Electronics > Repair
Tektronix DM501A repair
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Rax:

--- Quote from: David Hess on May 05, 2024, 12:46:07 pm ---I glue the needles in place so that they do not leak where they attach to the bottle top.

--- End quote ---

What do you use for this? I've considered taking this course of action, but been a bit apprehensive about contamination of the contact conditioner from the glue - so I'd be interested in your pick for glue.
Rax:
What I do have at hand from the MG range of products is 404B and 401B (obviously the latter doesn't apply).

I'll give a shot to the 801B product, though the price is on par with the Cadillac of this stuff (Deoxit).
David Hess:

--- Quote from: Rax on May 06, 2024, 05:10:19 am ---
--- Quote from: David Hess on May 05, 2024, 12:46:07 pm ---I glue the needles in place so that they do not leak where they attach to the bottle top.
--- End quote ---

What do you use for this? I've considered taking this course of action, but been a bit apprehensive about contamination of the contact conditioner from the glue - so I'd be interested in your pick for glue.
--- End quote ---

The clear Gorilla Glue works perfectly for it.  Screw the needle in place, and then fill the gap between the needle mounting and ridge on the bottle cap with glue.  Any clear polyurethane glue should work for this.

Since the needle mounting is already sealed, there should be no contamination at all.  My complaint is that the needle mounts tend to work loose and leak over time, and the glue prevents that.
Rax:
Somewhat going back to this project.

Given I have five defective units, I started being more systematic about repairing them. To that end, I am now putting together a log of itemized defects and some important measurement data points (such as PS rails levels).

I'll possibly post those results here, but some preliminary observations:

* Most units seem to experience a systematic issue - even if they measure very well in the (DCV) 200mV and 2V range, they have large offsets in the rest of the ranges. This tells me that the Caddock precision resistors arrays are busted, probably due to abuse from the user.
* DCA seems to be perfect in all ranges in most units, which is a bit weird as I believe the DCA gets derived from DCV. I need to take a closer look at how, though.
* The unit I both washed with IPA and conditioned the switches of seems to fare the best. Weirdly, it didn't do that from the beginning, but the treatment seems to have needed some time to take full effect...
Kleinstein:
The DCA part should use the 200 mV range for the voltage function. So it does not include the Caddock divider.

If the ranges with divider have an offset, this points to high input bias current for the amplifier or leaky switches, not so much a bad resistive divider. Leaky switches are a thing that can happen.
How much offset could be the question. If it is just a little offset / rel. low leakage (e.g. < 1 µA) one can often identify leaky fets by local warming. In most cases leaky fets also react strongly to temperature.

A bad divider would give a bad scale factor and maybe unstable readings from a loose contact.

edit:
Just realised the DM501A does not use much JFET switching but more relays / mechanical switches.  Still it is bias / leakage current current to cause an additive offset.
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