KB Power Supply 120-12-8 repair

[CapLeaker]

I have this KB Power Supply 120-12-8 here on my bench for repair.

Complaint: Had a transceiver hooked up to it and it will hum on transmit.

So, plugged that PSU in, hooked up a different radio I have kicking around, turn it on and I already hear a 120hz buzz. Tried to TX and the radio resets as soon as I press PTT. Hmm, O.k. so I hooked up my scope to see how bad the output capacitor actually is. Here is the kicker, there was a whopping 6.92 VAC on it!
Question is: How bad can a Mallory CGS 12000 uF 25V, screw in terminal cap go? Let's see: How about down to 495.5 nF with an ESR of 2.8kOhms!

The repair:
Total recap of everything. The regulator board has two axial capacitors 40V 470uF and a small 16V 33uF cap. Then the PSU had a regulating problem and the output voltage wasn't stable. The 2N3771 pass transistor was fine. After a while the SCR would fire and roast that long 1R0 resistor. Lucky me I was there when that resistor started smoking and turned the power off, other than that there would be a few more broken components. Checked the components and all was good, so I did a reflow with plenty flux on the whole back side of that board.

Problem fixed. That ole thing is older than I am. Now it's probably good for another 40 years and I don't have to worry about it anymore.

[eccerr0r]

Re: KB Power Supply 120-12-8 repair

Those translucent blue plastic wrapped axial aluminum electrolytic capacitors -- I've seen enough of them failed that those are replace on sight By visual inspection, those caps are probably late 60s to early 70s.

I'm surprised the 12mF capacitor failed the way it did, how did you replace it, that thing must have costed an arm and a leg to get a replacement?

That thing only has one pass transistor...and how many amperes is this thing rated for?
8A?

[CapLeaker]

Re: KB Power Supply 120-12-8 repair

Those two axial caps were bad, but not like that little 16V 33UF or the big 12000uF 25V caps. The age of the PSU I am guessing about late 60's or 70's as well.
I've replaced that big 12000uF cap with Digi-Key Part number: 338-1252-ND 22000UF 25V for CDN$32.58. That cap has the same physical size than the old one. Yes, the PSU is rated for 8A and only has one pass transistor.

[stj]

Re: KB Power Supply 120-12-8 repair

Those translucent blue plastic wrapped axial aluminum electrolytic capacitors -- I've seen enough of them failed that those are replace on sight By visual inspection, those caps are probably late 60s to early 70s.

they still make caps just like that, i got some last year.
very good ripple handling for their size.

[CapLeaker]

Re: KB Power Supply 120-12-8 repair

those two axial ones I put in are made by Vishay and are still available at Digi-key #: 4230PHBK-ND, 10.000 hrs rated.

[Per Hansson]

Re: KB Power Supply 120-12-8 repair

You should post a link to it in here

https://www.badcaps.net/forum/showthread.php?t=69814

[CapLeaker]

Re: KB Power Supply 120-12-8 repair

You should post a link to it in here

https://www.badcaps.net/forum/showthread.php?t=69814

Done, as you wished!

[CapLeaker]

Re: KB Power Supply 120-12-8 repair

... aaaand look at that. Scanned the schematic for a 120-12-12. It is identical to the 120-12-08, just this has 2 pass transistors.

[eccerr0r]

Re: KB Power Supply 120-12-8 repair

Those two axial caps were bad, but not like that little 16V 33UF or the big 12000uF 25V caps. The age of the PSU I am guessing about late 60's or 70's as well.
I've replaced that big 12000uF cap with Digi-Key Part number: 338-1252-ND 22000UF 25V for CDN$32.58. That cap has the same physical size than the old one. Yes, the PSU is rated for 8A and only has one pass transistor.

Interesting. Yeah, $23.20 USD right now from Digi-key, expensive capacitor IMHO. I should double check my salvage pile of "big" capacitors and make sure none dried out...

they still make caps just like that, i got some last year.
very good ripple handling for their size.

What company? Definitely not a Japanese or CDE... I forget who made the ones I had, looks like some of them are made by Philips... not well known for their capacitors...

---

I wonder if I should go build a 12V linear PSU. I have a pile of parts that are serving no purpose and could be made into a linear PSU.

- big heavy transformer 16V (guessing around 8A+) and 28V (guessing around 2A). Now if there was an easy way to tell how many watts it could handle...figuring out the pinout was annoying.
- Large 12A bridge rectifier
- pile of salvaged 2N3771 and other TO-3 transistors
- Miscellaneous pine of small paint can sized capacitors from 56mF up...

Now whether to control it with a discrete circuit, use a LM723, or something else...

[stj]

Re: KB Power Supply 120-12-8 repair

BC brand, a Vishay group company i think.

[eccerr0r]

Re: KB Power Supply 120-12-8 repair

Weird, from the schematic, those two SCRs - the current limit one look like one should simply shut off the power supply. The voltage protection SCR should...blow the fuse. It shouldn't roast a resistor, unless that fuse is considered a resistor?

[stj]

Re: KB Power Supply 120-12-8 repair

for an scr to pass 30-60A, what would the base current be??
probably more than you think.

before you ask where the numbers came from, remember that to blow a fuse fast takes a minimum of double the holding current (15a)

[CapLeaker]

Re: KB Power Supply 120-12-8 repair

if the wrong fuse is installed it will roast that resistor. Wrong fuse was installed.

[eccerr0r]

Re: KB Power Supply 120-12-8 repair

I'm still trying to figure out which 1R0 is roasting when one SCR triggers. Neither of the SCRs (SPS225 or 2N5062?) when triggered should roast resistors, at least if that schematic is correct, as far as I can tell. The SPS225 should roast a fuse, but the 2N5062 should just shutdown unless the output is actually shorted, which could heat up but not roast resistors...

[CapLeaker]

Re: KB Power Supply 120-12-8 repair

you are correct, it is the SPS225 that should blow the fuse. However, if there is a big fuse installed, it will take that 1R0 out, because the fuse doesn't blow, if there is a regulation problem. The 2n5062 SCR is just a tiny lil thing and it will shut down the PSU.
Don't forget, this schematic is for a 120-12-12 PSU, some parts are different in the one I repaired.