A while back I bought a Elenco XP-720 off of CraigsList. It was $40.00 and seemed like a deal at the time.
I brought it home and started to play around with it and realised it is missing some major features such as Meters and an Output ON/OFF switch. So I quickly bought the meters and added an Output ON/OFF switch. The switch has a lot of bounce but im not a NASA engineer so it works for my needs.
When the meters arrived and I got around to putting them in I had to back trace the wiring to what went where. I noticed a few things such as the constant 5V+ section had a 16v rated cap when it outputs around 20v from the transformer and rectifier. So I switched it out to a 50v 105*C rated cap. I think I even used double the capacitance as well since after all it’s only a filter cap. The meters I bought were not what I expected. The voltage sense and power are tied together so after I get around 2.6V the meters kick on. I need to figure out how to add a voltage sense lead or get better meters.
I currently have the constant 5V+ disconnected. I was thinking of a way to get a 3amp variable voltage output but after looking at the schematic the input of the regulator is in a voltage divider path. I would have to rebuild the whole circuit for that to work right.
The left Meter is for the positive voltage and the right Meter is for the Negative voltage. Between the pots is the Output ON/OFF switch. The smaller switch between the meter and positive pot was going to be a current limit LED but I think I’ll use it for the constant 5V+ Output ON/OFF.
I goofed on cutting the holes for the meters. I used a Drill for the four corners for each hole and a Coping saw with a metal-cutting blade. Kinda made myself mad since I measured everything five times and still made them a bit big.
I’m planning to repaint the case black and use white lettering for the front panel. This way I can use some black silicone to fill the gaps of the meters. Unless I have to change out the meters for the proper voltage sense and power.
I also wanted to add some 10 turn pots but the pair I ordered from Ebay were not the correct value. The Ebay seller sent me 100K instead of 2K that was advertised.