Products > Test Equipment
Fool for the 8656A Sig Gen
jrharley:
HP8656A update on a Friday night!
A10 C13 & C14 replaced with brand spankin' new Vishay Tantalums now, because it had to be done. It could just be me, but the unit seems to "cycle up" faster now. Not sure if that's real, or just my perception.
Anyway, I've removed the A10 U6 chip, no problems. De-soldering gun is doing the deed nicely. The chip just fell out and the pads are clean. Love it when that happens!
After those moves, the power up exhibits a dark display and a 1.8 V .....5 V power rail. I'll remove U10 next and see what happens.
JRH
Tony_G:
Had a bad week so I'm looking forward to hearing some news from you - Thanks for making my day a little brighter
jrharley:
DRAT! Sorry to hear it Tony_G.
For what it's worth, I'm sure if you wait a minute, I'll do something so head slappingly stupid with this unit, that it would bring a smile to ANY techs face. :)
At any rate, I pulled the A10 U10 chip this evening. Same results, which is honestly not unexpected. That's not to imply that I actually know what the heck I'm doing here, just that my gut is telling me that I'm chasing an unlikely component failure. Incidentally, I've restored the J2 connector to original status and I'm thinking I keep it that way, with pin 2 back in place. I'm thinking it doesn't really matter if taking pin 2 out of circuit allows the 5V rail to get up. It's still got the problem. It's an interesting clue, but I think I move forward testing the unit with all wires in place.
The question I'm pondering now is, do I put U6 and U10 back in before moving ahead? Or do I pull the next victim with them still out of circuit. Maybe U3 should be done before I get to that question. Not sure.
I'm Also wondering if there's a safe way to "bridge" the missing attenuator contacts, to at least put the unit into a defined state. Not sure if that makes any sense or not. It just makes my teeth itch that there's no connection across that section.
As always, the feedback I get from you and others, is invaluable, and I really appreciate your interest in this long shot project.
"Chin up" Brother, hope things smooth out for you.
JRH
Tony_G:
Thanks - Appreciate it.
You're not chasing a "ghost" problem - You know that the 5V rail is being dragged down by something, what you are doing is methodically checking components that can cause the issue - So I wouldn't consider it wasted time.
One quick question - Did you check U2 (located somewhere in the unit, probably towards the rear)? Confirm that the unregulated voltage stays well above 5V and the regulated side drops when Pin 2 is reconnected on J2? It's the 5V regulator and has the part number LAS-1905 (should be 5V +/- 5%) - It's not unheard of for the failure mode to be dropped regulation under larger loads. There should be a vast array of TO-3 regulators that can be used in place - Parametric search time... I'd be inclined to bodge something in place that matches the headline specs to see if the same problem goes on - Maybe measure the current being drawn on the regulated side to see if it is a reasonable amount, that sort of thing. It should be capable of providing up to 5A (probably some more but spec'd only to 5A).
You've tested the caps on the rail per SS14 and the caps on A10 so you should be expanding on what might be causing the problem, again by methodically ruling out components that could contribute to the issue.
It really is "Lather, Rinse & Repeat" now. Sorry :(
TonyG
jrharley:
Thanks for the pep talk TG!
It's too funny that you mention U2! I was removing U3 last night, (no change by the way), when I happened to notice an IC labeled U2, down on the far corner of the A10 board. I hadn't noticed it before and I still haven't found it on the schematic, but it was real evident from the back of the board and obviously right in the power circuit. I dug into it a bit and it's an LM393 N, a voltage "comparator" apparently. (But still a regulator of sorts). I've always felt confident that the main regulator U2, located near the transformer, was fine, but I was unaware of this little guy.
So I buzzed out some voltages on it. The 2 outputs read 0 Volts basically. No output. Pin 8 read 23.4V. I'll check the inputs just for grins next, but it seems like the LM393 is knackered. This just might be a good thing, I'll report back when I discover something else.
JRH
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