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I have never encountered a bad YTO in a 8566, but this might be one. I just got this unit and had some time to check it out, assuming typical small problems. I'll have to bring a scope out to my truck next - it's the only place left where I can park enough equipment and have some flat space to actually work on it. I figured the unlock was for common reasons, and looked first with a counter at the 1LO output and found nothing. Since microwave counters can't get a reading sometimes if the frequency is flailing around, I next put a 436A and 8484A on to see if there's any RF power at all regardless of frequency - nope.

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I haven't exhaustively checked the possible reasons for no YTO signal yet, so all the supplies will be next up for ripple etc - the DC values seem OK but the scope will say for sure. I have to get familiarized again with this beast since I haven't worked on the main RF section guts in a few years. Then all the usual other stuff to check out in the connections, signal paths, and support circuits. So far everything looks normal control- and display-wise, except there's no input signals showing above the noise floor in any band, which makes sense if there's no 1LO at all. I think all the internal PLL and other indicator LEDs are showing normal conditions. The unit overall looks very clean inside, and complete. One odd thing is that someone in the past had put the little front panel SMA 2IF I/O link in the wrong spot - the IF out was tied to the 1LO and the IF in was terminated. I don't think this could have possibly hurt anything.

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Should the worst be the case, I have spare YTOs, but I'm hoping it's something simple in the circuitry to be discovered.

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Anyway, while I study and revisit the manual and start poking around, I figured I'd check to see if anyone has seen this sort of problem before.

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Ed

I swapped out the entire A13 (I think that's the right number) YTO/PLL assembly with one from an old 8566A carcass that I was using for another project. The YTO output was good and strong, but way off in frequency since the YTO is now different and the coil driver system is at the original's tweak settings, which I don't want to mess with unless I have to. It still shows YTO unlock warning - not surprising but it would have been nice if it came out close to right on and started working. Oh well. Also, some of the mounting screws were missing, so someone had been looking in here before.

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So, it looks like it's narrowed down to something in A13. I still have some swapping options to try out yet, then better measurements on the supplies and support circuitry, then probably a bunch of adjustments, depending on the true cause.

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Ed

The original YTO is indeed shot. I swapped in the one from the other A13, and it now runs and the YTO locks, but on a way wrong frequency. I'm assuming it has slipped a cog in the synthesis, that should be cured by proper adjustment, unless there's a circuit failure there too. The SA is running with external 10 MHz reference from the counter so I can get "exact" readings of the 1LO. I need to review the frequency plan and YTO adjustment process in the manuals.

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Ed

I tried to do the adjustment procedure for A20, but could not get the YTO frequency above about 2.1 GHz, and likewise with any settings of the SA. Fortunately I have on hand a nearly full parts carcass, so I found by board swapping the actual fault appears to be on A19, the DAC pre-tuning board. Now the unit has original A21 and A20 (but all messed up from trying to tweak it before), and a different A19, that all now seems to work. The A19 is a different board version - I don't know if it's newer or older than the original here, but it should be OK.

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I'll have to try the whole procedure for tweaking it up, unless I can quickly find and fix the fault in the original. Now that I know where to look, I will poke around in there and see what's up.

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Ed

I think it's fixed and running beautifully. The other A19 board was functional but significantly different on top and none of the labeling on the plastic cover matched the board, so I decided I should fix the original. I quickly found that the -12.6V reference was out (only about -1.7V), but it took a while to figure out since there are a number of things that could do it. The culprit was perhaps the absolute last thing I would have expected. I had a couple of usual suspect Ta caps, a couple of high-K ceramic caps, a bunch of negative opamp rail circuitry to make -24V from the -40V supply, and of course the opamp itself that regulates the reference from a TC Zener.

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After some test point wire tacking and operating measurements I found that the input to the opamp U3 was shorted to ground by C18, showing about 18 ohms. I couldn't believe this high grade plastic cap could have failed - it's not even a high stress spot at all, just extra filtering at the input against 5.11 kR from the Zener. C18 and its counterpart C22 in the +10V reference apparently needed to be high grade (I'm virtually certain these are PPF = polypropylene and foil) types for very low ESR, ESL, and DC leakage. I have some of these salvaged from other gear over the years, so I immediately recognized the type, and I can often tell by the density, which usually "feels" higher than film type caps. This is a relatively fancy and expensive part, and physically big (board space-wise), so there must have been good reason. A cap like this failing in a spot like this has to be an extremely rare random failure.

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I found an equivalent one in stock, popped it in, and voila, just like new. All the original boards are back in now. I assumed the original setup for A19 is still good enough, so I only did the A20 adjustments, and this time the procedure worked as it was supposed to. It seems to be all working now, but will need some more checkout to be sure. I am happy for now.

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Ed