Keithley 619 repair

[veedub565]

Hi I'm trying to repair an old Keithley 619 multimeter, just wondered if anybody has any idea's

I'm following the cal procedure as per the manual, I can set the 0.00v ok, and I can set the 190mv ref ok and it will sit there around 190mv fluctuates a bit but within spec.

What I'm having problems with is the 1.9v ref, I've reached the end of the trimmer pot adjustment and still can't get the voltage down below 1.9037v  (max spec 1.9003) bizarrely it will occasionally, and suddenly, drop to 1.895v (min spec 1.897v) sit there for a little bit and then go back up again.

I am using a DC calibrator to feed stable 190mv and 1.9v into it, and I've verified the voltage with a modern 6 1/2 digit multimeter. I have tried both input channels and get the same problem on both channels, so that rules out a problem with the input cards. The CPU part is common to both 190mv and 1.9v ref so I can only think there's an issue with the Filter/Mux board.

I've checked the obvious resistors and things, I pulled and tested the JFET attached to the 1.9v ref part and it tests ok with an Atlas tester.

[tautech]

Hi I'm trying to repair an old Keithley 619 multimeter, just wondered if anybody has any idea's

I'm following the cal procedure as per the manual, I can set the 0.00v ok, and I can set the 190mv ref ok and it will sit there around 190mv fluctuates a bit but within spec.

What I'm having problems with is the 1.9v ref, I've reached the end of the trimmer pot adjustment and still can't get the voltage down below 1.9037v  (max spec 1.9003) bizarrely it will occasionally, and suddenly, drop to 1.895v (min spec 1.897v) sit there for a little bit and then go back up again.

I am using a DC calibrator to feed stable 190mv and 1.9v into it, and I've verified the voltage with a modern 6 1/2 digit multimeter. I have tried both input channels and get the same problem on both channels, so that rules out a problem with the input cards. The CPU part is common to both 190mv and 1.9v ref so I can only think there's an issue with the Filter/Mux board.

I've checked the obvious resistors and things, I pulled and tested the JFET attached to the 1.9v ref part and it tests ok with an Atlas tester.

Just wondering.....
Checked PSU voltages and ripple are to spec? Dodgy caps?
Trimpot faulty, Skeletal or sealed type?

[veedub565]

Can't say I've checked the PSU voltages or ripple, but supply rails would be common to both 190mv and 1.9v reference's and the 190mv ref is ok. Still worth checking though and I will do that next.

All the caps on that board (Filter/Mux) seem common to both 190mv and 1.9v references, they look ok but haven't tested them. Had a quick look at other electrolytics and seem ok. 

Trimpot tested ok, not sure what you mean by skeletal or sealed type.

[tautech]

Can't say I've checked the PSU voltages or ripple, but supply rails would be common to both 190mv and 1.9v reference's and the 190mv ref is ok. Still worth checking though and I will do that next.

All the caps on that board (Filter/Mux) seem common to both 190mv and 1.9v references, they look ok but haven't tested them. Had a quick look at other electrolytics and seem ok. 

Trimpot tested ok, not sure what you mean by skeletal or sealed type.

As with any attempted repair, the PSU is critical to any equipments performance and many unexpected results can be traced to a "not in spec" or failing PSU. It is always the first thing you should check, failure to do so can have you "chasing your tail". 
Always confirm the PSU is to spec. Excessive ripple is the first sign things are bad, often caps are bad and visual inspection doesn't always identify culprits. Simple capacitive measurement checks are often sufficent unless it's a SMPS, then ESR checks are needed too.
For simple checks, use ~5-10% tolerance as an indication that replacement is required.

Skeletal presets are an "open" type, the tracks are exposed to dust and any humidity that then can cause reactions that can seriously contaminate and damage tracks. Noisey and non-linear adjustments are the result.

[veedub565]

Ahh ok thanks for the info, it's a sealed trimpot so no exposed tracks.

Not had chance to look at it today but will def check the PSU out.

[veedub565]

Well just an update on this, I haven't forgotten, just been busy with other things.

PSU checks out ok, all power rails look good. haven't pulled the caps yet to check them but, I did find a potential problem on the Mux/Filter board. As mentioned it has 3 references by way of 3 FET's switched in and out by some logic circuits, 190mv, 1.9v, and 0v. The gate of each FET in addition to being connected to the logic circuit is also tied via a resistor to bootstrap ground. Bootstrap ground is achieved from the junction between 2 zeners with +15v at one end and -15v at the other. One zener reads 18Mohm one way and o/c the other. The other (identical) zener reads 1.8Mohm and 15Mohm.

So looks like a zener problem skewing bootstrap ground and causing problems. Will update more once I put some new zeners in.