To prepare the K1801 for it’s new life the 22 year old electrolytic capacitors were replaced. The new capacitors are higher voltage, higher temperature and have a very long rated life.
The original Rubicon 1000ufd 16v caps were replaced with 35v capacitors. These capacitors are in a 9v circuit so the 35v caps have a good margin. There were two smaller 100ufd, 16v caps. These were also replaced with 35v units. I chose large caps for long life but they were a little difficult to fit inside the housing.
It’s now ready to go for another 30 years or more.
A preamp interface cable was built using a 14 ft length of CAT 6 7 network cable. This has the advantage that all 4 wire pairs are foil shielded and there is an overall outer foil shield.
The 4 command lines were placed in two of the shielded pairs. The commands are 1k gain, 10k gain, 5ufd filter, 50ufd filter. These are simple +-9v dc signals. The +- 9v power lines were in another shielded pair. The preamp output and the ground reference were the last two signals to be placed in a pair. The overall shield was tied to preamp case on one end and to the isolated ground at the interface card.
TiN did a great job designing and building the interface card. He built one version using the original very expensive transformers. Then he designed another version to work with much less expensive transformers. I should have just waited for the second version! I did not use the bias current cancellation circuitry and so far it has not affected the noise readings or stability. The card was working in my K2001, the cable was built, now I just needed to plug it into the rare K1801 preamp. After triple checking voltages and my connector pinout I plugged it in and it worked fine.
Adding the preamp to the K2001 or K2002 adds several new ranges to the DVM. For example now there is a 20uV, 200uV, and a 2mV full scale range for DC volts. The new 2m ohm, 4 wire ohms range should read down to u ohms now.
I let the equipment warm up a bit and followed the factory calibration procedure. I used an EDC 521 calibrator monitored with a K2002 DVM to provide voltage to K262 Low Thermal Voltage Divider. Luckily I had a Keithley low thermal cable to connect the K262 to the K1801 preamp. The K2001 built in calibration procedure has you use a K262 divider so that worked out well. After the 20uV full scale calibration on the new 20uV range. The readings were spot on at 20.000 uV, even the negative full scale reading was right on. The calibration limit is about 1 nV because of the noise floor.
The 200uV range was just as accurate. The 2mV range would not calibrate. It looks like my K1801 has an issue above 0.5mv (0.5ma output stage current) on the x1000 gain range. Oh well, I'm just using it for the more sensitive ranges anyway.
More test data to follow but the K1801 noise data looks as good as the A10 noise data.
Thanks to TiN and others, it looks like this project was everything I hoped it would be.