ICOM R71A Power Supply

[Dominic1967]

I inadvertanly powered up one of my R71As with AC while the DC jumper was disconnected. Now it no longer functions on AC, but works fine on DC. I bought a replacement power suppply from Randy's Bigapple59 eBay store and it worked right away. Problem solved. But it got me wondering...

What exactly went bad in the power supply? Is this common? I know not to pull out the jumper while the radio is powered, as it can blow out the meter lamp. The caps "look" fine, though I've done no testing. I'm curious if it's worth the trouble to repair, especially since another one of my R71As has the same issue- works fine on DC only. (I don't know its history.)

Thanks,
Dominic

 

[RFI-EMI-GUY]

If you try powering those power supplies without a load, the voltage jumps to 18-21 VDC unregulated, (by design) same topology in the IC-R7000 and IC-R9000. There is a bootstrap resistor across the pass regulator that drags the output filters up to what the diode bridge generates. By pulling the DC jumper and putting it back, while powered causes a surge that will blow the meter lamp and perhaps other stuff. I don't know how that would damage the power supply itself, but it should not be hard to troubleshoot that simple circuit. It is worth repairing if the transformer is intact. If not there are replacements for the transformer.

 

[ratboy]

I've changed 3 or 4 power supplies over the years on R71A's. They run amazingly hot on 110V. The last one I tried to fix it, thinking it was just the solder joints on the rectifier had deteriorated, but the "cube" was bad, and when I looked closely, a resistor had cooked a little. I don't remember which one it was anymore, and when I started checking things it was pretty much all bad. I just put a new PS into the radio, and it was fine. I've never actually had one that I felt was worth repairing.

I'm kind of surprised that someone hasn't come up with a redesigned replacement that maybe would have a fan in it to keep the heat under control. I wonder how many R71A's were made?

 

[RFI-EMI-GUY]

I helped a friend fix his IC-R7000 power supply. The toroidial transformer shorted out so I found him a compatible replacement. I think the R71A has a standard transformer, but the overall circuit design is much the same. It is a matter of the physical replacement and the VA rating required.