to it 
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Don't be ripped off by the big book dealers.  Go to the URL
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Watson A.Name - "Watt Sun, Dar

In article , Watson A.Name - "Watt Sun, Dark Remover" mentioned...

I bought a can of DeoxIT, I gotta take the PS outside when it's warmer and give it a bit of cleaning. See what happens tomorrow.

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###Got a Question about ELECTRONICS?   Check HERE First:###
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Subject: line with other stuff.  alondra101  hotmail.com
Don't be ripped off by the big book dealers.  Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com  You'll be glad you did!
Just when you thought you had all this figured out, the gov't
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