Fluke 332D repair - new issue

[cncjerry]

Today I received a new wire-wound resistor to replace the collection of resistors I had in the string.  You might remember I replaced two, 19.985K wire wound in parallel with a combination of resistors to make 9.9925k.  I put the new resistor in and started to peak R8 with the next lower decade and I got confused by the '8' in the decade and was turning the wrong resistor.  So I pulled the unit apart, realized my mistake, put it together again, and now I have current all the time.  I can dial it down or trip it, but something is wrong.

I pulled the unit apart and can't find anything pinched, shorted, etc.  If I measure from the +sense to the chassis (not the front panel), I get .08 ohms.  If I measure from the +sense/+output to the -sense/-output, they are separately strapped, I get 5.56 ohms.  I know the chassis floats on this thing so I don't know what is normal. The chassis is at full potential according to the stickers and my prior findings (shock).  It looks like the +sense might be at chassis level all the time?  I'm reading thru the schematics but can't tell the guard, sense and grounding scheme yet. 

I keep thinking I grounded something pulling the front off but I was really careful as there are a lot of wires in there.  I can't see anything wrong, can't get the measurements to change by pulling/moving wires, etc.

The other point of note, I was wondering if there is an override, ground, etc on the outputs that might be linked to the power/standby switch?  You have to disconnect the switch to separate the front half off the machine.  I didn't realize there was also a plate on the back of the power switch (those that have a 332 or 335 will know what I mean) so I am wondering if the power/standby is confused.

I've had it apart several times now.  It powers up without a fault.  If I keep the Voltage output low enough to not trip the current sense, then it works.  I can output up to about .3V (.300001) but once I go over 60mA then it trips.   .300000/5.56 ohms is about 55mA. I can also limit the current with the dial. 

So I guess either I shorted something or a maybe cap or something blew in coincidence with my playing around.

Any ideas?  The primary help I need is in the sense and grounding - what is supposed to be at chassis level, what should be at true ground, etc.  Separating the straps on the output doesn't seem to help.

Thanks

[dacman]

The internal shield is connected to +Sense.  The inside cover is Guard.  The outer cover should be earth ground.  Ensure any RFI/EFI fingers (near the power cord) are not damaged.  Ensure adjustment cover plate attaching hardware is not damaged.

There are (about, I'm guessing) 12 white spacers on both the top and bottom, and about 6 or 8 on each side.  These are important.  They can be destroyed if the unit is pulled out of its case in its normal operating position.  I like to stand units on their handles before removing or replacing the cover.  (I replace these with stick-on ones from McMaster-Carr, although they are softer.)

The power switch can easily be misaligned if the shaft is removed.  It has no detent without the shaft and must be manually aligned once the shaft is reinstalled.  Inspect it and make sure the contacts are properly lined up (and centered).  If you did not pay attention to the contact alignment of the power switch when reinstalling the shaft, there is a high probability that it is misaligned.

[cncjerry]

I take the cover off on the handles as you suggest and the spacers are all in good shape.

I noticed from the schematic it looks like relay K1 B contacts shorts the outputs.  I have a feeling that is the issue and I am going to explore that now.  Relay K1 B contacts short diodes CR2 and CR3.  There must be some low resistance on the relay contacts causing some voltage to get out (my 5.56 ohms).  It doesn't sound right powering up so maybe the switch is out of sync and not opening K1.

Thanks

[cncjerry]

well, I found the problem.  K1 is tripping because the wafer switch has a problem.  I can see some contacts that are all buggered up.  Maybe when I pulled the front, the long activating post hit the contacts.  Don't know how it happened or if I can fix it.  Might have to do some major surgery and swap in some contacts.  One is pushed back pretty far. 

Damn, damn, damn, damn. 

[cncjerry]

Looking at the switch I found that I probably wasn't the first one in there.  It's in pretty bad shape.  I am trying to bend the contacts back to functional state, never will be perfect.

I like this unit.  It't very stable when I leave it powered on, draws a ton of power though.  I'm looking for a parts unit for other reasons, now I'll have to find a switch.

[dacman]

I'd suggest also checking the electrolytic capacitors if you haven't done so already.

[cncjerry]

The prior owner replaced some and I'll go back in the in the near future. 

This thing weighs a ton.  I was working on it on my bedroom floor.  I had it apart about 10 times, what 10 screws per top/bottom plus 10 on the sides?  Even with a small electric driver it's a lot of work.  I'm sore on every pointed part of my arms and legs, hips, fingers, etc.  No shocks this time.  The only thing heavier was my 8568 and 8566.

Prior to taking it apart this time I had used my 3456a as a transfer so I was able to quickly reset the pots I messed with, one by accident.  I am warming it now for the final adjustment.

This thing is pretty stable though.  In the low ranges below 3V I got incredibly low stdev from my 3457a, like 270E-9th over a number of hours.