HP 6236B -20V ammeter not working

[belzrebuth]

Hi all,
I've just got a 6236B and it's ammeter is not working when it's set in the -20V mode.

The -20V output works and can supply current, and the other two outputs also work fine (ammeter works in those modes as well!)
It's only in the -20V setting that the ammeter is not working.
At first I suspected a dirty switch but then I noticed that the needle goes slightly below zero when I turn it to -20V so it's there it just does not display anything.

Any ideas on what might be wrong?

[wn1fju]

Glancing at the schematic, my guess would be one of the resistors in the resistor network, Z1, is faulty.  Check them all with an ohmmeter.  Hopefully one has opened up and a replacement resistor can be tacked onto the pins of Z1.  Obtaining a new Z1 is probably very hard.

[belzrebuth]

I measured some voltages and my -12.5 is -13.5V.
7.5V is at 7.45 and -6.2 is -6.25 so those are okay.
The resistor network is not open at 17,18 to 2, but resistances all over the place since I'm measuring in circuit.
Haven't checked it all of the pins of the resistor network yet.
Need to make a table or something.

I also checked Q11 as per the troubleshooting steps which BTW gets really hot with no load.

It's not shorted and checks out okay in my transistor tester.

The other thing maybe worth mentioning is that the current limit on both rails kick in at around 480mA or so.
It's not at 550mA as stated so either the previous owner set it there or there's a fault in the current limit circuit.
I doubt it though cause the +20V supply is working okay and the meter is pretty accurate in both voltage and current.

I'm pretty puzzled on what might be wrong with it.
The only symptom is that the current meter at -20V is not working but I don't like that -13.5V where -12.4 should be and the hot Q11 ..
Maybe the fault is more involved..

[wn1fju]

Seems real strange that all three supplies (6, 20, -20) would work if there is something seriously wrong.  I opened my 6236B up and took a few measurements:

1.  The -12.4V bias measures as -12.7V
2.  With no load and all three supplies set at half-scale voltage, Q11 was about 135 degrees F after 30 minutes.  The two adjacent power resistors, R69/54, got to about 250 degrees!
3.  With the meter switch set to 6V, the Z1 resistances in the meter circuit were:
   Z1A pin 4-17:  147 ohms
   Z1E pin 4-5:  ~3.6K slowly charging up through some external capacitances
   Z1B pin 16-17:  611 ohms
   Z1C pin 2-17:  609 ohms
   Z1F pin 2-18  650 ohms

Hope this helps a little.  You might have to pull Z1 and measure it out of circuit to be sure since the resistances in circuit depend upon the meter switch and parallel resistances.  A bad Z1 is still my guess.  But I've been wrong before.  And as I was putting the covers back on my unit, I turned it on and got absolutely nothing - totally dead.  Turns out I broke a wire going between the power outlet and the on/off switch while I was playing around.

[belzrebuth]

Thanks so much for doing that!
I really appreciate it.
Q11 is not *that* hot just hot to the touch but I need to measure its temperature to be sure I'm not overstating.

So my measurements on the resistor network:
4-17   -> 87Ω (not same but kind of close)
4-5     -> also charging and about 3.7k (same as you probably)
16-17 -> 611ohms
2-17   -> 210k (!)
2-18   -> 235k or so

with the meter switch set at -20
the reading start at 7k or so but they go down a bit each time I measure the same pins
2-17 is about 1.3k
2-18 is about 1.3k as well

but I'm pretty sure earlier I got these values:

2-17  220Ω
2-18  230Ω

with the meter switch set at -20

2-17 is about 220k
2-18 is about 230k

I really hope this network is not at fault here since it seems like a very weird custom part even though it's Dale branded.

[belzrebuth]

I think I should address the -12.4 problem first.
I also need to investigate whether the current limit is actually set to 480mA or it's just misbehaving because of another fault.
The top two pots are measuring 1.33k between their bottom two legs so if that's the factory setting there's a problem in the upper limit of the current as well.
I didn't know someone would be kind enough to actually open his own unit to measure else I'd ask for this measurement as well:P

[wn1fju]

I'm slightly confused by your last readings. 

I will concentrate solely on Z1-C (pins 2-17) and Z1-F (pins 2-18).  My readings were taken with the machine powered off.  I got:

Z1-C resistances in ohms (2-17):  610 (meter switch to 6), 610 (meter switch to +20), 148 (meter switch to -20)
Z1-F resistances in ohms (2-18):  650 (meter switch to 6), 650 (meter switch to +20), 188 (meter switch to -20)

This all makes sense.  For instance, note that point I3 and point C differ only by R28 (1.25 ohms), so let's just say I3 = C for this discussion.  Also note that the point on the schematic marked +7.5V differs from C by R70 which is 40 ohms.  In other words, +7.5V = I3 + 40 = C + 40.  So all the Z1-F measurements will simply be 40 more ohms than the Z1-C measurements for all practical purposes.

When the meter switch is set to 6 or to +20, measuring 2-17 is just Z1-C in parallel with (Z1-F + 40).  Z1-F is 251K, so pins 2-17 should only be slightly smaller than the Z1-C resistance of 612, hence 610.   When the meter switch is set to -20, measuring 2-17 is the previously mentioned parallel resistance, but now also in parallel with the meter and adjustment pot (~200 ohms).  So that's about 610 || 200 = 150 ohms.  Pretty close to what I measured!   Yours might be slightly different depending upon the ammeter adjustment pot setting.

By the way, you have buzzed out the meter switch and made sure all the connections are being made???  I've certainly seen my fair share of switch problems.

[wn1fju]

As for your current adjustment questions, I got about 1.3K ohms on the two pots. 

Short the +20V and Common output jacks, turn up the voltage slightly until it current limits, then try and adjust the pot.  On mine, I was able to move it from about
0.45A to 0.6A in about 1/8 of a turn.  If you can't get to the rated 0.55A, then I agree that something else is probably wrong.

[belzrebuth]

I can't get to 0.55A with the supply as is right now.Tried with my electronic load and voltage falls at 480mA at both the +20 and -20 ranges.
(+6V reaches more than 2.5A so that's okay.)
Since we've got the same 1.3k on the pots there should be something else preventing me to do so.

About Z1 (again) :
I get 100ohms and 140ohms from 2 to 17 and 18 respectively when I'm in the -20V meter so that seems okay on paper.
I sprayed some light contact cleaner on the switch.(no deoxit or anything too drastic)
I now get 130k on both +20V and +6V meter ranges from 2 to 17 and 18.
It's less than the 220k or so I measured earlier.
I can't measure all the switch pins because I don't have access to them untill I dismantle the whole right side of the chassis.
I only measured Z1 pin2 (which corresponds to a blue wire at the switch side) and it makes contact to a black wire when I'm in the -20V range.

Would it be wise to solder 2 560 or 680 resistors from pin 2 of Z1 to  17 and 18?
edit: No 2*560/680 more like 240 and 560 as stated in the schematics.
I don't have 251k and 612k at hand.
But then again I'm worried to so just in case I damage Z1 myself and the fault is somewhere else?

[wn1fju]

You said 130K from 2-17 when in the +20V and +6V range???  Can't be.  On those ranges, Z1-C (2-17) and Z1-F (2-18) are isolated from the meter circuitry.
You therefore have 612 ohms in parallel with 251K+40 ohms, hence about 610 ohms. 

Measure Z1 right at the Z1 pins.  Remove the bottom panel and probe the circuit there.  If there is something wrong with the meter switch/wiring, measuring at the switch might give you erroneous readings.

I would also buzz out the switch as I said before.  But do it from the meter to Z1.  For instance, Z1 pin 2 to meter negative should be a 0 ohm short when the switch is set to -20V.  Buzz it all out from both sides of the meter (accounting for R59 on the positive side) to all the relevant pins on Z1 (in all three switch positions).

I wouldn't go tacking resistors onto Z1.  You might inadvertently supply too much current to the ammeter and damage it.

[belzrebuth]

That's where I'm measuring all this time.
Right at the pins of Z1!

I wouldn't be able to measure at the switch anyways because I don't think there's not enough clearance and the cables are pretty tightly wrapped.
Still need to verify every switch connection though cause the first time I've had a play with the instrument the +20V ammeter was also stuck and I needed to turn the switch some times in order to get it functioning.
But this is different because now it clearly switches from one mode to the other.

I think I might need to desolder Z1 or at least pins 2, 17, 18 and try measuring out of circuit.
Really hope it's okay as it's unobtainium but sadly I don't see any other explanation for the -20V ammeter to not work.
If it's actually open circuited it might explain the arbitrary value of 130k I'm getting since I'm essentially measuring an open//anything and 7.5V is to common is only 40ohms difference as you said earlier.

[belzrebuth]

I got pins 2 and 17 and 18 out of circuit and I'm measuring 251.8k from 2 to 18 and 0.7k from 2 to 17.
So Z1 is probably alright.
And the ammeter works in the +6, +20 ranges...
So it's not the ammeter and it's not z1.
And the -20V is indeed providing current so it's just the ammeter and in this particular range only.
I'm losing it..
Really can't figure this out.

I never mentioned this before but when I got the unit which was reported as faulty(no -20V output) there was a badly corrored trace (open circuit) from C18 to the negative terminal of C17.As soon as I bridged that trace the -20V output came back.
I don't know if this could have caused any other damage just thought I should mention it.

[wn1fju]

It's got me stumped also.  Has to be a missing wire/connection somewhere.  Time to go into desperation mode and buzz out the entire -20V ammeter circuit.

1.  Check for 7.5V at Z1 pin 18
2.  Check for zero ohms from Z1 pin 17 to Output Common
3.  Check for zero ohms from I3 (R28) to ammeter adjustment pot R59
4.  Check for zero ohms from Z1 pin 2 to ammeter negative post

If they all check out, and you have already confirmed Z1 is OK, I don't see what could be wrong.  I am taking your word for it that the power supply does output correctly on the -20V side.

[belzrebuth]

1.  Check for 7.5V at Z1 pin 18  (OK , 7.35 or thereabouts)
2.  Check for zero ohms from Z1 pin 17 to Output Common (ok)
3.  Check for zero ohms from I3 (R28) to ammeter adjustment pot R59
R28 measures 0.3Ohm in circuit while the other two (R8, R48 measure about 1.3ohms)
That's a bit strange. (edit : measured R28 out of circuit and it's fine).

Other strange thing is that R59 is set fully clockwise to zero ohm, is this the way it's supposed to be?

4.  Check for zero ohms from Z1 pin 2 to ammeter negative post (ok)

I measured Z1 pin 2 and 17 again and it's 213k not 0.67k.
I get 0.67k when I'm measuring the pad of pin 17(common) to the floating leg (z1 pin 17).

[wn1fju]

Your readings just baffle me more and more!

R28 is nominally a 1.25 ohm resistor.  You said that it measured OK out of circuit.  But when installed, you are reading 0.3 ohms???  Can't be unless there is a short somewhere.  Lift one end of R28 (to remove it from circuit) and measure the resistance across the R28 pads on the board to make sure there really is an unintended short.  With R28 out of circuit, the resistance should be in the tens of Kilo-ohms.  If you are getting near zero ohms, start tracing through everything connected to Common in the -20V circuit.  Could be shorted capacitors, diodes, transistors op-amps, etc.  You want to look for any part that gives you zero ohms across it.

If there is an actual short on the board, I can't see how the -20V supply would work.  Have you verified that you are getting correct V and I at the output terminals with a small load attached?

I wouldn't worry about the R59 setting.  Someone probably tweaked it trying to fix the meter problem.  But do verify that the pot is OK (not open). 

Also, I'm not sure about your Z1 2-17 resistance readings.  With the meter switch at -20V, the resistance from Common to Meter negative post (i.e. across 2-17) should be in the 600 ohm range.  How could it be more than that when you are essentially measuring across a 612 ohm resistor (with other stuff in parallel)?

[belzrebuth]

I'm sorry I meant the 6v psu power resistor which is indeed 0.25Ω.
Τhat's on me, I just got confused.
So R28 is okay.
I verified the -20V supply operation with 200mA constant load.
I've used my electronic load for this, I can draw 200mA regardless of the voltage set so I guess that's fine.
So I've a 10V output and constant draw of 200mA and took some more readings.

The meter (+) is at 0.2565V and (-) at 0.2588 (so that explains why the meter needle sits a bit below the 0 mark when I'm at -20V range).
pin 2 of Z1 is also at 0.2588 (as it goes to (-) terminal).
pin 18 is at 7.45 and 17 at zero which are fine.
R28 is at 0.2563 and the other leg is at zero volts.

What's interesting is that when I switch to +20V range pin 2 of Z1 sits at 3.2V which pretty much tells me that there is almost a 1:1 resistor ratio in the divider.

So I desoldered pin 17 of Z1 again being super careful it doesn't touch the pad and measured its resistance again to pin 2 and it's 120k or so.
With pin 17 out of circuit resistance from pin 2 to common is 95.5ohm with the meter set to -20V and 115k when the meter is set to +20 or +6.
Resistance of pin17 out of circuit and pin 16 is 0.615k.
So my earlier measurement isn't valid is think.
Z1 may indeed be faulty at that particular spot only.

If I remove the leg of Z1 pin17 out of the pad completely and place a close valued resistor (2*1.2k in parallel) between pin 2 and common do I risk anything?
Pin 17 is not reference to any other pins except pin16 from what I see in the schematic.

[belzrebuth]

I don't think it's that simple though.
pin 17 is used three times.

there is a 17-4 resistor at 612οhm (measures fine)
there is also a 17-17 resistor at 612 (which also measures fine)
only the 17-2 resistor does not measure okay.
But these are independent, aren't they?
I mean if I raise the leg of pin17 the other two resistors won't mind that this leg is not at common, right?
I really want to get this supply going but I sure don't want to cause more damage!
Can't undestand how this resistor got damaged, must have been a really unlucky concidence.
And I'm really unlucky as well I guess for getting a hold of this particular unit with this particular fault..

[wn1fju]

I wouldn't lift pin 17 since the other meter ranges won't like it.  I would simply take a ~600 ohm resistor and tack it on top of Z1 between pins 2 and 17.  Since that is what it is supposed to be, and your previous discussions seem to indicate that 2-17 is currently reading high, I don't think you will cause any harm.  If that makes things better, you might have to play around with the value a bit since you have the existing (incorrect) 2-17 in parallel.  Worth a shot.

[belzrebuth]

Thanks so much for your patience wn1fju.
I tried that now and the meter works!
I'm still uncomfortable about the -12.4 which is =-13.5 and the 480 current limit but there's progress.

[wn1fju]

Glad it's working better now.

For your -12.4V problem, that circuit is pretty simple.  A 6.2V Zener drops Common down to -6.2V which also goes to the + input of an op-amp.  When things are working correctly, the -12.4V is divided by two equal resistors such that -6.2V appears on the - input of the op-amp.  The whole regulation of the circuit via Q11 and Q12 is achieved because the op-amp tries to adjust things so that + and - inputs are equal, i.e. produces -12.4V. 

If you are getting something other than -12.4V, two things could be wrong.  The first is that the Zener is no longer behaving like a 6.2V Zener.  But you said earlier that you measured -6.25, so that's not it.  The second thing is that the two divider resistors are incorrect.  And where are those resistors?  Z1, of course!  So you might check those Z1-J and Z1-K.  Wouldn't surprise me if they have changed values given your other problem with Z1.  A larger value for Z1-K than Z1-J would make the -12.4V drop down more negative like you are seeing.

As far as the current limiting, I assume you have already tried to adjust the two pots and still couldn't get to 550 ma.  Note that the current adjustment pots are both in series with 30K resistors.  And where are those resistors?  Z1, of course!  The current limit (-20V as an example) compares the voltage after the current sensing resistor, R28, to a reference voltage set by R26 and Z1-N.  If Z1-N has increased in value, then R26 will run out of room and not be able to get to the voltage that you would need when 550ma is across R28.

Seems to be a common thread here.  Also note that the two current limiting resistors, Z1-D and Z1-N, as well as Z1-K in the -12.4V circuit all share a common pin 5.  If there were some added internal resistance on Z1 pin 5, that would explain both of your problems.

On my unit, I could easily set the current limit to 600 ma on both +20 and -20V supplies.  I didn't go any higher than that because that's full scale on the meter.  But I would guess I could have pushed it somewhat higher.

[belzrebuth]

Thanks for the explanation.
I didn't try to adjust the current pots yet since I thought that if you were having yours at the exact same value there's a problem with my unit and not its calibration but I could try that, no problem.

I don't need more than 500mA anyways since I have other supplies for more power hungry stuff.
This is going to be used for breadboard projects and such so even 100-150mA is usually enough on the symmetric rails where the analog stuff is getting fed from.

I was more like trying to understand what's wrong with my supply and I think Z1 might be the root of all evil (as you mentioned in your first post!).
I don't think I've ever read about a similar problem online regarding this supply. It was all about capacitors and maybe opamps or esoteric parts like stabistors etc.
I never saw this Z1 thing mentioned anywhere and it seems kind of weird (to me at least) for a custom made precision resistor to drift that much, could be wrong though.

I'll try to take some Z1 measurements (again) and come back with the results.

To be honest if the supply is not struggling or something I don't really care about its internal reference values if its functioning correctly from a user's point of view.