Keithley 197 Repair

[alm]

Did you test the rectifiers with reverse voltages up to 20 V? The AC waveform looks like one of the diodes is breaking down like a zener.

As long neither the resistor or zener diode is getting warm, I wouldn't be too worried. R132 might be dissipating ~0.1W (forgot the exact value)? That's not something to be worried about, since the 197 is unlikely to be using 0805 or smaller resistors. VR105 might be dissipating 1/4 W max.

[staze]

Did you test the rectifiers with reverse voltages up to 20 V? The AC waveform looks like one of the diodes is breaking down like a zener.

As long neither the resistor or zener diode is getting warm, I wouldn't be too worried. R132 might be dissipating ~0.1W (forgot the exact value)? That's not something to be worried about, since the 197 is unlikely to be using 0805 or smaller resistors. VR105 might be dissipating 1/4 W max.

I didn't. Didn't really think of that... also not sure how I would. Apply 20V to the DC side (output) and see if there's anything on the AC side (input)?

R132 and VR105 are both getting over 38C (100F+ or so), so, warm... but not hot. R132 (330R) LOOKS like it's at least a 1W resistor, if not 3W or so (they're old school brown cylindrical ones (carbon?))... not sure why they never labeled wattage on resistors. So, they're getting warm, but not hot. I local shop has a replacement BR that I can pick up easily and swap in... see if that helps. Would a leaky diode result in the lower negative like side like this? I did take the BR out, and looked at the waveform coming from the transformer, and it looked perfect (as expected).

[alm]

Apply 20V (positive and negative) to the AC terminals and see if you get anything odd (like excess current/heat, or odd voltages on the DC side). Not sure how this would be related to the excess voltage, though. Based on the scope screenshot (a 20 Vpp AC signal) -19V seems perfectly reasonable. I think the only thing that's going to fix it is is changing the transformer winding or adding a load resistor to load down this winding. It sounds reasonable to me that they specced identical windings for the + and - 15 V windings, but since the current draw is much lower on the -15 V rail, it's at -20 Vpp instead of -15 Vpp.

[staze]

Apply 20V (positive and negative) to the AC terminals and see if you get anything odd (like excess current/heat, or odd voltages on the DC side). Not sure how this would be related to the excess voltage, though. Based on the scope screenshot (a 20 Vpp AC signal) -19V seems perfectly reasonable. I think the only thing that's going to fix it is is changing the transformer winding or adding a load resistor to load down this winding. It sounds reasonable to me that they specced identical windings for the + and - 15 V windings, but since the current draw is much lower on the -15 V rail, it's at -20 Vpp instead of -15 Vpp.

Huh, good point on the load... Just weird the manual indicates it should be -15V ± 10%. I guess my replacement opamps could draw that much less current... =/

When you say apply that voltage, do you mean VAC, or VDC? my PSU isn't dual rail, not sure that matters though. The waveform is actually 48Vpp.

I'll try swapping the BR (and testing the current one once I get clarification), and maybe call it good. All just kind of weird. =/

[alm]

I don't expect the replacement op-amps to make much of a difference. A replacement bridge rectifier of this size costs sub-$0.50, so it would be an easy thing to try. There's not a whole lot that affects the voltage from the transformer: input voltage, winding ratio, core, wire gauge and load. Since this is a shunt regulator, the current draw is going to be fairly constant at the voltage across R132 divided by 330 ohm for VR105 to maintain regulation.

You don't need a dual rail power supply. Just apply +24 VDC to the AC terminals, measure, and then apply -24VDC. Essentially a diode test on steroids, although you might as well measure the voltage on the DC side while you're at it. The scope screenshot suggests maybe conduction between the two AC terminals at +24 VDC. Or an open in the diodes conducting at the negative part of the cycle.

[staze]

I don't expect the replacement op-amps to make much of a difference. A replacement bridge rectifier of this size costs sub-$0.50, so it would be an easy thing to try. There's not a whole lot that affects the voltage from the transformer: input voltage, winding ratio, core, wire gauge and load. Since this is a shunt regulator, the current draw is going to be fairly constant at the voltage across R132 divided by 330 ohm for VR105 to maintain regulation.

You don't need a dual rail power supply. Just apply +24 VDC to the AC terminals, measure, and then apply -24VDC. Essentially a diode test on steroids, although you might as well measure the voltage on the DC side while you're at it. The scope screenshot suggests maybe conduction between the two AC terminals at +24 VDC. Or an open in the diodes conducting at the negative part of the cycle.

New BR has the same waveform on the input. =/ Weird. Any other possibilities? Otherwise, I'm just going to call it good.

[alm]

I don't see any reason to worry about this as long as nothing gets excessively hot and the regulated DC rails are in spec as far as levels and ripple is concerned.

[staze]

I don't see any reason to worry about this as long as nothing gets excessively hot and the regulated DC rails are in spec as far as levels and ripple is concerned.

No idea what spec is as far as ripple. The rails are "out" in that according to the spec, pin 12 of the power switch should be -15±10%, but it's at -19V. But like you said, that's likely due to there being that much less load on the negative rail. So who knows.

Thanks for all your help! I'll just call it good since it works, and I don't think anything is getting "hot".

[alm]

I said regulated rails. I don't particularly care about the voltage of the unregulated rails as long as they are high enough that the regulator stays in regulation (clearly the case here) and not too high that it produces excessive heat in the regulator.

[staze]

I said regulated rails. I don't particularly care about the voltage of the unregulated rails as long as they are high enough that the regulator stays in regulation (clearly the case here) and not too high that it produces excessive heat in the regulator.

Ah, then yes, they're all within spec. And R132 and VR105 do get warm, but not hot. Like you said, they're not dissipating much in terms of heat.

Thanks!

[PA0PBZ]

*Hello thread, are you awake?*

Just got myself a 197 for € 20 in an unknown state. Looks like new but has a simular problem, OV on all ranges and no negative rail. Cutting pin 4 of U107 brought it back so I ordered a handful of TLC's.
To be continued...

[staze]

*Hello thread, are you awake?*

Just got myself a 197 for € 20 in an unknown state. Looks like new but has a simular problem, OV on all ranges and no negative rail. Cutting pin 4 of U107 brought it back so I ordered a handful of TLC's.
To be continued...

Cool!

[rwzahora]

I recently picked up a used Keithley 197A.  The unit tests fine but the backlight is quite dim.  I read somewhere that this is a typical problem with this model.  Does anyone know if repairs can be done here short of replacing the display board?  Also, does anyone have schematics for the 197A which would show the backlight circuitry?  I have the manual for the 197 that was referenced earlier in this string which has the schematics for that unit.  I also have two versions of the 197A manual but neither comes with schematics.  If there is no easy backlight solution I suppose I'll go low-tech and do something like tape a key-chain LED light to my meter to illuminate the display when I need to read it.  I have to agree with other posters that an LED display is more useful on a multimeter (the 197A replaces an older Keithley 177 I had been using).

[staze]

I recently picked up a used Keithley 197A.  The unit tests fine but the backlight is quite dim.  I read somewhere that this is a typical problem with this model.  Does anyone know if repairs can be done here short of replacing the display board?  Also, does anyone have schematics for the 197A which would show the backlight circuitry?  I have the manual for the 197 that was referenced earlier in this string which has the schematics for that unit.  I also have two versions of the 197A manual but neither comes with schematics.  If there is no easy backlight solution I suppose I'll go low-tech and do something like tape a key-chain LED light to my meter to illuminate the display when I need to read it.  I have to agree with other posters that an LED display is more useful on a multimeter (the 197A replaces an older Keithley 177 I had been using).

Check voltage rails first and foremost.

I haven't been able to find a schematic for the 197A... but you might be able to contact Keithley and get a copy. It's referenced in the 197A manual toward the end (there are specific diagram numbers).

Might also want to start a new thread...

[rwzahora]

I contacted Keithley tech support about a schematic for the 197A.   They said that because of intellectual property issues they don't include schematics in their manuals.  Funny, but the schematic for the 197 is included in their manual at http://www.keithley.com/support/data?asset=11252.

[staze]

Yeah, that's BS. Could they supply the schematic at all?

Section 6.5 (page 6-1) says you'd want drawing number 197A-116 for the display board schematic, or 197A-110 for the component layout. -100 and -106 are the component layout, and schematic for the motherboard, respectively.

[PA0PBZ]

Just got myself a 197 for € 20 in an unknown state. Looks like new but has a simular problem, OV on all ranges and no negative rail. Cutting pin 4 of U107 brought it back so I ordered a handful of TLC's.
To be continued...

Replaced U107 and got the same as staze, the -6.4V was at +6.2. After that also replaced U101 (with TLC271, not the low power one but I don't think it makes a difference)  and the meter is fine again. Looks like a common error with these meters. Checked the calibration and it is still spot on (last cal was 1993 according to the sticker).

Thanks staze for doing all the hard work!

[staze]

No problem! Glad it's working. Could you, by any chance, check the -V rail off the bridge rectifier? Should also be pin 12 on the power switch.

Thanks!

[PA0PBZ]

Could you, by any chance, check the -V rail off the bridge rectifier? Should also be pin 12 on the power switch.

Of course, just opened it again and the -V on the bridge is -18.64V. The + on the bridge is 16.68V.

[staze]

Could you, by any chance, check the -V rail off the bridge rectifier? Should also be pin 12 on the power switch.

Of course, just opened it again and the -V on the bridge is -18.64V. The + on the bridge is 16.68V.

Perfect, or rather, right inline with what mine is (if not off from what the manual thinks they should be).

Thanks!

[timb]

I recently picked up a used Keithley 197A.  The unit tests fine but the backlight is quite dim.  I read somewhere that this is a typical problem with this model.  Does anyone know if repairs can be done here short of replacing the display board?  Also, does anyone have schematics for the 197A which would show the backlight circuitry?  I have the manual for the 197 that was referenced earlier in this string which has the schematics for that unit.  I also have two versions of the 197A manual but neither comes with schematics.  If there is no easy backlight solution I suppose I'll go low-tech and do something like tape a key-chain LED light to my meter to illuminate the display when I need to read it.  I have to agree with other posters that an LED display is more useful on a multimeter (the 197A replaces an older Keithley 177 I had been using).

*Casts Raise Dead (Level 12, Prepared) on Thread*

I got one of these awhile back on eBay, tore the whole thing down. It seems there's an EL strip [DD-48] that's glued to the display PCB which the LCD [DD-50] simply sits on top of. The whole thing comes apart easily because the LCD is sandwiched between a pair of zebra strips [CS-460-2] and the the main bezel [175A-305].

I think you could get some EL tape or a panel, cut it to size and install a small inverter (possibly even directly off the old backlight power rail) and it would be better than new! If the older 197 non-backlit display uses this same type of bezel arrangement, I suspect you can even add a backlight to it.

I might give this a go on mine (using red EL tape, retains night vision) and make a thread about it if anyone is interested.

[rastro]

Great idea, EL tape sounds like a good replacement.  I just took my 197a LCD apart to get some dimensions.  This component looks exactly like the EL tape in the vendors pictures.  One side has a pink color that's similar to the white EL tape I saw in a video.

There was some info printed on the part:
  LSI
  P/N 37269-1 Rev E
  014840 08/05/98
I was not able to locate any supply doing Google search.

Strip dimensions: 19.5mm X 87.0mm
Active area: 17.5mm X 83mm

The selections from Radio Shack/Adafruit/sparkfun/eBay seem to be about 14mm wide and one eBay seller indicated 8mm wide active area.  This comes up a little short for our purpose.  Perhaps it's enough to light only the center covering most of the numerals.  I'll probably stop by RS this week to see what their EL tape looks like.

[timb]

Why not two of the 15x100mm EL tapes from Adafrui and trim each one to 10x90mm? Since you'd be trimming the non-glowy side off each one you shouldn't be able to notice a seam in the middle of the LCD (especially since it's semi-opaque and EL is very soft light).

Or you could just cut one of the 100x100mm panels and call it a day: http://www.adafruit.com/category/50_81

[rastro]

I prefer the panel.  Less cutting/soldering and more consistent coverage.

[timb]

That's what I'd do too.

I'd really love to design up a little PCB that form fit in place of the LCD with an LED or VFD on it instead. You could use a small micro to convert the data without a ton of effort, I'd imagine.

A bit off-topic, but I don't really have a need to undertake something like that right now since I've actually got an old ISA based GPIB card on the way from eBay that I'm going to be hooking into an old Compaq Portal III lunchbox PC (the one with the awesome orange gas plasma display), so I'll most likely be using that to display (and log) data right from the 197A anyway. Turns out the card was made by CEC (sometimes included with old Keithley gear) and they have a really awesome driver/library/example package that shows how to easily write custom GPIB apps in everything from Turbo C, Visual Basic for DOS/Windows, Qbasic, QuickBasic and pretty much any other MS-DOS/Windows 3.11 language.

Since you can find these cards in ISA and PCI variants for well under $50, I'd like to write a nice little modular GPIB control suite that'll run under FreeDOS. That way you could turn anything from an old 286 to Core2Duo into a command center for vintage gear!