Repairing a Fluke 5200A

[Zenwizard]

I have a fluke 5200A on the bench with the associated high voltage amplifier. The fault is with the 5200A. It is popping fuses at least that is what the previous owner told me. I have hooked it up to a current limited supply and found that the supplies were really loaded. In following the trouble shooting guide in the manual I pulled the power amplifier board out of the unit and the unit power up normally. So I think the fault is in the power amp section. Something smells burnt and the repair tag said the magic smoke came out but I cant find any of the usual burn marks that would point out a fault. Is there any common failed parts on the power board of these things. I know this is not a normal bit of kit.

[Kleinstein]

One suspect would be tantalum caps, especially those old dipped drop form. If these are found one may consider a replacement even as preventive maintenance.

Narrowing down the error to the power amplifier is already a big step forward.

The other usual fault to cause a short are power semiconductors like diodes or transistors that get fused. One can usually test these in circuit with the diode test function of a DMM. If they show a dead short they are likely dead.

[4thDoctorWhoFan]

I agree with Kleinstein from the post above.

When I first powered on my 5200, two tantalum caps blew out on the 15V rails. 
If I remember correctly, they were on the reference board and not the power amp section but tantalums are tantalums which ever board they are on. 

[Zenwizard]

I agree tants are a necessary evil... but also a time bomb  . Well I started going through the troubleshooting steps for the power amp board and as soon as I turned the unit on I found the smoke I was looking for.

CR12 is baked
Q32 seems to test ok
C41 is suspect
R51 is suspect
Q40 tests ok
C44 is suspect
R 286 is fried
R287 is fried
R288 is fried
and not in the picture but Q29 is dead shorted on all 3 pins.

The other good news is the 5205A that this came with seems to be working on I did a quick test with a signal source and it tracked with Don't know about cal or spec yet but it is at least not a brick.

[TheDefpom]

I did a major repair video series on one of these, it might be of help.

One of the issues I had (apart from bad OP-Amps, Tantalums, Electrolytic's, Zener's) was a high voltage transistor in the power amp which broke down when voltage was applied to it, but tested OK on a tester!

I repaired every single board in my unit, sometimes more than once as a failure would cascade through it, I fixed the power supply 4 times I think.

https://www.youtube.com/playlist?list=PL2z3V9RkHQE0pdgzjmzo3Oa11TdnvxtE5

[Zenwizard]

Thanks for making that series TheDefpom. I have been watching it and it has let me know what I am getting in to. Which is OK cause I need the gear. I am going to spend some time in the schematic today and see what i can figure out given the parts that cooked. My power amp is a little different it has 2 of those weird cards on the back of it that have 5 transistors are those just a bunch of parallel transistors to "make" a power transistor?  For the power transistors if need be I have an out of cal but functioning 576 so I can check them for breakdown. Also it is really good to know there is not a ton of secret sauce in this unit I'll post back with my progress.

Zen

[Zenwizard]

TheDefpom, Where did you find the extension card for servicing the modulus that you used in the repair?

Zen

[Zenwizard]

Well it looks like all the parts that burned up were in the -190V supply regulator. I am thinking something ether cascaded or the current limit did not kick in.

[TheDefpom]

I would definitely be suspecting a leaky pa transistor, that is all that supply feeds.

My unit was basically shorting itself out, as it is a push pull system, and the negative side wasn’t turning off fully or was latching on, shorting itself to the positive side, which was then blowing the negative supply.

[TheDefpom]

There is/was a guy on eBay that made riser cards.

[Zenwizard]

I found the riser card but the one you used had header pins to help with the probing did you have to add them your self?

[TheDefpom]

Yes thats right, I added them to the various lines that required measurements, to make it easier to probe.

You will also need SMB extension cables and T connectors.

[Zenwizard]

I got the parts order in for the power board it still has a fault after repairing the - 190V supply regulator. This is a photos of the 2n5240's from the back of the supply. The settings on the curve tracer were not changed. I am very much an untrained eye on this device but to me it looks like the (1) transistors is starting to breakdown this is right at 300V keep in mind the curve tracer is not calibrated I have not had the chance to do that yet it is on the list. But to me it also looks like these transistors are extremely poorly matched.

[Zenwizard]

Well it seems some of the more rare transistors are having some issues. I need a

2N4888

and a 2N4240

Possibly a 2N5240

and 2N4298

I also smoked a 2n3906 again but the good news is those are cheaper then dirt.

[Zenwizard]

Does any one know a PNP equivalent to a 2n4298? I trying to get a replacement just in case. I am still waiting on some parts to come in from over seas they are currently stuck in customs. On the power board Q20 and Q23 are these weird array packages. They are a matched set each. In a later service manual there is a single t66 replacement for Q20 as a 2N4298. But there is no such replacement documented. Q23 is in the same position on the schematic but it is in the negative supply so I need a PNP flavor.

[Zenwizard]

I am happy to report that after a rebuild of the -190V supply and some aggressive checking and replacement of transistors on the power amp board it seems the unit is alive again. I have had it sitting on for the past hour or so and no smoke. Need to run it through a cal and check it is but all is good.