Multi-Elmac M1070 Power Supply Repairs After A Dumb Mistake

[WB5IRI]

I'm cheap, but sometimes cheap is not necessarily a good thing. Was missing a cabinet screw on my PMR8 receiver. Now, Multi-Elmac uses 1/4" long cabinet screws, and I didn't have any on hand. But I did have some perfectly good 3/8" screws that would fit, so I decided to use one of those instead of the right screw, which I would have had to locate and purchase. What's 1/8" difference, right? Well, that 1/8" difference shorted the B+, which fried the rectifiers in the power supply, the 50 ohm 10W resistor from the rectifier bridge to ground, and the power supply choke. Talk about letting out all the smoke! Plus the panic in my wife's eyes as she screamed, "Something's on fire in your shack!"

So, replaced the bridge diodes with 1N4007's, the resistor with a 100 ohm 10W (earlier versions used 50 ohm, later versions used 100 ohm), and then started trying to find the right choke. I settled on a Hammond 159P, (10 HY, 125 ma, 550 volt), and it works fine. The original is rated at 8 HY, but I could not find any other info about it, so I went for overkill.  The new choke is larger than the original, but it will fit in the same space on its side instead of upright. Just have to bend one of the mounting ears down, and you can use one of the original holes and hardware to mount it. Held by one screw instead of two, but it seems plenty secure.

Cost of purchasing the correct screw at the local hardware store: $0.02
Cost of using that junk box screw: More than $46.

Live and learn.

Doug, WB5IRI

[K1JJ]

But I did have some perfectly good 3/8" screws that would fit, so I decided to use one of those instead of the right screw, which I would have had to locate and purchase. What's 1/8" difference, right? Well, that 1/8" difference shorted the B+, which fried the rectifiers in the power supply, the 50 ohm 10W resistor from the rectifier bridge to ground, and the power supply choke. Doug, WB5IRI

Yep, at one time or another we all come across this situation in its various forms.

The worst version (though rare) is when one of the screws was designed to be a little longer than the rest and MUST go into a certain hole.  If we just "shotgun" pull a rig apart, we will never notice it.

About six months ago, I pulled the cover off the bottom of my modulator chassis . I found two metal-screws threaded right thru an 8 uf, 1KV oil filled cap.  I'd forgotten that I added this cap and did not mark the "no screws" holes.  

It can happen in many, many different ways.

T

[KB2WIG]

FWIW, I use the 1N5408 3A, 1KV.  $13.20 per 100 count from Mouser.

http://www.fairchildsemi.com/ds/1N/1N5408.pdf

http://www.fairchildsemi.com/ds/1N/1N4007.pdf

The 100 count price for the '007 is $4.00.

I like the 5408 for that extra one shot surge current rating. Its over 6X that of the '007.
My stash of '007 are saved for stuff like replacing 6al5 and other small fry. Well, forget the fry, we don't want any of that.

klc

[WB5IRI]

FWIW, I use the 1N5408 3A, 1KV.  $13.20 per 100 count from Mouser.

http://www.fairchildsemi.com/ds/1N/1N5408.pdf

http://www.fairchildsemi.com/ds/1N/1N4007.pdf

The 100 count price for the '007 is $4.00.

I like the 5408 for that extra one shot surge current rating. Its over 6X that of the '007.
My stash of '007 are saved for stuff like replacing 6al5 and other small fry. Well, forget the fry, we don't want any of that.

klc

I thought about using the 5408 but decided to go with the 4007 on the premise that if anything like this ever happens again I would much rather lose the diodes than have them hold in there longer and lose the HV transformer. While I haven't looked for one, I suspect it may be difficult to find. And yes, in this context, "fry" is a dirty word!

73, Doug