HP 6177C DC Current Source Repair and Restoration (COMPLETE) - Page 1
Forum section: electronics / repair

author: motocoder
author_rank: Unknown
timestamp: July 04, 2015, 04:36:43 pm
content: 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:

author: HighVoltage
author_rank: Super Contributor
timestamp: July 04, 2015, 04:55:43 pm
content: That is a very impressive Restoration and Repair and amazingly good looking instrument.Congratulations and thanks for sharing this.

author: motocoder
author_rank: Unknown
timestamp: July 04, 2015, 08:06:16 pm
content: Quote from: HighVoltage on July 04, 2015, 04:55:43 pmThanks. It is quite enjoyable to see this old test equipment restored for a few more decades of service.

author: robrenz
author_rank: Super Contributor
timestamp: July 04, 2015, 09:09:01 pm
content: Very nice job with excellent diagnosis documentation. Keep them comming

author: eas
author_rank: Frequent Contributor
timestamp: July 24, 2015, 09:56:25 pm
content: Thanks for posting this. I just ordered one of these off of ebay. We'll see what functional condition its in when it arrives...

author: motocoder
author_rank: Unknown
timestamp: July 24, 2015, 10:42:26 pm
content: Quote from: eas on July 24, 2015, 09:56:25 pmOh, 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.

author: coldframe
author_rank: Regular Contributor
timestamp: July 25, 2015, 12:41:45 pm
content: What a wonderful creature.Thanks for sharing.Quote from: motocoder on July 04, 2015, 04:36:43 pm

author: eas
author_rank: Frequent Contributor
timestamp: July 26, 2015, 07:01:28 am
content: Quote from: motocoder on July 24, 2015, 10:42:26 pm6177C, like yours.

author: eas
author_rank: Frequent Contributor
timestamp: August 04, 2015, 07:06:31 am
content: 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.

author: eas
author_rank: Frequent Contributor
timestamp: August 05, 2015, 11:19:55 pm
content: 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?

author: motocoder
author_rank: Unknown
timestamp: August 15, 2015, 09:24:51 pm
content: 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-WMouser Part #: 105-SR2511F-34SFor 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.

author: JuiceKing
author_rank: Regular Contributor
timestamp: April 01, 2016, 01:03:05 am
content: 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!

author: motocoder
author_rank: Unknown
timestamp: April 01, 2016, 03:45:28 am
content: Quote from: JuiceKing on April 01, 2016, 01:03:05 amGlad 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.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: June 13, 2020, 02:53:14 pm
content: Just got one of those too to be fixed and for completion wanted to post the manuals.I have two of them. First one is from the HP website, second one is from KO4BB

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: June 13, 2020, 02:54:41 pm
content: And the Ko4BBB manual for HP6177c

author: motocoder
author_rank: Unknown
timestamp: June 13, 2020, 03:30:22 pm
content: Quote from: Ordinaryman1971 on June 13, 2020, 02:54:41 pmGreat! Thanks for posting.

author: DJ_Rix
author_rank: Newbie
timestamp: June 14, 2023, 04:33:14 pm
content: Hi, i have bought this great psu recently. I am struggling with knob disassembly, hex screws are that tight that i slipped two imbus keys with no luck to unscrew. Any advise how to unscrew them ? Except drilling them out

author: HighVoltage
author_rank: Super Contributor
timestamp: June 14, 2023, 04:38:26 pm
content: Quote from: DJ_Rix on June 14, 2023, 04:33:14 pmUse a fitting torx instead, they are usually a little tapered and when pressed into the hex, they will not slip.That is how I removed screws that looked impossible to loosen up.

author: TimFox
author_rank: Super Contributor
timestamp: June 14, 2023, 05:09:56 pm
content: "Helipot" turns-counting knobs often use spline setscrews, rather than hex.New knobs usually came with appropriate spline wrenches.https://www.mcmaster.com/products/wrenches/l-keys-9/for-drive-style~6-flute-spline/?s=spline+wrenches

author: Stray Electron
author_rank: Super Contributor
timestamp: June 14, 2023, 10:03:52 pm
content: Quote from: DJ_Rix on June 14, 2023, 04:33:14 pmBe aware that some of those small HP knobs require a 0.050" allen wrench and not the slightly larger 1/16". Also people occasionally put a second set screw on top of the first so make sure that the set screws don't loosen up. That's not common in electronic knobs but it is in larger mechanical assemblies and it's possible that someone did it to your knobs.

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2024, 06:16:45 pm
content: nice this mod actually works, I can set it to the closest 100nA pretty easily. I think the pot needs replacing but it does work on 250uA for 81 modelI see the meter misbehave when I adjust it on all ranges. There is a bit of noise too, I need to do test with coaxial cable, closed enclosure, cleaned PCB (its totally dirty because I am waiting for more resistors and caps to replace everything) and replaced potentiometer.

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2024, 11:34:34 pm
content: Hmm I replaced all but 3 of the carbon resistors, and all the electrolytics. Now it looks like the adjustment of the potentiometer is smooth. Maybe its fine and the problem was elsewhere.

author: motocoder
author_rank: Unknown
timestamp: October 18, 2024, 11:34:40 pm
content: Quote from: coppercone2 on October 18, 2024, 06:16:45 pmGlad it works for you.

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2024, 11:47:34 pm
content: yeah I am waiting for a metal foil to replace the 1/4 watt rando resistor I put in there

author: coppercone2
author_rank: Super Contributor
timestamp: October 19, 2024, 12:48:07 am
content: totally silent after I replaced the 3 50nF disks with 50nF foils on the rectifier PCB

author: coppercone2
author_rank: Super Contributor
timestamp: October 19, 2024, 04:43:10 am
content: and after removing the main PCB to clean it off after replacing everything, I noticed it was super stressed by the screws and made a nasty sound when I loosened it (I did the repairs in chassis using solder extractor and carefulness). (I did test it before the removal, it was fine, just very dirty)When I replaced it, it stopped working. same problem as the other one. I cut out the cable harnesses to make sure their fine (upon giving up) and they are fine.This one was very cheap but I give up on this series, I still don't like how its built, you still can't just remove the boardsthe hell with itI like the harrison HP voltage sources (I have like 10, they all survived all sorts of maitence abuse), the current source can suck a egg. this peice of shit is giving me anxiety, and I know whats going to happen when I trust it on a expensive laser diode like I was planning on.the cracking noise is similar to what I heard when I desoldered a VFD on a sony stereo. It had like massive PCB strain. I see the board was either warped or just poorly drilled (that is, the assembler loosely put the studs and shit together, then tightened them all up, because during reassembly I noticed one of the studs was at least 3mm off, because I tightened them to the backer plate before adding the PCB), so more then likely the board was 'stretched' for a good 40 years and then I let it go and something happened. I am not investigating what happened because I am sick of this model series and I don't want to look at the schematics again o probe it

author: motocoder
author_rank: Unknown
timestamp: October 19, 2024, 09:00:57 am
content: I think it's more likely that the chassis was warped. A piece of equipment this old, purchased used, has a high likelihood of having physical damage.Regardless, these units are not poorly made, nor poorly designed, and quite a few people have them and like them. I think most, possibly all of the test equipment HP was making during this era was topnotch.I put a lot of time and effort into this post, so I would really appreciate it if the last post in the thread isn't you threadcrapping.i humbly request you be considerate and just move on.

author: coppercone2
author_rank: Super Contributor
timestamp: October 19, 2024, 03:37:21 pm
content: im not going to get attached to what is essentially a old gear pumpyour mod does work, and my luck is unusually bad (statistically) or there is just some flaw with these things breakingmy suggestion is don't pay out the nose for it, because it was on a analog devices photo, and there is a good chance you will be faced with a very difficult repairin the interest of taking luck out of the equation and finding a cure for HP rabies, I ordered a third one

author: coppercone2
author_rank: Super Contributor
timestamp: October 25, 2024, 03:48:32 am
content: you might be right, there is some really evil shit going on with the cable harnesses or the switches. the main cracker looks like it works.it looks like its some kind of haunting 👻so it looks like it might just be two metric tons of work