No repair can proceed, without sufficient time, and without the right spare parts. Time was very much occupied by other business recently, spares took time to ship from Greece to Germany… the 2N6211 transistors.
Here they are – the 2N3442 are China-made (ISC) transistors of rather recent production, the 2N6211 date back to 1992.
Not much to write about the further repair, mounted the transistors on the heat sink, soldered-on the cables, replaced the Zener diodes of the main board (series regulator bias), and switched the 6115A on. Success! Some minor calibration of the panel meter. Other than that, all in good shape.
Various things can be measured to verify the correct operation of the 6115A, here just a quick test checking all the ranges, linearity, and deviations. In short, it is very much more accurate than the 0.025% + 1 mV output accuracy specification.
After a full cool-down, checked the stability/drift after a cold start, with the output programmed to 10 volts. It is ramping up nicely, with some very minor “instability” during the first hour, but then stabilizing to almost perfect level. This is with no load. Sure it may degrade a bit under full load, fair enough (horizontal axis shows measurement number, period is about 0.3 s per measurement).
Finally, this is the working precision supply.
I enjoyed reading about this repair, as I’ve owned one of these supplies since 2006, when I got a used one for $72 delivered from ebay. It worked fine for about 15 years, then stopped working one day while I was adjusting it. I’m a retired HP scientist and most of my test tools are older pieces of HP gear. My HP 3400A RMS voltmeter was built in 1967 when I was in college — and is still working, unlike me .
A friend from the HP site I used to work at fixed the 6115A for me a few years ago and it has been stunning me with its stability and output accuracy, particularly when you realize the model was introduced in 1971. I marvel at the drift spec: 15 ppm + 15 μV over 8 hours. I’m currently using it for a 5 V supply for my prototyping board, then can quickly use it for a working lab voltage standard if needed. It also outputs up to 100 V, something rarely needed today — but once in a while it’s useful.
So nice to hear from you! I get tremendous values of older HP equipment, I have a 8753C VNA I use every other day. Some instruments I might just use once a year but better to have some spare equipment than lacking the right tools when needed! Whoever at HP decided to make the manuals public and have manuals with schematics, can’t be praised enough. Also there is a fair degree of standardization, with some parts units around often can do repair with original parts, even if it is a different piece of equipment.