Electronics > Repair

Keithley 197 Repair

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staze:
All,

After my success repairing a Keithley 199, I hopped on eBay and found a 197 for a song. It too is damaged (all readings show as oL). After some basic voltage probing, I THINK I have figured out the issue.

It would appear the negative voltage zener is toast, but I'd love a second opinion.

The manual can be found here: http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Keithley/Keithley_197.pdf

Schematics are included. On the first page of schematics for the board, it appears that out of the bridge I get a filter cap, then through VR104 a positive rail, and VR105 a negative rail. On one side of VR105, I get 0V (makes sense, it's grounded), on the other I get about 0.8V. I confirmed off the bridge rectifier I'm getting a +15V, and a -18V.

Everything I've checked that has a negative rail, all gets that 0.8V instead of the -15V ±10% it should be getting. Is it most likely that zener is toast, or am I overlooking something? Looking at the board, it appears it's a 1N47 15V zener (1N4744, or NTE145A). Is there a good way to test it short of what I've done? It does show 0.53V in one direction, and 1.96V in the other... so... guessing that's bad.

Thanks!

alm:
This copy lacks schematics. In general, it helps to be specific: at least give a page / figure number, and preferably post the relevant part of the schematic as an inline image. The easier you make it for people to help you, the more likely you are to get a response.

Any semiconductor on the bus might be pulling the negative rail down (up). Desoldering the suspect component, measuring it out of circuit, and if necessary replacing it is a good strategy. Sometimes it may be necessary to isolate parts of the circuit (cutting a pin / track) if there's a lot attached to that rail. Some manufacturers will even include wire links that you can cut and replace for that exact purpose.

staze:
Shoot. There are two version of the manual out there... guess I linked to the wrong one. =/

Okay, schematics attached. This copy here: http://www.utwente.nl/tnw/onderwijs/Practica_TNW/doc/apparatuur/multimeters/keithley197.pdf has the full schematics as well. The voltage rails are all on the first page, in the lower right quadrant. I see where the positive and negative rails are "created", as well as the +5V analog and digital rails, and the +10V rail (for many of the opamps on the board running on +10V and -9V), but I must be blind because I can't see where the -9V rail is created from the -15V that the zener in question should be providing. 

The digital stuff all seems to be working, since obviously it just runs on +5V. 

So is your thought that it's unlikely the zener has died? I figured something could be dragging it down, but given I can't find where the -15V actually goes to be converted to all the other rails...

If I pull the zener, what's the best way to test? Just run 18V (or whatever) through it, with a 100R resistor in there and make sure it breaks down at ~15V?

As always, thanks alm.

staze:
Ah, looks like the -9V rail is created in F2, on page 1 of the schematics. It looks like VR101 may be the answer... but I can't read a model number, and keithley seem intent on not listing actual off the shelf part numbers in their manuals... =/

alm:
The parts list from the first manual you linked to gives a description and part number for VR101: DIODE, ZENER 8.2V, IN756A [sic] (DO-7).

I'm not saying it's unlikely, just that anything connected to V- and a positive rail could be responsible for this. Were those diode measurements you gave for VR105 in circuit? That's not very reliable. You can indeed desolder one leg and test it with a resistor + voltage source.

If you suspect a component, then cutting or desoldering its connection to V- should restore the rail to -15 V (this of course does not apply to VR105). I'm guessing R132 probably gets quite toasty with ~16 V across it?

I haven't looked at the schematic in detail. If a lot is powered via the -9 V / -6.4 V rails, then you could try isolating the -9 V section and see if the V- rail comes up (goes down) to -15 V. For example by desoldering one leg of VR101.

If there are tantalum caps in there, then a shorted tantalum cap would definitely be one of the candidates.

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