Tektronix DM501A repair

[Rax]

Working on a few units I got, hoping to put together one or two fully working if the other ones may become parts harvesters.

They all have slightly different signs of trouble, but essentially nothing works well, starting with non-sensical display annunciations. I'll stick with one particular unit for now to not confuse the conversation with inconsistencies.

Unit #1, hence, exhibits this inconsistent display of measurements. Typically, applying a certain excitation would change the screen somewhat, but information displayed would still be inconsistent and hard to interpret.

Here's some data I collected this far on this particular unit:

• The core voltage measurement stages are not working well (as I assume all other measurement modes boil down to this). I've followed the troubleshooting tree for DC voltage and I see no voltage present at U1601-pin15.
• In checking the clock, I enclose what I'm seeing at the base of Q1435 and the OSC input of U1603-pin8.
• I reseated the display board (access is quite tight, so I didn't pull it completely out, just enough to be able to apply some deoxit to the board pins and then pushed it back in.
• The PS rails seem to be close enough.
• I haven't checked the electrolytic and tantalum caps on the board, but given the health of the rails, I am not prioritizing these at this time.
• I reseated all ICs on sockets. To flesh that out a bit, I pulled them, sanded the pins a bit with an abrasive pen, cleaned them with IPA, then applied a touch of Deoxit with a swab.
• I've resoldered Q1415. Its solder points seemed to be pretty severely starved from solder.
• Access is not easy, but I've parsimoniously washed the switches with Deoxit. In hindsight, maybe I should have just tried an IPA wash first, particularly as that's less risky in terms of contamination. So maybe I'll give it another go with IPA on the switches, and seek to limit contamination with Deoxit oils on everywhere else than the inside of the switches (which will not be easy, I don't think).

The troubleshooting tree for DC Voltage is not too clear to me. For instance, what to do in case the U1601-pin15 levels are wrong (or non-existent).

[David Hess]

They all have slightly different signs of trouble, but essentially nothing works well, starting with non-sensical display annunciations.

If I am reading your DSO correctly, the clock signal at pin 8 is completely wrong, and does not even meet the duty cycle requirements.

[CaptDon]

One thing that ALL of the 500 series modules have in common is crappy switch contacts. Flush the switches with spray contact cleaner and then just a light wiff of spray contact lube. I will get flamed for this but I used WD-40 sparingly after using a spray cleaner on all of my 500 series module switches and they work fine!! I have inherited so many modules out of various dumpsters, most marked as "Will not calibrate" and I never had to do an electrical repair on any of them!!! Only had to clean the switches and excersize them about 20 times each.

[xrunner]

One thing that ALL of the 500 series modules have in common is crappy switch contacts.

Concur. Clean all the switch contacts with appropriate cleaner.

[Rax]

I agree with you all (work on this this far has some promising effects) though I'm still unsure if the restoration of the switches - IPA shower, MG contact cleaner, Deoxit, everything I could throw at it - can restore it to satisfactory operation for a meter in even medium term...

Will post here as I progress.

[David Hess]

The only problem I have had with the low frequency cam switches is the arms being bent so they make poor contact.  After cleaning, I use tweezers to adjust the arms a little bit and they never fail after that.

The high frequency cam switches in their oscilloscopes sometimes have the same problem, and the same solution.

[Rax]

After cleaning, I use tweezers to adjust the arms a little bit and they never fail after that.

David - I am just wondering how you were able to access them to the extent to be able to do this on the DM501A (unless your recommendation is more of a general nature). Maybe there's a neat way I've not figured out.

On this meter, the relevant banks of switches (range and function) are sandwiched behind boards and I can't figure out a way to get in there enough to do any mechanical servicing such as you describe. I am barely able to comfortably bend IPA/contact conditioner can tubes to spray inside the switches. Particularly as I feel any residue left on the actual boards would be a bad idea.

[David Hess]

After cleaning, I use tweezers to adjust the arms a little bit and they never fail after that.

David - I am just wondering how you were able to access them to the extent to be able to do this on the DM501A (unless your recommendation is more of a general nature). Maybe there's a neat way I've not figured out.

I did it on my DM501s and DM502s and it involved removing the cam switch assembly.  The DM501A uses wiping pushbutton switches and on those, I just had to use contact cleaner and lubricant.  The DM501A switches *must be lubricated*, otherwise their wiping action will destroy the switch surfaces.

On this meter, the relevant banks of switches (range and function) are sandwiched behind boards and I can't figure out a way to get in there enough to do any mechanical servicing such as you describe. I am barely able to comfortably bend IPA/contact conditioner can tubes to spray inside the switches. Particularly as I feel any residue left on the actual boards would be a bad idea.

Instead of using a spray, I bought a needle oiler and filled it with my favorite contact cleaner and lubricant, so I could drip the solution into the front or back of the pushbutton switches without making a mess.

Smaller needles can be mounted to the bottles for finer control.  I glue the needles in place so that they do not leak where they attach to the bottle top.  Good hobby shops carry these if you want to find someplace local.

[Rax]

my favorite [...] lubricant

What is your favorite lubricant, David? I assume we're talking about some grease of sorts, not just a thinner agent, such as an oil.

[David Hess]

my favorite [...] lubricant

What is your favorite lubricant, David? I assume we're talking about some grease of sorts, not just a thinner agent, such as an oil.

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.

[Rax]

I glue the needles in place so that they do not leak where they attach to the bottle top.

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]

I glue the needles in place so that they do not leak where they attach to the bottle top.

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.

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.