A look inside #9

[SeanB]

https://www.youtube.com/watch?v=6Dr0xQMWovE

A look inside a Keithley 179A Bench Multimeter.

Quick tip for those looking for them is the name is often misspelt as kiethly.

Manual here...

http://projectlab.engphys.ubc.ca/wp-content/uploads/Keithley-179A-.pdf

[bitwelder]

I'm surprised that Martin didn't learn yet to recognize the date code on the ICs, as they are often pretty easy to read (in this class of devices).

For instance in this video the big IC  ICL71C03 shows  on the 2nd line a '8143' which would point to week 43 of 1982,
and the round AD536JH is marked 8202 (1982 wk 02).
The latest code I could spot is the 8219 from the AD542JH, so probably the instrument is from mid 1982.

[steve30]

Looks like a nice bit of kit Martin. Hope you get it going properly  .

Some of the really old multimeters are very well designed and made, and can be just as usable as modern ones. I haven't used a Keithley one myself, but it looks good.

Personally I'm a big fan of the ganged push switches. My Thurlby multimeter has them and they're really nice to use.

[SeanB]

Martin is still learning, so with dating he has not had enough exposure to various devices to be able to reliable decipher them. Plus he does like confirmation that what he is seeing is correct.

I would guess most of the issues with the meter ( aside from the bent pins) is in those range switches, which would benefit from cleaning and lubrication. As those switch blocks age they do tend to become noisy and high resistance. A wash with DeOxit and then some lube will help a lot.

With the unstable readings I would guess that the axial electrolytics are getting marginal, a new set in the power supply would not go amiss, or to use an oscilloscope to show the ripple voltage across them before and after to compare. I used to do that repairing avionics, running a scope probe over the power rails to show up ripple and noise that would otherwise not be visible on a meter. It would also show up the common failure of a single diode in the 3 phase bridge rectifiers, by the missing bump in the waveform for the open diode, or the big dip where it has gone short and is getting ready to kill the power supply. As the transformer windings had a high DC resistance a shorted diode did little until 2 went short and shorted a phase winding, and this would eventually smoke the power supply. It ran a little warmer with a single shorted diode but not much.

[calin]

This remembers me of my HP 3466A, very similar meter. I got it of craiglist and it had same issues ... I say Martin , check those gang switches. I bet that the contacts are not happening. It really does not work at all to open these switches to clean them. On my 3466 I used a good amount of cleaner and IPA to get the dirt out and blown compressed air  through each switch then a small spritz of Deoxit in each and then gave them a good bunch of pushes to work them out a bit. After that ... all working.

I posted some photos of the HP on Dave's forum here -> http://www.eevblog.com/forum/testgear/hp-3466a-restoration-dpdt-switches/

[retiredcaps]

There were several reasons I brought the Keithley to Martin's attention.

1) Martin lives in Ohio so he must get a Keithley!! 
2) for around $25 USD ($8 for the meter + $17 s/h) he can get a possible repair video (or more follow on videos)
3) schematics are available to help troubleshoot
http://projectlab.engphys.ubc.ca/wp-content/uploads/Keithley-179A-.pdf
4) DCV accuracy is 0.04% +1 - slightly better than a Fluke 87V
5) interesting to see how a meter made in 1982 holds up wrt to calibration (if fixable) after 33 years
6) get some new viewers who are interested in repair and/or can't afford $$$ for a new bench multimeter
7) it won't break the bank if the dmm cannot be repaired due some custom IC
8] possibly get the original designers of the 179A to comment or participate in some future interview

[retiredcaps]

Hi Martin,

Some comments and suggestions on your 179A video.

1) The Keithley 179A was sold for $379 USD MSRP according to an ad in Radio Electronics May 1983 issue (page 47)

https://archive.org/details/radio_electronics_1983-05

2) Some of the old DMMs that used those push button switches require a good cleaning with something like contact cleaner.  In addition, check the input jacks for dirt, debris, loose solder joints, etc.

3) There seems to be some debris on the pcb (example around C105).  A good Isopropyl alcohol bath may help with some of those erratic readings due to current leakage?

Make sure to let it dry thoroughly before using it especially with mains power.  A fan pointed at the pcb will help dry it out.

4) The date codes can sometimes be found on the big filter capacitor or the main IC.  For example 8143 means 1981 week 43.  0725 would mean 2007 week 25.

5) The ICL71C03ACP runs in either 2,000 or 20,000 count mode depending if pin 2 is open or not.  Datasheet is attached.

6) For those wondering why Marting would spend $25 on a broken meter, he actually paid $8 USD for the actual unit + $17 s/h for a total of $25 USD.  At the time I alerted Martin, there were several non working Keithley bench meters in the $4 and $8 range + s/h.

7) The nice thing about these old bench meters are the easily accessible test points so if something is wrong, it is easy to check the +5, +15 and -15V rails with a multimeter and/or scope.

8] These Keithley meters are not for everyone.  I hate the brown/tan color scheme and the lack of annunciators (example, is the switch on AC or DC mode).  But for $4 to $8, I would take this anyday.

When working, you get an accurate meter, pretty fast readings, decent size easy to read LED, fantastic manual with schematics and easy to desolder through hole components.  Additionally, you won't have to pay Fluke prices for them either.  :-)

Anyway, I hope you will find some time to do a follow up repair video.  If not, no worries.  However, when Chris comes over, I think he will give you a hard time for not fixing it!!! 

[SeanB]

Any links for more? I could do with one for that price.

[MJLorton]

Thanks very much to Retiredcaps and Sean for the feedback and information.

I'll put it back on the bench away from prying eyes and have ago at cleaning the contacts and checking the other bits you gents have kindly noted.

[retiredcaps]

5) The ICL71C03ACP runs in either 2,000 or 20,000 count mode depending if pin 2 is open or not.  Datasheet is attached.

Open meaning that is why pin 2 was purposely not in the socket? 

[SeanB]

5) The ICL71C03ACP runs in either 2,000 or 20,000 count mode depending if pin 2 is open or not.  Datasheet is attached.

Open meaning that is why pin 2 was purposely not in the socket? 

You would have to check the clock then, it changes quite a bit between 128kHz for 4 1/2 mode versus 12kHz for 3 1/2 digit mode.

[Dr. Frank]

Geez,

the fault is very probably located in the circuit area, denoted as "Attenuator".
Either OpAmp U101 = AD 542 (low IBias, low Offset) is defect (then maybe unobtainium), or the reed relais switchings is not working correctly, maybe not supplied with 5V.
Everything else like the A/D circuitry or PSU is NOT probable at all to fail.

Why?

DCI is working correctly, including A/D, giving stable readings as expected.
A/D circuitry (U103/U106) obviously is set to 200mV sensitivity for DCI.
The current shunts (0.1 Ohm .. 1kOhm) therefore need no amplifier, and the output voltage goes directly to the A/D chip (pin 10 on ICL7103). (Can be easily traced in the schematics, switch "A" for Ampère measurement pressed, only  )

DCV, Ohm and also ACV, ACI all use the "Attenuator" circuitry. All these fail, therefore the error has to be there.

As the 1V reference is used in these modes, also have a look on the ref switching, especially sector F1-H2, Q103.
You therefore may measure the reference value on pin 7 / U106, 100mV in DCI, 1V on DCV.
But as Ohm also fails, this will be OK, and the attenuator circuitry is really most probably to fail.

Btw: This pin2 which stood apart, simply determines 3 1/2 or 4 1/2 digit operation... obviously not connected.
And this instrument is from 1982. Learn seeing the datecodes!

Frank

PS: 200mV and 2V DCV also should work correctly, as they also bypass "Attenuator" circuitry. That is visible at 26:45 in the video, 200mV with open jacks give some readings, not overranging.
These 0.44... 0.22 mV readings indicate an input current for U101, pin2 of about 440pA.. 220pA, instead of a few pA only. Therefore I strongly assume, that this FET OpAmp is damaged.

Apply a short for 200mV and 2V, for stable zero reading, and then  apply 100mV and 1V (maybe from your nice 332B) and you will see correct and stable readings also, I bet.

Forget about cleaning the switches in first instance, that will do more harm, as you can hardly access the interior!

PPS: That forum Verification is a total crap!!

[Martini]

the fault is very probably located in the circuit area, denoted as "Attenuator".
Either OpAmp U101 = AD 542 (low IBias, low Offset) is defect (then maybe unobtainium)

I've seen a few on eBay. I have a couple myself.


Would be interesting if you finally fixed that multimeter.