Keithley 224 Programmable Current Source - Compliance voltage limit repair

If everything goes according to plan and I become the Supreme Leader of Earth I will oblige all TE manufacturers to produce manuals in the style of old Keithleys and HPs. I don't care if it takes 5000 pages, they will have to do it. My propaganda slogan will be: "Make TE great again".

I recently bought a used Keithley 224 with no output and the V-limit LED always blinking. Otherwise the instrument seemed to work fine.

The repair

The first thing one should always check is the voltage rails. ±125V, ±15V and +5V were good and stable.

Next I checked the output. There was no voltage present between HI and LO but there was between LO and GUARD. Going backwards there was voltage before K306. That means that K306 was not activated.

K306 and K307 are activated together and there were 5V on the coil of K307. This means that either the coil or the contact of K306 is faulty. K306 is a Coto 240-0197 reed relay (Keithley part no. RL-70). I tried to activate it externally with a magnet and BINGO! The contact was OK. These relays are hard to find and cost about US$60 at ebay (excluding postage). So I found a semi-permanent solution.

More problems

All current ranges seemed to be fine but the compliance voltage limits were way off. I first tried to adjust the limits from R319 but it was not possible.

Short explanation of the circuit

U308 is an 8-bit DAC that controls the compliance voltage limits. The output of the DAC is buffered by U309B (Vc). It is then amplified 21 times by U307B or inverted (U309A) and amplified by U307A. This way the output of U319 is kept within these limits.

U308 is an AD7523 R-2R ladder DAC. U310 is inverting the voltage reference (Vref) from U311 (the 10-bit DAC for the current setting). This voltage (-Vref) is then converted to current by R319.

For more information RTFM. It containd a nice "theory of operation" section.

Troubleshooting

I first checked Vc and the inversion/amplification stages. Everything looked OK. Then the reference. Here the absolute values of Vref and -Vref were not the same. Also the voltage between pins 2 and 3 of U310 was 0.7V. This means that probably U310 is toast.

According to the manual U310 is an AD3247 (Keithley Part No. IC-77). I could not find any information on this operational amplifier. So let's find a replacement. Here's my train of thought:

1) U310 essentially buffers Vref so bandwidth is not important.

2) Offset voltage is also not important (as long as it is stable) because the reference is trimmed by R319.

3) It should be able to handle the ±15V of the supply and fit the pin-out of U310.

I decided to replace it with my favorite op amp. OP07. I now need to replace all electrolytic caps, backup the firmware and my 224 is ready to work for another 20 years.