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)