Products > Test Equipment
Fool for the 8656A Sig Gen
Tony_G:
From the schematic, the 5V line carries over to a bunch of filtering capacitors on 8-86. I would start checking those before I started partying on ICs.
Just as an aside that reference mnemonic works like H 14 - Means the signal passes to point H on service sheet 14 - If you see H 15,16,18 then it means point H of those service sheets.
TonyG
jrharley:
Thanks TG!
It's hard to estimate just how much time you just saved me. I was chasin' a bogus Rabbit hole. This is such a great example of how invaluable experience is. I thought that small number 14 was something else, not the link to the next node in the chain. According to page 8-23, table 8, "Schematic diagram notes", that number 14 should be BOLD per the example. Yet, sure enough, when I followed to service sheet 14, there it was....... "Big as Ike", as my Dad used to say. "From the +5V supply". And yes, that is a bunch of caps on that line. I'll get to testing and report back.
Thanks for nudging me back between the white lines!
JRH
Tony_G:
NP - Happy to help - You'll find some of those numbers being "handwritten" on the schematics - If I had to guess I would say that this manual was produced at the time of moving from drafted schematics to computer-generated so you'll see these various anachronisms around all the schematics (I don't actually know but I'm sure someone over on the HPAK group would).
TonyG
jrharley:
Hey TG, 8656A update here, hope it's all good -
I did a one night rundown of the caps on string 22H recently. Most of the caps on this line are .01uF ceramic packages. I have assumed that from the appearance of the caps and the fact that the parts list doesn't identify the type, that they're ceramic. They look to be high quality for sure, still shiny. Since each one is associated with an IC pin, I'm gonna' call 'em "Bi-Pass" caps. Not inclined to suspect a ceramic cap, unless it's baked or cracked. That, in addition to their relative circuit significance, has led me to discount any role they may have in drawing down the 5V rail. (Now that I've said that, this will probably come back to bite my 'arse in 6 months!)
There are also a couple of 'lytics in the chain. C1 is 100uF and C8 is 2.2uF. I measured them in circuit at 1KHZ. C1 was pretty much spot on @ 102, with an ESR of .27ohms. C8 was a different story, but I'm not clear if it would matter at all for the disappearing 5V rail. It measured a whacky 102 uF. ESR was so-so at .65ohms. Again, maybe worth a second look, but I can't see this pulling the 5V rail down since it's not dead shorted.
I'm moving on down the chain to look at service sheets 15,16 and 17. Interestingly, things seem to be converging on the sheets that came up as significant in my "bogus rabbit hole" journey.
So that's where I am. More probing around currently underway.
JRH
bdunham7:
--- Quote from: jrharley on December 10, 2022, 11:33:20 pm ---There are also a couple of 'lytics in the chain. C1 is 100uF and C8 is 2.2uF. I measured them in circuit at 1KHZ. C1 was pretty much spot on @ 102, with an ESR of .27ohms. C8 was a different story, but I'm not clear if it would matter at all for the disappearing 5V rail. It measured a whacky 102 uF. ESR was so-so at .65ohms. Again, maybe worth a second look, but I can't see this pulling the 5V rail down since it's not dead shorted.
--- End quote ---
They're in parallel in circuit, so you're effectively measuring both each time, along with the PCB trace between them that adds a little ESL.
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