Author Topic: Fluke 5205a Repair  (Read 8115 times)

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Offline classicTEK

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Re: Fluke 5205a Repair
« Reply #25 on: October 17, 2015, 11:41:39 pm »
My apologies sir but are you saying that Q121 has been the issue the entire time or are you saying that we have a better clue now?

I am having a bit of difficulty understanding the language.

d
 

Offline classicTEK

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Re: Fluke 5205a Repair
« Reply #26 on: October 17, 2015, 11:49:39 pm »
I am learning so much about this gear and all of its operation.  It has been nothing less than a challenge at every corner, but well worth the effort and understanding of the design.  The next repair will be a walk down memory lane.

I am not looking forward to replacing Q121 as it is a matched pair with Q122 and to unknown specs...........and it is placed very awkward on the board.  This is the only reason I came searching for deeper possible causes and tried very carefully to word my findings as to not leave this information out.  I do have a fully calibrated TEK 576 Curve tracer to her\lp me nail down the match if needed, so that part is ok.

These are the other part of the circuit, at the input.



d
« Last Edit: October 18, 2015, 12:09:44 am by classicTEK »
 

Offline singapol

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Re: Fluke 5205a Repair
« Reply #27 on: October 18, 2015, 02:14:10 am »
Part replacement is a valid procedure but a "costly" one if one blindly go about it. What I am suggesting is understand how overload is triggered. As far as I know from the manual block diagram overload is from the preamp up to the tube driver V1. I have traced overload from J44 to J23 and it's accompaning schematic and it leads to the logic, slewrate sensor, input divider and input fet Q121. :o

You have to know the function of Q121 as per the manual. It is a "gate" for 5200A if used. It controls the input with it's associated circuits ( temperature compensation & gate drive ). you have to think like CSI the crime drama how it all started. ;D Forensic thinking? Ask yourself how would overload be triggered. Is it normal component failure or user induced by pluging 5200A and feeding 5205A an improper signal or voltage or stand alone usage.

PS the triggering ogfhigh votltage K1 relay can come from initial internal tests during turn on. Read minor and major fault and it's solutions.

EDIT: Also check for cracked solder joints with such old equipment as you go about inspecting any particular
circuit you think is suspicious. Actually it should be part of the initial visual inspection procedure. The fault may just be a simple circuit wiring break.


« Last Edit: October 18, 2015, 02:58:22 am by singapol »
 

Offline classicTEK

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Re: Fluke 5205a Repair
« Reply #28 on: October 21, 2015, 03:17:08 pm »
Just a quick note.............I have found a stash of (160) SD210's, they were really cheap.  When they arrive I will match two of them for what I believe to be the important feature, resistance from drain to source.  I think it should be 40 ohm as printed in sec. 3-59.  Only then will I entertain the thought of swapping out Q121 & Q122.  They are soldered to a VERY oldskool, huge ceramic lug that will sink a bunch-o-heat before letting go of the attached parts.  Thoughts?

Cheers,

david
 

Offline singapol

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Re: Fluke 5205a Repair
« Reply #29 on: October 21, 2015, 05:45:07 pm »
Just a quick note.............I have found a stash of (160) SD210's, they were really cheap.  When they arrive I will match two of them for what I believe to be the important feature, resistance from drain to source.  I think it should be 40 ohm as printed in sec. 3-59.  Only then will I entertain the thought of swapping out Q121 & Q122.  They are soldered to a VERY oldskool, huge ceramic lug that will sink a bunch-o-heat before letting go of the attached parts.  Thoughts?

Cheers,

david

I think gunning for Q121 maybe premature. :P It's important to know what indicator lights are illuminated
during the power on sequence. Do you experience what is in the the troubleshooting manual below?
If it is then checking steps 39 and 40 maybe the way forward.

Quote
Page 4-26

38  Are both the STDBY-RESET and OVERLOAD indicators illuminated and the FAULT indicator
   extinguished?

39  Check the Overload and Delay circuitry on the Logic Assy and it's input from the Overload
   Logic circuitry on the Preamplifier assembly. Repair as required then repeat the test
   begining at step 23.

40  When the STDBY-RESET, FAULT and OVERLOAD indicators are illuminated check the Overload Delay
circuitry on the Logic Assy and it's input from the Overload and Logic circuitry on the
Preamplifier assembly.Check the inputs to the comparators that control the Fault Gate and Trouble Flip Flop.
Repair as required and then repeat the test begining at step 23.

 

Offline classicTEK

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Re: Fluke 5205a Repair
« Reply #30 on: October 22, 2015, 12:56:04 pm »
It is VERY tricky to understand precisely WHEN the design engineers are refering to in the TS procedures.  Do they mean EXACTLY and ONLY when the warm-up period is complete or do they intend to mean when ALL of the initializing and power up and warm up process is complete?

I have ONLY the OVLD indicator come on at the END of the warm-up period, BUT then the fault indicator lights due to an OVLD condition existing for more than 1/2 of one second.

So I have been approaching the TS chart with this mindset as opposed to BOTH the OVLD and FAULT indicators coming on at the EXACT same moment after the warm-up period is over.

Also, the more I think about this issue I wonder...............is Q121 the ONE and ONLY SINGLE part that can go bad, causing the overload at the differential amp and be making me continue to look for bad parts in other parts of the circuits?  In other words, are we going round and round and round because something made Q121 go bad and consequently causing the OVLD or is there something else that is making the Q121 show a -360mV at the source and the 1mV at the drain?  What is causing the -2v at TP4 and the gate of Q121?
 

Offline singapol

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Re: Fluke 5205a Repair
« Reply #31 on: October 22, 2015, 05:19:02 pm »
It is VERY tricky to understand precisely WHEN the design engineers are refering to in the TS procedures.  Do they mean EXACTLY and ONLY when the warm-up period is complete or do they intend to mean when ALL of the initializing and power up and warm up process is complete?

I have ONLY the OVLD indicator come on at the END of the warm-up period, BUT then the fault indicator lights due to an OVLD condition existing for more than 1/2 of one second.

So I have been approaching the TS chart with this mindset as opposed to BOTH the OVLD and FAULT indicators coming on at the EXACT same moment after the warm-up period is over.

Also, the more I think about this issue I wonder...............is Q121 the ONE and ONLY SINGLE part that can go bad, causing the overload at the differential amp and be making me continue to look for bad parts in other parts of the circuits?  In other words, are we going round and round and round because something made Q121 go bad and consequently causing the OVLD or is there something else that is making the Q121 show a -360mV at the source and the 1mV at the drain?  What is causing the -2v at TP4 and the gate of Q121?



Quote
Page 3-5

3-60  Input FET Switch

3-61  Q121, the input FET switch interupts the input signal when the instrument is in
      standby or overload. Since the resistance of Q121 is part of the input resistance
      any change in temperature will effect the resistance of the FET and through
      it the gain of the instrument. The compensation circuit, Q122 and U25 with their
      associated components, is used to hold the resistance of Q121 constant.


I'm getting dizzy muself but we are getting closer to solving it. ;D

Gate pin 3 of Q121 gets signal from U25 of temp. compensation & gate drive circuit. FIG. 8-7
sheet 1 of 3. Pin 4 of Q121 ( substrate ) goes back to pin 1 source of Q122 ( SD210 ).
Can you see the connection now? Check TP 16 of U25 ( output to Q121 ).Trimpot R22 1K ohm to pin 2 of U25.
'
SD210 datasheet:

http://www.micross.com/pdf/LSM_SD214DE_TO-72.pdf

« Last Edit: October 22, 2015, 05:22:45 pm by singapol »
 

Offline classicTEK

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Re: Fluke 5205a Repair
« Reply #32 on: October 22, 2015, 05:51:00 pm »
Are you saying that the 812mV I am seeing at Q121-4 is what is driving Q121-1 to be -360mV?  Thank you for your understanding.

d
 

Offline singapol

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Re: Fluke 5205a Repair
« Reply #33 on: October 22, 2015, 06:29:29 pm »
Are you saying that the 812mV I am seeing at Q121-4 is what is driving Q121-1 to be -360mV?  Thank you for your understanding.

d

No...can't really say for sure, see my previous post...Q121- pin4 substrate is going into pin 1 source of Q122 or vice versa. The fault could be at Q122 or U25 and associated components. There is feedback between Q121 and Q122 , U25 to Q121.
Yes it's getting dizzy. ;D Not forgetting 5200 divider above too but leave that for the moment.
 

Offline classicTEK

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Re: Fluke 5205a Repair
« Reply #34 on: October 30, 2015, 12:46:59 am »
Hello Sir!!!!

I am just checking in to say I have been caught up in a series of misfortunes this week and have had very little time to get into the lab.  I will be back at it early next week and go from there.

We will win this one, I feel we are getting closer.  And if all else fails, I have learned tonnes!!!

cheers
 


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