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Fool for the 8656A Sig Gen
Tony_G:
For those following along at home, the A10 board delivers a 5V rail to the other boards in the SigGen:
The CPU board starts to get covered in Service Sheet 14 (the nomenclature being the H in the circle followed by the 14 to refer to the applicable Service Sheet).
JRH has said that the 5V rail only drops when he connects pin 2 of A10J2 up and that is the one that leads to the CPU board:
Now it goes to other places on A14 but there are a bunch of bypass caps on that rail and this is where I would start - Then I would go with Swainster's suggestion - If nothing else leaped out then I would probably begin to identify the relevant ICs touched by the 5V rail and work from there.
A somewhat slow slog if the unit is uncooperative in the fault-finding mission.
TonyG
jrharley:
Hey HP Sig Gen’rs,
Great feedback and insights as usual, really appreciated! It’s a little hard for this old Bodger to keep up with it all, but that’s not a complaint, it just takes me some time to go through things and take it all in. :P
I hear everyone loud and clear on solving the 5V rail, and I concur, don’t get me wrong. I'm afraid that my ignorance of this kind of equipment and circuitry, has my pea brain struggling to comprehend how an electrical gap in the attenuator section couldn’t be contributing to the symptoms I’m seeing. But I’ve been studying the attenuator schematic and I think I’m starting to understand why I shouldn’t be concerned. (Any insights on that front would likely be useful though). I think I’ll just continue to try and sniff out the problem with the attenuator cable out of circuit entirely moving forward, and just get over it.
That said, fabricating the missing attenuator parts is going to be a real task, in and of itself. That’s why I’m going to start the process of finding a shop that can do it. I’m thinking not just any 3-D print set up can do this properly. A precision scan will have to be done, the right plastic will need determining, etc. In any case, I won’t pull the trigger on fabrication until the 5V rail mystery is solved, but there’s some leg work to do on this aspect of the repair.
Tony_G – Thanks for the printing offer, much appreciated! We’ll see how it goes.
I think I’ve beat the components along the H string to death at this point, I’m convinced there’s no component level issue along there. :horse: All of those IC pins are showing a positive voltage of approximately the same value, 1.6 to 1.7V. By the schematic, I think it should be 5V or thereabouts. I'm thinking the issue lies downstream of this section. I’ve been looking at service sheet 15 for the next place to investigate, giving some “side eye” to chips U1, U22, U24 and U32.
Great test suggestions swainster, thanks! As for the touch test, I’ve done a good bit of that and haven’t found anything finger blistering yet, even after prolonged power up, but it’s a thing I’ll be constantly doing. Also, I measured resistance across all of those caps statically a while back and the caps were consistent in their readings, giving me some confidence that they’re solid.
Anyway, I’m taking a brief sidetrack with a cute little Spectrum Analyzer I acquired recently for a couple of days. It needs some mechanical stabilizing before I can shelve it.
Then I’ll get back to the HP. I’ll report back with any new developments as they occur, and always love the comments folks! Thank you!
JRH
Tony_G:
I don't think you need to go all "high spec" on those actuators initially - You could just use a micrometer/caliper and simple 3D modeling tools to create a model that would work - That would get you an "operating" attenuator - You can then get the unit all running and see if you need to step up the attenuator components to have something in spec later.
Good luck with the SpecAn - Look forward to hearing the next updates.
TonyG
jrharley:
Quick 8656A update to ponder here - Maybe someone can explain this observation:
When the unit is powered up now, with the A10J2 pin 2 CONNECTED, The 5V rail reads 5.4V for a couple seconds, until the attenuator latches in, and then it drops back to 1.8V. (what part of the attenuator is switching in, I'm not sure). If I power up with pin 2 unconnected, it behaves as before.
I can't escape feeling like this is telling me something, these things are dependent on each other in some way guys.
If any one can prove to me why the attenuator can't be affecting the 5V rail, I'm all ears.
I may go dark for awhile following my gut here. Bear with me.
JRH
Swainster:
Sounds good! I think the point is that the priority should be the problem with the 5V rail rather than the attenuator (which is mostly just a bunch of 24V relays from the control point of view). Of course, if the 5V rail problem can be traced to the attenuator then that is a different matter. Did you try unplugging the attentuator control input (J3 on the power supply board) to see if that makes any difference? Then again, erhaps the reverse power protection circuit is involved somehow - that appears to use a 5V relay
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