Author Topic: Keithley 2000 some buttons don't work  (Read 657 times)

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Offline exe

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Keithley 2000 some buttons don't work
« on: April 29, 2019, 07:35:37 pm »
Dear forum,

My K2000 is ill again :(. About a year or two ago I repaired the front panel: I replaced VFD along with one driver. As far as I remember everything worked back then. I didn't use my DMM much since then. It was stored in a box, and was relocated to another place by a car (I was gentle with it).

Today I powered the unit and found that some keys in the bottom row don't work. The unit works well otherwise, I don't see any other problem. Do you have any suggestions why that could be? I tried to trace the problem, but so far no luck. What I tried:
1) measured potential between contacts. On top row there is 4.8V between buttons contact. But now voltage between contact on buttons on bottom row. Yet a few buttons on the right work somehow. I don't understand this.
2) I tried to track traces from non-working buttons, they go to an IC marked as 2000-800A02. The contacts are fine.
3) I did quick inspection with a microscope, nothing suspicions at first glance, except a few strands in ribbon cable were broken where it connects to the board. I repaired this. It was first two contacts, one is unused, another one is gnd, but there are multiple gnd connectors, so I don't that was problem. After repair the front panel still behaves the same.

So, my best guesses are:
1) 2000-800A02 is broken (not sure what it does, but I think it's an mcu, sticker covers the part number :()
2) there is some other trace is broken which I haven't found yet

New ideas are welcome :).

Why I need the bottom row: I need to enable rs-232 interface for logging. It is enable by the button that currently doesn't work. That's how I found out that my K2000 has a problem.
 

Offline exe

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Re: Keithley 2000 some buttons don't work
« Reply #1 on: April 29, 2019, 08:01:34 pm »
I opened repair manual, it says buttons are scanned. So, I connected my oscilloscope and found that indeed there are strobes of very low duty cycle are present. That's why my DMM couldn't see it.

The strange thing is, strobes are present on non-working keys, but strobe peak voltage is ~1.5V as opposed to 4.8V on working buttons. I have bad feeling that the mcu is fried... Do you guys know if it's possible to replace it?

PS it's not the first time I have a problem like that. Before a display driver suddenly failed in a similar manner. This makes me thinking that I might be doing something wrong. May be I should not press keys with a metal screw driver as excessive current may flow from the mcu... I don't know as I can't find schematic for the display.
 

Offline picburner

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Re: Keithley 2000 some buttons don't work
« Reply #2 on: April 30, 2019, 04:56:48 am »
The schematic of the display/keyboard section is in the attached file.
In reality it is that of the Kei2010 but practically it should be identical to the Kei2000.
It's my reverse engineering not the original schematic.
 
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Offline Satbeginner

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Re: Keithley 2000 some buttons don't work
« Reply #3 on: April 30, 2019, 07:24:36 am »
Hi,

I have these manuals, there are some steps to check the keyboard and display board and some schematics.
I used these to repair mine. :-)

Some of the files were too big, so I put them in a shared link for now: https://drive.google.com/file/d/1Y0LBmTIR_JrkCdNe041IU4lpA1sFVMfc/view?usp=sharing

Un saludo,

Leo
You need a scope to repair a scope, and you need many multimeters to repair another multimeter!
*Tek 2467B, Tek 2465B, Tek 2465B, Tek 485, Tek 475A,  Keithley 175A, Keithley 2000, HP 3468B, HP 3457A, HP 34401A, PM 6671, PM 5716, Fluke 45, Fluke 75, Fluke 77, Fluke 79, AFX 9660BL, KPS 605D, etc. *
 
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Offline exe

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Re: Keithley 2000 some buttons don't work
« Reply #4 on: April 30, 2019, 07:48:18 am »
The schematic of the display/keyboard section is in the attached file.
In reality it is that of the Kei2010 but practically it should be identical to the Kei2000.
It's my reverse engineering not the original schematic.

Thank you very much! It matches what I see on the board (e.g, pull-down resistors, vfd circutry, etc).

Next step I'm going to try is to see if I can boost voltage of the pin with a bjt. Lame solution, I know. I'd rather reprogram mcu to use another ping (there are unused pins), or replace the mcu entirely, but I didn't find firmware for the display unit.
 

Offline picburner

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Re: Keithley 2000 some buttons don't work
« Reply #5 on: April 30, 2019, 09:39:31 am »
Some time ago you could buy spare parts (IC, switches, etc.) directly from Keithley/Tektronix, in fact in 2015 I bought some TF-245 parts for the Kei2010 at their headquarters in Germany.
If I remember correctly it seems to me that there is a minimum spending limit.
I don't know if it's still possible now, try sending them an email.
Good luck.
 
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Offline exe

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Re: Keithley 2000 some buttons don't work
« Reply #6 on: May 30, 2019, 12:27:28 pm »
Good news, everyone! I fixed it with a mosfet level shifter. May be I make a short video about it, dunno :). Basically, I boosted voltage from the broken pin. Had to find a mosfet that could be driven from 1.5V. Fortunately, I have plenty FDY301, the first one from the tape had Vgsth around 1.2V, at 1.5V it was open-enough :).

So, yeah, quite horrible approach, but it does work. I was able to recover full functionality of my DMM, configured rs232, now I can control it remotely. Here my journey into volt-nutting begins :).

PS I contacted Tek in Germany, they confirmed they have spare parts, but they never sent me a quote. I requested it twice, once from the online chat, and another one from the form on their website. I also talked to sales representitive in the Netherlands, they said they can help. I'm just a little bit worried that they will a their margin on top of the price. May be I'll contact them, will see how much it cost to buy the MCU. The part itself is around ~$10 or so on Digikey (I'm not entirely sure which part it actually is and what are possible replacements).
 

Offline picburner

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Re: Keithley 2000 some buttons don't work
« Reply #7 on: May 30, 2019, 01:40:03 pm »
The repair you have done works and it is what matters especially for amateur use.
To give you an idea of the prices of spare parts I paid, four years ago, the red perspex panel (the one in front of the display) 75€ + VAT.
 
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