Author Topic: Keithley 179 TRMS DMM Repair/Refurb  (Read 22021 times)

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Offline xrunner

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Keithley 179 TRMS DMM Repair/Refurb
« on: February 28, 2015, 06:49:28 pm »
Here we go again ...

Glutton for punishment I know, but here we have a Keithley 179 TRMS DMM gotten off Ebay. It has cosmetic issues of course, such as broken pieces of plastic and a peeling front overlay, but it also has an electronic problem of some sort.

The issue is that the display has random variations of intensity and glitches and little fits. I took a pic of it but it doesn't really show the variations you would see in real time. I also hear a faint buzzing in the back that sortof comes and goes. Anyway, let's see where we go with this baby. Be back later with more info.  8)





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Offline xrunner

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #1 on: February 28, 2015, 11:40:40 pm »
Obviously someone's been in this before. I don't like that because it's not a virgin repair, there's no telling what's been mucked with. As in the last meter I worked on, it looks like the +5V rail has problems. What's with all the meters I get having +5V issues?  :-//

But I'll press on. I love the wires going to the front test lead jacks.  :palm:





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Offline ivan747

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #2 on: March 01, 2015, 03:05:52 am »
I had the exact same problem with the digits on my Keithley 191. It was a cracked solder joint on the transformer, which is soldered directly to the PCB, which can explain why it cracked in the first place. Try that first.

I'd also clean that flux off and take it to a cal lab if budget allows. Screw that, I would resolder the parts completely. Someone messed with the low leakage circuitry. Let's hope they didn't mess with binned parts or matched pairs or anything like that. ***Is that metal can the voltage reference???!!!***  :-[ Go download the schematic off keythley.com while you can. You will have to leave your email address but it's okay, they don't spam.  Try to figure out what was botched/repaired.

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P.S. if you're going to restore the plastics, do tell us. I'm interested.
« Last Edit: March 01, 2015, 03:11:58 am by ivan747 »
 

Offline Vgkid

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #3 on: March 01, 2015, 04:04:00 am »
Good luck. That 4 wire resistor/relay(beside the switches, furthest from viewer ) doesn't look very good.
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Offline ivan747

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #4 on: March 01, 2015, 04:13:33 am »
Good luck. That 4 wire resistor/relay(beside the switches, furthest from viewer ) doesn't look very good.

That must be the current shunt. Not critical unless you care about it.
 

Offline SeanB

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #5 on: March 01, 2015, 06:54:04 am »
I have one winging it's way across the Atlantic to me. Saving 5l of pure methanol for cleaning the board in places ( not the switches) and then will see how it behaves. Has the battery removed by now, to cut the shipping mass.
 

Offline VK5RC

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #6 on: March 01, 2015, 09:46:55 am »
Looks like a bit of a challenge,  good hunting!
Thanks for sharing your work.
Whoah! Watch where that landed we might need it later.
 

Offline xrunner

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #7 on: March 01, 2015, 02:06:32 pm »
I had the exact same problem with the digits on my Keithley 191. It was a cracked solder joint on the transformer, which is soldered directly to the PCB, which can explain why it cracked in the first place. Try that first.

I'd also clean that flux off and take it to a cal lab if budget allows. Screw that, I would resolder the parts completely. Someone messed with the low leakage circuitry. Let's hope they didn't mess with binned parts or matched pairs or anything like that. ***Is that metal can the voltage reference???!!!***  :-[ Go download the schematic off keythley.com while you can. You will have to leave your email address but it's okay, they don't spam.  Try to figure out what was botched/repaired.

I got the schematic off the internet when I bought it, so I'd have it ready. If you go by the solder flux residue, it looks like what was replaced was a small bridge rectifier, a low power 15V regulator, and some signal conditioning components in the attenuator section.

Quote
P.S. if you're going to restore the plastics, do tell us. I'm interested.

Will do. I will have to re-attach plastic parts and repair others, and general clean-up.

Good luck. That 4 wire resistor/relay(beside the switches, furthest from viewer ) doesn't look very good.

It doesn't look good but that's just the way it's made. I thought the same thing but it just has a case that looks bad, it has not been damaged.

I have one winging it's way across the Atlantic to me. Saving 5l of pure methanol for cleaning the board in places ( not the switches) and then will see how it behaves. Has the battery removed by now, to cut the shipping mass.

Yea mine needed cleaning too. I need to get some methanol.

Looks like a bit of a challenge,  good hunting!
Thanks for sharing your work.

Thanks, I'll post more today as I investigate things.

In the meantime I need to repair the front panel, because the overlay has been mucked up by being re-assembled improperly. Some of the edges are curled up pretty badly. What I'm doing now is not gluing it but trying to impart a flat "memory" to the mucked up plastic overlay. I want the plastic to lay flat by itself before I glue it down, so it won't try yo pull off later.  :)

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Offline ivan747

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #8 on: March 01, 2015, 02:09:03 pm »
Any plans for scratch mark removal? I might follow your steps here.
https://www.eevblog.com/forum/testgear/scored-this-keithley-191-multimeter-on-ebay/

It's just that I have no idea how to do this sort of restoration, except for the electronics.
« Last Edit: March 01, 2015, 02:18:34 pm by ivan747 »
 

Offline xrunner

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #9 on: March 01, 2015, 06:42:30 pm »
Any plans for scratch mark removal? I might follow your steps here.
https://www.eevblog.com/forum/testgear/scored-this-keithley-191-multimeter-on-ebay/

It's just that I have no idea how to do this sort of restoration, except for the electronics.

Oh yea, gotta restore it.  8)

I'm trying to track down the display issue, hope to be back later today with some progress, gotta do some house chores though.

Oh, I just found while inspecting the board that the current protection fuse is blown. Every meter I get is like that LOL.  :-//
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Offline Len

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #10 on: March 01, 2015, 08:49:13 pm »
Oh, I just found while inspecting the board that the current protection fuse is blown. Every meter I get is like that LOL.  :-//

Every meter I use is like that at some point.   :-BROKE
 

Offline ivan747

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #11 on: March 01, 2015, 09:51:59 pm »
Oh, I just found while inspecting the board that the current protection fuse is blown. Every meter I get is like that LOL.  :-//

Every meter I use is like that at some point.   :-BROKE

Not my case. I'm proud I have only blown a single multimeter fuse in the last 3 years  :-+
 

Offline lowimpedance

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #12 on: March 01, 2015, 11:41:13 pm »
Another good oldie ! , I have a Keithley 177 re branded to a RACAL DANA 4003.
As Ivan noted check the transformer solder joints, not unusual to see dry joints on these older Keithley's !.
And I see you have those little spring loaded clamps too!. I have found many little jobs that require some holding pressure and these little things work like a champ, where the more traditional clamps just wont work.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline xrunner

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #13 on: March 01, 2015, 11:54:29 pm »
Another good oldie ! , I have a Keithley 177 re branded to a RACAL DANA 4003.
As Ivan noted check the transformer solder joints, not unusual to see dry joints on these older Keithley's !.
And I see you have those little spring loaded clamps too!. I have found many little jobs that require some holding pressure and these little things work like a champ, where the more traditional clamps just wont work.

Yea I did check the transformer solder but it seems to be OK, but I'm beginning to think this guy has a few tricks up it's sleeve.

Back to the display issue, I measured about 4V on the 5V rail, but I also suspect it has fast fluctuations. When I inspected the 7805 regulator (VR104), it looked like Hell. It was really black and looked like it had overheated, so I pulled it. Since the 5V devices were then isolated from the transformer input, I hacked in an external 5V supply and connected it's ground it to the system ground, so it would act as a substitute 5V supply. When I did that, all the display problems went away and it looked beautiful. So I'm very pleased with that.

I then replaced the 7805 with a new one on hand ... but that did not fix the problem.  :( Obviously, the issue is ahead of that component. It would seem the bridge is OK (supplying voltage) and I can't find anything wrong with it. So I'm still investigating what the cause is. Here is the schematic of the PS showing the +5V supply and +/- 15V supplies.

It's beer time now, so I'll listen to any suggestions anyone has and hit it again tomorrow.  :)




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Offline lowimpedance

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #14 on: March 02, 2015, 12:48:55 am »
Buzzing sound may indicate a short in the transformer perhaps!!. Although the +/- 15v were still coming from the transformer when you applied the external +5v ?.
There's not much to it though , transformer bridge electectro's and regulator.
Perhaps a bit of contact cleaner on the power and line selector switches to take care of any old age oxidation not causing intermittent contact.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline xrunner

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #15 on: March 02, 2015, 01:34:06 am »
Buzzing sound may indicate a short in the transformer perhaps!!. Although the +/- 15v were still coming from the transformer when you applied the external +5v ?.

Yep!

Quote
There's not much to it though , transformer bridge electectro's and regulator.
Perhaps a bit of contact cleaner on the power and line selector switches to take care of any old age oxidation not causing intermittent contact.

You're right - not much left.  :-//

It's got to be a switch or cap or bad transformer. I should be able to isolate it tomorrow. Man, if it's just a simple thing this will be a steal. That's how you can score with these Ebay finds. People give up and just want to dump things for a song. I hope by posting these refurb threads it will give people some ideas and if they have the same instrument, some help with their own troubleshooting.

Also, it is dangerous poking around these things when they are connected to the mains - so if anyone out there is just getting into the hobby -be careful! :-/O

Anyway - we will see!  8)
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Offline ivan747

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #16 on: March 02, 2015, 03:31:35 am »
The power supply section is pretty much identical to the Keithley 191. I too suspected a dried cap when I had a similar problem, but as I said, it turned out to be a cracked join. I don't know how you will be able to detect a short in the transformer, though. If the switch contacts aren't gold plated, my next bet is the switches.
 

Offline xrunner

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #17 on: March 02, 2015, 05:35:46 pm »
Fixed!

I focused on C108 - 2200 uF cap. Thought it may have been bad so I connected another large cap in parallel with it, but that made no difference. Then, having noticed how bad some of the solder joints were here and there, I tried another test. I connected the positive lead of the cap I had placed in parallel to the far end of the trace it was supposed to be connected to, and the problem was fixed. Looking at the board with a magnifying glass, I saw that the trace was completely cracked right where the + end of the original cap was soldered. I then simply ran a new wire to the far end of the trace and the problem was fixed.

You can see pics of that below. You can also see pics of C108, and how they nicely soldered it "upside down" so you can't see the markings on it. Why do they do that?

Now I will inspect the board further and make sure there are no other obvious issues. Then I can check whether it really measures what it's supposed to, and the cal, and see exactly where I stand on this - whether or not this baby is going to be a useful DMM, but it's looking pretty promising now. I then will proceed to cosmetically clean it up and fix broken plastic - of which I just found another part about to break off yesterday. I will show that and what product I use to fix this kind of plastic.







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Offline xrunner

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #18 on: March 03, 2015, 01:29:10 pm »
Found this on the bottom. Looking at the pinouts of the switches, it connects pin 9 of the "20" switch with pin 7 of the "200" switch. On the schematic, it does show these pins connected with a hard connection. It just doesn't look like a factory installed wire though. I'd be interested if anyone else has a meter like this with this wire.

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Online McBryce

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #19 on: March 03, 2015, 03:13:37 pm »
Wow, there are so many lifted pads in that last photo!! Have you checked every one of them for broken traces?

McBryce.
 

Offline xrunner

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #20 on: March 03, 2015, 03:41:42 pm »
Wow, there are so many lifted pads in that last photo!! Have you checked every one of them for broken traces?

McBryce.

Yes I did McBryce. After what I found above, yes, I inspected

every

single

one.

With a magnifying glass. However, the proof is in the pudding, so to speak, so later today I'm going to compare this meter to my Rigol and see how it reads before I calibrate it. Then I'll find out if anything more is amiss. But as of right now, this basket case is starting to look like a real steal. But that's just the electronics, I have barely even begun to restore it, so keep looking here for a lot more pics of the restoration.  :)
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Online McBryce

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #21 on: March 03, 2015, 04:03:10 pm »
I will. The 179 is the device I used in University, so I've been toying with the idea of getting one to restore for myself. Just to bring back the memories.

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Offline ivan747

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #22 on: March 03, 2015, 04:15:48 pm »
What could have lifted so many pads? It seems to me the lifted pads are only there on larger, heavier components. Vibration?
 

Offline xrunner

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #23 on: March 03, 2015, 08:04:00 pm »
What could have lifted so many pads? It seems to me the lifted pads are only there on larger, heavier components. Vibration?

I don't really know. If anyone else has one I'd like to know if the circuit board is in the same condition.
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Offline SeanB

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Re: Keithley 179 TRMS DMM Repair/Refurb
« Reply #24 on: March 03, 2015, 08:18:52 pm »
Dropped in a rack onto a concrete floor, so there is no case damage but the insides are heavily jarred. Or on a bench and had a few falls onto a carpeted floor.
 


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