Author Topic: HP 6177C DC Current Source Repair and Restoration (COMPLETE)  (Read 8153 times)

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Offline motocoder

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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.



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.



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:



And here's a pic of the parts that were replaced:

« Last Edit: July 04, 2015, 04:38:56 pm by motocoder »
 
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Offline HighVoltage

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #1 on: July 04, 2015, 04:55:43 pm »
That is a very impressive Restoration and Repair and amazingly good looking instrument.
Congratulations and thanks for sharing this.
There are 3 kinds of people in this world, those who can count and those who can not.
 

Offline motocoder

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #2 on: July 04, 2015, 08:06:16 pm »
That is a very impressive Restoration and Repair and amazingly good looking instrument.
Congratulations and thanks for sharing this.

Thanks. It is quite enjoyable to see this old test equipment restored for a few more decades of service.
 

Offline robrenz

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #3 on: July 04, 2015, 09:09:01 pm »
Very nice job with excellent diagnosis documentation. Keep them comming :-+

Offline eas

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #4 on: July 24, 2015, 09:56:25 pm »
Thanks for posting this. I just ordered one of these off of ebay. We'll see what functional condition its in when it arrives...
 

Offline motocoder

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #5 on: July 24, 2015, 10:42:26 pm »
Thanks for posting this. I just ordered one of these off of ebay. We'll see what functional condition its in when it arrives...

Oh, excellent. Did you get the 6177C or the 6181C?

Please consider posting your progress here.

BTW - I have been considering modifying my 6177C to add two lower current ranges. I believe it should be possible, although perhaps I may run into some practical limitations. To do this, would require replacing range switch S2, a 2 pole, 3 position/throw, with a 2-pole 5-position/throw switch. The extra two positions on the switch would add additional precision resistors, a 1.8K and a 18K into the current sense path (along with R1, R2, and R3). This should give us a 500uA and 50uA range.

The challenge I am having is finding a suitable switch. It needs to be Shorting (make-before-break) type, and be able to handle the 500mA of current that can flow through it, and be in a compact enough design to fit. I haven't had any luck finding that at the various distributors I am aware of.

 

Offline coldframe

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #6 on: July 25, 2015, 12:41:45 pm »
What a wonderful creature.
Thanks for sharing.  :-+


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.


 

Offline eas

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #7 on: July 26, 2015, 07:01:28 am »
Thanks for posting this. I just ordered one of these off of ebay. We'll see what functional condition its in when it arrives...

Oh, excellent. Did you get the 6177C or the 6181C?
6177C, like yours.
 

Offline eas

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #8 on: August 04, 2015, 07:06:31 am »
Mine came. Before turning it on, I partially disassembled it to fix buckled metal in the front panel. While inside, I noted that mine dates to about the same time as yours. The date codes I spotted were 1984 & 1985.

As I was straightening the bent metal, I thought I somehow managed to crack one of the wafers on the power/metering-mode switch, but in looking at the photos I took, I see it was already cracked. I glued it and hope it will hold up.



Once I got it back together, I turned it on and put it through its paces. The meter needs calibrating, but the bigger problem is that found that the current is 1/10th of the expected value in each of three ranges. I haven't done any troubleshooting yet.
 

Offline eas

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #9 on: August 05, 2015, 11:19:55 pm »
I started troubleshooting.

All the reference voltages check out. Next step was to check the guard voltage, which I find only traverses 1/10th the range it should.

First step of the guard supply troubleshooting is nice and clear, lift a leg on CR30, and short TP18 to ground. Less clear is when to reverse this step.  What did you do, motocoder?
 

Offline motocoder

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #10 on: August 15, 2015, 09:24:51 pm »
I added an extra current range to this supply, and I thought I would share the details here in case anyone else wants to do this mod. The standard current ranges on this supply are 500mA, 50mA, and 5mA. These are enabled via current sense resistors of 2, 18, and 180 ohms. The selector switch connects these resistors in series as you go down in range, so you get effective resistances of 2, 20, and 200.

To add an additional, 500uA range, only requires adding a 1.8K resistor and swapping out the 2-pole, 3-position switch for a 2-pole 4-position switch. The switch should be a Make-Before-Break (MBB), also called Shorting, variety as is the OEM part. If a non-shorting variety is used, the supply will generate voltage spikes when the current momentarily drops to zero in-between positions. I had some difficulty sourcing a suitable switch. The closest I could get was a 3-pole, 4-position switch. I just ignored the connections for the 3rd-pole, and this switch worked fine. IT is from Alpha (Taiwan), and is available from Mouser for $2.77:

Alpha Part #: SR2511F-0304-19R0B-E9-S-W
Mouser Part #: 105-SR2511F-34S

For the 1.8K resistor, I chose a 1/2W variety for increased robustness, although this is really not necessary.

The first picture below show a diagram I put together to map out the connections on the OEM switch and how this maps to the new design. Below that I have posted some pictures of the PCB mod and the wiring. I noticed there was an extra, unused hole in the PCB between R2 (the 18ohm sense) and R3 (the 180 ohm sense resistor). I scooted R3 over into this hole so that I could shoehorn in the 1K8 resistor. I had to cut a trace on the bottom of the PCB to do this. Then I connected two wires to the junction of R3 and the new 1.8K resistor, and ran those over to the switch. Use whatever connector you like there so that you can still remove the PCB if needed.




Additional Resistor Shoe-horned in. Not the best soldering job in the world, but it works...


The extra wiring. (Ok, I didn't have a suitable connector)


Extra range. I didn't feel it was worth the effort to re-do the faceplate template.
« Last Edit: August 15, 2015, 09:28:17 pm by motocoder »
 

Offline JuiceKing

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #11 on: April 01, 2016, 01:03:05 am »
Thanks for the repair tips! I just finished fixing mine, which--like yours--had a bunch of blown zeners and one bad output transistor. I also went ahead and replaced the ugly kludged electrolytics that an earlier repair tech had left behind. I think I paid more in postage for parts (took three tries!) than the darn thing itself. A fun project and a useful instrument. Things got 1000x easier when I bit the bullet and just bought the Artek scanned manual. The legible schematic and grey-scale layout diagram were well worth it!
 

Offline motocoder

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Re: HP 6177C DC Current Source Repair and Restoration (COMPLETE)
« Reply #12 on: April 01, 2016, 03:45:28 am »
Thanks for the repair tips! I just finished fixing mine, which--like yours--had a bunch of blown zeners and one bad output transistor. I also went ahead and replaced the ugly kludged electrolytics that an earlier repair tech had left behind. I think I paid more in postage for parts (took three tries!) than the darn thing itself. A fun project and a useful instrument. Things got 1000x easier when I bit the bullet and just bought the Artek scanned manual. The legible schematic and grey-scale layout diagram were well worth it!

Glad to hear you got it all working. They are really useful. The only thing I would like to change about mine is the negative voltage it briefly outputs when you turn it on. This is a "feature" discussed in the manual. I'd be OK if it was zero volts, but am worried the negative voltage will blow something in the device-under-test.
 


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