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.

[Shock]

With replacing the caps repair first.  A minimal replacement job would be if you had a few of the same series or brand caps fail showing a trend.
If a lot of them were contributors, or on their way out and economical to replace, and the device was going to be a keeper then replace what you can.  Same goes for transistors and also checking resistors for drift etc. It's a reliability, accuracy, cost, time balance.

[sync]

CR8, CR9 are not zener diodes. They are basically a string of three diodes to get a forward voltage drop of 2.4V. If you need to replace one then use three silicon diodes (1N4148) in series.

A zener diode has it's specified voltage on reverse bias and conducts (0.7V) on forward bias. Big difference.

[mos6502]

When you're checking caps, ESR by itself is actually pretty meaningless, because an ESR value that would be fine for one type of cap can mean that it's defective for another type of cap. A much better parameter is the dissipation factor, D, which the DE 5000 displays by default in the top display in auto mode. So, just set the DE 5000 to 120 Hz (for electrolytics), and check that the D is less than 0.25 (0.15 for low ESR caps, but you don't have those in your PSU). If the capacitance is also OK, the cap is good.

[oldway]

Are you using a slow blow fuse ?

[GreyWoolfe]

mos6502, thanks for the tip.  I will go back and check.  Sync, the fuses are fast blow.  I found a bunch of 1N4148s so I am going to change out CR8, 9 and 10.  I will be hitting up home depot tonight to get the parts for a dim bulb tester and I will update over the weekend what I find.

You guys are great, pointing a second time around newbie in the right direction.  If I get it working, I will be sure to post pics of the guts, the crappy bezel and what it looks like after cleaning.  This will go great with my Agilent 3610A, Elenco XP-720K and Astron VS-35M power supplies on my bench.

[Andy Watson]

... the fuses are fast blow.

Unless you've gone way over the top with the rating, the initial inrush current will probably be sufficient to take-out a fast blow fuse, fault or no fault.

[Shock]

... the fuses are fast blow.

Unless you've gone way over the top with the rating, the initial inrush current will probably be sufficient to take-out a fast blow fuse, fault or no fault.

On the schematic it's not specifically marked slow blow only 2A how did you determine it will fail regardless?

[Andy Watson]

... the fuses are fast blow.

Unless you've gone way over the top with the rating, the initial inrush current will probably be sufficient to take-out a fast blow fuse, fault or no fault.

On the schematic it's not specifically marked slow blow only 2A how did you determine it will fail regardless?

Experience.
And note, I only said probably fail. But, the chances are that Mr HP made the fuse specification as low as possible, consistent with it not blowing every time the unit is powered-up. Using a fast-blow fuse in place of a normal/slow blow fuse allows less energy into the circuit; not what you need when there's a transformer to magnetise and smoothing capacitors to charge up.

[Wytnucls]

Agree with Andy. I would replace that fuse with a medium or slow blow one, 'tout de suite'.

[Po6ept]

The HP fuse number cross references to Littelfuse, Newark, and Boeing numbers that are specified as fast-acting.  This isn't to say that a slo-blo wouldn't be useful to see if inrush current is the culprit...

[tautech]

Not much point if you use a Bulb tester.
Bulb will indicate the inrush and Cap charging, then dim if there is no load and all is well.

[Shock]

Not much point if you use a Bulb tester.
Bulb will indicate the inrush and Cap charging, then dim if there is no load and all is well.

You mean not much point using a slow blow or a dim bulb tester? Unless I'm not mistaken the issue present is a short causing somewhere above 2A blowing the fuse.

As far as I'm aware the best approach is either to get the DUT to power up in a current or voltage limited state.
Or depowered and cap drained state, test and replace caps then move onto checking the transistors with a possible sacrificial fuse to check in between.
Or depowered and cap drained state, isolating parts of the circuit testing resistances.

Is there any better way to approach this?

[tautech]

Not much point if you use a Bulb tester.
Bulb will indicate the inrush and Cap charging, then dim if there is no load and all is well.

You mean not much point using a slow blow or a dim bulb tester? Unless I'm not mistaken the issue present is a short causing somewhere above 2A blowing the fuse.

As far as I'm aware the best approach is either to get the DUT to power up in a current or voltage limited state.
Or depowered and cap drained state, test and replace caps then move onto checking the transistors with a possible sacrificial fuse to check in between.
Or depowered and cap drained state, isolating parts of the circuit testing resistances.

Is there any better way to approach this?

Bulb tester will power DUT in a Voltage and Current limited state and keep the magic smoke in!
Then use your knowledge to fault find the areas of the DUT where low/no voltage is present. Powered up or otherwise.
Poor mans Variac !

[megajocke]

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!

Your earlier measurements suggest CR8 and CR9 are perfectly fine. They wouldn't cause the fuse to blow if broken in any case.

My prime suspects would be rectifier diodes, filter capacitors, CR11, CR34, Q4, Q6 and Q7.

Removing Q6,Q7 and Q4 and testing with a dim bulb tester sounds like a way to go, but it might be easier to start by disconnecting the CR26-29 rectifier and use the dim bulb tester to identify whether the problem lies in the regulator circuit or the power circuit.

There was a mention of CR10 earlier, but I can't find it on the schematic.

[lowimpedance]

Your earlier measurements suggest CR8 and CR9 are perfectly fine. They wouldn't cause the fuse to blow if broken in any case.

My prime suspects would be rectifier diodes, filter capacitors, CR11, CR34, Q4, Q6 and Q7.

Removing Q6,Q7 and Q4 and testing with a dim bulb tester sounds like a way to go, but it might be easier to start by disconnecting the CR26-29 rectifier and use the dim bulb tester to identify whether the problem lies in the regulator circuit or the power circuit.

There was a mention of CR10 earlier, but I can't find it on the schematic.

As I said earlier with CR8,9 not to assume they were suspect !! so move on and check CR11 (((((NOT 10, NOT 10 !! ))))) , and having looked again at the circuit CR34 would be good to check too.
 Q4 6, and 7 were supposed to be checked as good! so testing with the bulb tester and Q6 and 7 out of circuit there is not much left, but would be important to eliminate the rectifier and filter section before moving on to other possible areas.

[GreyWoolfe]

Lowimpedence, I replaced CR 8,9 and 11.  I powered the PSU without Q6 and & with the dim bulb tester and the PSU came on.  I checked CR34 out of circuit and it appears fine.  My Fluke 27/FM in diode test shows .585 and open in reverse.  With the tester, I am showing 862 mV.  I put Q6 and 7 back in and the PSU came on with the tester.  I will probe the circuit around CR34 and come back with findings.

[pomonabill221]

I am watching your progress with GREAT anticipation!  Looks like you are making progress!
Keep us updated (as I am sure you will)!

[Po6ept]

You're making good progress.  Assuming the supply is jumpered as shown in the schematic, what happens if you remove the sense jumpers -R to -S and +R to +S?

As lowimpedence suggested, if you look at this as three separate supplies, what happens if you open taps 7 and 10 on T1 one at a time to isolate the problem to one of the three?

[TerraHertz]

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.

 
All the years I've been servicing stuff, and that never occurred to me. Doh. Thanks.
I generally just used a variac, and an AC ammeter, and wound the output up slowly while watching the current.

Too bad lightbulbs are soon to be completely unobtainable. Better lay in some more stock.

[GreyWoolfe]

Here  is another update.  MOS6502, I went back and checked the dissipation factor on C12, 13, 14 and 20.  The caps are still in circuit and the D was well over .25-- .7 and up.  Do I need to check the D with the caps out of circuit?  They are a bear to try to unsolder as the mounting tabs are also soldered.  I just got some wider solder wick so I can try if necessary.  PO6EPT, I opened taps 7 and 10, the PSU powered up with the tester and blew the fuse plugged into the mains.  I am now officially out of fuses.  I refuse to pay Radio Shack ripoff prices so I ordered in 20 fuses of eBay, they will be here by Tuesday.  I will go and get a pack of 2A slow blow to see if the inrush current is a problem.  More to follow.  Once again, thanks for all the assistance.  If nothing else, I will have some time to try to understand the schematic better.

[pomonabill221]

Would be nice if you could find a circuit breaker to use rather than eat fuses?  A fast trip 2 amp Heinman from a scrap piece of electronic gear?  Even a 3 amp you could use but that is pushing it a little.  NOT a thermal breaker but a mag trip.

I agree with buying ratshack stuff... they are $$$$!
Aren't you using the light bulb for current limiting?  If you are using a 40 watt lamp, the fuse should NEVER blow.

[Po6ept]

A guy I used to work with was of the opinion that the best way to fix shorts was to jumper over the fuse and deal with whatever smoked.      (Don't do this, of course.)  After a few burned circuit traces and transformers, he was invited to go work elsewhere...

I realize this is a bit silly to ask, but have you verified that all the jumpers are where they should be?  I ask because I have spent hours working on equipment only to find that the person before me did something that deserves a face-palm. 

With the T1 taps open at 7 and 10, there really isn't much there that should blow the fuse.  CR26-CR29, C16A, C13, C14, Q6, Q7, C20 are the things that immediately jump onto the must-check list. 

With the supply off and the caps discharged, what is the DC resistance between TP24 and the negative rail? How about at TP23?   Same thing at +R with the jumper to +S on and off.

You can sometimes unsolder one lead of a filter cap well enough to center it in the via to take readings.  That gets around having to unsolder the tabs. 

[lowimpedance]

 So with tap7 and 10 disconnected and thus only leaving the main supply windings  the fuse still gives up the ghost!. Is Q6 and 7 still out of circuit?.
If so there is very little left and if you have already replaced the rectifiers that just leaves the caps (or maybe the use of a slow blow fuse is required).
 AS Po6ept said try and desolder the cap pins and give them a circular wiggle in there holes to make sure there is no 'little' whiskers of solder still connected from pad to pin!. Check with you DMM that the pin is indeed now isolated from the pad. Then try powering up with the 40W bulb tester and again test for an appropriate secondary voltage - if the new fuse remains intact. If all looks good try again without the bulb tester, does it look like previous measurement or does the fuse pop. (lets hope at this point it does not!) If the fast blow pops try the slow blow to see if it survives.
 If the fuses survive try bodging in another modern cap of approximately the same value and try with bulb tester that you now get an appropriate DC across the bodged in cap. If it looks good then try putting Q6 and 7 back in circuit and so on with taps 7 and 10. The upshot being hopefully the original filter caps have to go.
 Just a thought perhaps the Radio shack fuses were dodgy/mislabeled etc. With the ebay fuses it would be interesting if they pop the same way.

On a side note I own a 6236B triple supply and it uses a fast blow 1A (240V mains!), just checked!.

[pomonabill221]

To keep you inventory of fuses, why aren't you using the 40 watt lightblulb?
IF the fuses are blowing WITH the 40 watt lightbulb, something is wrong with your wiring because a 40 watt lightblub will NOT blow a 2 amp fuse!!!

[pomonabill221]

Just another thought.... I know you are in the US and line voltage is 110 (I assume that is what you are using to power the supply).
Is the supply strapped for 110 and is it done correctly?
May seem like a stupid question... but stranger things have been done and overlooked!

[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?

[GreyWoolfe]

Lowimpedence, I did check that the secondaries were isolated from the primary.  I did not check to see if the secondaries were isolated from each other.  I will check that.  SeanB, I did replace all 4.  Actually I replaced all the rectifier diodes when I found 3 of them on 2 secondaries in pieces.  However, what I bought were "NOS".  I am not sure of the quality of them.

[sync]

Have you installed the diodes with the right orientation/polarity?

[pomonabill221]

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

True... but in the case of C16, that is across the transformer secondary BEFORE the bridge, so it will have AC on it.

[pomonabill221]

You could try..
remove tap 14, and using an ohmmeter (not on diode check), measure across the AC input of the bridge.
That would be on CR26 K/ CR29 A to CR28 A/CR27 K.  Basically you are measuring to see if the diode(s) in the bridge are shorted... probably not, but that would be what I would check next, although acording to your scope pic, you are getting full wave AC on C13/14.
There appears to be a VERY low resistance or a short, in the power section and that is what is dragging the whole supply down.

Now we need to go through that section.

Just a quick glance at the circuit and I would question CR34.  That is one of the few components that is directly across the output and seems to be there to protect against reverse voltage being applied from the outside world.  Also, C19 (paralleled to CR34)
This is just a guess (hunch) but you could just measure across the output jacks with you ohmmeter as well.  I don't think you should see very much of anything as far as continuity, but this test might also confirm whether or not there is something going on on the output side of the supply.
This test should be easy (for starters).

Another thought.... see if the terminal strip on the back is strapped correctly.  Someone could have missed a terminal and placed a short across the output.
Also, C20 might be shorted... you can check that by removing either the jumper on A10/S+ or A9/S- and probe A9 to A10.

I am trying to find the easiest and "most possible" that I can think of before really diving into the supply.

[lowimpedance]

Since the thread is getting rather long some tests and checks have already been done, and I too cannot remember all .
The output strapping was checked as well as C20 etc , cannot remember if CR34 was checked (It should be of course being right across the output!).
 As Sync asked .. you did put the diode bridge back with the correct polarity?. What diodes did you use (part number) ?.
When you had the transistors removed what was the DC volts measured on the filter caps (correct polarity) ?.

While the taps 13 and 14 are isolated can you load those taps with a resistor (say 100 ohms /10W) for a short time to measure if the secondary volts drop significantly. Just to satisfy the winding is able to hold up under a reasonable load.

Do you have another or can borrow a power supply with current limit control?.
I think it had been suggested already!. Connect an external supply at around 25V across the filter caps (transformer taps 13/14 disconnected!).
Set the current limit low and then slowly increase to see if the hp6200 supply comes up or continues to draw too much current.
If it looks good ie no excessive current then do the same procedure again with the bias supplies operating (with bulb tester in place and only the main taps 13/14 disconnected). Does the supply start to work or does it still drag too much current?.

This repair is becoming a real mission! , When you look at the cct. there is not that much in it.
 lots of learning at the least.

Maybe a nice photo of the main bridge and filter caps area of the PCB.

Just a last thought, there is no chance a previous owner had done something silly like put a screw or some such in the wrong place causing a mechanical short, did the supply look 'fiddled with' when you first received it?.

[GreyWoolfe]

I want to thank everyone for their help.  Unfortunately, the saga is over.  I have been working on the psu without case which is basically the pcb and front and rear bezels.  I had it standing up on it's side-don't remember exactly why.  I didn't secure my office door properly, and my stepdaughter's (damn) cats got in and while playing around, managed to knock the psu off my workbench.  PCB cracked almost in half.   The lessons I learned is to make sure all DUTs are sitting flat and make sure the office door is secure.  I'm thinking dinner tonight is linguini con gatto al fresco (linguini with fresh cat)   That is why I prefer dogs as pets.  Time to move on to the next project.

[pomonabill221]

OOOHHHHHHH NNNOOOOO!!!!!!!!!!!!!!!
SO sorry to hear this!!!!!!
This was really getting interesting!!!!  and I TRULY hoping that you(we (all of us here)) with our inputs and suggestions, could find the problem!!!
This REALLY put a damper on my day!  (strange that an inanimate object, and a person that is trying to repair it, could have such an effect! guess I need to get a partener!) hehe

Well you can't say "we" didn't try!  hhmmmm...cat on a stick!

[SeanB]

Put the kit back together and post it to me.....

[robrenz]

Just pretend that you bought it that way and it's just going to be a little bit bigger challenge. If it is just a double layer board all the traces can be jumped across the crack on both sides. Don't let those felines get the best of you.    Come on, you can fix this thing.

[lowimpedance]

Bit of a shame  , Now you have spares for another 6200B (got cheap of course!) .

(Looking at the bright side BTW)

[GreyWoolfe]

Bit of a shame  , Now you have spares for another 6200B (got cheap of course!) .

(Looking at the bright side BTW)

Actually, I am considering getting another, but one that works 

[pomonabill221]

Bit of a shame  , Now you have spares for another 6200B (got cheap of course!) .

(Looking at the bright side BTW)

Actually, I am considering getting another, but one that works 

Now THAT'S the spirit!!!  Just think, you have spare parts as well!

[orbiter]

Damn that's unlucky.  I was following this thread with interest and was sooo disappointed when I read that the cats had killed off your chances of a repair :-(

Sent from my HTC One using Tapatalk Pro.

[charles]

So, I may be able to keep the broken power supply love alive. I recently bought a 6200b off ebay. It worked until I turned the voltage to maximum, around 40V. I heard a click and the gauge on the front went from reading 40v to being off the scale below zero, I.e. all the way to the left. It still worked, i.e. it was outputting 40v and could still be adjusted from 0 to 40v but the display was broken. Also, the thing will no longer go into the 5v range, i.e. the 5v range goes from 0 to 40v. So, I measured the voltage across the terminals of the gauge/meter and found that it was reading approximately -150 mV and would go up to about -100 mV when the desired voltage was increased. Any thoughts?

Also, as a heads up I am a noob but a determined noob. To prove this statement, I have an embarrassingly stupid follow up question. In my haste to l look around inside, I decided to take out the meter knob, I.e. cut all its 4 million leads and set it aside. I took pictures before doing this but unfortunately not very good ones. Is there any chance someone still has this supply and could tell me how this knob is wired up?

[Rigby]

 

[free_electron]

The cat may know...

[Andy Watson]

The cat may know...

+1

[charles]

Re-read original "joke" that I wrote in this post and decided it was too macabre and needed to die... So I removed it. Anyway, I really would appreciate any (preferably non-feline) help that anyone is willing to give.

[Andy Watson]

If you have not already done it, read the whole of this thread and find/download the links to the service manual(s). Good luck ... you're certainly going to need it!

[charles]

 

I fixed the 'major' fault! This is the first time I have ever fixed a piece of electronics before! Hooray!



I started by following the 'though shalt measure voltages' and found that the voltages were a bit out, at least according to the datasheet. Namely, my +6.2 volt rail was +6.4 and my +12.4 rail was +12.51. So, I looked at the regulator circuit, wasn't sure how it worked exactly, so I read up on zener based regulators and then simulated the circuit in LTSpice. Interestingly, the simulated regulator agreed with the voltages I measured and not the voltages on the datasheet. So, I left that line of inquiry for the time.

Then, I went and looked at what I should have in the first place, namely the meter circuit. I read up on differential amplifiers then simulated the circuit. I compared my simulated voltages to the voltages in the meter and the base of one of the transistors was way off (Q12). So, I replaced it with a new transistor and bingo, the meter works again!

Thanks to everyone who provided advice and links to manuals.

Now, the remaining thing that is slightly weird is that the 20v and 40v range selection seems to be way off. Specifically, when unloaded, the 20v range seems to be able to go to 40 volts and the 40v range seems to go above 50 volts (I don't really want to see how high in case I blow something up). I don't really understand the mechanism which limits the voltage in the first place, so I will need to read further into the schematic. However, it appears to be at least usable in its current state.

Anyway, I am happy with my mini-victory.

[megajocke]

The voltage range of the voltage control doesn't change with the range selected. It's just that you are not guaranteed the power supply will be able to supply more than 20 V over the full load range in the 20 V range and likewise for the 40 V range. It's not just that you have more ripple and voltage drop at the smoothing capacitor but one also has to account for line voltage variations. The rating is 105-125 V (or 210-250 V) so the worst case would be full load current at 105 V line voltage.

The voltage range of the control is determined by the +6.2V reference voltage, the voltage control pot resistance and the R12//R13 resistance.

There are some good description in the service manual:
http://cp.literature.agilent.com/litweb/pdf/06200-90001.pdf

From the table of specifications you can find that the ouptut voltage is 1 V per each 200 ohms of programming resistance. Therefore I'd expect the voltage control to have a 50 V full scale range. This also matches calculations using the R12 value well. I can't find any information on what the R13 value is, but it is likely it is (or could be) selected for a precision 200 ohms/volt programming gain.

That your 6.2 V voltage is 3 % off and 12.4 V is 1 % off is perfectly normal. The reference diodes (1N821) used to provide the reference voltage do not have an initial tolerance better than 5%. Their main feature is that they are temperature stable, which is the requirement here.

[charles]

Hi megajocke,

Thanks so much for your very detailed reply!

The point about the line voltage ratings clarifies the situation greatly. Thank you! That being said, do the guaranteed power supply voltage ratings also indicate the maximum output range that should be used? In other words, if I put the range on 40 volts and crank the voltage to maximum, say 55 volts, is it likely that I will see the magic blue smoke?

As to the voltage control, I will need to look at this some more. If I understand your comment, I should look at the +6.2v reference that then passes through the voltage divider composed of R12 and R10, then to -S. Now, if I let the middle of the voltage divider be S, then I have S=vin*(R10/(R12+R10)), where vin = S + 6.2V. So, I can solve for S and get a value. This value matches the figure of 1 volt/200 ohms across R10.

However, this feels a bit like magic because it's not clear where the voltage S comes from. In other words, if I were to draw a schematic of this system, it would contain a voltage source of unknown value +S connected to the middle of the voltage divider and the minus terminal of a voltage source of value +6.4. Then, the desired voltage +S is sort of 'wished' into existence so that the equations are satisfied. To put it another way, this circuit requires a seemingly magical battery that knows the exact voltage +S that it needs to be at that moment and then sets its voltage to +S accordingly.

Thanks again!