Fluke 6082A Repair

[smgvbest]

Hello

I picked up a this Fluke 6082A off eBay as a repair project.  I got it pretty inexpensively and as a learning project.   RF is not my area.   I'm a digital woman.  When I got it and powered it on the fan was atrocious and did a quick oil of it's bearings which helped long enough to get a replacement ordered.    Powering on the unit I am greeted with the messages  327, 328, 329,330,332,333 errors then once clear of those I also get a 244.

The 327-333 are a problem in the sum loop circuit.
the 244 is a sum loop not phase locked.   by the service guide fix the 244 first so that's where I am starting.

so a few pictures of the Fluke as I know you all love equipment pictures.   I've taken plenty.   I have to.  my eye's are as good as the where and photo's help allot. So I can post a ton if there is interest.

I've done some basic troubleshooting like checking the supply voltages.  All in spec.
Started going through the troubleshooting section.  I do get distracted when I see something interesting (but not always relevant)

I'm going to break up the post so the attachments make more sense.  so TBD

[smgvbest]

So i started opening the unit up.   tons of screws everywhere.  I'm sure those of you used to RF equipment this is nothing new.   to me it is.

Before I go to far into this write up I want to say.  My goal is to learn here so I appreciate things pointing me in the right direction not answers.  RF as I said is not my thing,  analog is not my thing.  just ask my electronics instructors back at American River College.

So,   I started with the block diagram from the service manual
The Sum Loop is comprised of the A12 Sum Loop and A9 SUM LOOP VCO
As inputs to the A12 Sum Loop there is the A14 FM, A5 Course Loop, A3 Sub Synthesizer VCO and indirectly the A4 Sub Synthesizer (which is where I think the problem is)

I started looking at the Inputs to the A12 Sum Loop board
80Mhz from the A14 FM
576-960MHz from the A5 Course Loop VCO
16-32MHz from the A3 Sub Synthesizer
and 480-1056Mhz from the A9 Sum Loop VCO

the A5, A14 and A3 looked good but the A9 did not.
See attached captures from the SA

[smgvbest]

Here's the rest of the schematic.

[smgvbest]

Here's the actual A12 Board along with the A14 FM and A13 Controller

Now, looking at the block diagram,  the bad signal comes from the A9 Sum Loop VCO board but the A12 board sends control signals to the A9 board.   Those signals are J5 Steering and J6 Phase Lock
Looking at the J5 signal it has a lot of jitter on it.
J5 takes in SUMSTEER, SUMVCO[4-6], SUMAUDIO, SUMCOMPHO-7(which is driving a DAC  So not we need to trace back these signals.

So my first question is does it seem like I'm following a correct method to track this down?

Now as I mentioned earlier, I've done more tracing and I'm actually on the A4 Sub Synthesizer board and think I found the area where a problem is in the Loop Amplifier which follows the Phase Detector on that board.

I'm trying to get down what I've done so far but I've not caught up to where I am at this point.   However the J5 Steering is the one that's gotten me over to the A4 board now.

 

[smgvbest]

Been doing more checking and I did find a problem on the A12 board.   At Q6 it should have ~3.6v,   My actual reading there is 0.4mV.   Q6 is a BFR582 (doing this from memory at the moment) which is an obsolete part.  I found some on eBay and ordered them so before I do anything else I will replace this part.  Q6 feeds the LO signal into the mixer  where is mixes with the RF from Q8 (80Mhz) which is good to produce the IF.  Q5 feeds Q6 and U4 Pin 3 which goes on to produce the SUMLV signal.   All other components in the RF section of A12 check out.

[smgvbest]

I replaced Q6 (MFR581 is the correct part) while it did improve my signal so it was a problem it was not the problem.  the 244/327-333 still presist.
I took a detour (just because the check was quick) and the A9 board which is the Sum Loop VCO checks out fine and the funny thing.  I removed the A9 board to get a closer look at it and after I replaced it the 244/327,328,329,330,333 disappeared.   hummm.  faultly connection at J9 maybe to the A12 board???   

So I did some basic adjustments and it is outputting a good signal now from 100Khz to 2112Ghz.   I calibrated the OXCO and now its off no more that 1.5Hz at 2112Ghz and in the mHz range in the lower frequencies.   

I don't have a power meter so I used the DSA815's power reading to adjust it.   may not be perfect but it's close enough for me, for now.

I ran a SPCL 06 for a full self test and a 332 (still) and 335 (new) shows up so it's not 100% fixed.   332 is still in the sum loop.  the 335 is with the Attenuator reverse power protection.   SO i'll look at those next.

[smgvbest]

Spoke too soon.  the 244/327-333 are back after being powered off for a bit.

[tango17]

No one posts here,  but you.  Nonetheless,  don't stop posting. We're rooting for you and I enjoy your reports. Best of luck in the repair.

[smgvbest]

Thank you,  I figured since I said I wasn't looking for help unless I asked for it is why no one is posting.   I'm relearning allot of stuff I forgot since school including RTFM     

Went back the A9 Sum Loop VCO and retested it.   U1 does read very different than U2-4 so could have a problem but the signal out of the VCO seems good overall.   

On the A4 Subsynthesizer I do have a question.  just to understand what I see
At TP5 is the output of the low order digit generator.

The pic A4TP5DSA is the 20Mhz signal as seen on the SA
pic A4TP5DS is the same signal on the MSO1104
the UUT us set to 804,001,499 per the manual for testing

The SA signal i'm not sure of.   The Oscope definitely has noise on it.   is the DSA showing the same noise?  the DSA makes me think the output is not good and the Oscope confirms it but I'm just sure what the SA is showing.

It's directly out of the U23 which is a custom gate array.  so I'm hoping it's not the problem here.   though I am curios about it clearing up for a time yesterday.   I'm thinking about reseating that chip just to be sure it's not something like that.

Actually just did that.   no good but when checking i also noticed that signal on the scopes only happens in the 1-999hz ranges of any higher frequency.  if it's 000hz it's much cleaner.

[smgvbest]

Anyone have any feedback on what the SA is showing?