HP 6200B power supply blowing fuses

[Jay_Diddy_B]

Hi,
 I recently repaired a HP6033a, which is a completely different technology. It is a switching supply that was blowing input fuse and occasionally the main rectifier.

Initially the main capacitor checked o.k. When tested with a multi-meter, they tested on the ohms range they were open. When tested for ESR they were also good.

It was only when I tested them with a high-voltage supply I found that one of the two capacitors arced internally at around 60V. These were 200V capacitors.

Replaced the capacitors, and the supply is healthy.

With the 6200B disconnected from the line, connect another bench supply across C13 and C14 and turn up the bench supply. See how much current the capacitors take from the bench supply.

You can re-assemble the whole unit. Disconnect one end of CR28 and C29. Connect a bench supply to C13. Set the Bench supply to 30V and you should be able to operate the 6200B.

These are some troubleshooting tips you can try.

Good luck !!

Jay_Diddy_B

[pomonabill221]

Been a while since the last post here... what's going on with the supply?  Any updates yet?
Hopefully you have made progress!

[pomonabill221]

So what's going on with the supply?
It's been a while since we heard from you.... curious how things are going?

[GreyWoolfe]

Finally found free time, so how is it going?  Not good.  I removed C16A, 13, 14 and 20.  T1 tapse open at 7 and 10--fuse blows on mains, fine on dim bulb tester.  Removed Q4,6,7-fuse good on tester and mains.  Put Q7 in-fuse blows on mains, good on tester.  Removed Q7 and put in Q6, fuse good on tester and mains.  Left Q6 in and put Q4 in-fuse blows on mains and stays on with tester.  Removed Q4 and put in C14 -fuse blows on mains, good on tester.  While the caps were out, I tested C16A, 14, 13 and 20 with the following:

C13-490uF--529uF, D= .036, ESR 0, Q= 27.4, theta= -87.9
C14-490uF--540uF, D= .033, ESR 0, Q= 30.1, theta= -79.8
C20-80uF--91.6uF, D.112, ESR .2R, Q=8.89, theta= -83.5
C16A-.05uF--45.7 nF, D= .024 ESR=1.1M, theta -88.3

The caps were checked with my DER EE D5000

The additional problem I am running into now, by swapping components in and out, I am starting to lift pads.  I am beginning to wonder is this should go to the to be stripped shelf.   The next thing I am going to try is to lift the cathods on CR28  and reattach pin 7 and 10 on T1 and see what happens.  If the fuse stays good, I'm thinking that the next step would be to reattach CR28 and 29 and lift the anodes on CR26 and 27 and seea if the fuse blows.  I will just leave C14 in while I do this because, as I said, I am starting to lift pads and I am not quite ready to scrap this.  I will update further.  Any additional comments/suggestions would be appreciated.

[Andy Watson]

Stop witch hunting and start fault finding! Using the mains fuse as a test indicator is going to get expensive and tedious. If the unit comes-on with a 40W lamp in series, keep it in series, that should be enough power to raise the voltage in the unit to a state where you can use you voltmeter to probe the levels.

[pomonabill221]

Thanks for the update!  I was kinda hoping that you solved the problem, but I guess not... sorry to hear it.
I agree... keep the light in the mains and start probing different points and see if things seem close to being correct.  Both AC and DC voltages.
How bright is the light?  It should be a pretty dim glow to barley noticable.  If it is bright or slightly dimmed, something is drawing too much current.
The fuse should NOT blow with the light in the circuit... after all it is only 40watts and the fuse (if a 2 amp) should be good for around 200 watts.

[pomonabill221]

Also look for solder splashes that might be shorting or ground things out.  With all the desoldering you have done, there is a possibility that a small solder blob was left behind.

Start at the transformer, look  at the AC voltages on ALL windings of the transformer to see if they are within reason (and if there is one that is 0 volts or very low, that is possibly where your problem is so start tracing there).

I wouldn't do any more component removal until you actually make some measurements.

You might want to include resistance to chassis WITH POWER REMOVED, to see if you have continuity to chassis.  Also check the AC input to chassis.. the transformer may have a fault and is grounded to the chassis.

Do NOT take anything for granted until you actually take some measurements!

[GreyWoolfe]

In any of the testing with the tester, the bulb glows brightly so it is definitely drawing too much current.  Tonight, I will try to work on it.  I will put all components back in and check all soldering.  The schematic does indicate some voltages.  I will test and make notes and bring them to the group.  Pomonabill221, I won't know if any of the AC voltages I will see on the secondaries of T1 are correct or not as the schematic doesn't give them.  I did, at one point, unsolder all the taps and check for voltages and voltages were present.  Looking at the pcb, there are test points that are labeled as such, but there is nothing to tell me what I should be reading at them.  All I have is the schematic in the user manual.  I have not been able to locate a service manual for it.  I am not sure if one even exists as the sites I checked under google that said it had the service manual, really was the user manual.  Now it is off to pretend to work so my boss can pretend to pay me.

[pomonabill221]

OK... the bulb glows brightly!  that means that there is SOMETHING drawing TOO much current.
Ya gotta find what that is (obviously) or the fuses will still blow.
HHmm... no voltage notifications anywhere... too bad BUT there STILL should be voltages.
It might be kinda hard to determine what they should be, but as a guess you could look at the working voltages on the caps and use that as a VERY VERY rough guide as to what the PEAK Ac voltage would be.

The caps will try and charge to the peak of the full wave rectified voltage (not rms).
Remember.... this is a VERY rough way to determine the peak voltage.... caps are usually use below their rated voltage.]
For instance.... cap rated working voltage=63vdc,  FWB output could be 50.  MOST manufacturers will NOT use the rated voltage of a cap as that is a bad design method, and really could shorten the life.

Just use that as a guide.
Just looking at the schematic I would guess:
voltage across C10 at 25 volts?
across C12 at 50
across C13 and C14 at 60-65?

These are ALL BIG guesses by just looking at the cap's voltages, and the max output of the supply.  They could be alot different, BUT THEY HAVE TO BE SOMEWHERE IN THE BALL PARK.
For instance, if the voltage across C12 is, say 15 volts, I'd say that is wrong.
Also, if you can see if the diodes are getting warm/hot with no load and the light bulb.

Do ALL your testing with the light bulb for now....

Check to see if the transformer primary is strapped correctly on the primary side.  It may look correct, but REALLY look at it... do NOT assume that it is ok.
If you can, completely remove ALL secondaries by lifting one of each secondary lead, and check what the open circuit voltages (AC) are... the light bulb should be barley visible!!!  If it still glows bright, your problem is somewhere in the transformer/connections.

Check the diodes in the bridges with an ohmeter in the diode check function (should be something one way and open the other in a bridge).  IF you get 0 or close to 0, one of the diodes is shorted.
You don't need to remove the diodes for this test... but it is a quick test and won't find WHICH diode is bad.

Don't give up yet.... you just need to be a little more methodical and don't get carried away by unsoldering/testing/unsoldering/testing.... that isn't going to narrow down where the problem is (and it just starts damaging things).
AND TAKE GOOD NOTES!!!!  It's easier to go over your notes after you have done testing, so you can look at the schematic and compare your notes.

[sync]

Just looking at the schematic I would guess:
voltage across C10 at 25 volts?
across C12 at 50
across C13 and C14 at 60-65?

C10  24V
C12  10V
C13  52V

From the service manual schematic from http://www.home.agilent.com/agilent/redirector.jspx?action=ref&cname=AGILENT_EDITORIAL&ckey=831254&lc=eng&cc=US&nfr=-536900193.536898400.00

[GreyWoolfe]

Sync, you are a blessing.  I bow down to your superior Google skills.

[sync]

It's just some experience with service manuals of older HP gear. There are different version sometimes. And you can get a lot of the manuals directly from www.agilent.com. Just type the model number in the search field, go to the product page and then to documents.

Hopefully this doesn't change with Lucky Keysigth.

[lowimpedance]

Pardon me if I missed it, did you say the 40W globe glows with all testing ...ie the transistors removed and no filter caps ??.
 Also when you put the transistors back in did you have a suitable filter cap in circuit when power was applied (noted you had removed the caps prior and did not mention they were put back until after the transistors!).
 Curious that the fuse blows with Q7 and not Q6 (being they are in parallel!).
Perhaps not having the other bias supplies operating is giving a red herring!

Yes it would be a good idea to re connect the transformer taps and essentially all other parts and test with the bulb tester in, to measure all suspect areas for a clue.

There are different version sometimes. And you can get a lot of the manuals directly from www.agilent.com. Just type the model number in the search field, go to the product page and then to documents

Always a good idea to check here as some other useful documentation can also be found, however quite a few of their manuals for older gear sometimes stop short of the schematics, and don't always expect to get different versions of the manuals or manual change information. But then thats the same with other web sites anyway 

[pomonabill221]

THAT'S GREAT!!!!  Found a manual with voltages!
I was just guessing as best I could at the voltages..
It should be TONS easier to troubleshoot now!
Let us know how it turns out!

[GreyWoolfe]

Lowimpedence, yes, the bulb glowed brightly without C13, 14, 16A and 20, and Q4,6 and 7 in place.  And taps 7 and 10 on T1.  I didn't get a chance to work on it last night beside soldering all the bits back in.  I am going to try to get some measurements tonight and compare to the service manual.

[GreyWoolfe]

I have had a chance to do some checking and I want to share my results.  I started measuring at where the power cord comes into the psu.  With the dim bulb tester off, I read 125VAC hot to neutral.  with the tester on, I read 29.2VAC.  I'm assuming that the voltage drops because the current draw is higher than it should be.  I then tested T1.  Tap 1-2 and tap 3-4 both show 19.15 VAC.  Tap 12-13 is 1.917V, tap 13-14 is 5.52V, tap 12-14 is 6.43V.  Tap 9-10 is 1.095V, 10-11 is 1.117V and 9-11 is 2.211V.  Tap 6-7 is 3.056V, 7-8 is 3.086V and 6-8 is 6.18V.  Next I checked the voltages across C16A (11mV), C12 (34mV) and C1097mV).  The capacitors are fine according to the DER EE.  I then did the reference circuit troubleshooting and found both VR1 and VR2 open.  I started on the High Output voltage troubleshooting.  Step 1 voltage between +S and A6 is 582mV and there is no open strap between A7 and A8 and R10 is not open.  Step 2 has voltage between +S and 12 as 987mV.  Q1A and Q1B test OK as does R3.  Step 3 has voltage of -1.9V.  Q3 tests OK.  When I checked C5 with the LCR meter, it read 5.23nF and the schematic says it should be .001uF.  I also measured a D-8.27, Q-.12 and Theta -6.8.  I checked the input to the voltage selector switch and found it to be 3.189V.  The output on 40V setting is 179mV and the output on 20V setting is 145mV.

I am not sure what I am really seeing here.  There is no doubt that there is a short since the fuse blows when plugged into the mains.  I see that there is an almost 100V drop at the power cord where it comes into the psu.  Since the tester is a current limiting device, is there a possibility that there is an issue with the transformer since the input and output voltages from all 3 secondaries are so low and the voltage readings at C16A, 12 and 10 are almost nonexistent.  Add in that I had to replace 2 blown in half diodes in 2 of the 3 secondary rectifiers,

[pomonabill221]

I have had a chance to do some checking and I want to share my results.  I started measuring at where the power cord comes into the psu.  With the dim bulb tester off, I read 125VAC hot to neutral.  with the tester on, I read 29.2VAC.  I'm assuming that the voltage drops because the current draw is higher than it should be.  Yes
I then tested T1.
 Tap 1-2 and tap 3-4 both show 19.15 VAC.  Strange that the H-N is 29.2vac yet the voltage at the transformer is only 19.15vac!  Are you sure you measured it correctly?  the voltage at the transformer should be the same.

Tap 12-13 is 1.917V, tap 13-14 is 5.52V, tap 12-14 is 6.43V. seems ok..
 Tap 9-10 is 1.095V, 10-11 is 1.117V and 9-11 is 2.211V. seems ok

Tap 6-7 is 3.056V, 7-8 is 3.086V and 6-8 is 6.18V.  seems ok

Next I checked the voltages across C16A (11mV), C12 (34mV) and C1097mV).  WHO!!!! Since you have SOME ac these caps must have some DC on them!  There will be about 1 volt drop across the diodes, but the caps should have something on them.  Try looking at their AC voltage.  If they are being overloaded, they cannot filter and should have full wave ac on them. C16 WILL have ac on it though... it is before the bridge.

The capacitors are fine according to the DER EE.  Try using a plain old ohm meter in the ~10K range, they should show a charge up, to see if they are leaking or if they charge (make sure they are discharged before using the ohm meter)... just to verify your cap checker

  I then did the reference circuit troubleshooting and found both VR1 and VR2 open.  I started on the High Output voltage troubleshooting.  Step 1 voltage between +S and A6 is 582mV and there is no open strap between A7 and A8 and R10 is not open.  Step 2 has voltage between +S and 12 as 987mV.  Q1A and Q1B test OK as does R3.  Step 3 has voltage of -1.9V.  Q3 tests OK.  When I checked C5 with the LCR meter, it read 5.23nF and the schematic says it should be .001uF.  I also measured a D-8.27, Q-.12 and Theta -6.8.  I checked the input to the voltage selector switch and found it to be 3.189V.  The output on 40V setting is 179mV and the output on 20V setting is 145mV. I think you are going into the supply too far right now... gotta find out why the primary supply is so low first.

I am not sure what I am really seeing here.  There is no doubt that there is a short since the fuse blows when plugged into the mains.  I see that there is an almost 100V drop at the power cord where it comes into the psu.  Since the tester is a current limiting device, is there a possibility that there is an issue with the transformer since the input and output voltages from all 3 secondaries are so low and the voltage readings at C16A, 12 and 10 are almost nonexistent.  Add in that I had to replace 2 blown in half diodes in 2 of the 3 secondary rectifiers, That is a possibilty... can you isolate all the secondaries and measure their voltages WITH the light bulb tester still in the circuit?  The light bulb SHOULD NOT glow bright at all,.  If it does, your transformer may be bad... Let's start with that.  You could lift 7,10, and 14... that would isolate the secondaries... lift all of them at the same time.

Even though I say "seems ok" the voltages are NOT ok,, just the fact that there is something there although very low.

[GreyWoolfe]

I agree on the variance in AC voltage form H-N and at the transformer, it does seem weird.  I can isolate the taps and check the outputs with the tester and see how the bulb glows with the psu turned on.  I also have a  Velleman manual multimeter so I can use it as a sanity check against the LCR meter.  I might also fire up the o'scope and use it in conjunction with the Velleman.   I will try to get some playtime with the psu tomorrow night and will post the results.  Pomonabill221, thanks for the advice.  I am also going to try to read some more on the service manual section on pus operation to try to get a better understanding how the supply works.

[pomonabill221]

Great!  That is very strange about the H-N voltage differences though... puzzling!
Hopefully, the transformer is ok,, but I would start there since the voltages are a little strange.
Good luck and let us know!

[pomonabill221]

Another question...
Where is the light bulb (tester) physically in the circuit?
I was assuming that you come out of the wall, hot side to one side of the light bulb, the other side of the bulb to the hot on the power supply input.
The neutral goes from the wall to the power supply input.
OR
Are you putting the light bulb in parallel with the fuse HOLDER and removing the fuse?
Either way would work fine, but the results where you say "With the dim bulb tester off, I read 125VAC hot to neutral.  with the tester on, I read 29.2VAC."  was rather confusing (to me at least).  Where were you measuring the 125vac hot to neutral?

[Jay_Diddy_B]

GreyWoolfe,

If you have an LCR meter you can measure the inductance of the windings on the transformer. If the inductance is low it indicates that the transformer has a shorted turn which could be the reason that the fuse is blowing.

Make the measurement with the secondary windings open, disconnected.

I would expect a reading of 3 - 20 H on the 120V winding.

Let us know what you measure.

Jay_Diddy_B

[lowimpedance]

In your opening post you say you isolated all the transformer secondaries and the fuse did not blow!

Lowimpedence, yes, the bulb glowed brightly without C13, 14, 16A and 20, and Q4,6 and 7 in place.  And taps 7 and 10 on T1.  I didn't get a chance to work on it last night beside soldering all the bits back in.  I am going to try to get some measurements tonight and compare to the service manual.

. Do you remember roughly the voltages you measured, are they the same now with the secondaries disconnected?
 If the bulb glowed brightly with all the parts (caps and transistors in the main PS circuit) after the rectifier and the taps 7 and 10 open you either have not wired the bulb tester correctly or the transformer is shot.
 As pomonabill221 asked where in the cct. have you connected the bulb tester?
Are you certain the transformer is definitely wired correctly !.

some photo's perhaps !

[GreyWoolfe]

Here is an update.  The voltage is the same at the H-N and the primary at about 28.9VAC.  The error was between the seat and the multimeter.  The tester is wired correctly.  I unsoldered taps 7, 10 and 14 and powered the psu up with the tester.  The bulb glowed very dim.  I soldered 7 first and then 10 and the bulb stayed dim  I made contact with tap 14 in circuit and the bulb glowed brightly.  I used my o-scope to check if there was an AC voltage on C13 and 14.  As you can see by the picture there is.  Pomona, I meant to say with the psu turned on and off plugged into the tester with the tester on.  Sorry for the brain cramp.   Lowimpedence, at this point I am sure that the transformer is OK.  I was so excited to see a dim bulb, that I failed to measure voltages at the secondaries when they were unsoldered.  I guess you can say that the dim bulb and I traded places .   I just went back and measured taps12-14 in and out of circuit.  Here is what I found:
Tap 13-14 out of circuit 22.49VAC, 4.57 VAC in circuit
      12-13                    10.10        1.94
      12-14                    33.1          6.6
If there is AC on the caps and Pomona believes that this is caused by overloading them, is the overload due to the existing short or an issue with the rectifier diodes?  I don't see how it could be the diodes but I could be wrong.  Remember that I replaced them when I found 2 of them broken in half.  These were bought as "NOS" and did test fine with the octopus before being installed.  If the issue is a circuit short, what would be the best way to isolate the various sections of the circuit to try to find the fault?

[lowimpedance]

Just one extra check while the transformer is isolated, can you test the secondaries are all isolated from each other and from the primary too.
No resistance measured on your DMM, (would have been better with a megohm meter but I guess you do not have one).

[SeanB]

AC on caps means shorted diodes. Did you replace all 4?