Power Designs 6050C Power Supply Display Upgrade

Cleanup, repair, cal­i­bra­tion check, and dis­play upgrade of a Power Designs Inc. 6050C Universal DC Source.

I thought I was done restor­ing Power Designs, Inc. pow­er sup­plies at least for the next two to three years. I had run out of space on my test bench shelf, and real­ly did­n’t need anoth­er pow­er sup­ply. I already had a 6050C that worked great but the alu­minum front pan­el had been bad­ly dam­aged by the pre­vi­ous own­er try­ing to remove some stub­born stick­ers with a screwdriver.

Along comes an Ebay auc­tion for a 6050C with a near­ly per­fect front pan­el and a “Buy it Now” price of $35 USD, includ­ing free Priority Mail shipping.

Needless to say I am the own­er of anoth­er PD 6050C pow­er sup­ply. The Ebay sell­er not­ed sev­er­al issues with the sup­ply, and based the price on it not being ful­ly operational.
Luckily the sell­er did a great job of pack­ing the sup­ply, and it arrived a cou­ple days later.

First order of busi­ness was to take it apart and clean it up.

After clean­ing and check­ing for visu­al dam­age, it was time to do a gen­er­al check of items such as trans­former wind­ings, fus­es, in-cir­cuit capac­i­tance checks, diodes and bridge rec­ti­fiers, pow­er tran­sis­tors for shorts, prop­er place­ment of jumpers, 120V AC wiring and ground continuity.

All gen­er­al items were in good con­di­tion so it was time to pow­er up for the first time. I plugged it into my “Watts Up?” pow­er mon­i­tor and flipped the switch.
For me it is always a bit unnerv­ing to pow­er on a piece of equip­ment in unknown con­di­tion for the first time. Always hop­ing that noth­ing too excit­ing will hap­pen, but ready to kill the pow­er at a momen­t’s notice.

Nothing excit­ing hap­pened. The dis­play lit up and there was a volt­age on the out­put ter­mi­nals that matched the dis­play, all good things.
As I adjust­ed the volt­age up to the range that the sell­er said was prob­lem­at­ic, I noticed the dis­play was start­ing to show errat­ic read­ings. The volt­age on the out­put ter­mi­nals was very steady though, with no issues all the way up to the max­i­mum setting.
Also when switch­ing from volt­age to cur­rent using the front pan­el push­but­tons, the mode indi­ca­tor was not chang­ing, along with the dis­play dec­i­mal point which also should have moved over one digit.

Time to get out the DeoxIT D5 con­tact clean­er and thor­ough­ly clean the grit and oxi­da­tion out of the switches.
Cleaning the switch­es cor­rect­ed the errat­ic volt­age read­ing on the dis­play, the mode indi­ca­tor now matched the push­but­ton set­ting, and the dec­i­mal point was now work­ing properly.

Next step, load test­ing the unit at 25%, 50%, and 105% of rat­ed out­put cur­rent. The PD 6050 series along with sev­er­al oth­er Power Designs, Inc. pow­er sup­plies use a UNIPLY® design. Basically the pow­er sup­ply is divid­ed into sev­er­al dis­tinct ranges. Each range has its own DC pow­er source which is auto­mat­i­cal­ly select­ed accord­ing to the out­put volt­age and cur­rent require­ments. Because of this it is nec­es­sary to check each dis­tinct range for prop­er operation.
AC out­put volt­age rip­ple on all ranges was at or below the min­i­mum range on both my 6.5 dig­it bench meters so I was quite hap­py with my Ebay catch.

Now that every­thing was work­ing great, it was time to fix things that real­ly did­n’t need to be fixed.
First item was to replace capac­i­tor C8 on the main con­trol board with a high qual­i­ty Vishay BC 470 uF 40 volt axi­al capac­i­tor rat­ed at 10,000 Hrs @ 105°C. This is the rec­ti­fied DC fil­ter capac­i­tor that sup­plies pow­er for VR1, VR3, and IC — U1 which is part of the cur­rent and volt­age reg­u­la­tor amplifiers.

Second item was to upgrade the mul­ti­me­ter dis­play and con­trol board.

Compared to the PD 6050D dis­play the dis­play on the PD 6050C is some­what small. It uses a 0.3″ high 7 seg­ment red com­mon cath­ode LED in a 14 pin DIP package.

I was able to find a larg­er 0.4″ high LED dis­play in the 14 pin DIP pack­age that was pin for pin com­pat­i­ble with the orig­i­nal displays.
The new dis­plays are made by Kingbright and are Super Bright Gallium Aluminum Arsenide Red, mod­el #SC04-11SRWA. Digi-Key part #754‑1485-5-ND

The new dis­plays are very effi­cient and very bright. Way too bright using the exist­ing cur­rent lim­it­ing resistors.
I replaced the nine 47 ohm resis­tors with 750 ohm 1/4 watt resis­tors and the five 200 ohm resis­tors with 4.7K ohm 1/4 watt resis­tors. The dis­play is still a lit­tle bit brighter than the orig­i­nal, but is now at a rea­son­able brightness.
The rea­son for the dif­fer­ent val­ues of the orig­i­nal cur­rent lim­it­ing resis­tors is that the 3 dig­it dis­plays are mul­ti­plexed, and each dig­it is only on for a lit­tle less than 1/4th of the time. Where the 5 seg­ments of the Mode dig­it are direct­ly dri­ven from the selec­tor switch, and are on 100% of the time.

A side ben­e­fit of the new high effi­cien­cy dis­plays was reduc­ing the load on the lin­ear reg­u­la­tor sup­ply­ing the mul­ti­me­ter boards. After replac­ing the dis­plays the max­i­mum board cur­rent used with all LED seg­ments turned on was 30 mA, and still brighter than the orig­i­nal dis­play. The max­i­mum cur­rent of the orig­i­nal con­fig­u­ra­tion was 254 mA, with 13 mA of that total being used for the non-LED circuitry.

I also upgrad­ed the fil­ter capac­i­tor on the mul­ti­me­ter board with a Vishay BC 2,200 uF 16 VDC axi­al capac­i­tor rat­ed at 8000 Hrs @ 125°C. This now match­es the ref­er­ence cir­cuit from National Semiconductor for the ADD3501CCN, IC U102 vs the orig­i­nal 1,000 uF capacitor.
For good mea­sure I also added a Wakefield-Vette mod­el 271-AB top mount heatsink with ther­mal com­pound to the exist­ing heatsink on the LM340T5 lin­ear reg­u­la­tor U101.
With the new LED’s and cur­rent lim­it­ing resis­tors the addi­tion­al heatsink is not need­ed, but when I was test­ing the orig­i­nal LED’s with all seg­ments on the reg­u­la­tor was dis­si­pat­ing 1.75 watts and was quite hot.

R104 adjusts the ref­er­ence volt­age on VR101 to 2.490 volts, mea­sured at R105 clos­est to VR101.
R103 is for the meter 0 volt adjustment
R106 is for the meter 60 volt cal­i­bra­tion and sets the VREF on pin 18 of the National Semiconductor ADD3501CCN IC U102. Should be close to 2.000 volts.

I may have to find a new home for my orig­i­nal PD 6050C, but I am also think­ing about devel­op­ing a new 4 dig­it plus mode dis­play that would be able to dis­play cur­rent at a mil­liamp res­o­lu­tion, and dis­play volt­age at a 10 mV res­o­lu­tion. Possibly a project for late sum­mer when it is too hot here to spend much time outside.

Additional pic­tures of the Power Designs, Inc. 6050C
(click on pic­tures for a larg­er view)