Electronics > Repair

General question about repairing an early 1960s constant current source

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Long time lurker, first time poster here.

I recently purchased an Electronic Measurements Co C612 constant current supply at a local surplus shop. There just happens to be one for sale on ebay at the moment. Link below.


When I turn mine on, a couple of the tubes start glowing, but there is no output. I googled the numbers on the vaccum tubes and it seems like I can source replacements. It also has, what looks like a selenium rectifier and some germanium transistors in it. It has around 60 components in total. I googled the germanium transistors and could find datasheets, but no replacements.

I guess my question is: Is 1960s technology repairable? Can germanium transistors be replaced with silicon?

My background, I love to repair electronics, my last project was fixing a dead Fluke 192B scopemeter I bought off ebay. I am 32 years old and missed the vacuum tube era, I don't really know anything about tubes and would love to learn a little about them. So, this is more about an opportunity for me to learn about something I do not know, and if i can repair this, learn about tubes and have a working constant current supply.

I had the idea I would take it apart, draw a schematic and test the components. Does anyone on the EEV blog repair this kind of stuff? In general, is it possible to source replacements for germanium transistors, etc? Sorry to ask such a general question, but I don't want to waste all the time to draw out a schematic and do a bunch of testing to find that there is no replacement for germanium, selenium, or something else in this unit.

Thanks for the info,

some times you dont need an exact match, it comes down to how the control loop is implemented, and that is best determined by drawing up a circuit diagram, you may even find some can be directly swapped or a working germanium may be able to replaced and can then be used in the more sensitive place

I repaire gear from the 60's, a lot of my gear is from that area and most gear came in not working or not used for decades (so that often means restauration anyhow)

Most gear from that area is not complex. No IC's and most times no unobtanium special things (but in rare cases it happens, hybrids in a HP-8601, special made swithes that lose their contacts in a HP8407 VNA)

But a lot of gear was still tube-like build. And with that, I mean that they often  used rather high voltages. 
Germanium is often replaceable with silicium but not always. In a GR1608 I replaced all leaking Germanium transistors by silicium ones. That was no problem but look in the schematics and you know. calculate or measure the voltages on the pins and calculate the current, look if it matters if Vbe becomes 0.7V instead of 0.4 and be carefull around oscillators, germanium has much higher paracitic capacitance.

Be carefull desoldering components if they used the darkbrown cardboard like pcbs. The track sometimes come loose if you only think about soldering.

For the rest, most times very easy to repair and most times very well build with large reserves. Engineers back then really knew what they were doing (they did not have to learn about so many fields and components as today)

My experience with vintage selenium rectifiers is that they are high failure items - and if you experience one failing, the odor emitted from the overheated selenium will never be forgotten. 

Electrolytic capacitors are another likely failure point in 40-50 year old gear, and the usual advice is to bring the AC voltage up slowly the first time the device is powered up after sitting on the shelf for decades. Use a VARIAC or the like and start with 0V and increase to full line voltage in small increments over the space of an hour or so. This allows electrolytic capacitors a chance to reform their dielectric layer.

http://www.pa4tim.nl/?p=1385 about testing and reforming old caps.

My experience with bad caps is limmited to gear from the early 90's to today. The number of caps I had to change in everything I own from the 60's and older or used to own is very, very little. Exception here are the first HP's from Japan (caps < 10 uF only) and Philips gear.
Every resoration here starts witrh reforming the caps by desoldering one leg and connecting them to a tester. I sometimes use a variac with lightbulb in series if it is to much work. Just restored a 1962 HP-122AR. This I used the variac  because the caps were to hard to reach and had a lot of wires and resistors soldered and most had 2 or three caps in one housing. And no delicate IC's or rare tubes inside. Even my Tek 547 still has all the original caps. B

But I agree that they need TLC , If you just plug the instrument it in the mains after several years there is a big change you need to replace caps.

Never smelt Selenium, is it worse as popping caps ? Once opened a aluminum  can in a ex army (dessert version )marconi LF generator from 1954. It had a spare filmscale inside from celluloid. I think my eyes burned for two day. That was discusting.

I know they are not very reliable (relative, if they fail after 50 years ;-) ). I have one very big Russian HV power supply from a transmitter that has selenium cells (i think) . They became polulair in the 1930's, used a lot in old radios from the 50's (a friend told me, I do not much with old radios) but in my HP, Tek, GR and Philips gear from that area I do not think I have ever seen one. Can you tell more about this. I always like to learn more about component behavour. I do lots of tests/measurements on components, and read books about them ( I measure with things like curvetracer,  bridges, vna's, I evenfound a special transistor risetime plugin from tek last year, very cool. ).

I only know them as the big things with cooling fins. I have a Triumph speedtwin 5T from 1954 that had something that looked like selenium but I think it was a zener (forgot, replaced it with a more modern solution 15 years ago before I knew much about electronics)


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