Products > Test Equipment
Fool for the 8656A Sig Gen
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jrharley:
Thanks Tony G!

Appreciate you weighing in. I'll get the Artek manuals to supplement my file, nice to know of that site.
Onward....


JRH
jrharley:
Greetings HP Guru's
Update on the 8656A project. Mostly, I've been learning how to read the HP schematics and trying to relate them to what I'm seeing in this beast. Kinda' slow going. Some progress though on a few fronts. I've determined that, even though it looked like someone had removed the high stability timebase option 001 upon first look, the unit never actually had the option installed. Maybe they tried to install it and decided not to. Hard to tell. But the factory label doesn't list the option and I don't see any indication it was ever installed, so I think that is settled. I was investigating this with the idea that maybe if someone removed option 001, they could have left a glitch that was contributing to what I was seeing on power up. That appears to be a dead end.

I swapped out a new microprocessor. It had only a very subtle effect on the display operation. I can go into that subtle difference later, but wanted to post a picture of the display on power up to you guys to see if anyone recognizes the "symptom". Keys are unresponsive - all of them. Obviously the circuitry driving the LED segments is good.

I'll be continuing to try and learn things on this, but check out the power up display and let me know what you think. Much appreciated!

JRH[/img]
Tony_G:
I don't have an 8656A - Only the 8657B which, as an aside, now has an FM problem that is on the whiteboard to get around to fixing...

Anyway, I'm assuming that you did the normal check power rails thing (including ripple because they specifically call that out) and if they're all good then you have pretty much moved from the troubleshooting steps in BD1 (as they tell you to bail before checking outputs, etc if the unit is frozen) - As an aside, it would really help out if you provided the results of the step checks there as they have very specific results for the internal indicators on things like the Power-on Check (I don't know about anyone else but it'd help me out at least to understand where your unit is failing in the start-up process).

Anyway, I'm working from reading the manual here so forgive me if I'm not adding any value, it looks like you should be exiting BD1 in the "Power-up Check" section. I think that you're failing section 1 there (or somewhere near that) so you're instincts are right, I think, in that it is a digital issue. So in BD4 it seems like you're failing the "Power-on or Reset Sequence" part.

According to the manual from the Keysight website, you should be looking at Service Sheet 14 or 16 - However it appears that the manual they have is missing pages so I can't really provide much help (they don't have the start of the section) and, as is typical of the instruments of the era, they assume that you have a signature analyzer to test the board - I'm at a bit of a loss to provide any more commentary.

My recommendation though is to use the service manual to grab all the test point data you can and then, assuming that the manual from Artek with the missing pages doesn't provide any more insight, drop a post on the HPAK group asking if anyone has suggestions. Link to the group is https://groups.io/g/HP-Agilent-Keysight-equipment

Sorry,

TonyG
jrharley:
No, this is great Tony G! I really appreciate your feedback.

I'm going to spend a little more time with service sheet BD1. Checking for ripple makes some sense, I missed that. Bottom line though is the power up test is a bust for sure. The unit doesn't step through any of its self diagnostics on power up, it just goes straight to the display you see in the photo.

When following the service instructions, you're right, you quickly run up on the need for a signature analyzer in order to follow the steps. I'm keeping my eye out for one, but for the moment, my test equipment will have to consist of a 250MHz 'scope, a decent DMM and a simple Logic Probe. I also want to check the triacs and diodes in the crowbar protection circuit, but not really expecting that to be the issue since it's tied to the attenuator operation. You never know though.

I will definitely post on the HP site, that's a great resource potentially, so thanks for that. By the way, I think I've watched a video you did on the 8657B. Nice job. There are a lot of similarities in the designs so it was informative and I'll be returning to it for more insight.

So anyway, thanks again for the help and I'll keep you posted with any relevant developments.

Best,

JRH
Tony_G:
I'd probably start probing the microprocessor lines then - All the standard stuff, clock, address, etc - You're sort of stuck working outwards from there.

One thing that the 8657B has is a hardware-implemented NOP instruction to enable ROM testing - I've never used it but I think there is a switch somewhere on the processor board that switches from "run" to "test" (or words to that effect). You might want to make sure that it is set to run mode. Probably already is but worth checking. Assuming that there is such a switch on the 8656A. In fact, if the implementations are close enough, it might help to get the 8657B manual and see what it recommends in terms of direction for tracking down boot issues.

Good luck - I look forward to hearing what you find.

TonyG
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