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