Power Designs C500 Precision DC Source

Cleanup, repair and Calibration of a Power Designs Inc. C500 Precision DC Source.

ower Designs Inc. C500 Power supply front 50V
Power Designs Inc. C500 Power sup­ply front 50V

Another lucky Ebay score of a used Power Designs Inc. C500 pow­er sup­ply. The sup­ply was described as “Powered Up Only, Untested”, with very poor blur­ry pic­tures. It seems that when­ev­er I see blur­ry pic­tures on Ebay the sell­er does­n’t want you to know how bad of shape the item is in, so it was def­i­nite­ly a gam­ble bid­ding on this unit.
The pow­er sup­ply did arrive most­ly well packed, and as expect­ed was very dirty when unwrapped. The biggest prob­lem was the acrylic meter face had been dam­aged by some type of sol­vent mak­ing read­ing the meter next to impos­si­ble. One of the back feet was miss­ing, and I could hear what I hoped was its attach­ment screw rat­tling  around inside.

Output volt­age   0 — 100 VDC, cur­rent 0 — 0.5 Amps
Regulation   0.001%
Ripple and Noise   100 uV P‑P
Stability per 8 Hrs.   0.001% +100 uV
TC per degree C   0.001%
Source Impedance   Less than 0.002 Ohms at DC

First order of busi­ness was to see if the pan­el meter was repairable.

PD-C500 Front Panel Meter Bezel Damage
PD-C500 Front Panel Meter lens Damage

The meter body and move­ment were in good shape along with the rest of the front pan­el. I was able to remove the acrylic lens from the body to work on it which made the repair much easier.
I start­ed the acrylic repair by wet sand­ing with 1000 grit sand­pa­per on a pre­ci­sion flat. I then moved to 1500 grit wet and then fin­ished the pol­ish­ing with Novus plas­tic pol­ish #2 and then #1.

PD-C500 Front Panel Cleaning, starting with the upper left corner.
PD-C500 Front Panel Cleaning, start­ing with the upper left corner.

This C‑500 pow­er sup­ply was built in the lat­er part of 1985 and had 32 years of accu­mu­lat­ed grime built up on the front pan­el and a sim­i­lar amount of dust inside the cover.
The decade volt­age selec­tor switch­es were very dirty also and dur­ing test­ing had vari­able con­tact resis­tance. A good clean­ing with DeoxIT-D5 and then DeoxIT Gold fixed that problem.

PD-C500 Bottom Front Kelvin decade switches and back of meter
PD-C500 Top Front Kelvin decade switch­es and back of meter. Zero and Cal trim poten­tiome­ters low­er right.

I replaced the miss­ing and exist­ing back feet with a close match from Digikey Part# RPC1130-ND.  Made by  Essentra Components SFF-028, Santoprene Black. I did end up cut­ting off the the expand attach­ment on the foot and used 4–40 screws.

Essentra Components SFF-028, Foot Cylindrical
Essentra Components SFF-028, Foot Cylindrical

After clean­ing, and some basic resis­tance and capac­i­tor checks it was time to pow­er up the unit.
The PD-C500 pow­ered up with no issues, and after a 30 minute warmup I decid­ed to check the calibration.

Looking at the open bottom of the C500 and series regulator transistors
Looking at the open top of the C500 and series reg­u­la­tor tran­sis­tor connections
Looking ate the open bottom of the C-500 and main circuit board with LM399H
Looking at the open bot­tom of the C‑500 and main cir­cuit board with LM399H ovenized pre­ci­sion zen­er shunt ref­er­ence in the white package.

Voltage cal­i­bra­tion is fair­ly easy, and requires a 5 1/2 dig­it volt­meter with bet­ter than 0.1% accuracy.
(Warning, cal­i­bra­tion requires oper­a­tion with the cov­er off and high volt­ages present inter­nal­ly and on the out­put terminals.)
Start by un-tight­en­ing the lock­ing nuts on R42 and R43 potentiometers.
To cal­i­brate set the decade switch­es to 0 volt out­put and rotate the front pan­el vernier con­trol ful­ly coun­ter­clock­wise. Set the “Zero” volts adjust (R42) so that the volt­meter reads 0 volts +/- 50 microvolts.
Tighten the lock­ing nut on R42 and re-check the zero reading.
After per­form­ing the Zero cal­i­bra­tion per­form the max­i­mum volt­age adjust­ment. Set the decade switch­es to the max­i­mum out­put volt­age, which is 100 volts DC on the C‑500, and rotate the front pan­el vernier con­trol ful­ly coun­ter­clock­wise. Make sure the least sig­nif­i­cant dig­it is set to “10”, not “9”. Adjust the “CAL” poten­tiome­ter (R43) for a volt­meter read­ing of 100.000 volts DC +/- 0.001 volts.
Tighten the lock­ing nut on R43 and re-check the 100.000 volt reading.

Zero cal­i­bra­tion on my PD-C500 was only slight­ly off (-250 uV), and at 100V it only required 1.5 mV of adjust­ment. Noise and rip­ple at 50% load was around 12 uVAC.

Once again just shows that Power Designs Inc. made some very fine equip­ment 30+ years ago and is still usable today with just a lit­tle care and maintenance.

PD-C500 Front angle view operating at 50V no load
PD-C500 Front angle view oper­at­ing at 50V no load


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