Philmore PS123 Power Supply (8): 4th Unit

 I now have four of these antique Philmore PS123 power supplies.  The first I got from KA5VZE as a basket case, back in May of 2023.  That is the unit covered by most of the posts in this blog.  I first attempted to redesign the unit with an LM338K.  That was a long saga where I wasted a lot of energy designing and building the LM338K unit, only to find I couldn't get the LM338K parts. I did get some LM338Ks, but they were counterfeit units that wouldn't handle over 1 amp... 

OK then...  I did a 2nd redesign, changing from a PNP pass transistor  to an NPN 2N3055.  This worked out ok, I modified the board I built for the previous LM338K circuit into the 2N3055 circuit.  On initial testing it appears to work ok but I haven tested it up to 3 amps yet.  See this link for info. 

In the process of this work I found  second and third PS123 units on Ebay.com.  The second was an unused unit in the box, see link. This was very nice since the original factory wiring and parts were intact and it included manufacturer's one page instructions contained a schematic.  This unit cost me \$10 plus \$20 shipping.

The third unit was ordered at the same time as the second and when it arrived it appeared to be operating well, and contained original parts and wiring, but with the 2SB411 PNP pass transistor replaced.  It cost \$15 plus \$15 shipping.

I found another PS123 unit on Ebay, it was described as "not working" for parts only.  I paid \$5 for the broken unit plus \$12 for shipping.  I figured the 18VAC 3 amp transformer in the unit is worth that much.  Here's a photo of the unit as it arrived. 

Upon inspection I found that 1) a wire from the transformer to the diode bridge had broken off,  2)  the base drive transistor heat sink was in contact with the transformer,  3) the emitter lead was in contact with the unit's case at the transistor.  

     I repaired these physical faults, plugged the unit it and turned it on, then observed a wisp of smoke.  I pulled the diode bridge out and tested it.  It was good.  I then pulled the pass transistor out and found it was shorted.  

The 2SB411 germanium transistor is obsolete of course and the cheapest replacement is in the \$15 range.  I could probably substitute a silicon transistor like the MJ2955 (PNP complement to the ubiquitous 2N3055).  But, I'd have to deal with avoiding counterfeit devices again. 

Instead, I ordered some LM2596 based buck converters from Amazon.  

The LM2596 uses a 150kHz switching regulator which should be able to take the 18 to 26V output of the diode bridge to supply 3 amps at 13.8 volts.  I'm looking forward to trying it out.