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Heath Kit!!! Bench Supply

Heath Kit!!! Bench Supply

Heath Kit!!! Bench Supply

I turned on my office Heath kit bench supply to turn on my LED light for the 100th time and POOF! The LED merely blinked. The fuse blew.
It does this after only being built and put in service by me recently (12/6/1984). The quality of modern products, huh?

I'll be fixing it and will report what was the problem.

It's a fixed 5V and two variable 0~20V triple supply.

Here's a link to the schematic:
Heath Schematic (jpg)

Anyone want to theorize what failed? I'll check your theories first.

You can see the failed fuse. A replacement fails instantly.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Heath Kit!!! Bench Supply

My guess: One of the TO-5 [edit TO-3, thanks Scotty!] cases, insulated washer failed. Hot case shorted to chassis.

My other guess: Not that. Something else.

RE: Heath Kit!!! Bench Supply

Does your local Radio Shack still stock canned smoke?

"Why not the best?"
Jimmy Carter

RE: Heath Kit!!! Bench Supply

C2, the 12,000uF to the left of the xformer?


"It's the questions that drive us"

RE: Heath Kit!!! Bench Supply

I'd have guessed an output filter cap shorted, like analogkid, but see no signs of case bulging or disc rupture...though the fuse may have prevented much heat rise?

I also like VE's suggestion, but my thought was that the heat sink compound finally gave up the ghost, and one of these transistors just plain overheated...again, but we see no signs of smoke...so dunno.

RE: Heath Kit!!! Bench Supply

Motorola TO3 transistors... that dates it! My guess: electrolytic cap went bad, taking some of the larger semiconductors with it, or preset went bad with same outcome.

I have a much earlier Heathkit supply, the IP-27 single-channel which gives 0-50V in 5V coarse steps with fine adjustment. Mine is also dead and lying in the 'repair one day' pile along with my beloved and much-abused Farnell LT30/2 supply. I did manage to repair my newer (but still old) HP6675A bench supply which was sick a few years ago after a proper run around looking for awkward-sized capacitors.

Are you familiar with http://www.vintage-radio.info/heathkit ?

RE: Heath Kit!!! Bench Supply

I replaced the bake heater element on my 1974 Frigidaire electric oven/range in March 1999 and it failed again this past weekend, so I commiserate with you on the poor quality of modern products.

RE: Heath Kit!!! Bench Supply

Ok, third possibility. The main circuit board looks bowed outwards in your first photo - any chance of the solder joints on the backside of the board making contact/shorting to the case/housing? I'd look for signs of contact on the inside of the cover panel.

RE: Heath Kit!!! Bench Supply

Don't expect that modern crap [recently (12/6/1984)] to last more than a few decades. Go for the pre-75 brown boxes instead. They were designed to last. Nice, sturdy handles, too.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Heath Kit!!! Bench Supply

Here we go. Thanks for the great suggestions.

First check was the for V1EBLL's T03 crunch-throughs. No Joy. All have MΩ to the case.

Using my 6-1/2 digit DMM I studied the BIG cap. It seemed to "charge up to 220 ohms" which is the discharge res that's across it. Then when I poke the DMM selector to VDC I can watch it discharge from 0.8V down to zero in a reasonable RC.

The same sort of result occurs if I go across the two BIG A and B bulk supply caps. However, across the smaller C108 (A supply) C208 (B supply) while it's sort of the same it isn't as similar as two identical supplies should be - I'm thinking.

Since all 3 supplies are completely independent and can all be hooked in series if one had a mind to, it's time to figure out which one is the problem.

Easiest is to remove IC1 the pass element and regulator for the 5V supply. A couple of screws and some wiggling and it's out.

Jack in a crap 1/2A fuse I have a fist-full of and..... %$*!@%*@#E!!!! my eyes!! It was like looking at a Class B Super Giant star.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Heath Kit!!! Bench Supply

Next, I've unsoldered the two wires supplying the FW rectifier to the "B" supply.

New fuse... averting vision switch-on.

Progress. Fuse living. The meter selecting knob happens to be set to "A" supply and reads 15VDC!!

Now to find the culprit.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Heath Kit!!! Bench Supply

Ha. I was right!

"My other guess: Not that. Something else."


RE: Heath Kit!!! Bench Supply

Yes... Yes you were VE1BLL. .

I studied all the caps in supply B against all the caps in A and all the transistors with all the transistors. Using the diode mode I checked all the diodes. D202... Shorted. That'll load the fuse up nicely.

So I pulled it:

And this is what I get measuring it with a Diode Test (Forward Voltage Drop) a whopping 0.00058V. It's the same with either polarity!

And of course measuring it with an ohmmeter:

It appears Heath Kit in an effort to hide its designs from competitors had everything marked with their own codes so I can't see what diode that really is. I stuffed a 1N4007 in and called it a day. A 1N4007 is a little smaller than the one I pulled so it may fail sooner than later but I left long leads on it to help cool it.

BTW thanks JG2828 for the manual link!! It came in handy. I looked for troubleshooting in the manual and somehow overlooked what you pointed out. :/

Dang, btrueblood. That is very observant of you to notice the bow in the circuit board. It is there and very pronounced. Still the board clears the side of the cover by about 3/8".

All is happy now and the supply is back doing lighting duty for me.

Thanks for your help everyone!

*****================= Break =================*****

Something to note about web browsers and for you to throw into your 'this browser is better than that browser' database. While typing this in Chrome the power failed. I lost everything. Teeth gnashing blue-air. Head hung in depression. You've all been there.

Power comes back 10 seconds later. I have to reboot my desktop. Open Chrome again and it sez the usual "didn't shut down correctly wanna reload everything?"

"Sure", I think I might as well get back to Tips and start over. Sure enough it comes up at the top of this forum. I scroll down to do an entry and ALL my typing is there. I was all

Keith Cress
kcress - http://www.flaminsystems.com

RE: Heath Kit!!! Bench Supply

According to the manual, D202 should be a 1N4002 (1A, 100VR). You replaced it with a 1N4007 (1A, 1000VR) You should be good to go for another 20 years...

RE: Heath Kit!!! Bench Supply

All those fuses! You know there are people in China that don't have fuses. Best way to test something in that condition is to place a 60-100W lamp in series with the AC. That allows you to make voltage readings and generally not cause components to smoke. I have a HEATH power supply with germanium transistors I converted to silicon. Being T-03 that still makes it dated.

RE: Heath Kit!!! Bench Supply

JG2828; Why did I not think to check the part in the manual.. Besides having left it home. Thanks again JG2828.

Opera; I was disappointed I wasn't able to smoke many many more of those fuses... I hate those things I have. They were pulled out of a product that had been HumiSealed so they're all covered with acrylic spray. They look like crap and don't make contact if put in at the wrong position. And, they make fishing thru them(150) for the value needed a royal PITA.

Once I had the Heathkit going I had to find an actual 1-1/2A fuse. I must have looked at 200 fuses with a magnifier without finding the one I needed. I then embarked on sorting my 400 fuses in frustration. After going thru every one of them I'd struck-out. Now with sorted piles I needed to put those little plastic separators into my plastic drawer file boxes to keep them sorted. While methodically going thru the drawers looking for the one with the excess pile of dividers I opened one with a Mouser bag full of 1-1/2A fuses!!! That's how I got back on track.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Heath Kit!!! Bench Supply


" I have a HEATH power supply with germanium transistors I converted to silicon."

whoa, atomic transmutation - what's going on, cold fission in your power supply??!!

RE: Heath Kit!!! Bench Supply

I did what the China sellers on ebay do. I just relabeled them to what I wanted.

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