Fluke 8502A - fuse blown

[Rax]

Hi all,
Going through some of the other units I got - see my other thread(s) - there's also a Fluke 8502A that worked for a minute, then blew its 1/2A fuse.

I've been going through the PS with one of those universal testers, and most electrolytics check OK, except C7 (which gave weird, either non functional, or resistive readings). Pulled it out and measures OK out of circuit. I will likely simply replace all electrolytics soon, but in troubleshooting this there's also a C10 right across C7, just very near the regulator's legs (of course). This is a tantalum 5.6uF (and I'm generally deeply suspicious of tantalums...). Due to where it's placed, checking it and/or taking it out will likely mean replacing it - what have others used for this? I am tempted to go for a 4.7uF film cap (maybe even further bypass it with a .1uF or so). Datasheet says 25uF for an electrolytic if not going tantalum, but I'd rather go film there.

Not at all sure this may cause my fuse meltdown (I'd say not likely), but other things like the safety caps seem to check OK. Any suggestions for what to check or look for are very welcome!

[RichardM]

Hi, we are on a similar journey, see me recent post on the 8502a.

I replaced the three tantalums near the back heat sink and regulators with 10uf tantalum caps and I seem to have the correct voltages now. After powering the unit on the first time, 2 of those old 5.6uf caps let out the magic smoke. The negative voltage regulator was also dying and these are socketed and easy to replace.

[Rax]

Richard,
Yes, I was just reading your notes... As soon as I posted my thread, I went back to the listing of all topics, and for a second I wasn't sure which thread was mine...

I am tempted to do an all-recap, but only after I conclude it's worth going through the expense. So far so good, as during the minute or so it was working, I didn't see much obvious signs of trouble. Not a lengthy examination, for sure, but at least I get good heart to keep at it.

On those tantalums - I never stock any, as almost all my work is in audio or FM, and dread their reliability record, but I'd be tempted to replace them with film caps to keep the esr low. I have WIMA caps that are very miniature for their values - essentially, almost the overall size of the electrolytics for given value, and the same typical lead spacing - and may use here. I am not yet sure these are blown, but I'll try to correct things I suspect or measure out of whack, as hopefully they correct whatever blows its fuse, as I don't have the comfort to fire it up and measure for issues at my leisure. I got a variac and a "dim-bulb" fixture, but not sure if I should try it here.

Looking forward to your progress!
Radu.

[RichardM]

Hi Radu,

Your repair may be much smoother if it was working at some point. Mine came in a non booting state.

I went with tantalums as there is polarity shown on the circuit board and schematic I think.

Richard

[Rax]

Hi Radu,

Your repair may be much smoother if it was working at some point. Mine came in a non booting state.

I went with tantalums as there is polarity shown on the circuit board and schematic I think.

Richard

Richard,
If you could drop a quick indication of how you gained access to those parts (7912, tantalum caps). Does the heatsink pop out? Been looking at this many different ways, but before I start disassembling too many parts, some of which I may not need to touch, I though I'd ask you as you've just done this.

Thanks!

[meanwhile: loudly slaps forehead, repeatedly, and hides in a corner for a while]

Jeez, I guess reading the SM (which I finally could print today, after getting the darn toner order) was too much to ask of me... It slides right out from the back!

[Rax]

Alright, so tonight I got as far as replacing all those three regulators - U6, U7, and U8 - and their caps (used 22uF low esr lythics, per datasheet), and now I have solid +5V, +12V, and -12V coming out of those. I forgot to how much I hate the PCBs of the time... barely got away with lifting just a small number of pads (checked continuity everywhere and I think I'm OK). Brittle as heck.

Also, the SM is not very pretty.

****************************************************************************************************************************

Yeah!... Tonight I got further than what I just said above. PS seems to be sorted out. Turns on, no fuse burn or anything.

But now, when I switch it to measure VDC/AUTO, it goes into "Error 5." If the SM would be legible, I'd figure what the heck that is.

[Rax]

Given Error 5 is "R^2 A/D Error - the recirculating remainder analog to digital converter module is faulty or not installed." I can't find this module in the SM (an index would have been nice...). I did remove the "R^2 A/D Converter" module and cleaned the contacts (deoxit) and reseated a couple of times. Still the same issue ("Error 5"). Not right away, and may need triggering by switching modes, but it occurs ultimately.

[Rax]

Not much positive progress to report... Tried all available modes and there's fault codes to most:

• ohms gives me "error 3"
• all others, immediately or after a while (which may include measurements) ultimately give me "error 5"

Worst of all, I don't think I ever got it to credibly measure anything. The startup screens are correct and uneventful, but sooner than later an error code will pop up. And the instrument doesn't seem responsive to the trigger of a stimuli applied to it.

Adding insult to injury, the SM is dead quiet on workflow around "error 5." For that matter, any error codes. Please point the page to me if I'm wrong... Would love to be wrong.

Thanks all!

[RichardM]

Hi Radu, sorry. Work got in the way for the last few days.

Unfortunately I have not seen error 5. I have only had seen errors 2, 6  & 9.

There is a flow chart in Table 4.6 worth following if you haven't already in addition to the accompanying tables.

My own repair is going around in circles!! I think I have cleared the signal conditioner module and now believe it may be my filter module as the measurement from RT2 to RT5 is not right. Another rabbit hole to follow

Richard

[Rax]

A bit of an update/summary.

• Power supply - repaired and adjusted, including confirming 16VAC on the 60Hz ref.
  
• AD Module - replaced the 470uF axial on the analog PCB (which was bad, though not exploded or shorted, just completely out of spec), and confirmed the rails and Vref (7.0000V). I observed some oddity where some ground(s) (pin 30) seemed for a while - which I couldn't replicate later - elevated by about 0.3V from the rest (TP6 Ref Common) - which is visible on the included scope plots (collected at TP5 to observe switching). Redoing the plots referenced to TP6 restores symmetry around 0V/x axis, although currently all grounds on the analog board of AD seem to be at same potential. Would love to hear thoughts on what's going on at TP5, but seems OK to me.

Evaluating further steps, but not sure what it is I should look at next. Also, feedback and interpretations of this data are appreciated. If possible, comps with similar readings from known good units would really be great.

[Rax]

Progress!! 

[Rax]

Update on my status.

As the picture included a few days ago shows, it seems my work on the PS and AD module restored operation on DCV (and ACV) most of the time. And yet, "Error 5" (when operating on DCV, especially in high res mode) and "Error 3" (when operating in ohms mode) can still occur. I am currently focusing on "Error 5," and leaving "Error 3" for later.

When this happens, TP5 hangs either on a high state of -7V, or 0V (instead of the switching behavior I placed screenshots of above; during "Error 5," it flatlines). After trying this extended weekend to provoke the error so I'd figure what triggers it, it seems to be the "high res" mode tends to by far. I'm actually pretty stable if I simply up the scaling of the measurement as far as it goes. If I go to "high res" though, Error 5 is pretty quick to occur. Not sure if some else has dealt with this and has any recommendation to what to look at first.

Also, I'm looking for a compatible extender/riser card (I'm told the 5100B work), which would allow me to poke further into the functionality of the AD board.

[bdunham7]

If you take the isolator board out of its shell, what part number and revision level is on the PCB?

[Rax]

If you take the isolator board out of its shell, what part number and revision level is on the PCB?

486415 Rev A

Thanks!

[RichardM]

Update on my status.

As the picture included a few days ago shows, it seems my work on the PS and AD module restored operation on DCV (and ACV) most of the time. And yet, "Error 5" (when operating on DCV, especially in high res mode) and "Error 3" (when operating in ohms mode) can still occur. I am currently focusing on "Error 5," and leaving "Error 3" for later.

When this happens, TP5 hangs either on a high state of -7V, or 0V (instead of the switching behavior I placed screenshots of above; during "Error 5," it flatlines). After trying this extended weekend to provoke the error so I'd figure what triggers it, it seems to be the "high res" mode tends to by far. I'm actually pretty stable if I simply up the scaling of the measurement as far as it goes. If I go to "high res" though, Error 5 is pretty quick to occur. Not sure if some else has dealt with this and has any recommendation to what to look at first.

Also, I'm looking for a compatible extender/riser card (I'm told the 5100B work), which would allow me to poke further into the functionality of the AD board.

Well done. Unfortunately I have not been able to get any further with my problem. I have non-zero readings at boot and they do not change with input to the front jacks.

I am back looking at the A/D board now and was wondering if you are able to probe a few pins on J1 or J2 on the back of the analog board using J6 are common ground ? Apparently the pulses need to be 2us max rising and falling but I am seeing much greater times when rising.

This is becoming an obsession

Regards

Richard

[Rax]

Update on my status.

As the picture included a few days ago shows, it seems my work on the PS and AD module restored operation on DCV (and ACV) most of the time. And yet, "Error 5" (when operating on DCV, especially in high res mode) and "Error 3" (when operating in ohms mode) can still occur. I am currently focusing on "Error 5," and leaving "Error 3" for later.

When this happens, TP5 hangs either on a high state of -7V, or 0V (instead of the switching behavior I placed screenshots of above; during "Error 5," it flatlines). After trying this extended weekend to provoke the error so I'd figure what triggers it, it seems to be the "high res" mode tends to by far. I'm actually pretty stable if I simply up the scaling of the measurement as far as it goes. If I go to "high res" though, Error 5 is pretty quick to occur. Not sure if some else has dealt with this and has any recommendation to what to look at first.

Also, I'm looking for a compatible extender/riser card (I'm told the 5100B work), which would allow me to poke further into the functionality of the AD board.

Well done. Unfortunately I have not been able to get any further with my problem. I have non-zero readings at boot and they do not change with input to the front jacks.

I am back looking at the A/D board now and was wondering if you are able to probe a few pins on J1 or J2 on the back of the analog board using J6 are common ground ? Apparently the pulses need to be 2us max rising and falling but I am seeing much greater times when rising.

This is becoming an obsession

Regards

Richard

Richard,
Here's a view of TP2. I am using a 10:1 probe, but the reading on the screen of the DMM changes, so I know the probing affects the instrument. So take the risetime value with a grain of salt (this is a quick read before work... not much time for thinking this through). But I think what you can take from this is that the risetime and fall are the same.

[Rax]

Here's the plot

[RichardM]

I meant to say use TP6 as common ground, apologie. Is that what is shown in your image ? The rise and fall times are much greater than 2us ?

Thanks again, Richard

[Rax]

I meant to say use TP6 as common ground, apologie. Is that what is shown in your image ? The rise and fall times are much greater than 2us ?

Thanks again, Richard

Richard,
Sorry, I may have misunderstood your request. What's included above is a read off of TP2 (as referenced to TP6, of course). The probe is loading the circuit, so the voltage reading is truncated while the probe is attached and the waveform looks like you see it (I assume the risetime is much slower than what it actually is without the probe loading the circuit). FWIW, there's no malfunction while doing this, as soon as the probe goes away the reading restores and no errors are being thrown out.

Could you please explain what you need measured again? A picture is worth a thousand words, so that would be gorgeous, if at all possible. I will do my best to help. Thanks.

[Rax]

At this point, I have a pretty high degree of confidence "high res" triggers "Error 5." I left it run measuring about 13Vdc with all digits possible outside "high res" for just about 24hrs, the error never came up. I switch it to "high res," it locks up on "Error 5" right away (may take up to a minute). I did the same with my AP box - for a calibrated AC level - and it locked up when I requested "high res."

This happens on both ACV and DCV.

[RichardM]

I meant to say use TP6 as common ground, apologie. Is that what is shown in your image ? The rise and fall times are much greater than 2us ?

Thanks again, Richard

Richard,
Sorry, I may have misunderstood your request. What's included above is a read off of TP2 (as referenced to TP6, of course). The probe is loading the circuit, so the voltage reading is truncated while the probe is attached and the waveform looks like you see it (I assume the risetime is much slower than what it actually is without the probe loading the circuit). FWIW, there's no malfunction while doing this, as soon as the probe goes away the reading restores and no errors are being thrown out.

Could you please explain what you need measured again? A picture is worth a thousand words, so that would be gorgeous, if at all possible. I will do my best to help. Thanks.

Hi Rax

My explanation was not very clear that's for sure.

I have attached a photo of the Analog side of the A/D twin board. J1 and J2 are circled in red and TP6 is also. The Fluke is turned around so the back of the unit is closest to me and obviously the cover is taken off the A/D module. With the scope ground attached to TP6 I am looking to see what you get when you probe the pins on say J1 (easiest to reach). Most pins will show pulses and the rise/fall should be <2us. My rise times are much greater than 2us.

Just fyi, I have also attached scope readings for TP2,3,4,and 5. I don't see much at all at TP2. With the exception of TP5 the results don't appear correct, although I am unsure what to see at these test points ?

Thanks again, Richard

[m k]

What manuals you are using?

Shouldn't be a problem if A/D board it the interesting one.

[RichardM]

What manuals you are using?

Shouldn't be a problem if A/D board it the interesting one.

I have the Instruction Manual. I still there something else available ?

If it is the A/D board I thought it would be a big problem as I doubt you can easily find things like the opamps ?

Regards

Richard

[m k]

I meant that the schematics is not a problem since it seems to be always the same.

Instruction Manual is good.

I have 4.
Instruction Manual 1978 rev 7/81
Instruction Manual 1981 (AT)
Operating and Service Manual
Operating and Service Manual (Artek)

Original 1978 manual I dont have but not Artek O&S has old Isolator schematics.
Yet that O&S has new general pictures, including Ext Trigger connector and it's not dated.
Artek version is missing all but -01 options but it has single page schematics.
1981 vesion has AT stuff only.

[RichardM]

Success!!!!!

I pulled Q27 and Q9 for the A/D board which are dual n-channel jfets and Q27 tested badly on the curve tracer, basically no curves at all on both sides. It’s late at night but I thought I would just try to replace Q27 with 2 J113 jfets that I had and the meter has come to life. I guess there may be flow on effects but for now I will run the meter until something fails I guess. I will try to run a calibration tomorrow before ai get too excited. I may try to find a data sheet for the old jfet but the markings and manual don’t provide much of an indication of the model and maker.

Thankyou to all who assisted with suggestions etc

Richard

[Rax]

Success!!!!!

I pulled Q27 and Q9 for the A/D board which are dual n-channel jfets and Q27 tested badly on the curve tracer, basically no curves at all on both sides. It’s late at night but I thought I would just try to replace Q27 with 2 J113 jfets that I had and the meter has come to life. I guess there may be flow on effects but for now I will run the meter until something fails I guess. I will try to run a calibration tomorrow before ai get too excited. I may try to find a data sheet for the old jfet but the markings and manual don’t provide much of an indication of the model and maker.

Thankyou to all who assisted with suggestions etc

Richard

Richard - great work!!! So awesome when a repair succeeds. Well done!

BTW - I've replaced C13 in mine, which is the only electrolytic on the board, so suspect by default at this age, but also measured out of spec. May be a good idea to check it and replace if off. I think this improved things quite a bit on my AD module.

I actually wonder if my unit has issues in the same area (maybe Q27 failing intermittently?), as "Error 5" has to do with the REMAINDER (though I don't think you've seen this particular error, have you?). Did you install the two J113s in physical contact? I'm curious what will be your permanent solution for this part. I wonder if any of the dual JFETs from LS would work.

[RichardM]

Success!!!!!

I pulled Q27 and Q9 for the A/D board which are dual n-channel jfets and Q27 tested badly on the curve tracer, basically no curves at all on both sides. It’s late at night but I thought I would just try to replace Q27 with 2 J113 jfets that I had and the meter has come to life. I guess there may be flow on effects but for now I will run the meter until something fails I guess. I will try to run a calibration tomorrow before ai get too excited. I may try to find a data sheet for the old jfet but the markings and manual don’t provide much of an indication of the model and maker.

Thankyou to all who assisted with suggestions etc

Richard

Richard - great work!!! So awesome when a repair succeeds. Well done!

BTW - I've replaced C13 in mine, which is the only electrolytic on the board, so suspect by default at this age, but also measured out of spec. May be a good idea to check it and replace if off. I think this improved things quite a bit on my AD module.

I actually wonder if my unit has issues in the same area (maybe Q27 failing intermittently?), as "Error 5" has to do with the REMAINDER (though I don't think you've seen this particular error, have you?). Did you install the two J113s in physical contact? I'm curious what will be your permanent solution for this part. I wonder if any of the dual JFETs from LS would work.

Thanks Rax

I will test C13 again now you mention it. Thanks for the tip.

No, I don’t recall seeing Error 5 but you may want to check those jfets anyway. I found the one that does work a little noisy on the curve tracer as well. I didn’t tie the two j113 together but that’s prob a good suggestion. It’s a bit tricky as the legs/pin out were inconsistent and needed to be bent at strange angles. I am considering pulling out Q9 again and capturing the curve tracer curves and then searching for a suitable match long term. For now I will see how it performs.

Richard

[Kleinstein]

The J113 may be a reasonable choice. The input is not that high in impedance, so input leakage is not an issue. The J113 are reasonable low noise. For the pin bending it may help to know that the drain and source are interchangeble.
It may need some selection for a reasonable low drift for the pairs (probably in circuit, or if the drift is too high adjust one of the drain resistors)

The needs for Q9 should be similar.

If really desparate and performance really suffers, there would be the last resort of replacing the JFET pair and follwing OP with just a single modern high performance OP.

[RichardM]

The J113 may be a reasonable choice. The input is not that high in impedance, so input leakage is not an issue. The J113 are reasonable low noise. For the pin bending it may help to know that the drain and source are interchangeble.
It may need some selection for a reasonable low drift for the pairs (probably in circuit, or if the drift is too high adjust one of the drain resistors)

The needs for Q9 should be similar.

If really desperate and performance really suffers, there would be the last resort of replacing the JFET pair and following OP with just a single modern high performance OP.

Thankyou for your expert advice. I did notice that the curve tracer gave the same output regardless of how I connected the drain and source. As a novice I still thought they needed to be connected as per the datasheet. I may not need to bend them now, thanks.

Not sure whether to also replace the other dual opamp ? If it aint broke.... ?

[RichardM]

From tears of joy to tears of despair!!

The last module I put back into its plastic shell and installed back in the Fluke was the filter module. Unfortunately the TO99 opamp I replaced was sitting a little too high and touched the tin shielding inside the plastic case and out came the magic smoke!! I hope its just a case of replacing 2 burnt resistors and ordering another opamp for $20..... Sigh

I wish there was another high voltage opamp that was equivalent to the LM1436H.

Richard

[SilverSolder]

From tears of joy to tears of despair!!

The last module I put back into its plastic shell and installed back in the Fluke was the filter module. Unfortunately the TO99 opamp I replaced was sitting a little too high and touched the tin shielding inside the plastic case and out came the magic smoke!! I hope its just a case of replacing 2 burnt resistors and ordering another opamp for $20..... Sigh

I wish there was another high voltage opamp that was equivalent to the LM1436H.

Richard

A good trick is to take a "Before" picture with your phone, and then try to make the repair look the same.  You rarely go wrong by having the new parts installed exactly the same way as the old ones - and it is a great sanity check (did I put that ferrite bead back on the correct leg, etc.)!

[m k]

At this point, I have a pretty high degree of confidence "high res" triggers "Error 5." I left it run measuring about 13Vdc with all digits possible outside "high res" for just about 24hrs, the error never came up. I switch it to "high res," it locks up on "Error 5" right away (may take up to a minute). I did the same with my AP box - for a calibrated AC level - and it locked up when I requested "high res."

This happens on both ACV and DCV.

How you measure that Error 5?
Since it's high res can it be that summing part it the problem?

Simplified analog part of A/D in O&S page 52/354.
Explanation in previous page.
https://www.manualslib.com/manual/888772/Fluke-8502a.html#product-8502A

Digital of A/D in O&S schematics pages 353-354/354
ID0 is Reset
ID1 is AutoZero.
ID2 is Buffer In and Remainder In, topmost NAND with inverted inputs is OR.
ID3 is Channel Y.
ID3 inverted is Channel X.

So topmost Tr of pictured U17 is both AutoZero and Buffer In.
It's pretty likely fully operational.
It also possibly shouldn't happen if Error 5 happens before it.

Anyway, with those data lines you should be able to analyze how intermittent that High Res Error 5 is.
For now it seems that you have a temp and noise situation.

[Rax]

At this point, I have a pretty high degree of confidence "high res" triggers "Error 5." I left it run measuring about 13Vdc with all digits possible outside "high res" for just about 24hrs, the error never came up. I switch it to "high res," it locks up on "Error 5" right away (may take up to a minute). I did the same with my AP box - for a calibrated AC level - and it locked up when I requested "high res."

This happens on both ACV and DCV.

How you measure that Error 5?
Since it's high res can it be that summing part it the problem?

Simplified analog part of A/D in O&S page 52/354.
Explanation in previous page.
https://www.manualslib.com/manual/888772/Fluke-8502a.html#product-8502A

Digital of A/D in O&S schematics pages 353-354/354
ID0 is Reset
ID1 is AutoZero.
ID2 is Buffer In and Remainder In, topmost NAND with inverted inputs is OR.
ID3 is Channel Y.
ID3 inverted is Channel X.

So topmost Tr of pictured U17 is both AutoZero and Buffer In.
It's pretty likely fully operational.
It also possibly shouldn't happen if Error 5 happens before it.

Anyway, with those data lines you should be able to analyze how intermittent that High Res Error 5 is.
For now it seems that you have a temp and noise situation.

m k - thank you very much for this overview. Would I need a logic analyzer to look at these data lines? I am not versed in this type of work. Not sure if what I need to look at is visible with a scope (HP 54522A). Some other instrumentation available at hand but could use some guidance on what would be useful.

Would you also elaborate on the "temp and noise situation?"

Thanks a bunch!

[Rax]

m k - further on your points. I've had a hunch on the summing nodes (Q11-Q21) due to the triggering of the error by "high res," along the lines of your train of thought, I think.

Also, when "Error 5" occurs, TP5 hangs on either 0V or -7V (most often -7V), and stops switching:

When this happens, TP5 hangs either on a high state of -7V, or 0V (instead of the switching behavior I placed screenshots of above; during "Error 5," it flatlines).

That said, I am unsure of next steps.

[m k]

You can store what happens in ID7 but I'm not sure how to trigger it.
And if I understand it right it takes quite a long time to complete.

Temp part of temp and noise is because you said that some times the error is not coming right away after selecting High Res.
Noise part is because if the error is staying away without High Res it's not very big and maybe it's even an adjustent thing but there are not much to adjust.

Somebody should verify my understanding.
Every D/A converter part is used more than once in every value generation and one or some switching reference parts are used only once?

[Rax]

You can store what happens in ID7 but I'm not sure how to trigger it.
And if I understand it right it takes quite a long time to complete.

Temp part of temp and noise is because you said that some times the error is not coming right away after selecting High Res.
Noise part is because if the error is staying away without High Res it's not very big and maybe it's even an adjustent thing but there are not much to adjust.

Somebody should verify my understanding.
Every D/A converter part is used more than once in every value generation and one or some switching reference parts are used only once?

m k - thank you for elaborating your thoughts (and I appreciate their "seeking form," which I mean in the best way of "thinking on this with me"). The noise part could also be leakage, and I'm thinking of the FETs in the summing node (or maybe differential inputs elsewhere?). The temp part... gut tells me no, because the fault occurs just too quickly after creating what I've determined to be conducive conditions, at least lately.

It'd be great learning to acquire and interpret the data/control lines. I wonder if this could be supplied with affordable hardware.

On a different track, I grabbed a parts unit off of ePay and may try to continue isolating by swapping modules from the donor unit. Are the codes on the modules cases enough to match them (for instance, "fast A/D Converter" = 383984), or codes from the PCBs are required for safe swapping? I'm thinking maybe software accounts for some variations and don't want to upset the unit. Of course, interdependent adjustments may also throw things off, though I assume that depends on the exact module replaced.

[m k]

Maybe I understand it a bit better now.
All parts are used many times or all the time and switching time is what makes it closer to goal.
(I have that HP Journal but it's in the que)

With noise I didn't mean ripple but more generally something that flips the error flag up.

My understanding of the value generation is seeking it using iterations and flipping comparator U7 is telling it's too far now.
That bit value is then put out to controller using ID7.
You can try trigging it with U6 pin 13 of digital board.
(right of text of down of 4 D-flops of upper left corner)
If you get much less than 20 bits you're probably getting only partial value and next part is completely its own thing and next pin 13 down/up.
It's also possible that you're getting same bit values.
The pin is operating so that first D-flop's R is up and Q is down, then R goes down allowing next S put Q permanently up, clock is never happening so D is also never happening.
With storing the trigger channel also you can see how long you should get bits.

Why High Res.
Last reference is so close to input that comparator misbehaves.
If iteration starts from center and comparator indicates less in both directions the value is exactly there in the middle.
But if comparator starts indicating more in both directions it's an error.

I don't understand those completion part offsets.
(3-73 of O&S)
Finally Q4 and Q8 are opened, Q5, Q6 and Q7 closed and offsets stored.
Where, the sample is complete and circuits are autozeroed.

The operation of the meter is pretty slow compared to what is available now.
So pretty much any logic analyzer will do.

I have many never used China gadgets.
Two months shipping brings up anybody's inner fortune teller.
Anyway, one of those is USB logic analyzer and no idea how it works, close to
https://www.sparkfun.com/products/15033

A/D 383984 seems to be the only module version in all models.
Much more likely you are upsetting the software with almost anything else.

[Kleinstein]

The gain from step to step is x 16. So From the 5 bit DAC (plus maybe the sign) only 4 bits should be really used in the later cycles. Changes are the highest bit is mainly used in the first conversion and should not be in used in the later cycles. It should still not be a problem with high res mode, but not the normal mode.
 
The residual signal is amplified from cycle to cycle. So the comparator would not see a weaker signal later. A signal just at the edge may happen (just random at any stage), but this should not get the ADC to an error state. Worst case get a metastable state and a bit error. I don't see a much higher frequency for the high res mode here.

A point that could upset the ADC could be if the gain from cycle to cycle is too large and from this it may need the highest bit. In theory this could be fixed with some math, but my feeling is the old ADC is not using numeric error correction but would need the trimmers to be set right.

The sampling type ADC is a bit different from modern DMMs with there integrating ADC. This makes the old Fluke meters a bit noisy / sensitive to hum  or slow settling if they use the analog filter. The meter was not that bad for it's time.

Normally TP5 should get a 0 V only for the auto zero phase. So stuck at 0 would mean something like a stuck control sequence, not somerhing like noise or an offset in the analog part. Maybe have another check on the supply for the digital side.

[SilverSolder]

[...]I grabbed a parts unit off of ePay and may try to continue isolating by swapping modules from the donor unit. [...]

That is a great idea and a huge troubleshooting time saver with these units, given how easy it is to swap modules.  Even if you get both of them working, it never hurts to have two DMMs (and the ability to swap modules between them!).

I believe the A/D converter is one of the boards that works in pretty much any version of the instrument, so if it was mine, I would go ahead and try the "new" one.

[Kleinstein]

For the new unit, I would still first check the supplies: a broken regulator can do quite some damage.

The modulales should not interact very much. So the adjustments are local to the modules and not much need for a new adjustment with swapping modules.

[Rax]

For the new unit, I would still first check the supplies: a broken regulator can do quite some damage.

Good point, it's also typically my first point of evaluation, as it can give me a first indication of any possible big faults in the modules themselves (such as a short, etc.). It's neat that with these units, the PS can be pulled out of the unit very easily and be verified (and then adjusted) by itself.

[m k]

Instruction Manual's 4-52 has A/D Converter calibrations.

First there is to check that between TP7 and 6 is -7V.
Next is zero adjustment, pretty easy also and without any meters, just a good short needed.

[Rax]

Instruction Manual's 4-52 has A/D Converter calibrations.

First there is to check that between TP7 and 6 is -7V.
Next is zero adjustment, pretty easy also and without any meters, just a good short needed.

Thank you, m k. This far, all -7V references where I measured are what I'd see as dead on. On the AD it's -7.0000V on my Fluke 8840A (occasionally going -7.0001V, which may account for minute environmental or otherwise variations in either meter in my mind) - BTW, the calibration test equipment per SM requires a 0.01% voltage accuracy DMM, while the 8840A is I believe 0.005%. On the ohms module, for instance, it's -6.9999V, sometimes going -6.9998V. I think the manual says +-0.0001V is acceptable. I've looked a bit, but I've not yet figured out whether there's one main -7V reference for all modules (with local adjustments adjustments) or each has their own.

I will check in the new modules, but, overarchingly, I will ever be amazed at how well these instruments hold with age.

I think most posters said the calibrations and adjustments on different module are typically independent of each other (they don't interact), which is great to know and a very positive scenario.

[SilverSolder]

Instruction Manual's 4-52 has A/D Converter calibrations.

First there is to check that between TP7 and 6 is -7V.
Next is zero adjustment, pretty easy also and without any meters, just a good short needed.

Thank you, m k. This far, all -7V references where I measured are what I'd see as dead on. On the AD it's -7.0000V on my Fluke 8840A (occasionally going -7.0001V, which may account for minute environmental or otherwise variations in either meter in my mind) - BTW, the calibration test equipment per SM requires a 0.01% voltage accuracy DMM, while the 8840A is I believe 0.005%. On the ohms module, for instance, it's -6.9999V, sometimes going -6.9998V. I think the manual says +-0.0001V is acceptable. I've looked a bit, but I've not yet figured out whether there's one main -7V reference for all modules (with local adjustments adjustments) or each has their own.

I will check in the new modules, but, overarchingly, I will ever be amazed at how well these instruments hold with age.

I think most posters said the calibrations and adjustments on different module are typically independent of each other (they don't interact), which is great to know and a very positive scenario.

The reference is brought out on the backplane bus, so any module that needs it has access to it.

[Rax]

The reference is brought out on the backplane bus, so any module that needs it has access to it.

Awesome, thank you for confirming that. Then I'd say in my main unit the -7V Vref is confirmed working within spec 100%.

[m k]

If you need more spares I have
https://www.ebay.com/itm/174945154128
discounted in my cart.

USA reroute is possible, everything else is unknown.

Rerouted.

[Rax]

Anyway, one of those is USB logic analyzer and no idea how it works, close to
https://www.sparkfun.com/products/15033

I got this: https://www.amazon.com/gp/product/B07K6HXDH1/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1. ***Luckily,*** in the US we have the big elephant in the room and things can be had (delivered) sometimes the same day.

I've got it running on my Windows 10 laptop (PulseView/Zadig) - seems to be purring 100%. Now I wonder if anyone can point me to the applicable reading material. I'm entirely not sure this is good for the job, or if it is even compatible with the hardware on the Fluke. Any pointers greatly appreciated - thanks a lot in advance.

This is at least until I get the unit for parts and try swapping got get ahead with this (early next week).

[m k]

Somewhere was mentioned that some parts are groupies.

In the case like that you should change all of those that are selected to that group.
Not much more than 2 components I guess.

Calibration is of course the next thing in that situation.

E,
Sigrok seems to be suitable.

You can always try installing it again.
Go to device manager and uninstall its drivers.
Then with luck reinstalling picks better usb side base drivers.

E2,
I mean the device.

[Rax]

Well, drat. I got the parts unit with spare modules, and swapping the AD module still returns Error 5. I'd have thrown a couple of coins on the table this was gonna fix it. I may try other likely modules as see what's happening, but I'm a little taken aback and will be mulling over this for a bit.

[Rax]

You can always try installing it again.
Go to device manager and uninstall its drivers.
Then with luck reinstalling picks better usb side base drivers.

No need, I probably wasn't clear but the install went very smoothly. I think it's working OK.

I just have no idea how to connect it, where, etc., but not knowing which end to grab this "using a logic analyzer on a DMM" issue, I thought I'd ask to be sent to read somewhere the basics if that resource exists. I'm not sure how complex the topic is (this is pretty far afield for me, but eager to learn), so I wanted to be respectful of everyone's time. Maybe it's doable through simple instructions and then I'll just get right on to it.

[bdunham7]

Well, drat. I got the parts unit with spare modules, and swapping the AD module still returns Error 5. I'd have thrown a couple of coins on the table this was gonna fix it. I may try other likely modules as see what's happening, but I'm a little taken aback and will be mulling over this for a bit.

This is not an uncommon situation for this era of Flukes.  Having spare modules will quickly narrow down your options.  I'd try the isolator first and then the controller next, and of course remove any unnecessary modules while testing and swapping.  Is it an identical parts unit?  Does the parts unit work?  Swapping modules into a working unit is sometimes actually more helpful, especially in cases where it turns out all the modules are fine!  There's still the backplane board and PSU to worry about.  Or you could have two bad ADCs...

[SilverSolder]

Well, drat. I got the parts unit with spare modules, and swapping the AD module still returns Error 5. I'd have thrown a couple of coins on the table this was gonna fix it. I may try other likely modules as see what's happening, but I'm a little taken aback and will be mulling over this for a bit.

My money is on the controller, but it's just a guess! 

I have had all kinds of weird behaviour due to an EPROM getting "weak bits" that sometimes worked, sometimes didn't, depending on whether there was rainfall in the Outer Hebrides on that day or not...   -  Swapping controllers should not be an issue as long as the two instruments are the same model e.g. 8502A.

[Rax]

I'd try the isolator first

Right on the money!!! BINGO!

After the AD, I tried the DC conditioner, and then the isolator, and Error 5 is gone, gone-gone-gone, baby!

Now checking AC ranges and will keep you guys posted. There's also Error 3 with resistance measurements, so that's next tackle.

Yeah! Can hardly sit. Thanks a whole bunch to everyone that's tagged along in this.

[SilverSolder]

Winner, winner, chicken dinner! 

[Rax]

This far, it's holding perfectly in both DCV and ACV ranges.

Now, on to Error 3 in ohms. I'm tempted to try replacing the ohms module - a hunch - although Error 3 blames the DC Conditioner.

• If it is the DC Conditioner, I think I should just be able to do 4-50 at p.4-8 - which seems simple enough
• If I'd start by replacing the Ohms module, I just figured the module I got with the replacement is a different #: mine is 651778, replacement from parts unit is 383968. No luck, huh?... 

[Rax]

Well, you know what, the DC Signal Conditioner is also a different module number (mine 716266, replacement parts unit's 383901), which may explain some of the non-sensical things I've seen with it on (some ranges completely off, others exact, and an Error 4 one time). The reason I used it is that my 716266 module has a sticker on it stating when it was replaced, it looks newer and of different plastic. So I assumed a repair job at some later point. I wager the original part was 383901 at this particular production time.

Anyway, if anyone has tried these replacements across "different number" modules, I'd love to hear that.

[SilverSolder]

This far, it's holding perfectly in both DCV and ACV ranges.

Now, on to Error 3 in ohms. I'm tempted to try replacing the ohms module - a hunch - although Error 3 blames the DC Conditioner.

• If it is the DC Conditioner, I think I should just be able to do 4-50 at p.4-8 - which seems simple enough
• If I'd start by replacing the Ohms module, I just figured the module I got with the replacement is a different #: mine is 651778, replacement from parts unit is 383968. No luck, huh?... 

Over time, people may have swapped all kinds of boards in and out of these things, so you could have ended up with a board for a 6.5 digit model in there.

In my experience, the Ohms modules are all compatible - whatever number is printed on them.  There may be different accuracy specifications depending on whether the board is for the 5.5 or 6.5 digit models, I don't know...   but they all seem to work. 

Personally, I would try the swap and see if the "parts board" loses the error messages...

[m k]

Anyway, if anyone has tried these replacements across "different number" modules, I'd love to hear that.

What software versions you have?

Ohms Conv
383968 02AT from manual
651778 05A from picture

DC Sign Conv
383901 02A from manual
716266 00A from picture

Are those isolator layouts equal or different, what are their numbers?

Your logic analyzer is like a scope, just pick a triggering channel and start collecting.
The ad says it's 24MHz, should be a plenty, I think I have the same.
Maybe you saw it free running and somewhere is a stop.

The meter has many grounds, maybe digital board and logic ground L is the way.

Try using opposite autozero U34 pin 12 as a trigger.
Include ID7 and B to F, last one maybe ACK TP2.

It's also possible that you just had a wrong configuration, software included.

[RichardM]

My meter is back up and running thank goodness after a self inflicted "magic smoke" incident.

I am sure you have already done this but I have found that giving each of the motherboard slots and edge connectors an intense clean makes a big difference. I put my meter back together and started getting error 8, error 4 etc. I then pulled the modules and drenched the motherboard connectors in alcohol/contact cleaned and scrubbed and mopped the excess up. No errors since, fingers crossed.

Richard

[Rax]

I am sure you have already done this but I have found that giving each of the motherboard slots and edge connectors an intense clean makes a big difference.

Richard - definitely, my first breakthrough with this unit (after the fairly expected PS rebuild) was cleaning up the edge connectors on the cranky modules (contact cleaner, though alcohol first sounds like a good idea - as long as it's a single malt! ). Only then I realized the SM actually requires removing all modules and cleaning the interior of the unit EVERY 90 DAYS (see p.4-4). I don't recall seeing this requirement in any other SM, but YMMV.

[RichardM]

I am sure you have already done this but I have found that giving each of the motherboard slots and edge connectors an intense clean makes a big difference.

Richard - definitely, my first breakthrough with this unit (after the fairly expected PS rebuild) was cleaning up the edge connectors on the cranky modules (contact cleaner, though alcohol first sounds like a good idea - as long as it's a single malt! ). Only then I realized the SM actually requires removing all modules and cleaning the interior of the unit EVERY 90 DAYS (see p.4-4). I don't recall seeing this requirement in any other SM, but YMMV.

Hi Radu,

The only connector I forgot to clean was the power supply one. After getting random re-starts I decided to clean that also. However, I think the power supply may be overheating so I am running it open with a fan blowing on the regulators to see if it restarts. Probably now is the time to do a full re-cap and swap all the regulators out. I will then start placing each option module in 1 by 1 to see if they are the cause of the restarts. This is an all absorbing repair!! Fix one thing and another random problem appears.

Can I ask whether you have removed the whole motherboard ? I would love to immerse the whole thing in water/vinegar to remove every pesky piece of grit. Mine was rather dirty inside when I got it.

Cheers

Richard

[SilverSolder]

I am sure you have already done this but I have found that giving each of the motherboard slots and edge connectors an intense clean makes a big difference.

Richard - definitely, my first breakthrough with this unit (after the fairly expected PS rebuild) was cleaning up the edge connectors on the cranky modules (contact cleaner, though alcohol first sounds like a good idea - as long as it's a single malt! ). Only then I realized the SM actually requires removing all modules and cleaning the interior of the unit EVERY 90 DAYS (see p.4-4). I don't recall seeing this requirement in any other SM, but YMMV.

Every 90 days is definitely excessive...   don't forget that disassembling/assembling the units introduces wear too!  -  in other words, if it ain't broke, don't fix it...

[Rax]

Every 90 days is definitely excessive...   don't forget that disassembling/assembling the units introduces wear too!  -  in other words, if it ain't broke, don't fix it...

Very good point, but I think it's a good indication how important it is to check this as a source of weird, headachy, manifold issues that don't make much sense (especially together). I've sorted out some issues on mine independent of Richard who sorted out issues with his by doing the same. Seems well advised, particulalry as the SM makes a clear note of this.

I don't think I'd do this cleaning every 90 days myself for the reasons you mention, but it'll always be the first thing I'll check on another broken unit I'd cross paths with.

[Kleinstein]

Cleaning every 90 days would be excessive, but cleaning after 90 months may be a good idea.
 Connectors are definitely on the list of more likely failure points, just like electrolytic caps and sockets for chips. If not moved they don't fail so often, but shipping often has enough vibrations to stress the connectors.  Beside the connetors itself there may also be solder joints at the connectors that many show failure from stress.

[bdunham7]

Can I ask whether you have removed the whole motherboard ? I would love to immerse the whole thing in water/vinegar to remove every pesky piece of grit. Mine was rather dirty inside when I got it.

I field-stripped my 8505A, including mobo out, did some repairs and cleaned everything including the connectors and the PCB connection pads.  It wasn't really dirty, but it has been trouble-free since.  It's a fair amount of work, but more time-consuming and demanding of patience and organization, not difficult and frustrating.  I would do that early on, even if you have to go back in it is a lot easier the second time!

[Rax]

Can I ask whether you have removed the whole motherboard ? I would love to immerse the whole thing in water/vinegar to remove every pesky piece of grit. Mine was rather dirty inside when I got it.

Hi Richard - no, I personally have not. I removed all internal cased modules and I pulled the PS. The front panel board stayed in place, and there's also a harness (forget which) that stayed hooked on the motherboard. In all honesty, as this is still an open project for me, I have not cleaned every and each module. But maybe I should and that may purge the few stray gremlins I still have hiding in the darkest corners of the unit.
Radu.

[1audio]

The plastic housings etc. are so fragile I think any board cycling needs to be for good reason. On one 8506a-ct I got the housings were more fragile than eggshells. Really disappointing, also with the dead AC converter.  -Does anyone have a converter module they would sell? This one had a fried converter chip- unobtanium.

[m k]

What chip, Amplifier U1?
Practical hands would construct an alternative in "no" time.
(I'm not that practical)

I don't think I'd do this cleaning every 90 days myself for the reasons you mention, but it'll always be the first thing I'll check on another broken unit I'd cross paths with.

90 days may be overkill for some but more sales for other and better "long" term accuracy.

Some cases you should be extra cautious.
Those are situations where quality is low and used metals are easily oxidised.

Worst case is that you wont be able to continue without new connectors.
Typical examples are a pin header and chip socket from '80s.
This also includes a spring effect of female part, it just couldn't hold it.
Back then far east started injecting stuff en masse and quality was not eighter needed or wanted.

Later came shrinking of everything.
That wasn't bad by it self but sometimes mechanical things couldn't keep up.
Like pins were too short or ribbon cable connectors were pressed badly or wire was too weak and so on.
Up side was that circuit boards were better and socketed chips were a small minority, sockets also were better.

[RichardM]

Hi All

Does anyone know what voltage lithium battery is needed for the Calibration Memory Module 384016 ? Mine is missing.

cheers

Richard

[Rax]

Does anyone know what voltage lithium battery is needed for the Calibration Memory Module 384016 ? Mine is missing.
Richard

Richard,
Here's a pic of mine (sorry, a bit blurry, but legible) and an excerpt from the SM. Not sure which part # applies to yours.
BTW, it holds 6V like there's no tomorrow, which is amazing to me given the age (and potential years of sitting in storage somewhere).
Radu.

[m k]

Is your manual a hard copy?
My pdf manuals are missing those pages.
Content is listed but pages are missing.

Whole picture of the module would be nice.

In case somebody happens to have that 02AT version of Thermal RMS, its picture would be nice also.

[RichardM]

Thanks Radu, much appreciated. It’s the last thing to repair on mine with any luck.

Richard

[RichardM]

Thanks Radu, much appreciated. It’s the last thing to repair on mine with any luck.

Richard

Not the greatest photo, only had the iPad.

[m k]

Much better than my manual picture, it's actually so bad that you can't really locate components with it, maybe Rax has a better page.
Other side of schematics is a bit better so included.

Your module seems to have 10k to ground, better, I don't like nicad.

Have you said you controller module number somewhere?
At some point the 8502A controller must have been updated so that Cal memory is included.
I can't see any other reason why Cal module is missing from every 8502A pdf manual I can find online.

BTW,
What was that high DC problem, a relay?

[Kleinstein]

From the list of options and the calibration instructions it looks like the 850x meter seem to support adjustment with old style trimmers as well. So the cal memory is actually optional and not all meters may have it.

It would make sense to integrate the cal memory into the main controller, this should be visible with a battery there. I somewhat doubt that had a EEPROM solution.

[m k]

My 8505A manual is Rev.2 2/91 but it's component pages seems to be written with solid typewriter style letters.
It's controller has U20 512x8 EEPROM X2804AP-45.
There's also a note that some jumpers are not installed on 8506A controller.
So no idea how old the controller actually is.

8502A manual Rev. 7/81 has a single module controller.
It's noted to still has an expanded memory possibility but included dual module PROM/ROM board has all normal ROMs.
That single module controller is noted to be included in 5100 series also so who knows what have been available.
(can't include the page, M$ W10 pdf printer hang, manualslib have them all)

8500A manual Rev. 5/82 then stiil has a Cal module and included schematics are from there.
(can be from documents.pub)

Found a separated option -04 file.

[RichardM]

I haven't established what the high DC problem was/is. I was also thinking it may be the K2 relay but it looks so fragile I was loathed to touch it. I may have to open the casing of the relay and try a bit of contact cleaner ? Has anyone opened these relays ? Above 20V my unit just seems to drift around at about 0.5 - 1V above the DC input voltage. That makes it unusable. Another thing I have noticed recently when all the modules are in I only get FFFFFFFF displayed unless I turn the filter on ?

I have attached a new image of the calibration module with my attempt at a 3D battery holder and my module numbers for those interested.

Controller 384024 (2 sided)
Fast R2 A/D Converter 383984
Current Shunts 383943
OHMS converter 383968
Active Filter 383976
AC/DC Converter (averaging) 383919
Signal Conditioner 383901

Richard

[SilverSolder]

I haven't established what the high DC problem was/is. I was also thinking it may be the K2 relay but it looks so fragile I was loathed to touch it. I may have to open the casing of the relay and try a bit of contact cleaner ? Has anyone opened these relays ? Above 20V my unit just seems to drift around at about 0.5 - 1V above the DC input voltage. That makes it unusable. Another thing I have noticed recently when all the modules are in I only get FFFFFFFF displayed unless I turn the filter on ?

I have attached a new image of the calibration module with my attempt at a 3D battery holder and my module numbers for those interested.

Controller 384024 (2 sided)
Fast R2 A/D Converter 383984
Current Shunts 383943
OHMS converter 383968
Active Filter 383976
AC/DC Converter (averaging) 383919
Signal Conditioner 383901

Richard

Since the error is so big, you could try tracing the DC voltage step by step (module to module) with another DMM, until it goes bad?

[RichardM]

I haven't established what the high DC problem was/is. I was also thinking it may be the K2 relay but it looks so fragile I was loathed to touch it. I may have to open the casing of the relay and try a bit of contact cleaner ? Has anyone opened these relays ? Above 20V my unit just seems to drift around at about 0.5 - 1V above the DC input voltage. That makes it unusable. Another thing I have noticed recently when all the modules are in I only get FFFFFFFF displayed unless I turn the filter on ?

I have attached a new image of the calibration module with my attempt at a 3D battery holder and my module numbers for those interested.

Controller 384024 (2 sided)
Fast R2 A/D Converter 383984
Current Shunts 383943
OHMS converter 383968
Active Filter 383976
AC/DC Converter (averaging) 383919
Signal Conditioner 383901

Richard

Since the error is so big, you could try tracing the DC voltage step by step (module to module) with another DMM, until it goes bad?

Ok thanks. I am pretty sure the error is at the first module (signal conditioner with telays) but will check again. The error is not that large at the output as it is divided by 8 or 64.

Cheers

Richard

[Rax]

Found a separated option -04 file.

Yeah, that's it... You beat me to posting it on here. Been "away" on a different project for a bit.

[m k]

We've got something white from above.

I haven't established what the high DC problem was/is.

Your Current Shunts module number is in 8505A manual somewhere.
Are its functions operating as they should?

The manual has a misleading Theory of Operation Figure 3-2 Analog Signal Flow.
Signal paths RT1 and RT3 should go to input side of the block.

DC amd AC voltage modules are an exclusive pair.
Then ohms modes can hijack DC input and current modes both.

So keeping multiple measuring situations connected when changing modes can reveal something.
But still, keep energy levels low, just in case.
Maybe easiest is Kleinstein's way, just putting something passive to inputs.

Checking voltages over relay contacts is also a way.
They should swap between something and nothing so fast that bad one must be obvious but I don't have experience how low currency does it.
High current is much easier, they finally just stop clicking, for a reason or another.
I have opened bigger relays, and cleaned, and filed and what not but it's always more or less unreliable and temporary or first aid solution.

You clearly need few meters to fix one.

[m k]

Now, on to Error 3 in ohms. I'm tempted to try replacing the ohms module - a hunch - although Error 3 blames the DC Conditioner.

If you get past Error 4 without Ohms Conv. and you have Current Shunts you can try it.
Richard's thread has a "picture" how the signal goes, it's the same with Current Shunts.

If there is still Error 3 you must have something wrong in mainboard and possibly that slot.

[m k]

All modules have two paths, singnal path and access path.
Errors are from access path and from its ACK or data in.

Error 3 means that since DC Sgn Cond is required the process can't continue.

Asumption, initially errors are happening in order, bigger number later.

So if Ohms is 4 and DC Sgn Cond 3 and error goes away when Ohms is removed the cause seems to be an address line.
If it is a data line then it should be visible earlier.
Active Filter(Error 2) uses same data lines as Ohms(Error 4).

DC Sgn Cond uses IC0, 3 and 4 when Ohms uses 1, 2 and 3.
Active Filter then uses IC1, 3 and 4 and is before those others.

IC line is active when it goes up but inverted in software.
The machine uses incomplete addressing so unused lines are irrelevant.
Software is also lifting only nesessary ones.

So IC1, 3 and 4 can go up.
Then IC0, 3 and 4 should go up.

So Ohms seems to be messing with IC0.
Easiest would be that eighter of Ohms slot connectors have something that shorts IC0 with IC1 when module is present.
Active Filter can pass since IC1 is up then, but next time it's down and takes IC0 with it.
Didn't check how possible grounding of IC0 could be manifesting.

[Rax]

All modules have two paths, singnal path and access path.
Errors are from access path and from its ACK or data in.

Error 3 means that since DC Sgn Cond is required the process can't continue.

Asumption, initially errors are happening in order, bigger number later.

So if Ohms is 4 and DC Sgn Cond 3 and error goes away when Ohms is removed the cause seems to be an address line.
If it is a data line then it should be visible earlier.
Active Filter(Error 2) uses same data lines as Ohms(Error 4).

DC Sgn Cond uses IC0, 3 and 4 when Ohms uses 1, 2 and 3.
Active Filter then uses IC1, 3 and 4 and is before those others.

IC line is active when it goes up but inverted in software.
The machine uses incomplete addressing so unused lines are irrelevant.
Software is also lifting only nesessary ones.

So IC1, 3 and 4 can go up.
Then IC0, 3 and 4 should go up.

So Ohms seems to be messing with IC0.
Easiest would be that eighter of Ohms slot connectors have something that shorts IC0 with IC1 when module is present.
Active Filter can pass since IC1 is up then, but next time it's down and takes IC0 with it.
Didn't check how possible grounding of IC0 could be manifesting.

Thank you, m k. Will ponder on this for a bit. The unit is for now on the shelf, parked temporarily essentially alongside another project, but I'm hopeful holidays break will mean I can toggle the projects a bit.

A reasonable explanation may be that I have to finish cleaning up the contacts on all modules. I've not messed with the motherboard inserts, just counting that the tiny excess from the edge connectors will condition those contacts too.