Fluke 5200A question

[FTM777]

I am (slowly) bringing a 5200A back to life (not a task for the faint of heart!).  I am puzzled by the monitor circuit on the A3 power supply main board that generates a not INT OP signal.  It takes the output from the winding that supplies the 5V regulator (around 8V?), rectifies it and has a 20uF tantalum capacitor.  That is then divided by two using two 680 ohm resistors in series.  The output from the junction of the two resistors is connected to a 4.3V zener feeding the base of a pnp transistor (2N3906).  Provided the power supply remains on, the emitter of the transistor sits at close to 5V.  Here is the puzzle, there is also a feed to the base of the transistor via a 6.8K resistor from the -15V supply.  These resistor values are the same on the board.  As far as I can figure, this -15V feed cannot affect the output of the transistor (given the resistor values).  So how is this circuit supposed to work?  (It obviously does otherwise Fluke would have changed it).

Nick

[TheDefpom]

You mean the A5 assembly.

I'm sure you have seen my 19 part video series repairing one of these... I had to repair the power supply a few times !

The INT OP signal disables the calibrator output by turning off a relay, that transistor sensing arrangement detects the 5V rail being overloaded (don't ask me how I know), and also as designed, to detect the power being turned off to disable the output to prevent a surge on the output.

As the transistor is a PNP type it needs the -15V rail there to turn it on, the 5V rail is there to make sure it is turned off normally, if the 5V rail drops too low from excess current draw the gate will drop below 0V being pulled down by the -15V rail, and turn the transistor on, R43 from the -15V rail is a 6.8K.

If I remember rightly I had to replace that transistor in my unit as it had failed.

[FTM777]

Thank you Defpom, yes I am working through your youtube videos.  I have just finished part 4 (#295), this seems to have become a bit jumbled, it starts with the power supply repair and then about half way through it jumps to the Reference supply also about halfway through that.  It then returns to the beginning of the reference supply repair.  Not a major issue, just a bit confusing.  Just starting part 5.  Every bit of knowledge will help me keep at it.
I am with you about protection of the power supplies.  Some of them have current limiting (a couple with a u723), but the + /- 15V(o) only has some resistance in the feed to the control transistor and those resistors are underrated.  The -15V(o) supply to the Reference had a fault which caused those resistors in the -15V(o) supply on A5A1 to smoke and took out the transistor (Q2).  I am considering changing Q1 & Q2 (and removing the other relevant components) for 3 terminal regulators (maybe 50mA limited).
I have been using the fault finding table (starting on p4-21) once I managed to get the basic thing working without smoke.  That has proved to be brilliant if a bit tedious.  Keep up the good work
Nick

[TheDefpom]

Yeah some of the series jumped around as it was how I was working at the time, I would get one board done or partially done, and find a board it linked to was also faulty, so then I would go and work on that, so I could go back and finish working on the first board, sometimes it was also because I was waiting for parts to arrive so I could keep working on the unit, I go back to the power supply a few times, because the other faulty boards kept blowing it up.

Every single board in my unit was faulty, there was not a single board that didn't have a problem.

[Rax]

I seem to recall some components are placed in marginal design situations, by which I mean they're stretched near their max ratings. Then there's the necessary recap for a unit of this age. And then also carbon comp power resistors that are drifted and just about ready to really fail with every hour of operation.

I wonder if putting together a list of "must-do"s would be helpful for everyone getting one of these. Mine is working OK, except the second digit would sometimes jump up by four counts... A bit weird. It'd typically correct itself with a restart, but I assume sooner than later I'll have to go back in and properly restore it. If I do that, it'd be great to have a punch list to go through, where I'd know the unit is good to go at the end of that work, and that expected failures were prevented before actually occurring.

[TheDefpom]

I wonder if putting together a list of "must-do"s would be helpful for everyone getting one of these.

Basically replacing all the electrolytic caps, replacing (upgrading the V rating where possible) all of the dipped tantalum caps, cleaning all the card edge connectors, cleaning all switch contacts, replace fan (or if possible the just the fan bearings), check/replace old carbon composite resistors.

Mine is working OK, except the second digit would sometimes jump up by four counts... A bit weird.

That is probably the front panel switch itself, I had a similar problem with mine which was due to a switch being dirty.

[Rax]

Basically replacing all the electrolytic caps, replacing (upgrading the V rating where possible) all of the dipped tantalum caps, cleaning all the card edge connectors, cleaning all switch contacts, replace fan (or if possible the just the fan bearings), check/replace old carbon composite resistors.

That sounds like a good list (though I'm personally not typically replacing by default all dipped tantalums, other than those I determine to be placed in marginal design, such as not derated enough; I think that's their typical failure mode, otherwise tantalums can pretty much last forever). When I get back in there again, my main urgency will be to replace all power carbon composition resistors. I don't really think CCs should have ever served power duty. I think all of them should replaced in sight with a unit of this age (where they're common), just like Schaffner line caps and leaked electrolytics.

That is probably the front panel switch itself, I had a similar problem with mine which was due to a switch being dirty.

I don't recall if I tried this, but it's definitely a good idea to closely consider.