Author Topic: Keithley 177 power supply  (Read 1324 times)

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Offline dml

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Keithley 177 power supply
« on: August 21, 2018, 11:17:21 pm »
I'm trying to debug a Keithley 177 microvolt meter and noticed a discrepancy between continuity testing and the schematic.

From what I can tell all three voltage regulators should be connected sharing a common 0V rail - but I'm reading no continuity between the common pins of the 5V regulator and the other two (+/-15V).

The +/-15V regulators are connected at the common pin, as you'd expect for a split supply.

When the unit is powered up I read a significant voltage difference between the common pin of the 5V regulator and that of the other two. Same situation versus the shield - there's an offset of several volts. It seems as if the 5V circuit isn't referenced at all to the rest of it - only to the front panel.

The display is getting 5V (flashes 0.000) but it's acting like the input limits are exceeded in all modes with no input. Seems like a fault with common 0V - if that's what it is - could matter here.

I have played with the ganged switches but I don't see how a problem with that could explain the discrepancy. Been wondering about less obvious connections via the shields. Will look at that next.

I inspected both sides of the board and being populated this isn't easy - some parts are obscured e.g. under the rectifiers. It appears to be a double sided board with thick tracks so it should still be possible to find a 'hidden' track without too much trouble. Still I do not see where these regulators are supposed to be connected to share 0V.


So before I go removing components to look for hidden tracks and breaks - does anyone know if this is a schematic bug or a fault in the unit? Or a missing heatsink or clip? While this does seem like a fault, it would be a pain to remove more components to find all units are connected the same way.

A previous owner apparently lifted 3 pads and created a few other issues. There might be more problems here but I'd like to have the board agree with the schematic before making more changes. :)





 

Online coromonadalix

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Re: Keithley 177 power supply
« Reply #1 on: August 21, 2018, 11:43:52 pm »
you have teardown video of Dave, it may give you an inside look ??

I would thrust the service manual, all the fixed regulators have an common ground ...  tried to find an good and clean service manual  but no luck

You have a thread here:
https://www.eevblog.com/forum/testgear/looking-for-keithley-177-dmm-schematic/
 

Offline lowimpedance

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Re: Keithley 177 power supply
« Reply #2 on: August 22, 2018, 03:46:35 am »
Ahh reviewing the link reminded me that I did hunt down a manual for the 177 so for the benefit of all, here is a better copy of the schematic and parts layout.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline floobydust

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Re: Keithley 177 power supply
« Reply #3 on: August 22, 2018, 04:10:06 am »
If the unit experienced a ground fault, I might expect an open GND trace. That's an overcurrent between input LO and earth-ground.

But this seems odd, earth-ground does not connect to the chassis, only to the power transformer shield? The whole unit floats?

I would continuity-check the grounds are connected.
 

Offline dml

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Re: Keithley 177 power supply
« Reply #4 on: August 22, 2018, 09:03:59 am »
you have teardown video of Dave, it may give you an inside look ??

I would thrust the service manual, all the fixed regulators have an common ground ...  tried to find an good and clean service manual  but no luck

Thanks for the links - I had seen those but maybe not been through everything.

I have also seen Dave's video. It has been helpful with a few things but not with this issue. Still I'll give it another spin in case I missed something!

Knowing that the schematic is good and the regulators should share a common ground is useful because that's not what I have here. I'll probably start removing some parts to get a better view of that area next.

 

Offline dml

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Re: Keithley 177 power supply
« Reply #5 on: August 22, 2018, 09:06:48 am »
Ahh reviewing the link reminded me that I did hunt down a manual for the 177 so for the benefit of all, here is a better copy of the schematic and parts layout.

That's fantastic - much better than the crappy version I had!  :-+

[EDIT] I refer here to the original scan which is unreadable - not the photo of the manual from ebay, which has been pretty useful!
« Last Edit: August 22, 2018, 09:21:37 am by dml »
 

Offline dml

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Re: Keithley 177 power supply
« Reply #6 on: August 22, 2018, 09:20:17 am »
If the unit experienced a ground fault, I might expect an open GND trace. That's an overcurrent between input LO and earth-ground.

That makes a lot of sense. And something that can be checked easily. I'll report back what I find.

But this seems odd, earth-ground does not connect to the chassis, only to the power transformer shield? The whole unit floats?

I would continuity-check the grounds are connected.

I haven't mapped out all four grounds properly yet but will do it next.

I do know that the 5V circuit (with its 0V rail) is floating with respect to the chassis/shield and at least one of the other 0V rails. This includes the display board which uses 5V and appears to 'work'. So the 5V circuit seems completely isolated from the rest of the unit!

I have attached a pic showing where there appears to be a continuity break in the schematic (a different one I was using, also from this forum). I just haven't identified yet where that common connection/track is on the board. There are a few more things I can map out by following the exported 0V line [7]



Thanks for the suggestions. Will play with it later today and see what turns up.

 

Offline dml

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Re: Keithley 177 power supply
« Reply #7 on: August 23, 2018, 11:49:53 am »
I removed the components obscuring the traces, cleaned it up a bit and chased the 0V lines around the board for a while. Long enough to understand I need another approach :)



[EDIT] the orange tracks are 0V for the 5V supply. blue is 0V for the split 15 supply.

The design appears to go out of its way to avoid joining the 0V lines for the 5V and split 15V supplies, anywhere near the supply source. There's enough ducking and weaving to say that much.

So my next guess is that both supplies are meant to be referenced much closer to the A2D section. Just a guess at the moment - will need to spend more time studying the board.

If not that, it's supposed to happen inside a component or switch. But I don't see that being likely.

Or the schematic is wrong.


It's clear that previous repairs haven't treated the board well. Four pads lifted and one track, plus some sketchy solder joins in places. I lifted one pad myself just by applying solder to wet it at 300c.


Re: Ground fault - there is continuity between the front panel -VE input terminal and the shield and the split 15V supply 0V rail. But not the 0V rail for the 5V supply. So no change here.



« Last Edit: August 23, 2018, 12:03:09 pm by dml »
 

Offline dml

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Re: Keithley 177 power supply
« Reply #8 on: August 23, 2018, 12:41:21 pm »
The plot thickens - I have now found 3 different circuits all supposed to be using the digital GND (5V supply), originating at the 5V regulator common pin. But there is no continuity between any of them.

The A2D IC is connected to the 5V regulator circuit GND. The clock circuit is not - but is connected to the shield. And there is an inductor K101 which has one side connected to the same GND but no continuity between that component and either of the other two! It also appears to be open -> toast.

So I'm going to assume the schematic is correct and there are one or more holes and other busted components hidden in the board. I just need better eyes.


I did notice that during a few power-on tests, the display would show a single zero, which remained frozen. Power cycling would show the zero on a different segment, almost at random. But always a zero. It seemed like a problem with the clock not multiplexing the display properly. Other times it would flash 0.000 until power cycled again.

Seems like a floating clock circuit floating could create some crazy issues like that.
 

Offline floobydust

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Re: Keithley 177 power supply
« Reply #9 on: August 23, 2018, 06:05:50 pm »
K101 is a reed relay? I was looking at http://doc.xdevs.com/doc/Keithley/177/ pics

What is the battery in this?
Frequently I have seen people install a battery and touch the battery (+) to the chassis or shield, which melts ground traces or vias or jumpers etc. as there is no fuse.

I would find the Keithley single-point ground node; where they chose to have it.
They might have placed ferrite beads or inductors to the ground point, and these went open circuit.

The clock oscillator sounds like it is not starting up.
« Last Edit: August 23, 2018, 07:50:59 pm by floobydust »
 

Offline dml

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Re: Keithley 177 power supply
« Reply #10 on: August 24, 2018, 08:42:40 am »
K101 is a reed relay? I was looking at http://doc.xdevs.com/doc/Keithley/177/ pics

You're right. I was thrown by seeing only two axial leads. There are two underneath as well :)

That explains the switches nearby in the schematic and my test 'result' for the two visible pins.

In any case the tests at other locations still indicate some kind of ground fault.

What is the battery in this?
Frequently I have seen people install a battery and touch the battery (+) to the chassis or shield, which melts ground traces or vias or jumpers etc. as there is no fuse.

These can take a lead acid battery pack which sits above the main board. My unit doesn't have a battery pack - instead it has some sort of expansion port board with an edge connector hanging out the back. I have removed this currently.

I would find the Keithley single-point ground node; where they chose to have it.
They might have placed ferrite beads or inductors to the ground point, and these went open circuit.

The clock oscillator sounds like it is not starting up.

I'll try to find that ground node. I didn't see many inductors but I'll have a closer look at the parts list & schematic later today in case it gives any clues to the ground node position.

Thanks!

 

Offline dml

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Re: Keithley 177 power supply
« Reply #11 on: August 30, 2018, 07:20:21 pm »
This continues to be a fun case.

I didn't find a common ground point on the board. It seems like there should be one, but if so then it's incredibly obscure.

Here's what I did find:

  • The schematic shows the main ADC device with two different grounds on two different pins (analog = 11, digital = 15). That is suspicious, but not in a major way. (There is no continuity between the two).
  • All of the analog ground points indicated in the schematic do check out on the board and there is continuity between them all.
  • Some of the digital ground points indicated in the schematic have continuity with the analog ground instead of the other digital ground points, conflicting with the schematic. That is super suspicious. It includes most of the ground points for the clock circuit and crystal and also includes relay K101.
  • One of the 'features' of this unit is an apparent problem with the clock circuit, causing the display to show one digit only,
     as if trying to multiplex but getting stuck


Taken at face value, this suggests a break in the digital ground routing. A break that isn't visible on either side of the board, in a routing which seems quite limited - and I already followed round the board twice without finding any gaps.


For now I'll just keep looking for the broken trace, via or whatever...
 

Offline dml

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Re: Keithley 177 power supply
« Reply #12 on: August 30, 2018, 07:41:56 pm »
After all that, it turned out to be the only thing it could be.

Golden rule of repair - first place to look for problems is everything done by the previous repair.

Somehow (and it was quite a feat) the last attempt at re-capping this thing had soldered an electrolytic in such a way that the pads on both sides had separated from the via and while the lead was 'held' by the solder on both sides, it wasn't connected on both sides. Bonus points for that.



Having removed the cap and disturbed those pads, there is now continuity between the grounds.

Time to repopulate the board for some tests :)

 

Offline The Soulman

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Re: Keithley 177 power supply
« Reply #13 on: August 30, 2018, 07:56:57 pm »
 :-+
 

Offline dml

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Re: Keithley 177 power supply
« Reply #14 on: August 31, 2018, 05:53:47 pm »
It seems this unit is a bit of a troll.

I cleaned & repopulated the board, confirmed continuity again, fired it up and this time got a nice 0.000+/-1 reading which updates normally.

However it ignores inputs in all modes in most of the ranges. It shows the wrong value in the remaining (lower) ranges. a 1V input in the 2V range shows 0.25V. Higher ranges show 0V.

After a lot more messing around, checking all of the (many) switch connections, following the path from the input to the A2D, checking the testpoints etc.... it appears the A2D is toast. Not sure if it's both chips but U105 is definitely gone. The 1.2V internal reference is reading 6V.

Back on the shelf until I find a replacement.
 

Offline dml

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Re: Keithley 177 power supply
« Reply #15 on: September 05, 2018, 05:46:38 pm »
The replacement IC arrived for this Keithley 177 - the ICL8052ACPD, which is the half of the ADC paired set.

The price floor for these is a bit unwelcome but given the low cost of the faulty DMM in the first place, it was still worth a shot.


So I replaced the chip, fired it up and the unit appears to be working. At least, Volts and Ohms are reading correctly in all but the highest ranges, which will require more than 5 minutes to confirm. Will try current mode later too.

So a bit of extra hassle and cost involved - but not very much - and a useful bit of gear is back from the dead.

:)
 

Offline lowimpedance

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Re: Keithley 177 power supply
« Reply #16 on: September 06, 2018, 12:30:31 am »
Nice  :-+, It is a neat DMM and should serve well into the future. And one less for E waste.
A good lesson in fault finding and satisfying result.......... enjoy.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 


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