Electronics > Repair
Tektronix DM501A repair
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Kleinstein:
The multiplexter in the LD120 would be more a thing of thermal drift and offset, not occasional wandering. That is more a contract / switch isssue.

For the MUX replacement one does not need low R_on, but more low leakage. There is a large resistor in series anyway. So the DG419 is more on the low R-on side, but an lower leakage alternative like DG213 usually comes with more switches and pins.
David Hess:

--- Quote from: Rax on May 30, 2024, 10:34:40 pm ---Any reason for choosing an "11 ohm through resistance" part? By which I mean the DG419. The 417 has 6 ohms, I'd be inclined to think less resistance is better here?...
--- End quote ---

The DG419 was the lowest leakage part that I could easily get at the time.  The Texas Instruments TMUX6119 should be better.

The DIP package DG419 does make it easier to air-wire however for lowest leakage.
Rax:

--- Quote from: Kleinstein on May 31, 2024, 09:22:11 am ---The multiplexter in the LD120 would be more a thing of thermal drift and offset, not occasional wandering. That is more a contract / switch isssue.

--- End quote ---

I agree. I'll persist on conditioning the switches - hopefully, they're feasibly fixable.
Rax:

--- Quote from: David Hess on May 06, 2024, 12:54:08 am ---I have had excellent results with MG Chemicals 801B - Super Contact Cleaner With PPE (Polyphenyl Ether).  Its solvent is naphtha which is plastic safe, and the PPE oil does not creep across surfaces unlike silicone oils minimizing contamination.

--- End quote ---

From my experience with it this far with 801B on these switches, it really needs a generous amount of time to work and cure (have the volatile parts evaporate, etc.). Differently put, it seems to me it took a day or maybe a couple before I saw the meters behave as expected (nominally). Before that I saw all sorts of leakage and slightly weird readings. Not sure what your experience from this perspective is, and my own data is still trickling in.
Rax:
On further thoughts with these units, I came to find what everyone has been saying - the switches on these meters are a VERY weak point. And I don't capitalize easily.

I can't really fathom why Tektronix used such cheap, flaky switches. I've not seen similarly underwhelming parts with competitors - HP, etc. This far, I have one unit out of five I regard now as fully functional - a couple others waiting for 801B to do its work - but given the time they need to have their switches fixed it's by a long shot a labor of love.
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