Mastech Power Supply not staying calibrated

[SurprisingRice]

Here is the power supply i recently won on ebay.

https://www.ebay.com/itm/Mastech-HY3005-3-Triple-Output-DC-Power-Supply-2-x-0-30V-0-5A-5V-3A/332431631114?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I have a problem with it. Specifically the output voltage varrying heavily from what the displays show.
Internally there are some 10K pots to adjust the displays slightly, i was only able to get it to about 0.1 of a Volt accuracy last night, i woke up this morning and it was about 0.4-0.5V off from what the outputs read to what the displays read.

this is alittle better then what it came with, a 0.7V different from output to display.

Is there anyway to fix this issue? or do i have to live with it?

[ArthurDent]

I bought one of the other HY3005-3 supplies (that had a listed problem) from the same seller. After fixing the display problem (a cap.), I adjusted the supply. Here are 2 photos of the adjustments I used.

On the back of the meter board inside there are two 10-turn trimpots for voltage on the 2 channels and two 10-turn trimpots for current on the 2 channels. These just adjust the meter readings and do not affect the actual output. While measuring the output with my DMM I adjusted the voltage pots so the readings on the DMM and those on the supply were the same then repeated the procedure with the ammeters with a heavy load on the outputs. Initially the meters were quite a bit off but it was obvious I wasn’t the first person to work on this supply so I don’t know what others may have done inside before I got the supply.

On the two p.c. board on either side are two hard to reach single-turn pots that adjust the output voltage. These are kind of touchy but with patience and a long screwdriver they can be adjusted. The voltage adj pots are the ones on the right at the bottom of each board where there are two pots and I didn’t check what the left pot is for.  One of the boards also has a small p.c. board with a 10-turn trimpot to adjust the +5 output as well. I turned the knobs on the front panel pots to maximum and adjusted the internal pots to 32.2V and the pot on the small board to 5.1V. 

I’ve always had great luck buying as-is items from this seller and this purchase was no exception. As to the Mastech power supply, this is the first one of these I’ve owned and I am not impressed. Although it doesn’t look that bad from the outside, it seems to be a low end supply as far as quality and design goes. As far as output under load goes, it isn’t as stable as any of the other supplies that I have and even though it has 2 fans to look impressive, the internal heatsink for the pass transistors is a flat piece of aluminum that doesn’t look like it could dissipate enough heat at high loads. It is still worth what you paid for it as a general purpose bench supply.  Also, before reassembling the supply it would be wise to re-oil the fans. While I was working on my supply, one of the fans started squealing until I oiled it.

[jaycee]

Swap those single turn trimmers on the mainboard for 10 turn ones. Replacing the capacitors across the output terminals with nice low ESR ones will help a lot too.

Ultimately these things are built right down to a price. I'm almost tempted to buy one just for the transformer, then scrap the rest!

[suryaputhra]

Good info to adjust the display readings.