The Radio Shack Micronta 22-124 12V, 2.5A power supply is a relatively common find at the hamfests. This power supply is usually available in the either the working, kinda-working, and broke. I bought the mostly-broke with crappy capacitors version (25 VDC output vs. the specified 13.8 VDC, with lots of ripple). The outside was a little rugged but I didn't mind that as I planned a thorough cleaning and repainting. The figures show the front and back of the as bought-unit. I paid $3.00.
Figure 1 - The Micronta 22-124 Power Supply, as found (front). [Micronta_22-124_PS_01.jpg]
Figure 2 - The Micronta 22-124 Power Supply, as found (rear). [Micronta_22-124_PS_02.jpg]
Figure 3 - The Micronta 22-124 Power Supply (inside). Note the exposed 120 VAC mains lines, metal chassis and no safety ground. I'm sure it's OK. [Micronta_22-124_PS_11.jpg]
Figure xxx shows the schematic. Although I didn't deeply investigate, I suspect the large pass transistor (Q2) was shorted. The workmanship was reminiscent of the 1970s and a little scarey. The inside revealed exposed 120VAC wiring inside a metal chassis with no ground connections.
Figure 4 - The Micronta 22-124 Power Supply Schematic. [Micronta_22-124_PS_09.png]
Around the same time I made this stellar purchase, I ran across several videos regarding the inexpensive DPS series of CC/CV power supply modules from RD Tech. Particular videos include:
Figure 5 - The DPS3005 Power Supply Module [Micronta_22-124_PS_13.jpg]
The DPS3005 features both constant voltage and constant current modes. It also features a few memorized states. It can take in 30VDC and provide 5A of output current.
I originally planned on completely replacing the electronics of the semi-working 22-124 with a new transformer, bridge rectifier and smoothing capacitors, then feed that DC into the DPS3005 power supply module. I removed the electronics and old paint from the 22-124,
Figure 6 - The Micronta 22-124 chassis, Electronics and Paint Removed [Micronta_22-124_PS_04.jpg]
I found the space inside the chassis lacking for a convential linear circuit (transformer, bridge rectifier and large filtering capacitor). Fitting this amount of electroncs into the chassis along with the DPS3005 looked like a problem if I wanted to source any decent amount of current.
Pondering, I eventually stumbled over another common hamfest acquistition: the cardboard-box-of-laptop-power-supplies. Laptop power supplies are fixed voltage DC supplies which often come in 65W and 90W versions. Also, they're usually UL listed and heavily insulated to keep your average consumer from electrocuting themselves. In quantity, and at the end of the hamfest's rainy day, they go for as little as $1 each. The output voltage varies depending upon the laptop, but 20V is pretty common. I purchased this 65W beauty, which produces 19.5V at 3.3A:
Figure 7 - Surplus Laptop Power Supply, 19.5V at 3.3A [Micronta_22-124_PS_05.jpg]
These power supplies are small and self-contained so I can mimimize the exposed 120VAC wiring in the Micronta chassis. I opened the sealed power supply with a hacksaw:
Figure 8 - Opened Laptop Power Supply [Micronta_22-124_PS_06.jpg]
and mounted it (using hot melt glue) inside the Micronta case. Nice fit!
Figure 9 - Laptop Power Supply in Micronta Box [Micronta_22-124_PS_07.jpg]
After running the fuse, power switch and SPS3005 connections, I had an operational, high-functioning power supply with both constant-current and constant-voltage modes. Here's the front.
Figure 10 - Final Result (No Paint) [Micronta_22-124_PS_08.jpg]
Here's the finished unit after a coat of paint. The result looks pretty good.
Figure 11 - Final Result [Micronta_22-124_PS_10.jpg]
If I had it to do over again, I would have left the power supply in its double-insulated case. I would alsa add a small fan although I'm not sure it's necessary. Maybe on the next iteration.