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.

[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.

[David Hess]

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.

There is nothing wrong with the Caddock precision decade dividers.  High offset on ranges which rely on the decade divider is caused by excessive input bias current from the multiplexer, or damage to the input protection diodes.  The multiplexer is a known failure point in these designs, but it can be replaced with an external multiplexer and low input bias current precision operational amplifier.  I have the parts but have not quite gotten around to fixing mine.

Later versions of the DM501A used the alternate chipset which supported an external operational amplifier.

[Rax]

David - thank you for this.

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.

There is nothing wrong with the Caddock precision decade dividers.  High offset on ranges which rely on the decade divider is caused by excessive input bias current from the multiplexer, or damage to the input protection diodes. 

By the "multiplexer," I assume you mean the ADC? (U1601/LD120) And if yes, you'd recommend replacing it?

The multiplexer is a known failure point in these designs, but it can be replaced with an external multiplexer and low input bias current precision operational amplifier.  I have the parts but have not quite gotten around to fixing mine.

Later versions of the DM501A used the alternate chipset which supported an external operational amplifier.

I seem to have two out of five having this revision applied. But they still exhibit issues similar with the rest of them - namely, that the 200mV and 2V ranges work OK, but the rest of them don't. There may be one difference in the way these two groups of meters are defective in the 20V through 1000V ranges - more specifically that the one having the revision mentioned above is completely dead on those higher ranges, and the DCA is completely dead too. The "non-revised" ones are defective in a different way, where the 20V through 1000V ranges are responsive but wrong, and the DCA works OK.

I may be switching here to a more invested approach - remove the smaller board attached to the switches (desolder) to gain good access to the switches for cleaning and conditioning (which is what the manual requires) - and am not sure how to resolve the input bias issues. For starters, as I explain above, I currently assume the fault to be located in U1601/LD120.

[David Hess]

By the "multiplexer," I assume you mean the ADC? (U1601/LD120) And if yes, you'd recommend replacing it?

The problem is with half of the LD120.  The input goes through a multiplexer, then into a voltage follower, and then comes out before going back in on a different pin, which is why it can be repaired.  The multiplexer gets damaged because it is directly exposed to the input.  You could find another LD120, but they are not common.

iMo posted a diagram showing what is required here.  It is not complicated.

I bought some OPA140 operational amplifiers and DG419 multiplexers to fix my DM501A, but have not gotten around to it yet.

I seem to have two out of five having this revision applied. But they still exhibit issues similar with the rest of them - namely, that the 200mV and 2V ranges work OK, but the rest of them don't. There may be one difference in the way these two groups of meters are defective in the 20V through 1000V ranges - more specifically that the one having the revision mentioned above is completely dead on those higher ranges, and the DCA is completely dead too. The "non-revised" ones are defective in a different way, where the 20V through 1000V ranges are responsive but wrong, and the DCA works OK.

The modification was to fix a different problem.  The built in CMOS voltage follower lacks enough common mode rejection to support 4-1/2 digits.  The modification replaces the LD120 with the LD122, which lacks the built in voltage follower, but keeps the multiplexer, so an external precision voltage follower can be used instead making the ADC more accurate.

[Rax]

Thank you, David, this is very helpful. I'm new to these meters, and the background issues they have, so thanks for catching me up with what needs to be known and what needs to be done.

The problem is with half of the LD120.  The input goes through a multiplexer, then into a voltage follower, and then comes out before going back in on a different pin, which is why it can be repaired.  The multiplexer gets damaged because it is directly exposed to the input.  You could find another LD120, but they are not common.

iMo posted a diagram showing what is required here.  It is not complicated.

I bought some OPA140 operational amplifiers and DG419 multiplexers to fix my DM501A, but have not gotten around to it yet.

I'm flirting with the idea of doing the mod (I have some OPA140s and OPA189s handy, alongside other parts I'd need...). But it's more invested than what I was planning for these. May be fun though!

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?...

The modification was to fix a different problem.  The build in CMOS voltage follower lacks enough common mode rejection to support 4-1/2 digits.  The modification replaces the LD120 with the LD122, which lacks the built in voltage follower, but keeps the multiplexer, so an external precision voltage follower can be used instead making the ADC more accurate.

Makes sense from the standpoint of what you helped clarifying for me. Thanks again.

[Rax]

One unfortunate, and a fortunate thing happened today. The first one is that I dropped the best performing DM 501A in the batch (the only thing with zero reaction to input excitations was the DCA) on the floor (from my lap, so not a tall fall), and the second one is... I dropped etc. etc.

Initially, the plug seemed pretty shaken with zero readings on most inputs. After reading the manual (nth time) I had a hunch and reseated the display module and things came back to normal. By normal, I mean actually, everything works 95% now (all modes and ranges!...).

I regard this outcome being the result of:

• This is one that I both washed with IPA, and conditioned with David's recommended contact conditioner.
• The fall must've helped with whatever mechanical issue the DCA was inflicted with.
• This unit is one of two with the revision David mentioned being applied to the later units (and earlier ones that probably visited the Tek shop and had it applied, and I'm pretty sure the other one I have that has the revision is also my oldest unit).

Now, I'm saying 95% because there's a bit of "wander" of the indication of a measured value. It doesn't happen all the time, and when it does, I have to re-actuate the range switches (up-down-up...) to restore the correct indication of the meter. I'm tempted to blame this for some leftover flakiness of the switches. But I have a nagging gut feeling there's some flakiness of the input circuitry - maybe the multiplexer? - which creates or just adds to this wandering.

[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]

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?...

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]

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.

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

[Rax]

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.

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.