I just completed restoration of an HP 6177C DC Current Source, and I thought I would share the results here in case anyone else needs to restore one of these. This is a very nice current source, with a range of 0-50V, 0-500mA. I like this current source over some of the alternatives because of its compact size, non-lethal voltage range (0-50V), and current ranges (5mA, 50mA, 500mA) that I find practical for my typical needs.
When I received the supply, the power light would come on, but there was no output. Also, the front panel meter was pushed back into the supply. Once I was able to get a readable copy of the manual for this supply, I discovered that the manual is really quite excellent. It has a detailed theory of operation section, readable schematics, and a series of troubleshooting flowcharts.
The supply has a couple of circuit boards. The main board houses almost all the components. There is a small daughterboard that holds the big filter caps. The meter, switches, current adjust, and voltage compliance adjust pots are on the front panel, and the heatsink in the back has the output and driver transistors. Here's a picture of the main circuit board.
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If you look carefully at this pic, you can see the discoloration (browning) that is typical of scorched components. Seeing that, I expected to find lots of burned out components, but this did not turn out to be the case. I guess it is some property of whatever material the board is made of.
Here is a pic of the big TO-3 transistors on the heatsink. No sockets on these transistors; the wires were soldered straight on.
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Before diving into the troubleshooting flowcharts, I checked out some of the obvious stuff. The first thing I noticed is that the internal fuse was missing. I replaced this, but it blew immediately. A quick check of the series output transistors showed that at least one of them was shorted on. So I removed the heatsink and checked all 4 transistors there, discovering that 2 of them (IIRC, Q29 and Q30) were bad. I replaced these with Central Semiconductor's 2N3716. After replacing these transistors, the fuse was no longer blowing, and I could actually see the meter move a bit as I adjusted the current setting. However, it was not moving over the full range, so time to really dive into the manual and understand this thing better.
The first recommendation in the troubleshooting section of the manual was to check the various DC voltages that are used as references and bias sources for the supply. I discovered that there were 2 zener diodes that were not working. VR12, a 5.6V zener was completely blown and had a breakdown voltage of < 1V. VR6, a 16.2V zener was a bit out of spec. I replaced both of these, and though it fixed several of the power rails, the supply was still not working. I noticed that the -16.2V rail was getting pulled up to around -9V. However, the zener diode that is used as a shunt regulator for this rail was fine. The manual mentioned that a number of faults elsewhere in the circuit could impact this voltage, so I decided to move on to the next step in troubleshooting.
The supply has a "guard voltage" which is essentially a 0 to -1V adjustable reference voltage that is compared with the voltage across one of three current sense resistors (one sense resistor per range). It's called the "guard" voltage because it is also used to surround the output conductors so as to minimize leakage current. See the manual for a better explanation, but I thought this last aspect of the design was rather elegant. Anyway, the guard voltage circuit can be isolated and tested, and after doing so, I could see that it was not working. Going through the flowchart for this section led to a step that said to "Check VR3, replace U2". VR3 is a 4.5V zener that had failed in a shorted condition. I replaced VR3. I actually did replace U2 as well (with an LM301), but I don't think this was necessary. After this change, and undoing the steps to isolate the guard supply, everything started working!
So, with the electrical components all working, it was time to do some maintenance on the electro-mechanical components. I cleaned every thing with QD electronic cleaner, cleaned the switches with DeOxit, and re-lubed the switch gear. However, the current adjustment potentiometer, a Bourns 10-turn unit, was really sticky. I opened this up to see if I could repair it, but this ended up being a destructive change. So $24 later, I had a new pot, swapped that in, and all the switches and pots work great. I also swapped out the turns counter for one I had salvaged from another project, and which was in much better physical shape.
The pushed in meter was, fortunately, just a clip that had popped off. I was able to push this back in without cracking any plastic. However, I noticed that the Line/Power light holder had broken loose. This looks to have been glued on, so I just used some 1-minuteepoxy to re-attach it.
Last step was to clean up the case. I wet-sanded the (ala robrenz) the machined metal sides to remove burs. The vinyl panels were cleaned with goo-be-gone, soap, and IPA. The vinyl was then treated with Macguires Ultimate Black to protect and restore the original sheen, and the metal was given a light coat of WD-40. I could definitely do more clean-up, and perhaps fabricate another front panel template like I did for my 6115A restoration, but I think it is good enough for now.
Here's the final result:
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And here's a pic of the parts that were replaced:
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