Fluke 5205a Repair

[classicTEK]

Hello All,

I have come upon a very nice Fluke 5205a Precision Amplifier and it is a lil under the weather. I have spent more hours than I care to count trying to track down this problem to no avail. I thought the best place to get troubleshooting help would be here. If someone would be so kind as to reach out and help me get this thing going again that would be great!!!!

It is being driven into overload and shutting down to save itself from certain death, but I cannot figure out why. What I mean by that is she will turn on and go through her initialize circuitry and process, but determine an overload or fault and then go into protect and flip the relay so the output goes to zero. I think I can see where and how it is going into protect mode, but I cannot nail down the issue causing it to do that. I think the Slew Rate Sense Amplifier is sensing a differential between the input and a sample of the output. When it detects any more of a difference than 125mV, she goes into the save me mode. That difference at Q104-2 & 5 on the preamp board is currently 750'ish mV. I believe this to be caused by U123 getting clobbered with a total of 365mV at U123-2&3 which pegs my output to near 12v at U123-6 and consequently showing up as 7v at the collector of Q148 which is the feed to the differential amp that is telling the system to shut down.

Something in this part of the circuit is feeding U123 with a difference of 365mV and I thought this circuit, the way it was designed, demanded that the difference should be no more than a couple mV max. My thoughts are that this HUGE difference at the input to U123 is causing the 7v I am seeing at TP1 and consequently, the .7v I am getting at one input of Q104. Now either this input is high and I need to know why, or the other input to Q104 is low and I need to know why.

I have traced the logic from the overload indictor and I seem to be ok there, but run out of understanding at U42.

I just now forgot to reconnect the output to the Power Amp Board and the voltage at the input of U123 were both 312mV. I reconnected the coax to the power amp and the voltages difference of 365mV came back to the input of U123.

I am getting valid logic and measurements that all support that the OVERLOAD circuitry is working correctly and keeping it in STBY, but I cannot locate the crux of the issue causing it.

Maybe the output recipient that is spoke of in 2-31 is in fact the power amp in this case instead of another piece of equipment since she is next in line from the preamp circuit. Maybe it is the power amp that is putting to much load on the pre and forcing her to kick back to STBY.

Hmmmmmmm..........


I just read over 3-72, if I am understanding correctly, I need the voltage at TP6 next to the Zener String to be -85v. Is this correct thinking, as I just discovered I am getting +7v. When in the warm-up mode this holds at 7v. ASAP after the warm-up mode the voltage very quickly drops to -127v and then I hear the relay click and the voltage returns to +7v after it goes into STBY mode.

Any thoughts?

If anyone is interested in my brain teaser, PLEASE.............for the love of all things good, PLEASE reach out and help.

Manual in PDF:

http://assets.fluke.com/manuals/5205A___imeng0000.pdf

If you need anything to help you help me, then do not hesitate to ask.

PS........part of my problem is that I simply do NOT want to end up unconscious against the wall from a sudden jolt. This thing is BEEFY.

Cheers,

David

[singapol]

After a quick read of the manual I think you have jumped the gun and made wrong assumptions. This is a complicated circuit so your best friend is the troubleshooting flowchart table 4-3. Also the preamp block diagram page 3-9, a careful inspection is need. If you follow the flowchart step by step with understanding you should be able to zero in the cause.

Note in step 2: Front panel LOW and GND terminals are connected with shorting link.

Your best clue is looking at the front panel indicators like OPERATE and the STANBY/RESET, FAULT and OVERLOAD indicating lights. Their sequence of activation is a big clue. Follow the flowchart for example
from step 30 onwards.

If you think it's the slew rate sense amp which is part of the overload block which also includes ONESHOT, GATE DRIVE, S200A FET SWITCH and 10:1 DIVIDER AMP. You need to think through carefully. In otherwords
it points to the input circuit so there are many things to check.

PS As you are aware there are lethal voltages of +/- 2500V DC and output can supply up to 1500VDC.
  Hope  you are familiar with tube circuitry.

EDIT:

I have spent some time studying manual and schematics.Like all good practice I assume power
supply voltages have been checked, especially control logic +5V , +28V.

There is a safety interlock switch S2 to take note of when cover is off?

Quote by classicTek

I just read over 3-72, if I am understanding correctly, I need the voltage at TP6
next to the Zener String to be -85v. Is this correct thinking, as I just discovered I am
getting +7v. When in the warm-up mode this holds at 7v. ASAP after the warm-up mode the
voltage very quickly drops to -127v and then I hear the relay click and the voltage returns
 to +7v after it goes into STBY mode.

Any thoughts?

If there is -85V and the junction of R151 and R158 and pin3 of U157 is zero volt means any
change of zener string voltage will affect U157, Q161 and optocoupler U160.Signal is then
passed to Q177 which drives the regulator tube.So if there is no 1900V then tube section
can't work resulting in no output.

[classicTEK]

Hello Singapol,

Thank you for taking the time to look at this with me.  True it is a VERY lethal weapon if not given due respect.  For much of this work has been hands off while power is applied.  I also use my battery powered DMM and do NOT touch it while the amp is powered up for measurements.  In this way, I should be safe.

I started with the step-by-step of the TS table and have come to the point that it is telling me to fix or repair the problems in the overload circuit then start back at step 23.  I have not been able to locate the trouble spot, so I am trying to do that now so as to continue with the table.  The interlock switch has been pulled up to disable its function.  I checked all of the power supplies first and they are all VERY close, if not spot on.   I do need to check the 1.9kV, but other than that all are good.

I have at times been using just the AUX supplies power cord as it at least minimizes any possibilities of the HV being involved in the TS.  Still VERY careful though.  When I use just the aux cord the OVERLOAD indicator comes on AFTER the warm-up period and the FAULT indicator only comes on IF I put it to OPERATE, but then the FAULT indicator will go out if I place the knob back to STBY.  If I use both the AUX & HV power line cords, the OVERLOAD indicator comes on AFTER the warm-up period, and is quickly followed by the FAULT indicator and keeping the amp in STBY.

All of my work to this point has been done while the amp has set itself to STBY.........WITHOUT me trying to set it to OPERATE.  Even if I disable the OVERLOAD circuitry by shorting TP-18 to ground(TP-19) the OVERLOAD indicator will go out, but my voltages do not seem to change after I do this.  I keep this disabling to a minimum and ONLY long enough to check measurements.

I have checked the logic surrounding the OVERLOAD circuitry, but only with my oscilloscope probe and not a logic probe.  So I set my scope to trigger on the falling edge of the pulse to look for Pin 8 of U41 to go low and clear the Trouble FF.  All of the CLR's and PRE's are coming at the right time, I THINK.  I have an HP logic probe on the way to make it easier for me to see things without having to stare at my trigger light to flash ever so quickly when the transition happens.

I also believe the amp is not being told to OPERATE by any other device and that sort of eliminates the Dual Control Detection circuitry.  The 5200a is NOT connected so I have been trying to make this work in stand-alone or LOCAL mode if you will.

It is challenging, but I will win this one and have learned a TONNE of schtuff along the way.

[classicTEK]

It seems that it may take me longer to recognize things that do not LEAP out at me on the page.

In a more determined effort I re-read the Theory of Operation for the 120th time.  After your comments about the 3-72.  It had NEVER leapt off the page as it did this am.  It states, VERY clearly, the output is sensed and sent back to the overload circuit.  This circuit looks for and reports on any HV sensed or low voltage inside the amplifier.

This would makes sense why even when I disable the overload circuit, that the voltages do not change............DUH!!!!!

DOE!!!!!

Considering all of the latest info, reading and thought.............it is time to dig in and measure the two killer voltages.  I will get out the 20kV HV gloves and see what I come up with.  I will also pull the tubes and run them through a few tests to verify full functionality.  Then put everything back and se where we stand.

Thank you for the jolt of clarity!!!!!

Cheers,

David

[classicTEK]

Ok Folks,

A little update after changing out C1 in the HV relay driver circuit due to high ESR.

I am no longer getting the fault indicator after the warm up period is over and the OVERLOAD indicator comes on.  However I am still getting the OVERLOAD indicator, BUT no relays clicking on or off either.  That has to be a clue...............any thoughts?

I gathered the proper tools to safely measure the HV and something is not allowing K1, the HV relay to kick in and allow the HV circuits to produce as they should.  So I am NOT getting either of the three HV's.  I need to verify what signal that HVRD is being fed from the Logic board and see if that is telling the opt-isolator to shut down the system or what............

the saga continues, but I am making progress.

d

[singapol]

Ok Folks,

A little update after changing out C1 in the HV relay driver circuit due to high ESR.

I am no longer getting the fault indicator after the warm up period is over and the OVERLOAD indicator comes on.  However I am still getting the OVERLOAD indicator, BUT no relays clicking on or off either.  That has to be a clue...............any thoughts?

I gathered the proper tools to safely measure the HV and something is not allowing K1, the HV relay to kick in and allow the HV circuits to produce as they should.  So I am NOT getting either of the three HV's.  I need to verify what signal that HVRD is being fed from the Logic board and see if that is telling the opt-isolator to shut down the system or what............

the saga continues, but I am making progress.

d

Good that you are making progress.It happens to the best of us when we don't read
carefully. 5205A is one big feedback loop so it is challenging to troubleshoot.
.

Troubleshooting

Step 30 (cont): ...If the OVERLOAD indicator illuminates check the disabling curcuit
 between the Warm-Up Delay circuit and U8-11 on the Logic Assy.Repair as required then
repeat the test begining at step23.

The above refers to startup power sequence and control logic. Does the OverLoad indicator
also connect to the OverLoad curcuit? Looking at the block diagram, OverLoad box in dashed
lines points to Slew Rate Sensor Amp, One Shot, Gate Drive, 5200A Fet Switch, 10:1 Divider..So if -85V is missing you have to go backwards to this section. The logical arrow comes from the Input Amp.

[classicTEK]

"So if -85V is missing you have to go backwards to this section."

By this you mean the section that refers to the -85 volts?

[singapol]

"So if -85V is missing you have to go backwards to this section."

By this you mean the section that refers to the -85 volts?

No point checking for -85V because it assumes high voltage relay is engaged. So we have to assume that no high voltage is fed to pre amp section. Can try tracing Logic control circuit but I think a faster way is checking the tube circuits. No worries you are checking using ohm and diode test of DMM. No high voltage power is
applied ( Pull the HV plug). Check high voltage caps are discharged...although manual says it takes 30 seconds for bleeder resistors to drain.

Check -1900V tube regulator, driver tube and top and lower output power tube/ screen regulator. They are hybrid circuits with tubes and solidstate curcuit control. I bet the problem is in one of these parts because you have not tested these.

[classicTEK]

Good Morning,

I have come to the point where K1 will kick closed after the warm-up light goes out and the warm-up period is over, but the OVERLOAD light still comes on too.  It is however not closing the contacts and then kicking the them back open due to an overload, as before.  I am getting 120v AC into the HVRD circuit and it is making K1 close, but I am getting no voltage between TP501 & TP502, TP504 & TP506, and TP505 & TP506.  Is it simply a bad K1 HV relay that is causing my problem, or could it be the tube circuit as you suspect?

As I said, ZERO voltage across any of the HV's, but power into K1.  I am only getting 24.5vdc at the base of Q3, is it possible this is not as much as is needed to not only close K1, but to give it enough to make the rest of the circuit start conducting current to the HV Transformers?

d

[classicTEK]

UGH!!!!

Scratch that, blown mains fuse cause the finger slipped.

So it is kicking the contacts open just after they close after the warm-up period.  OK, into the tubes.

d

[dacman]

What are you using to drive the 5205A?  Have you tested the electrolytic capacitors?  What does C175 on the PRE AMP test at?  What happens if you short the BNC input on the back?

[singapol]

Good Morning,

I have come to the point where K1 will kick closed after the warm-up light goes out and the warm-up period is over, but the OVERLOAD light still comes on too.  It is however not closing the contacts and then kicking the them back open due to an overload, as before.  I am getting 120v AC into the HVRD circuit and it is making K1 close, but I am getting no voltage between TP501 & TP502, TP504 & TP506, and TP505 & TP506.  Is it simply a bad K1 HV relay that is causing my problem, or could it be the tube circuit as you suspect?

As I said, ZERO voltage across any of the HV's, but power into K1.  I am only getting 24.5vdc at the base of Q3, is it possible this is not as much as is needed to not only close K1, but to give it enough to make the rest of the circuit start conducting current to the HV Transformers?

d

3-10 HIGH VOLTAGE RELAY DRIVER

3-11. Power applied to the instrument cannot reach the High Voltage Transformer
until K1 is energized. P56-8 brings in the control signal from the Logic Assembly.This
signal goes low approximately 30 seconds after the Power switch is set to ON position.
Prior to that time the Optical Isolator U1 has been cut off, preventing K1 from energizing.
Once low, U1 conducts, cuts off Q1, which causes the current source Q2 to turn on Q3 and
energize K1. Q2 also supplies current to zener VR5 to provide a regulated 110V to the
relay coil, optimising the dissipation of the relay under varying voltages. Diodes CR1 through
CR4 and C1 provide a 110V supply to relay driver.


                         CAUTION!

               The Relay Driver circuitry is located on the
               Primary side of the HV Transformer. Extreme care
               should be taken when connecting any test instruments
               to the circuitry.

Take note Interlock switch S2 must be closed to complete K1 relay circuit.

If K1 and S2 are open there will be no high voltage at the test points TP501, etc.
See schematic Fig. 8-3 sheet 1 of 3.Base of Q3 should be 110V as VR5 is a 110V zener across it.

 Edit:

There are 3 K1 designations and 1 K2 all with different part numbers. Surprisingly the one connected
to high voltage relay is listed in the control logic assembly schematic. Anhother K1 in the Pre amp assembly
and pair K1,2 in the power amp assembly.

K1 is discribed as Relay,dry reed and the K1,2 pair as reed switch.  I guess you have to settle this relay circuit
before going further and  . The power supply is taken from the bridge rectifier CR1-4 at connector J61. No indication of what AC voltage is going into bridge rectifier.

[classicTEK]

The voltage required at CR1-CR4 originates at the AUX line mains cord and is fed to the bridge and all AUX voltages thereafter.

I have 117-120'ish coming out across the bridge output.

d

[classicTEK]

The Latest,

So I pulled all of the tubes from this unit as well as another parts unit that I had so I could compare readings.  Due to the type of tube, the best I could do were continuity checks between pins.

9 of the 12 all tested identical. The other three, all from the unit I am trying to repair, had continuity between pins 1 & 5.  None of the others had this same continuity.  These three tubes were V1, V3, and V4.  V2 tested the same as all of the rest.

This begs the question.............why are these three shorted between pins 1 & 5?  Is it something in the circuit that is bad, shorted, open, or otherwise that is causing this condition in these tubes or should I simply swap these SUPPOSEDLY bad tubes out for good ones or do I need to find out what caused this failure?

I do not want to jump off this cliff before checking and end up with more bad tubes cause it is the amp that is making them short.
d

[singapol]

The Latest,

So I pulled all of the tubes from this unit as well as another parts unit that I had so I could compare readings.  Due to the type of tube, the best I could do were continuity checks between pins.

9 of the 12 all tested identical. The other three, all from the unit I am trying to repair, had continuity between pins 1 & 5.  None of the others had this same continuity.  These three tubes were V1, V3, and V4.  V2 tested the same as all of the rest.

This begs the question.............why are these three shorted between pins 1 & 5?  Is it something in the circuit that is bad, shorted, open, or otherwise that is causing this condition in these tubes or should I simply swap these SUPPOSEDLY bad tubes out for good ones or do I need to find out what caused this failure?

I do not want to jump off this cliff before checking and end up with more bad tubes cause it is the amp that is making them short.
d

You have provided an important clue,
 
resistance between pin 1 and 5 of 4X150A.Take a look at the links to the datasheet.
Pin 5 is not used ( it's an internal connection) so I guess an internal arcing had ocurred.
What would cause this arcing? Well pin 1 is the screen grid or G2.look at fig. 8.9
sheet 2 of 3.To the left of the power tubes is the screen voltage regualtor circuit.
Check this circuit.At TP304/TP325 there is 450V.There is also K1,K2 relay for sense high
and output low. Check the bias current circuit that links TP304/TP325.

Take note there is also another pair of 4X150A before the power tubes V1 is the driver
for the power tubes and V2 is the 1.9KV regulator. I would concentrate on the
screen regulator circuit as it connects to pin 1 screen grid.
 PS the " shorted tubes" should not be used and deemed damaged.

http://frank.pocnet.net/sheets/049/7/7034.pdf

http://frank.pocnet.net/sheets/164/4/4X150A.pdf

[classicTEK]

I am getting the 450 necessary between TP315 and TP325.

I will win this one..........it is just a matter of time.  I have learned a ton about many things along the way, so this is all good!!!

d

[classicTEK]

Question.........if I am supposed to be seeing -85 volts at TP6(zener input), shouldn't that be the same voltage at TP4 all on the preamp board?

[singapol]

Question.........if I am supposed to be seeing -85 volts at TP6(zener input), shouldn't that be the same voltage at TP4 all on the preamp board?

No...TP4 is the output of the preamp Q148. -85V is at junction of R151 5.6M and R158 1M. Leave this for the time being as it implies having -1.9KV present which is not.

Another important test point TP 309 of upper tube card (power tube bias). please read 3-102 power amplifier assembly and see fig.3.3. TP 309 shoud be 30mA? Schematic 8-9 sheet 1 of 3.

[classicTEK]

I do not have a current probe so I disconnected the strap and put my DMM in series, 33mA.  If I am understanding the procedure in the calibration section 4-75 step (e), this measurement is supposed to be made while the HV line voltage is at 25% (approx 29v) and three jumper pcb's installed(sec 4-64).  These jumper pcb's ensure that the tubes maintain a safe bias voltage while the HV is at 25% for the reading.  I do NOT have these.

So I took the reading at regular line voltage and got 33mA.  This is supposed to be 30mA +/- 1mA.  It tells me to adjust R172 to bring this within tolerance, but I did not change it considering I do not have the jumper pcb's and I checked it at full voltage.  I can investigate further if necessary to make these jumpers or something that will suffice.

Also, I can only see this 33mA reading for a second before the OVLD sys sends it to OVLD and shuts off the HV supply again.

GRRRRR!!!!!

:  ))

d

[singapol]

That's good it means the bias is in the ballpark figure. What is overloading? Looking at the block
diagram the overload box is in the preamp circuit. I think the overload/fault could be there.Keep digging. 

[classicTEK]

oh yes, more digging!!

:  )

[classicTEK]

These are the voltages I am seeing in the differential circuit that I originally thought might be the problem.  I have been around many corners on this journey, but I seem to always end up back here.  Thoughts?




d

[singapol]

I have been examining the schematics and manual just now and the slew rate sensor may be part of the story. Maybe I did not read the manual properly. What is this overload? I had assume it was coming from the power tube output but apparently it is not. On further reading and comprehension it seems this overload signal has to do with the preamp but there is no clear indication where overload signal "originates".
Or at least that's what I thought.

Clues come in fig.8-1 sheet 1 of2. At the left bottom corner you can see overload 5,6 at J44 linked to J23 5,6.
The actual schematic is at fig. 8-5 logic pcb assembly sheet 3 of 3 in the middle right starting at P44 ,5
to TP401, U42, U8,13, U5 to overload LED indicator.

Go back to fig.8-1 sheet 1 of 2, you will see NEG HV at J44 to J23 again. I think they are related.
Now go to fig. 8-7 sheet 1 of 3 Pre amplifier assembly , there at bottom left you can see overload and
NEG HV. Trace NEG HV all the way to U52 opto coupler, U62, Q62, TP20 and TP21. FRom R59 to slew rate sensor ,5200 DIVIDER and BUFFER and finally to Q121. You are at the signal input of 5200A then to Q128 preamp circuit. So there you have it, overload actually refers to preamp so we have been on a wild goose chase. Read carefully the Theory of Operation and you will find the problem.

My guess is in the input and preamp and logic as the manual says to rectify the circuits concerned with the indicated fault.

PS at the slew rate sensor circuit you can see at TP13 overload trigger signal and TP18 overload disable.
Good hunting!

[classicTEK]

Possible............now please go back to my very first post and see if this is making any more sense.

cheers,

d

[singapol]

What I can say is think creatively. If you can't go by the front door, try the back one. 5205A is one big feedback
loop.You have power supply, logic and active circuits not that easy to troubleshoot unless you have a clear and definite clue or stategy. You have to think what about if the circuit component failure can lead to false triggering of panel indicators. Is there a real fault in the first place? You would have found it by now.