Keithley 600A Electrometer Repair

Repair and cleanup of a vin­tage Keithley Instruments 600A Electrometer.

Keithley Instruments 600A Electrometer after repair, cleanup, and calibration.

After search­ing online for a lit­tle over a year I came across this Keithley meter on an Ebay auc­tion. I had been hold­ing out for a repairable dig­i­tal dis­play Keithley mod­el, but with the start­ing bid and ship­ping on this vin­tage mod­el I thought it would be worth a try. Luckily I was the only bid­der on this unit and picked it up for under $40 USD.
Shipping took 10 days from the West coast, so I had some time to pre­pare for it’s arrival.

Keithley 600A cor­ro­sion around D‑cell bat­tery holder

One item that would need to be replaced from view­ing the auc­tion pho­tos was the dual D‑cell bat­tery hold­er. The pho­tos indi­cat­ed lots of cor­ro­sion and elec­trolyte leak­age in the past, so I ordered a Keystone Electronics 176 bat­tery hold­er which arrived before the meter. I also ordered a full set of bat­ter­ies for the unit.

Keithley Instruments 600A Electrometer as shipped, front view

From the angle of the auc­tion pho­tos, I did not notice that the meter bezel was pushed in past the front pan­el. Other than that the meter arrived in good shape in the con­di­tion that I expect­ed.
Next order of busi­ness after it arrived was to open it up and check the con­di­tion of all the inter­nal com­po­nents, and for any obvi­ous problems.

Broken sol­der con­nec­tion to Zero Check mid sup­port terminal

The first item I found was a bro­ken wire to the “Zero Check” mid sup­port ter­mi­nal. The next item was a ter­mi­nal block short­ed to the out­er case of the “Set Zero Center” poten­tiome­ter which is con­nect­ed to case ground.

Terminal strip touch­ing poten­tiome­ter hous­ing on back panel

Battery elec­trolyte on top of resis­tors and old flux on sol­der con­nec­tions.

Overall the inside of the unit had a lot of resid­ual bat­tery elec­trolyte that had leaked and then flaked off onto the sides and com­po­nents. It took near­ly a day and a half of clean­ing and check­ing before I was sat­is­fied.
The prob­lem with clean­ing Electrometers is that many of the con­nec­tions between com­po­nents and their phys­i­cal mount­ing points need an extreme­ly high resis­tance either between com­po­nents or ground for accu­rate mea­sure­ments. The Keithley 600A Electrometer uses PTFE bush­ings and stand­offs which pro­vide a very high elec­tri­cal insu­la­tion of > 1018 ohms. To pre­serve this high resis­tance all sur­faces of the PTFE must be con­t­a­m­i­na­tion free.
Other areas that must be con­t­a­m­i­na­tion free are the input con­nec­tor and the glass enve­lope of the vac­u­um tubes where the leads enter the glass. 

PTFE “Teflon” bush­ings and gold plat­ed con­tacts of selec­tor switch. That 1 ohm resis­tor lead is very close to the alu­minum standoff

My weapons of choice for clean­ing PTFE and gold con­tacts are 99.9 % Isopropyl alco­hol, and then DeoxIT Gold on the con­tacts. I usu­al­ly start with a phys­i­cal clean­ing using the Isopropyl alco­hol and a soft bris­tle tooth­brush, and then mul­ti­ple rins­es with the alco­hol and dry­ing with com­pressed air between rinses.

New D‑cell bat­tery hold­er installed after cleanup

After per­form­ing oth­er var­i­ous repairs includ­ing the meter hous­ing, it was time to install the bat­ter­ies.
The oper­a­tions man­u­al called for 1.34 V mer­cury D‑cell bat­ter­ies and 8.4 V mer­cury bat­ter­ies which are non-exis­tent now days. I end­ed up using some Tenergy Centura low self-dis­charge Ni-MH D‑cell and 9 volt bat­ter­ies. Both had sim­i­lar volt­age spec­i­fi­ca­tions to the mer­cury bat­ter­ies and are recharge­able. I went with the low self-dis­charge type to reduce the need to open up the case every few months.
The oth­er hard to find bat­tery is the No. 413 type 30 volt units. The 600A requires two of these (B4-B5) which are expen­sive, but avail­able online.

Keithley 600A Electrometer left side after repair and cleanup

I did end up using some DeoxIT D5 on the front and back switch con­tacts, and fol­lowed up with sev­er­al more rins­es with Isopropyl alco­hol on the PTFE mounts.
The main board is vibra­tion iso­lat­ed at four points to reduce micro­phon­ics in the vac­u­um tubes mount­ed on the board. The cop­per wires from the input con­nec­tor and range selec­tor are made from fine braid­ed wires and are looped to also reduce vibra­tion con­duct­ed into the main board.
For ship­ping there is a met­al angle brack­et that sup­ports the main board and is removed for oper­a­tion. The brack­et is attached thru the hole in the back pan­el above the “Normal-Fast” selec­tor switch.


Keithley 600A Electrometer Right side after repair and cleanup. 5886 elec­trom­e­ter tubes under shield

After warm­ing up for 30 min­utes I per­formed a cal­i­bra­tion on the Volts set­ting, then sev­er­al cur­rent and resis­tance checks on the meter. I cur­rent­ly do not have any equip­ment or stan­dards to check the upper ranges of this meter, but all mid-range checks were accu­rate.
I did get a read­ing of over 100 Teraohms ( 1014 ohms ) with no leads attached.
Not bad for a meter over 55 years old made in the ear­ly 1960’s.

The oper­a­tions man­u­al is avail­able from the Keithley website.

Keithley Instruments 600A “Zero Check”, “Coarse Zero”, and range switch­es
Keithley Instruments 600A back panel
Keithley Instruments 600A meter face and bezel

10 Replies to “Keithley 600A Electrometer Repair”

  1. Hi, I just bought one of these, but the man­u­al talks about using mer­cury bat­ter­ies, which are no longer avail­able (I found the 30V one, though). Are you sure that it’s sup­posed to use the 9Vs and D cells, because the man­u­al that I found online says oth­er­wise. Thanks

    1. Hi Mike,
      I am sure that is NOT sup­posed to use stan­dard 9 volt and alka­line D‑cell batteries.
      In my post I stat­ed, “I end­ed up using some Tenergy Centura low self-dis­charge Ni-MH D‑cell and 9 volt bat­ter­ies. Both had sim­i­lar volt­age spec­i­fi­ca­tions to the mer­cury bat­ter­ies and are rechargeable”.
      I have been using this elec­trom­e­ter for the past 20 months with no issues and have recharged the bat­ter­ies twice in that time period.

      Greg (Barbouri)

      1. One of my man­u­als came with a pen­ciled-in extra resis­tor, in series with each of the 2 D‑cells. If you add a well cho­sen resis­tor, you should be able to use stan­dard 1.54 V alka­line D‑cells. The peanut tubes each draw 10 mA of fil­a­ment cur­rent, and each cell dri­ves 2 fil­a­ments, so a 10 Ohm resis­tor should work well there.

        I have not tried this, but it might be a good solu­tion for those D‑cells. I got stick­er shock last time I priced those in NiCad or NiMH. How well this works would depend on how flat the dis­charge curve is on the alka­line D‑cells.

  2. I have a cou­ple of these that I’m just in the process of clean­ing up and get­ting going again. Yours appears to be an even ear­li­er mod­el, with sock­ets for the peanut tube leads. Later ver­sions are all sol­dered, to avoid any chance of vary­ing con­tact resistance.

    You can match your own pairs of 5886 elec­trom­e­ter tubes by set­ting up a test jig with a NiCad cell fil­a­ment sup­ply, a 9 V bat­tery, and a 620 kOhm cath­ode bias resis­tor. This is 2x the bias resis­tor in the 600A, because you’re only test­ing one tube at a time. Tubes that test with the same V across that resis­tor are a matched pair, but dif­fer­ent Keithley mod­els used pairs with dif­fer­ent Vs. You could also change the bias resis­tor in your 600A, with­in lim­its, if you can’t find a matched a pair that works well with your instrument. 

    I see that the clamp for your 9 V bat­ter­ies is held down by thumb nuts. None of mind came to me this way, and my units don’t have enough head­room to clear. If the knurled nuts short out against the case/cover, that shorts out the float­ing ground of the 600A. In that case, just use nor­mal hex nuts.

    My 600As have a “tapered” cov­er that over­hangs the meter if it installed cor­rect­ly. If it’s installed back­wards, the over­hang is in the rear. I don’t see any front over­hang in your 3rd pho­to, but it’s pos­si­ble that the ear­ly units did not have this feature.

    I see an extra car­bon comp 10 MOhm resis­tor tacked onto the top of the coarse bal­ance switch (rear pan­el.) The resis­tors at each end of that switch should be 430–450 kOhms, but peo­ple often fudged that to get mis­matched 5886 tubes to work. It’s not the best workaround.

    The spring that adds extra con­tact force to the bat­tery hold­ers is a real­ly good idea. They made a big dif­fer­ence in con­nec­tion reli­a­bil­i­ty in mine. I had to make anoth­er spring to replace one that was miss­ing out of my 2 units.

    Also, in the bat­tery hold­ers, even if you replaced the hold­ers, it’s a good idea to SOLDER the riv­et to the lug in each bat­tery ter­mi­nal. Those tend to go resis­tive over time if you don’t.

    Your hold­ers show some­thing wrapped around the upper ends of the side plates. What is that? It looks like a rub­ber band, but I can’t see well enough to be sure.

    1. The thumb nuts on my unit have good clear­ance from the case side, so no issues with shorting.
      You are cor­rect with the tapered cov­er. The pho­to shows how the meter was shipped to me with the cov­er reversed. If it had been installed cor­rect­ly it might have pre­vent­ed some of the dam­age to the meter bezel in shipping.
      Good to know about that 10 MOhm resis­tor on the coarse bal­ance switch. It looks to be orig­i­nal to the unit, but will check into it.
      There is a rub­ber band on the D‑cell hold­er, for a bit more hold­ing force, as the spring would not work for the new style holder.

      I am get­ting ready to recharge the bat­ter­ies this month so will also sol­der the riv­ets to the lugs of the bat­tery holders.
      Will also be replac­ing the NEDA 210 bat­ter­ies with some 3D print­ed hold­ers with CR2354 cells. 

      Thanks,
      Greg (Barbouri)

      CR2354 30 volt battery holders

      1. I like your replace­ment for the #210s. Would you con­sid­er sell­ing a cou­ple of them?

        I’d rec­om­mend adding a round­ed gus­set at each end of each side rail, to strength­en and stiff­en that cor­ner. Sharp inside cor­ners that are under ten­sion tend to be places where cracks start.

        I bought a cou­ple of the #210 replace­ments. They work, but they are some­what expen­sive. We’ll see how long they last. Your solu­tion has the advan­tage of being able to replace indi­vid­ual cells easily.

        BTW, one of the 600As that I just fin­ished going thru, seems to work well, except that it lacks the large cen­tral knob. If any­one has a parts unit that they’d be will­ing to sell me a knob from, please try to con­tact me thru this web page. Greg has my email and should be able to pass your mes­sage on to me.

  3. Hello,
    I just fin­ished repair­ing my 600A (also from eBay, for about 40€): one of the 450k resis­tors on the coarse grain zero adjust had lost inter­nal con­tact (gone to infinity).

    I am using also NiMH recharge­able bat­ter­ies for the heat­ing sup­ply (C‑cells (baby) since I did­n’t have D‑cells (mono) around — plen­ty for the 20mA heat­ing cur­rent). I am using 8.4V recharge­able NiMH 9V-block bat­ter­ies for B2a and B2b, and 9V alka­line blocks for B6 and B7. For the 30V bat­ter­ies, B4 and B5, the pre­vi­ous own­er had already pre­pared hold­ers for alka­line 23A bat­ter­ies (12V), two of them in series. But the result­ing 24V were too lit­tle, so I wedged 4 x AG12 coin cells (1.5V each) into the hold­er as well, increas­ing the volt­age to exact­ly 30V. I think this is a rel­a­tive­ly cheap bat­tery solu­tion, which works well.

    The 600A works like a charm now! I am so hap­py — I had want­ed a high imped­ance meter for many years!

    Would you make your 3D-print mod­el for the coin cell hold­er avail­able for down­load? Maybe on thingiev­erse or TinkerCAD or just here?

    Bye
    Helmut

      1. Thanks for shar­ing the coin cell hold­er file, but I just noticed that there’s a typo in the name. It says CR2345 instead of CR2354.

        I’ve down­loaded the file and cor­rect­ed the name.

        Jim

  4. i have also just recieved one of these. its got a lit­tle bit of rust and some weird white par­tic­u­late with­in the case but oth­er than that it seem to work ok. ive put in full sized alka­line d cells into the unit (i am a bit wor­ried if the cur­rent is too much after read­ing some com­ments) and ive tak­en apart some 23A bat­ter­ies for their inside cells and shrink wrapped 20 of them in series to make the prop­er 30v. seems to work ok though they have about 1/4rd of the capac­i­ty of the 413 cells. my zero check knob snapped off and need to be glued back togeth­er. I have also been deal­ing with the issue of noise on the low­er sen­si­tiv­i­ty ranges. a fluc­tu­a­tion of about 4ish mil­li­volts. Im not sure if its nor­mal but it takes a long while for my unit to set­tle down as well. it takes about 15–20 min­utes for the nee­dle to stop drift­ing to less than 1 millivolt/ minute. ill add in a series resis­tor to the alka­line bat­ter­ies and hope that helps. any­ways thanks for the arti­cle. it real­ly helped me out

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