[ARC5] 28 volt transformer question ?

[Robert Eleazer]

I have a ARC-116 220-400 MHZ AM transceiver I bought on ebay years ago and a ARC-114 FM 30-75 MHZ transceiver I got for free.  I just knew I could build a 28 VDC power supply for them with stuff I just had laying around.

My first attempt used a 26CT V transformer I bought from Fair Radio in 1974.  Turned out that it was more like 31 VCT with 122VAC input.  I ran though several three terminal regulators before figuring out that it was never going to work; the input voltage to the regulator was too high.  I finally built a regulator circuit featuring a pass transistor and that worked if I used a Variac to drop the input voltage.  Then I removed the transformer and installed an XL connector and hooked in a much bigger electric wheelchair charger a neightbor gave me.  It worked then.  I added a mike premap using one of the kits that Fair Radio sells.  It was a rather awkward package, though.

Then I found a discarded stereo amp and built another power supply on a Lear aircraft radio power supply chassis.  After using some of the transformer extra secondaries as reverse buckers, putting in a regulator IC, adding a really big pass transistor and heatsink salvaged from the stereo, adding a fan, and then built a copy of the mike preamp circuit; it worked fine, too.  Handles 3 amps 28 VDC output without breathing hard.

But it ain't easy to turn commonly available parts into 28 VDC power supplies, even given cheap IC regulators for $1.00 and really high output pass transistors and transformers available in trashpiles by the road.  I do not wonder at all why the guys in the 40's through the 80's did not try to run the gear on 28 VDC; they did not even have the junk we can get now.

Wayne
WB5WSV          

   

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