Teardown / repair: Keithley 619 Dual channel multimeter

[free_electron]

Found this on ebay for a nickle and a dime (well, 80$) : keitlhey 619 dual channel electrometer. I always wanted an electrometer so this was it. Known to broken but at that price worth the gamble.

Front panel

http://www.siliconvalleygarage.com/eevblog/619/619front.jpg

These are older machines but constructed very nice and modular so easy to work on

http://www.siliconvalleygarage.com/eevblog/619/619guts.jpg

GPIb is handled by a motorola 68488 in cerdil package.

http://www.siliconvalleygarage.com/eevblog/619/619gpib.jpg

The brains of the outfit is a 6802 and two 2716 eproms. Four static nibble-wide static rams of the  2114 type (1024 nibbles) make th emain memory.

http://www.siliconvalleygarage.com/eevblog/619/619cpu.jpg

The i/o controller board handles display muxing and keyboard scanning. it also creates an optical isolated serial interface for both channels.

http://www.siliconvalleygarage.com/eevblog/619/619io.jpg

Digital portion of the dual slope a/d convertor i smade using TTL and cms logic Nothing custom here. simple counters do the work.

http://www.siliconvalleygarage.com/eevblog/619/619digital.jpg

The analog portion of the A/d is partially built on teflon standoffs to guarantee low leakage. Das mashinen is nicht fur das gefingerpoken !  in plain english : keep your greasy fingers out !

http://www.siliconvalleygarage.com/eevblog/619/619analog.jpg

Oh joy ! The reference is made using the well known LM199 buried zener with built in thermostat.The metal can AD542 is the input opamp for the a/d convertor.

http://www.siliconvalleygarage.com/eevblog/619/619ref.jpg

Each channel has its own driver board and high voltage supply. they mke +260 and -260 volts for the feedback driver.

http://www.siliconvalleygarage.com/eevblog/619/619driver.jpg

the input boards employ guarding and double shielding techniques .

http://www.siliconvalleygarage.com/eevblog/619/619input.jpg

Teflon stnadoffs galore ! Even the reed relays are special constructions. with separate shielding cans The metal can AD515 electrometer-opamp does the work.

http://www.siliconvalleygarage.com/eevblog/619/619teflon_thumb.jpg[/img]
[url]http://www.siliconvalleygarage.com/eevblog/619/619teflon.jpg]http://www.siliconvalleygarage.com/eevblog/619/619electrometer.jpg[/url

Same warning as witht the A/d convertor : keep your greasy fingers out. This stuff is sensitive they even recommend waiting at least an hour after soldering in this section to allow thermal equalisation...

[url]http://www.siliconvalleygarage.com/eevblog/619/619teflon.jpg

Backside with the two input channels

http://www.siliconvalleygarage.com/eevblog/619/619twin.jpg

THe machine was dead. at powerup it would show its serial number and firmware revision and then hang. a quick check of the internal supplies showed one at 1 volt that should have been 5 volt. Unplugging the boards fed by that rail ( i suspected a short in one of em ) made that rail shoot to 13 volts. oops. not good !
Turned out the to-3 can regulator for that real was dead. with no load it would simply pass its input voltage. hang 1 kiloohm of it and it collapsed to 1 volt. a quick dig in the scrap bin and i found a donor board that had one.
power up and all was well.
i checked readings against a calibarated 34401 and the machine is well within spec on the ranges i could check.

[grumpydoc]

Sigh. There are times I really, really wish I lived in the 'states.