Elenco XP-720 Modifications

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.

20170610_193011The 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.


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