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All,
I think I posed this problem before, but to be clear and simple: this problem took months to fully develop, initially when turned on the 2712 would act insensitive as it is now, but coming to life within 10-15 minutes each time. Of course, finally it stayed deaf. Anyone seen and fixed this symptom? Is this a symptom of a drifting or failing YIG oscillator?

Cheers,
Howard Hoyt

Howard, Reading your original note and application, my guess is partly blown input mixer.

We had simi;lar application and similar symptoms decades ago, but with 2710 not 2712.

Perhaps heat sensitive. Did you try hot/cold air (heat gun, freeze mist) on suspect parts like mixer or YIG?

See the details of disassembly and troubleshooting proceedures in full service manual.

Bon Chance,

Jon

Hi Jon,
I took the cover off of the RF Input and 1st Converter module and tested the two dual-diodes CR256 & 257 at the input (see schematic and pcb : /g/TekScopes/album?id=294504)

All four diodes test good in circuit. The quad mixer diodes CR130 cannot be tested in circuit, they are soldered directly to a PCB stripline below (see picture) and removing them would be quite the challenge.

I am thinking since CR256 &267 are good, the failure is not due to an overload, and is likely the YIG LO injection level being down 28 dB, so I will find a way to check that out.

Thank you all for the hints and troubleshooting ideas!
Howard Hoyt

Hi all,

I could not find any bad semiconductors in the RF input circuit, and quite frankly the 1st Converter PCB is to put it mildly, inaccessible so I have not checked any semis in it. To verify the YIG and LO, fortunately this 2712 was equipped with Opt. 15 which routes the LO to a rear-panel jack. I do not have an RF voltmeter spec'ed up to 3 GHz, but using a very trusted (but old) HP 410C RF probe and interpolating with the output of a calibrated Agilent E4424B RF Gen, it looks to be around 8 dBm at that jack (unterminated) and it was sweeping from a bit over 2 GHz to over 3 GHz as it should, so It appears the YIG and 1st LO signal are likely correct. The LO is fed directly into the stripline mixer and I verified the semis up to that point.

I am at a loss at this point in time without total disassembly to get to the RF motherboard which, if anyone has seen the 2712 construction, is a (expletive deleted) nightmare to put it mildly. I have worked on some difficult RF assemblies in microwave STLs and transmitters in the past but this one beats all. I guess some packaging engineer took fitting all of this in a small portable case as a challenge, it may have been the guy who designed the Tetris game...

It is unfortunate when you are falling behind on work because you are trying to fix your tools...

Thank you,
Howard Hoyt

Dear Howard, a pity:

Have you checked all SMA/B/C type hardline/flex coax connectors for poor contact?

See disassembly proceedure in serv manual?

Jon

The 0.1uF will pass nasty voltage spikes right into the SA and without a voltage limiter post 0.1uF, there may have been progressive damage occurring. Just because a diode looks good at dc, it may still work poorly at speed. I don't see the usual Tek goodness in these circuit board photos either. Can you get some sort of signal gen or little VNA? Sometimes it takes a SA to fix a SA.

Hi all,
I want to inform the group that Japp is a genius...well, he understands the 2712 as if he had designed it! His experience with his (and other) 271x SA failures was exactly what mine experienced. I replaced some very hard to get electrolytics which had severely degraded in the 1st LO Buffer and 1st LO Interface Board and after re-normalizations the 2712 passes all internal tests and is working as it should.
Thank you Jaap!!! What a resource for this group!

Cheers & 73,
Howie / WA4PSC