Keithley 175 repair and inside pictures.

[Wim_L]

I recently got my hands on a defective Keithley 175 multimeter. The display wasn't working right anymore, with some segments failing to activate. The soft buttons to the left of the display also didn't seem to work consistently.



I found a scanned manual online, for those who'd like to see how it works. (includes block diagrams):
www.utwente.nl/tnw/onderwijs/Practica_TNW/doc/apparatuur/multimeters/keithley175.pdf

Anyway, on to the pictures... First a quick overview of the case. It's a shell in two parts, made of plastic, held together by four screws in the bottom. Quality construction, of course, threaded metal inserts for all screws. To open, remove the screws, turn it around, and lift the top off. That also releases the handle.



While the top part of the case is just a plastic piece, the bottom part includes shielding, and is also where the mains wire enters. No IEC connector here, it's a fixed wire. There's also a shield covering half of the main board, held in place by a strong metal clip on the main board, and two of the four screws (plus plastic standoffs) that hold the case together when it's closed.







Another notable feature is that they print some specifications and instructions on the bottom of the case. Not sure why, it seems the top of the case would be more practical, maybe they figured it would be less likely to suffer damage there.



(apologies for the sometimes low quality of pictures. No tripod, poor lighting, and some were taken with a rather poor quality spare compact camera after the SLR battery got too low)

[Wim_L]

Moving on, with a detailed look at the main board! First, a few views from the top and sides...







There's also a somewhat unusual model of fuse in there. Apparently, it's supposed to work when the resistor in there gets too hot, and melts a solder joint which is then pulled open by the spring.



Then, the section at the top, plenty of switches there. Mostly connected through the PCB, but one can see the occasional connection to the top of a switch too. The black part at the right is a fuse holder, the fuse can be replaced from the front panel. It's a fuse for up to 2A ranges, the 10A range is unfused!



Then, the input section. Input sockets that accept shrouded banana jacks are on the front panel, which remains attached to the main PCB when the meter is opened. We also get a good view of a current shunt just below the inputs.





Finally, the bottom of the board... No real surprises there. One rather prominent spring is visible, connecting the main board to the shield inside the bottom shell of the case.

[Wim_L]

Getting to the problem area now... The display board! It's located right above the switch section.



It's held in position by three screws, two of them easily accessible, and one in a terribly inconvenient spot squeezed in under the ribbon cable. Still, one can get to it, it's just inconvenient.



Then the display board itself in detail, just the ribbon cable connector and one chip on this side.



The other side has the 4 contacts for 4 soft rubber front panel buttons, and two rows of contacts at the top for the zebra strips leading to the LCD display. I cleaned all of those contacts and the buttons with isopropyl alcohol.



Then the display itself... It's a unit composed of several parts, here shown assembled, that clips onto the display PCB. Two little plastic pins match positions on the board for exact positioning. The zebra strips are visible as the two black lines above the LCD here, pressing down on contacts on the LCD glass. I cleaned those thoroughly with isopropyl alcohol as well. I didn't have spare zebra strips, so these would have to do. Take care when reusing these, as one side (the LCD side) will be flat, while the PCB side will have little indents from being pressed onto the raised contacts on the PCB. Don't turn them around, or those indents may cause bad contacts on the LCD side. Figure 6-2 in the manual linked to in the first post shows how this unit fits together.



And finally... Success! All segments show as active when powering it up in diagnostic mode (hold dB button while turning the meter on, for details, see section 5.4.2 in the manual).

[Wim_L]

Finally... The big question. Does this thing still work right? I don't have any meter that exceeds the 175's capabilities to calibrate it, the best I have around is a fairly new Fluke 28II. But that should be good enough to check if it's at least reasonably close.

First a few tests on DC volts...



And DC amps. Not too bad...



Then, AC volts. I tried a few waveforms, and various frequencies too (not all tests shown), it was always pretty close to the Fluke for the various waveforms, so we can assume the RMS converter is doing its job. (Manuals for different revisions of the 175 suggest not all models have an RMS converter, some are average responding)



And an 180 ohm resistor (compared to the Fluke reading, though the last two digits weren't all that stable, probably the resistor heating too much during the measurement), 4.7Mohm, and a 4.7 ohm 1% resistor with lower than 100 ppm / deg C tempco in relative mode.



Which concludes the overview, I'm out of pictures here!

[saturation]

Great job and details.  Thanks for posting!

[Smokey]

Awesome job on the photo tear-down and repair.  I just scored one of these off ebay on the cheap.  I was really pleasantly surprised to see the build quality considering what some of the other 30 year old equipment I have looks like on the inside.  It's amazingly clean and modern looking without any serious hand wiring and excessive bodges.  The only things that are suspect are, as you pointed out, the seemingly un-fused high current jack and the non-removable power cable.  I can live with that though.  Good job Keithley.
The unit I got has a B1 firmware, so it's probably only about 25 years old (HA!).  So far everything appears to be working correctly, but I don't have anything more accurate to test it against yet.  It does seem to track with my Fluke 112 and Chinese cheapie.  The calibration is all digitally stored so I guess I could tune it in if I came across something more accurate.  I wonder if you could even still get something like this professionally calibrated and certified, not that I would or that it would even be worth it.  Just curious.

[thgreer44]

I have a Keithly 175 that reads 30% high on all DCV ranges (tried calibration routine - can adjust, but not enough). Meter reads resistance OK, so problem is something prior to Input Buffer Amp and A/D, which are common to DCV and resistance measurements. Manual says "The Model 175 voltage and current measurements are based on comparing the unknown signal with an internal -2 V reference voltage” [accuracy & precision not specified] and that "the -2 V reference is made up of a highly stable zener diode (VR101), an op amp and a resistive voltage divider [1N4577, Vz = 6.35 V, fed by a constant current source; R120 resistor network resistance values specified but not tolerance]. Can't find data sheet on 1N4577 so don't know Vz tolerance.

Any experience with this problem? Is it likely that Vz or R120 would change with time? I don’t want to buy replacement part(s) from Keithly unless I know they are bad.

[Bored@Work]

Can't find data sheet on 1N4577 so don't know Vz tolerance.

Datasheets are all over the Internet on these datasheet collection sites and distributors.  6.4 V +/- 5%, tighter tolerance selections (1%, 2%, then centered around 6.2V) available. With a low tempco 0.13 mV/K.

[Smokey]

Hmmmm.....
I wonder what the limiting factor in the accuracy of a meter like this is.  Is it the reference or the ADC resolution?  Could you significantly increase the accuracy by putting a modern precision reference in place of the old one?

[Bored@Work]

Hmmmm.....
I wonder what the limiting factor in the accuracy of a meter like this is.  Is it the reference or the ADC resolution?  Could you significantly increase the accuracy by putting a modern precision reference in place of the old one?

If it is well designed, then all the components are balanced. I.e. they are all "good enough" to get the required performance. So replacing one component will just mean that another one becomes the limiting factor with not much increase in overall performance.

There are of course cases and reasons for an unbalanced design. Using an existing module or subcircuit in a new design where it is better than needed, because it is known to work and reduces inventory. Or it is so cheap that it doesn't make sense to use something with less performance. Or in preparation of upgrade options. Or simply accidentally.

The only way to find out is to study the circuit and device.

[idpromnut]

@Wim_L: any ideas what the difference between the 175 and 175A models was/were?  I'm getting a 175A in the next few days.  Great old meter

[robrenz]

Very nice Job!

[Smokey]

@Wim_L: any ideas what the difference between the 175 and 175A models was/were?  I'm getting a 175A in the next few days.  Great old meter

I actually have one of the 175 and 175A now.  I have a bit of a Keithley bench meter Ebay addiction.
The 175A has a backlight and removable power cord.  Off the top of my head I think the specs are also marginally better, but not significantly.  They may also not be as old

[Wh1sper]

It seems the LCD has only an aluminum foil at the back, so maybe one could try to remove and place a back light?

[idpromnut]

Well I received my 175A today, and it looks like it will be another repair project!  I will start up another thread with this one, it's a bit of a doozy