Fluke 8506A , need help with repair  2.0 - Page 1
Forum section: electronics / repair

author: GigaJoe
author_rank: Frequent Contributor
timestamp: January 31, 2024, 12:37:24 am
content: if no reason to read everything here is shortcut:last - 2 switches on AC Volts attenuator board faulty , need replacement by , probably - TMUX6119https://www.eevblog.com/forum/repair/fluke-8506a-need-help-with-repair-2-0/msg5337092/#msg5337092I got another 8506A ,starting from voltage option ...only board inserted:-- DC signal conditioner-- Active filterinput shorted:-- all ranges OK around 0 ; but 200V ( DC sig. cond range X10) shows -11.3V ;input 100mV :-- all ranges OK, approx 0.1V ; but 200V -11.2VDC signal cond output , at all ranges correct result , it multiplying or dividing correctlyActive filter output matched with dc signal cond.I'm kinda lost ....... Is it A/D converter bador controller counting wrongi may suspect someone with calibration mode screw it upany ideas? help ....

author: bdunham7
author_rank: Super Contributor
timestamp: January 31, 2024, 01:41:21 am
content: Short the input, flip the CAL switch on, change the range to 200V and press the ZERO DC/OHMS button. See what you have and the flip the CAL switch back off.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: January 31, 2024, 03:02:33 am
content: yes !!!i actually dig the manual found the same sequence to erase constants ... in CAL mode; so volts back to the normal state ( that constants irrelevant now )Ohm board malfunctioning , my next step ....but AC board gives "HHHHHHH" on all ranges .... last time when I look at , it was some customs chips i'm worried it completely busted ....

author: GigaJoe
author_rank: Frequent Contributor
timestamp: January 31, 2024, 06:08:24 pm
content: Ohm board back in businessnow question:OP97 replace to: ( what i have )OPA197OPA189OPA188 (preferred option)ADA4522 - 1and i wonder , how bad it would be if I solder SOIC in the same way , a bit more accurate , and avoidind SOIC to DIP converterleaking seems crucial for this circuit for 100M range

author: Kleinstein
author_rank: Super Contributor
timestamp: January 31, 2024, 07:37:49 pm
content: Of the listed types the OPA188 is likely the best. Chances are it would work good for smaller resistors, but maybe not as good for very large ones. The OPA197 could be good for large resistors, but not good for the small one because of the drift / LF voltage noise.The leakage should not be that critical. Much of the bias/leakage current can be considered part of the test current. The really critical parameters are more the drift of the leakage / bias current and in part the low frequency current noise.The current noise specs in the OPA188 datasheet are too good to be true. I would still expect them to be slightly better than with the OPA189.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 02, 2024, 03:09:10 am
content: ok ... now it a dificult parts .. AC converteri'm not so much understanding in details, how it operates ... so need some helpthe output of AC conv. - 2.1Vit has 2 boards , so i disconnect it .and shorted input on second board - same outputhere is full schematic for amp board:https://i.ibb.co/1vTLqWC/8506-A-True-RMS-A11-A2.pngTP6 output +2.1Vhere is voltage map of front end amp. and frankly im a bit lostis Q1 faulty? Or receiving a wrong signal on right transistor as feedback ...or below "guard driver" with strange opamp ... it influence both Q1 ...

author: bdunham7
author_rank: Super Contributor
timestamp: February 02, 2024, 03:17:51 am
content: Quote from: GigaJoe on January 31, 2024, 06:08:24 pmYou can still get OP97 in PDIP8 from Rochester or fleaBay and that is what I would do. Otherwise I'd just use a DIP8-to-whatever adapter just for neatness.

author: bdunham7
author_rank: Super Contributor
timestamp: February 02, 2024, 03:33:12 am
content: Can you check the voltages at 1 and 5 of U1 (the non-ground sides of C6 and C7)? Does the voltage at pin 3 of Q1 vary if you manually select different ranges?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2024, 08:04:16 am
content: For the OP97 a more direct replacement would be a LT1012, that may still be available as DIP.For the AC amplifier it looks there is an issue forther down the amplifier, not necessary Q1. Q1 still gives out a large signal, that shoud saturate the output. Still the output is only 2.x V.Another point is that somehow some 5.8 V come from the FB divider. So there is likely something wrong too.There maybe something loading the amplifier too much pulling it way up. One should be careful not to desroy the thermal RMS converter part. Ideally there would be a way to isolate the amplifier from the converter part and only check the amplifier first. So far I don't see such a way.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 02, 2024, 04:31:54 pm
content: Thanks, for valuable input about opamp, but let postpone it for a while ( sorry im so driving to figureout with AC now ... ( I may have LT102 btw ... )Capacitors .. I simply removed .. caps seems ok , and voltage +15.169 ; -14.978 So power feed looks ok, and symmetrical.measuring output at TP6 as 2.1500 , it didn't change to last digit at all , when switching ranges from top to bottom ....?-1: may i consider that digital U4 fry and it throwing wrench to back feed with +5.X V on input Q1? ( im not certain how ... )?-2: Guard driver , how to removed it from equation? remove Q34 , U3,3 - ground ? (U3,2 - actually ground)?-3: how about cut U1,3 and loop it directly to Q1,3 - direct back feed , and should we see "0" ? - meaning that parts working wellSo voltages: (chip, pin)U1,3 2.228U6,9 2.148U6,6 2.1625U6,10 2.750U6,5 2.750U6,2 -0.344U9,6 2.1511 (output)I may wrong but it seems flowing from beginning to end ...UPD:- I disconnected digital cable between boards , so only ground are common , well slots as well ...Voltage drops to 1.94 - 1.96 and floating 0.5hz between - that completely differ behavior now , it never floats and was kind of steady slowly down at warmup ....if disconnected , U4 100K pullup resistors should set output to "1" ? , then fet switches should act .... still no precise picture in my head about feedback voltage ...( but this jumpy output correlated with floating U4 input ..... that 8 mosfet on amplifier switching leaking +5 from U4 high output ?)( BTW , +2 volts then going to board called DC signal conditioner , that board X10 , so input to DAC +21V , above DAC 20V range, and 'HHHHHHH' blinking , seems AC board operate within +- 2V range , so maybe it saturated )

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2024, 07:06:01 pm
content: A broken U4 chips is a possibility, though to backfeed it would also need a bad diode used JFET (Q13,Q15,Q17,Q27,Q35). I don't think this is the issue.Another possiblity would be that all 5 channels for the feedback are turned off, e.g. U4 outputs stuck at a negative output. The Q1 input could than drift up from leakage current.The guard driver could also pull up the FB path. What is the guard output voltage (U3 pin 6) ?The voltages at the thermal converter (U6) at least look good, so the converter seems to work as intended. Pins 5 and 10 with the same voltage suggests that the control loop is balanced.

author: bdunham7
author_rank: Super Contributor
timestamp: February 02, 2024, 07:50:40 pm
content: I would recommend you back up a step and try going through the AC converter calibration steps starting on page 4-21 of the manual. Ignore the equipment requirements and just make do with what you have. Go through until you get to the step that you cannot complete. I'd think that this would probably be the second step, "amplifier zero", but you never know--it might fail at step one. Knowing this might help decide where to go next.Next, maybe go through the troubleshooting table starting on page 4-39 with the same idea--when you get to a dead end, you might be in the area where the trouble is.I'll try to think about this a bit more when I have time. I have two of these units, one working great and the other not (and beyond all hope), so I am curious.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 02, 2024, 08:42:41 pm
content: consider I move everything back as it was ...then it blinking 'HHHHHH' on every range - ( even 1000 V) with shorted input , basically overload ...what kind of calibration are we talking about ?even now with +1.9V output it shows 650 V on input ....what i will do:I will remove U4 and U15 , test it standalone , and leave it , then all input would be highU6 in socket - I will remove with caution, it will not affect the circuit ... so voltage to control would be U1,3 or TP3with all input high , my understanding would be lowest gain for amplifier , and voltage across all fet switches suppose to be equal as all of them in "ON" state( im even thinking to short U1,3 and Q1,3 ( cutting backfeed circuit off) - in my understanding , it should be "0" to verify Q1, Q2, U1, U3, U5 - all this circuit OK ) if not, still half of circuit eliminated )

author: bdunham7
author_rank: Super Contributor
timestamp: February 02, 2024, 08:52:57 pm
content: Quote from: GigaJoe on February 02, 2024, 08:42:41 pmRead the steps. The first portion of the calibration procedure is not calibration, but rather adjustments of various circuits like the ground equalizer and amplifier zero, etc. It's not tweaking pots or setting constants. If you can set the ground equalizer as per the instructions, that rules out a certain portion of the circuit, then the same thing for each successive step.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 02, 2024, 09:16:02 pm
content: listen, i'm perfectly understand it ...but it not the case when you have 2.1V output and ac shows "HHHHH" as overload statelike R35 balance "0" as microvolts on output .... not really when 2Vthe AC board in a broken stage currently

author: bdunham7
author_rank: Super Contributor
timestamp: February 02, 2024, 10:23:55 pm
content: Quote from: GigaJoe on February 02, 2024, 09:16:02 pmYour DC volts works correctly, right? Reread that first step about the ground equalizervery carefully! The AC output being broken has nothing to do with it and you are not using the AC reading for this step.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2024, 10:44:46 pm
content: I also think the AC board is somewhat broken. Removing U6 from the socket could be a good step, to bring it to safety. A faulty amplifier may overload the converter, though there are 2 safety mechanisms.Chanes are the part around U5 is currently active and protecting U6.With U6 removed it would be about debugging the AC amplifier first, not worring about the ADC and RMS converter part.Adjusting the zero balance for the AC amplifier will anyway most likely fail - so chances are that the path follwing the service manual / cal instructons would not be such a different path.The logic way for the amplifier offset adjustment should be done for 0 V at TP3, not a 0 V RMS reading at the ADC.Form the readings shown in the plan, there is a chance the U1 could be the problem - with a higher voltage in the FB the output shoud be negative, not positive.Sometimes if DC readings don't make sense this could also be an oscillating amplifier.The ground link is a good point -- seems to be a weak point in some Fluke instruments. So definitely wirth a check.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 03, 2024, 12:51:42 am
content: I did look at very beginning, to find a measurement ground, that "ground equalizer" thingi , very top small amp U13, that adjust AC ground over that white Teflon wire connected separately ... , due to floating ground. it gives me 20+ microvolts different. I consider it healthy operational , non adjusted.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 07, 2024, 01:38:49 am
content: Can it be Q2 - faulty? same voltage on left side ?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 07, 2024, 08:06:33 am
content: One can not exclude a problem with Q2, but the values look more like an issue with Q1 or Q4.Are the voltages checked with a scope ? Sometime an amplifier oscillates and this way can give rather odd looking DC readings that don't make any sense.

author: m k
author_rank: Super Contributor
timestamp: February 08, 2024, 10:44:56 am
content: Quote from: GigaJoe on February 02, 2024, 09:16:02 pmIs the input side sane, what is the real input voltage from the attenuator, can you change it from outside?Low left of Attenuator schematics, right of those relay coils is that U17, that is selecting attenuator buddies for relays, only one should be active.Low left of Amplifier schematics has switching drivers, those are also quite easy to check against the range table.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 08, 2024, 02:34:50 pm
content: there is no input , both coax conn. disconnected.10K resistor - shorted inputdigital disconnected, U4 removed ,U6 - removedQ26 - shortedat U1,3 - output 2.8V ( or something )currently Q2 is suspected , will replace it by 2 regular will see result ...and need remove Q34 so this circuit will be isolated ...

author: Kleinstein
author_rank: Super Contributor
timestamp: February 08, 2024, 02:55:34 pm
content: Not having U4 is a problem as this controls the JFET switches and would have them on.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 08, 2024, 03:01:09 pm
content: that why Q26 shorted - 10k in feed back.anyway disbalance on input pointed to this area ...let me play a bit ...

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 08, 2024, 11:54:41 pm
content: Q2 ,Q5 appears to be good, transistor tester recognize it , measure coefficient .. seems OKI remove Q1 , carefully shorted all legs first wit intention to test it , ( can i hook it in transistor tester '' or how to test it right way )then i remove Q3 Q4 .... I dont know ( i'm so dumb . or that thingi so special , or it just failure ..... )transistor tester recognize it as 2 resistors .... with different value for each transistorsDMM measure diode :C - E - 0.7V one way 2.4V opposite way ; between transistors that voltage also differentB -E - 0.3V one way for one and 0.6 for another ...is it something special? i can not findout any description about ..... and how to test it ...

author: Kleinstein
author_rank: Super Contributor
timestamp: February 09, 2024, 10:11:26 am
content: The test readings for Q3 / Q4 looks suspicious, like bad parts.I don't think the transistors need to be special types. At least for a first test normal types like 2N3906 should be OK. One may want a reasonable high ft and good gain.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 09, 2024, 02:33:15 pm
content: how it looks like , depend on how it sit onit def, not a transistor any more ....second oneit also has unusual pinout - BEC?: can I test Q1 fet in transistortester - without damage ?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 09, 2024, 05:35:49 pm
content: It depends on the tester if one could also check the JFETs and if they are safe (not too much current or voltage). The better ones should also be OK with JFETs, especially those with a relatively low threshold as typical for amplification. Some of the cheap ones can't detect the higher threshold ones (e.g. some 2N4391).Q1 should be 2 separate FETs to test.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 09, 2024, 05:49:23 pm
content: so ...if i hook it over 10K resistor on 10V let say .then gradually apply up to , let say 4V over 100K resistor , i have change in voltage drop on transistor. and that curve should be symmetrical for both transistors ? something like this

author: Kleinstein
author_rank: Super Contributor
timestamp: February 09, 2024, 07:26:16 pm
content: With just a resistor on one side the resistor should be on the source side, that is the more negative side. The 100 K would be protection for the gate and one could get away with a fixed gate at ground. The rising source voltage sets a suitable negative gate source voltage. The JFETs are usually symmetric and source and darin are usually pretty symmetric.Depending on the source resistor one gets more a test on Idss (with a small resistor) or on the threshold (with a large resistor). For a good pair both are correlated and should match.For a working JFET pair one should see a voltage drop of some 1 V over the source resistor with good symmetry between the FETs. The actual voltage depends on the threshold and could vary a bit: low frequency amplifiers usually prefer a low threshold like 1 V, while more RF amplifiers often have more like a 2-5 V threshold.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 10, 2024, 11:04:50 pm
content: so.. Q1 - that dual jfet (or whatever ... ) - it fry , left side - infinite resistance no matter what on input, right side - around 70 ohm - no matter what on input.trans. tester dont recognize pair as well ...label - DN1725any ideas for replacement ? even 2 x single, to get it started \ running, as i may expect something else get blown, then whole AC can get tossed awayand that rusted U1 amp .... (gosh ... )is it possible , just throw away whole this schematic Q1 , U1 , and hook some modern opamp, and simply adjust amplification level that this circuit suppose to? it seems the easiest way to fix it ..

author: Martian Tech
author_rank: Regular Contributor
timestamp: February 11, 2024, 03:18:41 am
content: Some specs on that DN1725 (fromhttps://www.wbparts.com/rfq/5961-01-242-9424.html):NSN 5961-01-242-9424N-CHANNEL JUNCTION TYPE-25.0 MAXIMUM DRAIN TO GATE VOLTAGE50.00 MILLIAMPERES MAXIMUM GATE CURRENT500.0 MILLIWATTS MAXIMUM TOTAL DEVICE DISSIPATIONAlternate part numbers:Fluke 495036Solitron FD1819Intersil ITS31129Not really very helpful though. I can't find any available (to a normal person) under any of these part numbers.Maybe LS844? (but only rated at 400mW max)

author: Kleinstein
author_rank: Super Contributor
timestamp: February 11, 2024, 09:24:00 am
content: The LS844 looks reasonable and it is available. 2 separate JFETs may be an option too - e.g. mmbfj309 would be a candidate. The amplifer runs the JFETs with quite some current.To reduce the power one could likely reduce the current a little.With new FETs one would have to change/ trim the R1-R6 and the R10-R13 parts too. These are there to trim the TC and offset. So matching of the FETs should not be that critical.Replacing the amplifier with just a modern OP-amp is tricky, as the amplifier also includes the protection part around U5.If really needed one could think about replacing U1 and Q3+Q4 with an NE5534 (the compensation part has the intermediate signal available for the protection). So one would still need the front-end JFETs.One may get away with a 2 stage OP-amp solution, with an fast JFET OP for the input and NE5534 for the output.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 11, 2024, 05:41:20 pm
content: Great ... Thank you !!! LS844 seems looks quite fit consider current circumstances ....but ... before going to get JFET, i just realize, I didn't test first half of AC board ... A1, where all attenuators.If AC board installed ... even in a current condition. the control signals and input still coming to A1 board, means I should received sinewave output on J2 connector ..and ... switching ranges manually I should received attenuated signal on J2connector .... and (!!!) if I NOT ... then all going to the garbage bin , due to U5 , U6, U7, U8 are customized sealed bricks .... Ha!A1:https://i.ibb.co/JqS3frw/8506-A-True-RMS-A11-A1.png

author: Kleinstein
author_rank: Super Contributor
timestamp: February 11, 2024, 07:23:39 pm
content: U5-U8 are custom switch chips chips, but one could get away with a replacement - the manual gives the internal circuitry, that is not especially complicated. A change there would still requite a recalibration / trim of the capacitive part of the attenuators.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 15, 2024, 10:56:43 pm
content: so ... there is an attachment of schematic for attenuator board a bit above ...TP1 as ground ; J2 as output ...an output voltage on this connector : -19.XXXV , with exception on 30V range +0.5Vwhere it come from (-19V) ??my understanding U5 -U9 broken, or some of them , .... hmUPD:I disconnect Pin-3 on all commutators, the output:U5 0.0000VU6 0.0000VU7 -19.6VU8 +0.41V7,8 - in the garbage bin ..... so question of replacement .... is anyone knows a chip as commutator for small signals ? I think 5V P-P , 1Mhz, ttl signal on\offbtw, im kinda silly in chip switches, so basically i did click on TI web site, by lowest leakage and capacitance as --- TMUX6119 .is it really possible to hook it instead broken onesorry, have no any experience with such kinda chips at all ...beauty of malfunction fluke

author: Kleinstein
author_rank: Super Contributor
timestamp: February 16, 2024, 05:36:19 am
content: Getting -19 V looks odd: where is this voltage comong from ? I don't think the CMOS logic chips would be happy with Vss at -19 V. Are the +-7 V OK ?So there is still a chance that the problem is with the supplies somehow.The TMUX6119 looks at least reasonable. The supply would likely be the +-7 V. For the control signal one may have to see if the level is right.For any replacement of U5-U8 one may have the readjust the capacitors at the dividers.

author: m k
author_rank: Super Contributor
timestamp: February 16, 2024, 11:50:03 am
content: Guarded bus Vss is -20V, Vcc is -15V.