Need advice for a Fluke 8505A repair attempt

[Kinkless Tetrode]

Hi everyone,

I just got my long awaited Fluke 8505A from eBay today.  The seller did a nice job packing the shipment, but when I first picked up the unit I heard parts shaking around inside.  Not a good sign...    But being excited, and foolish, I immediately plugged in it and powered it up.  It started up just fine, no errors displayed, enumerated the option cards, and showed what looks like normal random measurements in Volts DC mode when no test leads are attached.  All the buttons respond on the from panel, but the measurement mode does not switch out of VDC mode.   

Ok, so when I opened up the case I found what was shaking around inside.  See image #1.

All the plastic clips and connectors on all the modules and option cards where shattered and the pieces were scattered throughout the entire unit like crumbled and broken potato chips.  There were no survivors.

The plastic fragments are just piled up everywhere, see image #6.   

It seems to me that these plastic clips holding in the modules have become brittle with age and when this unit was tossed around it all just crumbled apart.  The plastic front display is in good condition, so I am hoping that the damage is restricted to these plastic module frames, but I can expect to find cracked PCB, PCB traces, and solder joints.  I think that this 8505A wouldn't even power up at all if Fluke didn't build this thing like an armored tank, but I remain hopeful.

I would like to try to reconstruct the plastic tabs and frames on the module boards, as much as possible with superglue, before I move on to the electrical troubleshooting of the unit.  Unfortunately, like restoring a dinosaur fossil, I don't have any idea of what an unbroken module board plastic frame should look like.  Are there any fellow 8505A owners here who can post a photo of a good module frame?  It looks like each module has a long plastic cover that flips open to allow trim pots to be adjusted when the unit is serviced, but I am not sure.

I'd appreciate any advice the experts here can give me to get started fixing this 8505A.

Thanks,
KT

[MadTux]

Plastics on those Fluke 8505A/8506A is absolutely terrible.
Also got one in similar state, broken crap plastics all over.
Found some aluminium U-stock that I milled down to size as rail replacement, but there is unfortuately little space for rivets or similar fastening methods. Probably will spot-TIG-weld the rails to the aluminium frame, as soon as I get a working AC/DC tig welder.
Fortunately, have enough working HP/Keithley DMMs, so it's not a priority problem ;-)

[bdunham7]

I have an 8505A project and an 8500A that I bought for parts.  As luck would have it, the "parts" 8500A works absolutely perfectly, but doesn't do me much good for parts because it doesn't use the top clip arrangement at all.  The modules just slide in and IIRC, they are just held down by a pad inside the top cover.  Primitive, but in the long run, more practical. You probably don't have broken circuit boards, etc--but the module shells may be a bit crispy so be careful handling them and especially flipping the covers up for calibration.  My advice for now is just get all of the shards out and put them in a tupperware container for later.  Then just slide the modules in and get the unit to the point of working properly before worrying about the clips.  I'll try to find time to get you some photos if someone doesn't beat me to it.

[bdunham7]

Here you go.  You'll see I have one missing ,but there are two "spares" in the empty third-from-the-back slot.  You may find your best examples there.  As long as the channel parts are salvageable, I wouldn't worry about the top clips--your's are pretty far gone.  As far as how they work, you'll see that the clip for the module I've removed is slide straight out away from the center.  That's how they work--they just slide horizontally in and out.  The covers are flipped up by carefully prying where I've indicated.  They're crispy, and most of mine have been cracked or damaged over the years.

[Kinkless Tetrode]

Plastics on those Fluke 8505A/8506A is absolutely terrible.
Also got one in similar state, broken crap plastics all over.
Found some aluminium U-stock that I milled down to size as rail replacement, but there is unfortuately little space for rivets or similar fastening methods. Probably will spot-TIG-weld the rails to the aluminium frame, as soon as I get a working AC/DC tig welder.
Fortunately, have enough working HP/Keithley DMMs, so it's not a priority problem ;-)

So true, it is always good to have spare DMMs around.  There can never be too many DMMs on our bench.   

Sorry to hear that you have broken plastic also.  I hope your metal fix works out on those rails.

[Kinkless Tetrode]

I have an 8505A project and an 8500A that I bought for parts.  As luck would have it, the "parts" 8500A works absolutely perfectly, but doesn't do me much good for parts because it doesn't use the top clip arrangement at all.  The modules just slide in and IIRC, they are just held down by a pad inside the top cover.  Primitive, but in the long run, more practical. You probably don't have broken circuit boards, etc--but the module shells may be a bit crispy so be careful handling them and especially flipping the covers up for calibration.  My advice for now is just get all of the shards out and put them in a tupperware container for later.  Then just slide the modules in and get the unit to the point of working properly before worrying about the clips.  I'll try to find time to get you some photos if someone doesn't beat me to it.

Here you go.  You'll see I have one missing ,but there are two "spares" in the empty third-from-the-back slot.  You may find your best examples there.  As long as the channel parts are salvageable, I wouldn't worry about the top clips--your's are pretty far gone.  As far as how they work, you'll see that the clip for the module I've removed is slide straight out away from the center.  That's how they work--they just slide horizontally in and out.  The covers are flipped up by carefully prying where I've indicated.  They're crispy, and most of mine have been cracked or damaged over the years.

Thanks very much!  This is exactly what needed.   . It is fortunate that the plastic shells covering the modules do not play much of a functional role.  Good idea to save the pieces and puzzle them together later on. I can now clean it up and move on to getting the unit to take measurements.  Maybe a little DeOxit will do the trick.  ...fingers crossed. 

Thanks again for taking the photos, they're a great help.  I shall post updates on my repair journey.

KT

[SilverSolder]

Looks like the unit might have taken an impact from above?  Does the top cover show any signs of that?

[Kinkless Tetrode]

Looks like the unit might have taken an impact from above?  Does the top cover show any signs of that?

Yes, good observation, the top metal lid is quite roughed up. The surface is scratched, dented, and gouged in some places.  It appears to have had something heavy placed on top of it, perhaps it has been in storage for a long time.  I hope the PCBs are still held together and in tact.

KT

[SilverSolder]

Devcon 22045 Plastic Welder has rescued many broken plastic parts for me.  It smells to high heaven, but it really works - rough up the plastic a little first.  I'm pretty sure it would be strong enough to repair the clips...

[Kinkless Tetrode]

Devcon 22045 Plastic Welder has rescued many broken plastic parts for me.  It smells to high heaven, but it really works - rough up the plastic a little first.  I'm pretty sure it would be strong enough to repair the clips...

Thanks, I will look for that adhesive and give it try.

[Kinkless Tetrode]

Hi All, an update.

The clean up of all the broken plastic fragments has led me to take apart all the modules and circuit boards in my 8505A.  Following the procedure in the Fluke service manual, I have all the boards removed except for the motherboard.  But I have reached a road block.  In order to remove the motherboard from the system frame, the procedure is to remove the two front handles at the left and right corners of the 8505A.  There are three screws mounting the handle, but it looks like access to these screws will force me to peel away and destroy a plastic label of the Fluke company logo that is glued on covering the screws.  Please see the photos.

I really want to avoid damaging the Fluke label, so does anybody have any ideas what I can do?

BTW, this Fluke product is seriously built with quality by design.  Every screw that I removed, except for the ones for the case covers, was dipped in a Loctite compound to prevent any of them from coming loose.  And, the power supply boards are shielded within their own compartments within the system frame with heavy gauge steel.  Serious Quality with a capital Q!  Also, the 8505A design is totally modular and all the action takes place in the module boards.  The motherboard is pretty much just an isolated backplane bus connecting all the modules together.  Too bad all the modules had to be contained in the brittle plastic clam shells.  This meter has taken a lot of abuse and sat in pretty grimy conditions through its history.  Luckily it is constructed so strong.  The crushed BNC connector on the rear panel shows how much damage it has suffered, see the last three photos.

Also, I want to give the all the circuit boards an alcohol bath.  I this a good idea?

Thanks

[bdunham7]

I don't know if removing the motherboard is a good idea at this point--I haven't removed mine.  I just vacuumed it and blew it off.  Is yours visibly dirty or contaminated?  As for the Fluke labels, I did take mine off by gently warming them up, working a razor under the lower edge and very slowly pulling them off.  I'll use contact cement to reinstall them eventually.

Mine is on hold right now because I need to completely disassemble the right side to get at the front/rear and 4W ohms switches and circuitry.  I'd suggest you thoroughly troubleshoot yours before any more disassembly so that you know what to fix when you do take it apart.

[Kinkless Tetrode]

I don't know if removing the motherboard is a good idea at this point--I haven't removed mine.  I just vacuumed it and blew it off.  Is yours visibly dirty or contaminated?  As for the Fluke labels, I did take mine off by gently warming them up, working a razor under the lower edge and very slowly pulling them off.  I'll use contact cement to reinstall them eventually.

Mine is on hold right now because I need to completely disassemble the right side to get at the front/rear and 4W ohms switches and circuitry.  I'd suggest you thoroughly troubleshoot yours before any more disassembly so that you know what to fix when you do take it apart.

Thanks, bdunham7.  True, I don’t need to remove the motherboard, I can just blow it clean.
The plastic module clips are all hopeless to repair because they practically crumbled to into hundreds of fragments that are hiding everywhere, but it’s all cleaned now.  You saved me from my own over zealous urge to strip down the unit.   

[SilverSolder]

If you take the bottom cover off, you can pretty much inspect the motherboard for cracks or other issues...   Don your optivisors and take a look at the connectors, to make sure they didn't get damaged when the cards were hit...   With just a little luck, there will be no problem on the mobo!

Regarding washing the boards in alcohol...   Unless there is a reason to do it (i.e. they look contaminated or there is reason to believe the surface is contaminated), is it perhaps less risky to leave them as they are?
 

[COSMOS2K]

Hello:

The fluke 8505A is a good machine, but not the best of its time. I have 2 units in operation and one HP3456A in my laboratory, all work perfectly, but the 8505A take a long time to stabilize your measurement but the HP is almost instantaneous.

My units 8505A are 100% operative and with all the options installed, although there are some of them that are quite difficult to calibrate since calibrators are needed that in AC can generate 1000V at 500KHz to calibrate the Option 09A_AC RMS Converter.
If your intention is to calibrate it remember that in each adjustment field you have to delete the previous calibration data, this is explained in the manual.

Regarding the plastic containers, the same thing happened to me, just be patient to repair it, I would achieve it with Loctite and plastic pieces added, also mixing the cyanoacrylate with bicarbonate to fill. It is important that the covers are in place if you want to maintain a good stability in the measurements since these are connected to the ground through the copper tongue when closing them, and this is very important, as is the spring that makes contact with the aluminum shield, this is essential, I say it from my own experience.

I really use the HP3456A on a regular basis, it is more stable and accurate in the measurements, in fact the internal reference voltage in this is much more advanced and stable and ready to measure almost immediately, while the 8505 takes a long time to stabilize . I use it for intensity measurements and for simultaneous measurements when necessary.

As it is said in this thread, remove the lower cover and check the connections of the PCB connectors that may have deteriorated when hitting the top cover.

I will continue reading this interesting topic.

Sorry for my English.

Greetings.

[Kinkless Tetrode]

If you take the bottom cover off, you can pretty much inspect the motherboard for cracks or other issues...   Don your optivisors and take a look at the connectors, to make sure they didn't get damaged when the cards were hit...   With just a little luck, there will be no problem on the mobo!

Regarding washing the boards in alcohol...   Unless there is a reason to do it (i.e. they look contaminated or there is reason to believe the surface is contaminated), is it perhaps less risky to leave them as they are?
 

Good advice indeed!   

Of course, I was just too curious, and against perfectly good advice, I took the motherboard out and stripped the unit down to its bare metal frame skeleton.  I took a chance and carefully peeled off the FLUKE labels on the handles and it turns out that the labels are quite thick and the adhesive remained strong while leaving no residue on the plastic of the handles.  The labels were very easy to put back into place and it does not look any different than before.  So it was a very nice surprise.  Then, I was able to remove the hidden screws, removed the front frame, and released the motherboard out of the main frame.

The motherboard looks in great condition and carefully inspected it for solder problems but everything was nice and shiny. Skipping the bath, I gave the motherboard and air blast shower and then put it back into place.

**** Note for 8505A owners:
The 8505A has a wonderfully modular design and built of quality (non-plastic) material.  But the biggest design flaw that I can see is with the mounting of the Front/Rear Switch PCB Assembly with the motherboard on the right front part of the unit.  This F/R Switch board hides in its own shielded compartment on the right side.  Taking it out and putting it back into place was the most complicated part of the teardown process.  There is little space and no room for error.

When removing the F/R Switch board, first remove the three vertical column of buttons and their extender rods before pulling the board out from the back.  Be very careful, it will feel like the plastic extender rods will break, and there is a good likelihood that they might, but pull with a small twist to disconnect it from the pushbutton switches to which they are attached. (the plastic holding the spring actually fractured on the pushbutton for my Front/Rear input button, but the button still works so I shall leave it be.)  Then, to release the F/R switch board from the motherboard sockets you must pull up the entire board using a tab protruding in the front next to the three vertical buttons on the far right side.  Pulling up releases the board from the sockets (there are two sets, J3, J4, and J5, J9 on the schematic) of the motherboard allowing you to pull the F/R switch board out the back.  These sockets were a problem for me.

I could not easily reseat the F/R Switch board back into these motherboard sockets in the front.  They just would not insert back into the socket and there was little to no room to move it around.  You could only hold the board from the small tab in the front and the input connector in the back.  So I had to remove the motherboard and see why the F/R board would not seat into the sockets.

It turns out that socket J4 has a plastic block in blade position 2 that slides into a groove in blade position 2 of the F/R board.  The block ensures proper positioning of the board with the motherboard socket.  You can see this block in J4 of the motherboard in page 5-10 of the 8505A manual.  Well, looking closely, this small plastic block was worn down and had gotten wider from past removal and insertion of the F/R switch board into and out of the frame over the years.  The block was "blocking" the board from being reinserted into place.  Fortunately the motherboard was removed and I was able to squeeze this worn down plastic block to a smaller size with a pair of needle nose pliers and then the F/R switch board was able to slide back into place. A squirt of DeoxiT helped lubicate things and it finally fit back into place.

I had to remove the motherboard to fix this and I would never have been able to put the F/R switch board back into place without the motherboard removed.  The official procedure is to slide the F/R switch board back in from the back of the unit and blindly wiggle it around while holding the board from the back and the small tab in the front.  It is obvious to me that from the amount of wear and tear on the plastic block on J4 that other people had similar problems with it in the past and mashing down the plastic block in socket J4 in the process.  IMHO, this is the only real defect (a maintenance defect) with the 8505A, besides, of course, the totally awful crispy plastic that is used to contain all the modules.   

I hope this is useful to other 8505A owners.
*****

So, I was finally able to put my 8505A back together once again.  The power came back on and it all looked as before.  I cleaned all the connectors on all the PCBs with DeoxiT and I notice that the LED displays are much brighter than before.  Very nice.

But it still couldn't switch out of V DC mode.  The entire row of Function pushbuttons on the front panel had no effect.  So, I tested the panel buttons (with my trusty little Fluke 101 DMM) and the buttons all tested good.  Next I tested the diodes next to the buttons and found one that appeared to be open in both directions.  Odd... So turning power back on, I tested the diode for voltage and found zero voltage, so I pushed on it a little harder to make better contact.  At that moment I started hearing relays clicking and the Ohms reading came alive.   

Turns out diode C4 on the Display PCB was cracked in half and was an open circuit.  Pushing on it with my test lead caused it to conduct and make everything come alive for a moment.  The cracked diode disabled the entire row of Function pushbuttons on the front panel.  Looks like a clean crack in the glass of the diode.  I am guessing due to the past impacts this little machine has suffered.

So now I have a functional 8505A that can make all DC measurements.  Next problem, the system is not recognizing my AC Converter module for AC measurements.

And, the story continues...

[Kinkless Tetrode]

Hello:

The fluke 8505A is a good machine, but not the best of its time. I have 2 units in operation and one HP3456A in my laboratory, all work perfectly, but the 8505A take a long time to stabilize your measurement but the HP is almost instantaneous.

My units 8505A are 100% operative and with all the options installed, although there are some of them that are quite difficult to calibrate since calibrators are needed that in AC can generate 1000V at 500KHz to calibrate the Option 09A_AC RMS Converter.
If your intention is to calibrate it remember that in each adjustment field you have to delete the previous calibration data, this is explained in the manual.

Regarding the plastic containers, the same thing happened to me, just be patient to repair it, I would achieve it with Loctite and plastic pieces added, also mixing the cyanoacrylate with bicarbonate to fill. It is important that the covers are in place if you want to maintain a good stability in the measurements since these are connected to the ground through the copper tongue when closing them, and this is very important, as is the spring that makes contact with the aluminum shield, this is essential, I say it from my own experience.

I really use the HP3456A on a regular basis, it is more stable and accurate in the measurements, in fact the internal reference voltage in this is much more advanced and stable and ready to measure almost immediately, while the 8505 takes a long time to stabilize . I use it for intensity measurements and for simultaneous measurements when necessary.

As it is said in this thread, remove the lower cover and check the connections of the PCB connectors that may have deteriorated when hitting the top cover.

I will continue reading this interesting topic.

Sorry for my English.

Greetings.

Hello, COSMOS2K.  Thanks for the advice.  Your English seems perfect to me.  I am sorry to hear that you have problems with the brittle plastic also.  Yes, I also noticed the copper tongues on the lids of the module containers.  I will be sure to close the lids with the proper grounding contact.  Thanks.  The metal used is strong, but the plastic used is very weak.

I completely agree with you about the 8505A measurement speed.  I have only used it a little since I have it working now, but you are correct, it does take some time to settle down for a measurement, at least compared to my HP 3457A.  Your HP 3456A and my 3457A are very much faster to take a measurement and it is very noticeable.

It sounds like you have good experience performing a 8505A Calibration.  Once I get the Option 9 AC Converter working I would appreciate your good advice on how to get my unit calibrated also.  I am still reading the instructions on how to perform a calibration.

I also find it very amusing that I had to use my little Fluke 101 DMM to fix my Fluke 8505A.  The little brother helped restore his bigger older brother.   

Thanks,
KT

[Kinkless Tetrode]

At startup, my 8505A was not detecting my Option-09A AC RMS Converter and it was displaying Error 9 when I press any AC functions.  So I was all getting ready to fire up the oscilloscope and trace the power supply into the module.  But I happened to be looking at Section 8, page 8 in the 8505A instruction manual, and it shows a module slot table of valid slot positions for each of the modules.  Well, of course the Option-09A module was plugged into slot C. Slot C is reserved for the DC Signal Conditioner module and it is an invalid slot for the -09A module. Moving the DC Conditioner into slot C and moving the -09A module into slot D solved everything.

The 8505A now starts up recognizing the Option-09A module.  Golly, sometimes reading the manual takes all the fun out of fixing things....   

Now I have to learn how to perform a calibration on this 8505A.

Thank you everyone for all the good advice to get this DMM fixed and running!
Total repair part cost is $0.15 for a 1N4448 diode.

[SilverSolder]

Nice one!

These units can be calibrated both in hardware and software. 

One thing I've noticed is that the active filter modules are prone to dropping the reading by several uV when switched in...  I haven't had a chance to troubleshoot yet...

To see this effect in action:   just switch on Averaging and watch the reading drop a little...   

[Kinkless Tetrode]

Thanks!

I haven't played with the filter modules yet, but I will definitely give it a try and look out for that uV drop.  That is very odd.

Still swimming through the manual to review the hardware and software calibration procedure.

[SilverSolder]

As "luck" would have it, my unit has a number of strange problems, one of which is that the Guard terminal does not connect the GUARD and LO terminals when you push the Guard switch when measured from the front panel...   but, when you push the switch, the instrument behaves as if the LO terminal gets connected to chassis ground!  Comparing with a "known good" unit, this behavior is indeed cryptic, to say the least.

I took the lid off the unit and saw the challenge that Fluke has created for anyone wishing to take out the Front/Rear Switch assembly...   and then remembered your post, @Kinkless Tetrode, thank you for the detailed explanation!

[Kinkless Tetrode]

As "luck" would have it, my unit has a number of strange problems, one of which is that the Guard terminal does not connect the GUARD and LO terminals when you push the Guard switch when measured from the front panel...   but, when you push the switch, the instrument behaves as if the LO terminal gets connected to chassis ground!  Comparing with a "known good" unit, this behavior is indeed cryptic, to say the least.

I took the lid off the unit and saw the challenge that Fluke has created for anyone wishing to take out the Front/Rear Switch assembly...   and then remembered your post, @Kinkless Tetrode, thank you for the detailed explanation!

You’re very welcome!  I am pleased to hear that it has helped a fellow 8505/6A owner. 
And I look forward to hearing what else you find with Guard behavior.

BTW, I decided to skip the 8505A calibration after all because the DCV measurements are within about 100 uV of my HP 3457A and a few mVs of my Keithley 197.  A blind leading the blind calibration (and shameful for a wannabe volt-nut), but good enough for my tasks. The 8505A is one awesome DMM.

[SilverSolder]

Thank you Kinkless,  the Guard problem turned out to be related to the same problem you found:  a previous visitor inside the 8505a had broken the little plastic guide for the slot in the motherboard connector for the Front/Rear Switch Assembly.  Sadly, a tiny broken speck of debris from it was laying at the bottom of the slot, preventing the card from seating properly.  And yes,  I had to tear it down to atoms (almost), including removing the motherboard to even see the problem - let alone fix it.




I simply left that guide out altogether - it is far more trouble than it's worth,  it seems to me - and fitted the card carefully before tightening the three side screws.  Success! - the guard now works.

The ground issue...  is a little embarrassing.  I had used a screw for one of the instrument feet that sadly, protruded just far enough into the case to touch the guard... and grounding it.  Don't ask how long it took to diagnose that...

The net sum of it all...   a smiley faced 8506A behaving much better.

The patient was the top meter in the picture!




These were both adjusted ("calibrated" is going too far for my crude workshop) about 6 months ago.  The two seem to agree within about 5ppm most of the time.  They are not normally this close, they are on best behavior today after the surgery! :-)

[Kinkless Tetrode]

The net sum of it all...   a smiley faced 8506A behaving much better.

The patient was the top meter in the picture!

These were both adjusted ("calibrated" is going too far for my crude workshop) about 6 months ago.  The two seem to agree within about 5ppm most of the time.  They are not normally this close, they are on best behavior today after the surgery! :-)

Great to hear that the Guard is now resolved.  Very good catch, that sounds like a very tricky problem.  Definitely not an obvious fix at all.

Those are a couple of sharp and great looking 8505A twins!!  They are an ideal looking pair stacked one atop the other.  They read so closely that one cannot tell which of the two had the surgery. 

Nice job!

As for my 8505A, I hope to get it talking via GP-IB. That is my next project with it.

[COSMOS2K]

I've been absent for a few months, but I liked reading this thread again.

This is because of some old tricks and of course because of some instability of the ancient Fluke 8505A.

Well, because I want to leave the Fluke 8505A in the bank and remove the HP3456A since the latter despite being much more accurate and stable, its exterior furniture is much larger and takes up more space logically.

Because of this I have started to review the 8505A and I have discovered several failures, although some do not consider them really important if they are, especially in machines of certain years as is the case. The 8505A that I have has some instability in the measures with respect to the HP, so something is happening in the Fluke
.
Inquiring into the different modules that compose it, it is clear that the option modules are not the culprits such as the Ohm, Intensity, DC and AC Voltages module or even the AC / RMS module, because in reality instability is general regardless of the module that is being used Logically instability and as a scoop is within the indispensable modules of the equipment such as Fast ADC Converter and Active Filter.

The ADC has a lot of adjustments that also according to the manual have to be repeated several times because (example) the last setting changes the previous settings and vice versa therefore it is necessary to repeat, average and verify previous steps so that the calibration is perfect, so Otherwise, it will not measure with the same precision at different scales.

For this, the ADC has integrated the voltage reference of 7,000000 V, which is quite difficult to measure because when connecting any load it interferes easily having to do it with probes a bit special. In addition there are a series of intermediate adjustments to provide a good linearity since the resistances for reasons that we already know do not give it, so it carries these adjustments that are made exclusively on the 10V scale and with the calibration switch activated providing in this mode 7½ digits so we have better control of the settings.

Obviously this multimeter has a tendency to instability due to design reasons, logically compared to an HP3456A. These instabilities especially in the measures of tension have no solution, but I assure you that I will try to solve them in the best possible way just for leisure and slap a little.
If we look at its construction we realize that the signal that enters through its bananas goes a long way with a shield I would say lousy, and logically it goes to each of the measurement modules that it has installed. In case it reaches all modules, but not before going through the front panel PCB connections, then to the rear / front selector and guard PCB, it is a long way for a voltage of mV or uV, since all connections produce extra-thermocouple voltages that add or subtract from the voltage to be measured, obviously in voltages of some volts go unnoticed.
This logically is only by poking after work and entertaining, since I have 2 8505A units one of them cosmetically perfect, but both work correctly and are calibrated, so I will try it in the older unit.

My idea is to clean and improve all the contacts that come from the banana connections to the last module, passing through the front and through the 4 or 2 wire rear / front and guard selection card, as well as going through the main mainframe board , even being screened in the way that has been done, it seems to me insufficient since if the AC RMS module that accepts voltages up to 3 MHz is connected, all this network of distribution of signals to be measured will suffer important alterations, and in fact it happens if we put on or under the multimeter other equipment that does not have its power supply on the same side as the 8505A. I have to try to put Kapton tape on the entire path with at least 2 layers as insulation and then copper adhesive tape connected to the ground or chassis and see the behavior. The two layers of Kapton is so that the insulation is in line with the tensions to be measured.

If this work goes well I will do it in the complete unit with all the options, and will return to the work table, because I am in love with this multimeter.

On the other hand it has been very difficult for me to measure the reference voltages, it gave me the impression that my HP3456A and the test leads interfered with that voltage reference, so I had to use an oscilloscope tip with a 4mm banana adapter, apparently the problem is solved, but I would like to know from your experiences if you have measured these tensions.

Regards.