Keithley 150A Microvolt-Ammeter

[Krampmeier]

Hello,

I just got an old Keithley 150A Microvolt-Ammeter from a ham radio flea market. The unit seems to be in very good conditon, except the handle, which "decomposed". What a beauty for 10 Euro!

Lifting the lid revealed a rather clean and tidy inside, without missing parts or signs of modifications. I noticed several selenium rectifiers and electrolytic capacitors, so I didn't dare to power the thing up yet. I don't have experience repairing tube (valve) equipment, but I have seen enough episodes of "Mr Carlson's Lab" to know that such old devices don't like to suddenly get the full 240 V line voltage after sitting on a shelf for several decades. This is why I decided to slowly start the meter with an adjustable AC power source (which I have at work).

I am concerned about the capacitors in particular. The large black and orange ones are clearly electrolytics, and I don't know if I should dare to keep them in place at all. I'd hate to replace those beautiful things with modern caps, but I also don't want to fry the tubes because of shorted or (electrically) leaky capacitors.

And one more thing: There is some cadmium-tin solder inside the unit, intended for repairs. Do I have to assume that the whole thing is soldered with that kind of solder, or will it only be in the low voltage path? Some solder joints are marked with red-brown paint - could that be an indicator for the critical joints?

Does someone here have experience with this kind of stuff? Any advice on the first power-up and / or restoration from a more experienced fella?

[Gyro]

A lovely find. 

I'd be tempted to bring that up very gently on a variac. As you say, it is beautifully constructed and I'm sure replacing the caps with new ones wouldn't look as neat.

Once you have it warmed up to normal operating voltage I would carefully check for excess ripple voltage on the supplies - that's the true test of an electrolytic's condition, not necessarily its age. Modern caps will have lower ESR too, meaning extra stress on the rectifiers.

Yes, they would have marked the Cadmium solder joints in some way, they would almost certainly have reserved that for critical front-end circuit joints I think.

[Kleinstein]

It might be a good idea to replace the selenium rectifiers anyway. They do fail just from aging, much like caps, but if they fail chances are they will produce some nasty gases.

The typical replacement is a silicon rectifier and a series resistor. Modern rectifiers have way lower impedance and thus higher ripple currents.

[lowimpedance]

Nice old Keithley you have there, with tubes too  ., love the "selected and aged " tube.
I do suspect some one has done a 'Mod' to the original input connector, (which would certainly be unobtainium.), to shoehorn in a BNC !. At least its on a PTFE base.
Be careful cleaning the tube glass not to rub or wash off the tubes number.
Hopefully you wont need to touch the front end and have to use the Cadmium solder. The less sensitive parts would be just normal Lead/tin solder so no worries there.
There is a 150B at work (no tubes) which also has the little spool of Cadmium solder for repairs, not to keen to heat up the iron for that  .

[texaspyro]

I love the smell of fried selenium rectifiers in the morning... it smells like... uhh... turds from a rotten egg.   

Check out Derek Lowe's "Things I Won't Work With" articles on the subtle wonders of selenium compounds.  Here's one:

http://blogs.sciencemag.org/pipeline/archives/2012/05/15/things_i_wont_work_with_selenophenol

[drussell]

If you do need to replace those capacitors, I would re-stuff the original ones to keep the appearance like the vintage radio and TV guys do on nice units...

Guys like bandersentv have several videos showing various methods...  Pretty easy.

[Krampmeier]

I took the chance and powered the unit up very slowly while monitoring the temperatures of the rectifiers and capacitors. The big orange rectifier (likely the one for the tube heaters) reached about 60 °C after the unit was on full supply voltage for 20 minutes, but there was no bad smell at all and the input power was 50 W, just as specified. The mechanical chopper transformer was buzzing nicely, and obviously with a well working speed control.

Surprisingly, the measurements worked quite well! All the voltage ranges were almost accurate, stable and did not need any extensive zeroing.
The current ranges were not that stable, I had to adjust the "zero" after every range change and move my had to the right distance from the wires when I took the photo of the 5 nA measurement. A pleasant surprise though.

Of cause, I will still measure the ripple on the supply ranges, even though I do not have reference values for a "good" unit. I'll also I suppose something like 5 % ripple would be quite normal in such a device?

I would also like to replace the front connector, which is pretty oxidized and gives an unstable connection when moved. I'll look for one with teflon insulation and gold plating.
I am not sure if I want to use the cadmium solder though... Lowimpedance, you are right, this is not the original connector, which is not available any more. I don't mind having a connector that I can actually use!

Thank you to everyone who contributed in this thread! I'll keep your advice in mind when I do repairs or conservation on this or another old piece of test gear!