HP 6200B power supply blowing fuses

[GreyWoolfe]

I Hope the massive minds here at EEVBlog can help.  I picked up an HP 6200B power supply non working, for a small sum off of eBay.  When I replaced the bad fuse in it, it immediately blew the fuse again.  I unsoldered the secondaries of the transformer and tilted them out of the pcb to test them.  All the secondaries had voltages and didn't blow the fuse when powered.  I didn't record the voltages because the schematic gives no clue and no test points.  I resoldered the transformer and unsoldered 1 leg of all the rectifier diodes off of the secondaries.  I checked them with my octopus circuit tester and saw the appropriate waveforms on most of them.  When I unsoldered the leg on CR27 and CR25, they fell apart in 2 pieces.  I picked up some off of the internet and replaced CR24, 25, 26, 27, 28 and 29.  I figured if 2 fell apart, what is the possibility of failure with the others.  Upon applying power, the fuse blew immediately.  So right now, I am out of fuses and will have to hit up Rat Shack for more.  In the meantime, can anyone give me a clue?  A visual inspection of the board shows no obvious damage.  I included the schematic but I didn't take any pictures as I have an old crappy Kodak 2 MP camera and couldn't really get any good pictures with any kind of resolution.   The quality of the schematic is so-so as it is a scan of a scanned copy of the manual.  Any help would be greatly appreciated.

[nctnico]

I'd start with checking the primary circuit first. Filter? Transformers are not likely to cause problems.

[Jay_Diddy_B]

Hi,

There is a nice clean copy of the manual, including schematics here:

http://www.hparchive.com/Manuals/HP-6200B-Scheamtics.pdf

I would continue your troubleshooting by checking for a short circuit electrolytic either C13 or C14.

Good luck!!

Jay_Diddy_B

[lowimpedance]

Q4 shorted?, I would also check the pass transistors Q6 and 7. Also before applying power again check C12, 13, and 14 are still good.
If good is it possible to isolate Q6,7 from the circuit then apply power to see if the fuse pops, if not then check voltage on the filter caps is good (no excessive ac ripple to be sure the caps are actually still good). Then check the reference reg is good.

[Wytnucls]

When the fuse blows, one has to suspect a short in the load, causing a much higher current flow than normal. Look for unusually hot components on the board and if you have an IR camera, look for hot traces on the PCB. Best of luck.

[GreyWoolfe]

Thanks for all the info so far.  I just received my DE 5000, I will have to get out the manual and read so I can check the caps.

[GreyWoolfe]

Q4 shorted?, I would also check the pass transistors Q6 and 7. Also before applying power again check C12, 13, and 14 are still good.
If good is it possible to isolate Q6,7 from the circuit then apply power to see if the fuse pops, if not then check voltage on the filter caps is good (no excessive ac ripple to be sure the caps are actually still good). Then check the reference reg is good.

I removed Q4,6 and 7.  All I have to test them with is my octopus but they test fine as long as I am correct in measuring an open between base and emitter.  Q4 is a Motorola 3-063 (or 8026) and Q6 & 7 are RCA (HM 8006 or 4 225)  and I was unable to find any datasheets.  I checked C12, 13 and 14 in circuit with my DE-5000 after discharging all the caps.  C12 measured 517 uF and I measured 1043 uF for C 13 & 14 since they are in parallel.  I couldn't unsolder them as they are big Mallory metal can caps with big lugs soldered to the pcb and I don't have anything here that can unsolder them.  My wick is small diameter and my XD-985 doesn't have a large enough nozzle.  I will pick up fuses tomorrow and test without Q6 and Q7 and will report my findings.  If the fuse still blows, would it make sense to break the circuit at each bridge rectifier and then solder them back one at a time to see if I can isolate the fault to a particular part of the circuit?  I am not that good yet at reading schematics so I am not sure that this would work. 

[robrenz]

Make sure the current limit pot is not open.  I had a PD supply that blew a fuse every time the current limit pot went intermittently open.

[GreyWoolfe]

Thanks robrenz.  I will add that to my list of troubleshooting steps.

[lowimpedance]

Just a thought, does the rear terminal block have the correct strappings as per the manual for stand alone operation?. I would still suspect the filter caps (also the output one C20 if its strapped in cct (on rear term block!). Does the fuse blow violently or just sort of a little 'ping' .(I am assuming you have used the correct type of fuse?).
 One last thing , test CR 8 and 9 and 11 for shorts. (you do have a multimeter with diode test ??)

[oldway]

Also check C16A (.05µF 500V) and C10 for short.

[GreyWoolfe]

Lowinpedence, I have 2 multimeters that have diode test functions.  I will check the diodes after my work day is done.  I will also look at the strapping, if any on C20.  The fuse doesn't blow violently afaik.  The power light lights momentarily and goes out, so I would say with a ping.  Oldway, I will check the caps you mention.

[Shock]

Check the transistors as well.  You don't happen to have a variac?

[mos6502]

First off, put something like a 40W light bulb in series with the mains input. This will prevent you from blowing any more fuses (or semiconductors), and also serves as a visual indicator. If it glows brightly, the PSU is still broken.

[pomonabill221]

First off, put something like a 40W light bulb in series with the mains input. This will prevent you from blowing any more fuses (or semiconductors), and also serves as a visual indicator. If it glows brightly, the PSU is still broken.

DAMN.... you beat me to it!  I was thinking the same thing!  Saves on blowing fuses and still supplies a very low voltage if there is a very heavy load for checking things out and hopefully NOT cooking things!

[tautech]

First off, put something like a 40W light bulb in series with the mains input. This will prevent you from blowing any more fuses (or semiconductors), and also serves as a visual indicator. If it glows brightly, the PSU is still broken.

Standard mains powered DUT procedure after repair or for fault finding. And good for first power up of a new project.
I suggest all techs make a test box with bulb holder and mains output socket. Add a switch if you want. And have a small selection of bulb wattages.
Must have piece of kit IMO.
Helps keep the magic smoke in.

[GreyWoolfe]

Here is where I am at so far.  All the jumpers on the barrier strip are where they are supposed to be.  I checked the current limit pot.  It is a double ganged pot.  The first part of the pot, which I assume is the fine adjustment, has 10R stamped on it and measures 10.2R.  The course adjustment has 900R stamped on it but only reads 760R.  I checked it with 2 different meters and both were close to each other.  I am going to check CR8, 9 & ten.  Then  I am going to see if there is an extra lamp in the garage.  If not, I guess I will head to home depot and get what I need to build a test box.  Certainly cheaper than a variac. 

[Shock]

I was going to suggest a dim bulb tester as well, but while I was looking at the schematic to see where transistor failures could cause a short circuit, I fell asleep.

[don.r]

Dim bulb tester is a must have for anyone repairing AC equipment. Add a switch for convenience.

PS Don't forget to use an incandescent bulb!

[GreyWoolfe]

Don.r, thanks for clarifying.  I thought you were calling me the dim bulb tester .  I checked a couple of more things out.  C20 is 100 uF and it tests at 104 uF with an ESR  of .16.  CR8 and 9 are 1N485B small signal diodes and according to my multimeter ( Fluke 27/FM and a cheap Velleman) CR8 forward reading is 1.745 and open in reverse.  CR9 is 1.728 and open in reverse.  CR 10 doesn't exist.  It is too late now to go to home depot to get stuff to make a dim bulb tester.  I don't even have incandescent bulbs here.  We went to CFL a few years ago ( I know, but the lower electric bills are nice.  The lights are never completely off here).  Off to hang with the family.  They make me do it every so often.

Another question, even though C12,13,14 and 20 check out fine with the DE-5000, would it still be smart to replace them?  If so, that poses another question.  These are big metal can Mallory caps with what I am guessing support tabs soldered into the pcb (they have no traces)  Do I have to do like for like or can I substitute something else like regular electrolytics that I am used to as long as the value is correct?  My thought is that I should also try to match the ESR as close as possible to the readings off of the DE-5000.

[don.r]

Don.r, thanks for clarifying.  I thought you were calling me the dim bulb tester .

As my profile says...

Dim-bulb tester... works on me too

[GreyWoolfe]

OK, brain cramp on CR8 & 9.  They are not 1N485B diodes but GD8-001 silicon diodes rated at 2.4v @ 100mA.  Googling finds nothing.  In looking for for something similar, I found 1N4617-2.4v @ 200mA.  Could this be a reasonable substitution?  Assuming that CR8 and 9 are bad.  With readings over 1.7, I would think that they would warrant a replacement.  Someone correct me if I am wrong.  Don.r, maybe I really am a dim bulb tester!

[don.r]

OK, brain cramp on CR8 & 9.  They are not 1N485B diodes but GD8-001 silicon diodes rated at 2.4v @ 100mA.  Googling finds nothing.  In looking for for something similar, I found 1N4617-2.4v @ 200mA.  Could this be a reasonable substitution?  Assuming that CR8 and 9 are bad.  With readings over 1.7, I would think that they would warrant a replacement.  Someone correct me if I am wrong.  Don.r, maybe I really am a dim bulb tester!

Not sure about that but diodes and HP power supplies ring bells in my head! I remember someone else repairing one with bad diodes (on this forum) and they also had odd part numbers (hello HP!) and they were simply standard signal diodes or something like that so the bulb may have some light in it yet!

[lowimpedance]

If CR8,9 are not short circuit I would not then assume they are bad at this stage. (BTW I said to check CR11 not 10). The filter caps are only running at 120Hz so ESR is not critical as in switchers. If none of your caps measure short or low ohms with the multimeter then I would not jump to replace them just yet.
 Make the "bulb tester" and then power up the 6200 with Q6,7 out of circuit then see if there are any issues. If not then its time to risk another fuse, ie remove the bulb tester and power up again and if the fuse holds do some more measurements for the dc levels and any ripple etc.
 Next step would be to re connect the bulb tester and then put back Q6,7 in circuit and try again etc etc. You get the idea.
Keep plugin at it , you will crack it! (fix that is- not make it worse )

[Po6ept]

With readings over 1.7, I would think that they would warrant a replacement.  Someone correct me if I am wrong. 

1N4617-2.4v @ 200mA is a reasonable substitution for a 2.4V 100mA Zener.

You cannot test Zener diodes using the diode function on your multimeter.  The easiest way is to put a resistor in series with the cathode side of the diode and connect the pair across a voltage source with the resistor to the positive terminal and the anode to negative.  Check the voltage across the diode.  470 ohm 1/4W would be a good value with a 9V battery.

I'm also a big fan of using a bulb tester.