Fool for the 8656A Sig Gen

[jrharley]

Greetings EEVBlog forum - A newbie here, hope everyone is well and gettin' their nerd on. I'm a hobbyist focused on vintage audio and test equipment. Professional background is Architecture, but I've obtained a six year degree from UYT. (Thanks Dave, Paul, Tony, Clive, et al!).

I recently acquired an HP 8656A Signal Generator. Couldn't resist it for the price. Sold as a parts unit, but in pretty good shape overall. Serial number : 2146A02249, which I believe is a later model. Despite it's reputation, I'm going to give the thing a go. All 4 power rails appear to be good to go and the display lights up with basically nonsense. No response from any key. I went straight to the attenuator assembly for a quick look and the fault became obvious. The unit is missing two actuator rods and the actuator cover. The gold contacts for the actuator however, were bagged and stored in the unit. Since these parts are no longer made, my only options are 3D printing or getting lucky with an identical trashed unit that I could pull from. The thing that complicates the latter approach is that this attenuator assembly appears different than others I have seen, which have ball bearings on the forked actuator rods. Not so in my version. The ball bearings have been eliminated from the design.

So, finally getting to my question - Does anyone have a reliable source that could take a couple of the parts I have intact on other sections of the attenuator, scan them and 3D print me some duplicates? I've browsed around and it looks like the capability is out there. Tolerances and material selection are more detailed topics for discussion, but I need to start somewhere. Any suggestions?

Thanks in advance for any help! - JRH

[bdunham7]

Mechanical failure of the attenuator shouldn't cause anything other than incorrect/missing output levels.  You might want to sort all of the other problems out first before you go after that attenuator.  As far as the attenuator parts, I'm only familiar with the grommet/ball-bearing design, could you post good photos of what you have?  Is there a revision shown in a manual anywhere? 

[Stray Electron]

   Where are you located?  I think there is (was?) the same unit in a surplus place about 60 miles from me.

   FWIW I hope that you got it CHEAP!  I hate buying TE that other people have been into. There is almost always parts missing or or it's damaged beyond repair.  When it comes to Ebay or when buying in person, I always check to see if the warranty seals are unbroken on the covers. If they're broken then the value of the items goes WAY down unless they're giving me a guarantee that the TE is working.

[jrharley]

Thanks for weighing in Stray!

Yeah, I hear you. I did get it cheap, and wasn't expecting a working unit. My enjoyment comes from getting these boat anchors up and running again.
Appreciate the feedback. - JRH

[jrharley]

A very good point bd, and food for thought moving forward. I had assumed that since the missing parts left an open circuit in an essential part of the unit, that I ought to start there, but your comment makes a lot of sense. I still have a lot of studying of the intimidating schematic to do, as well as researching revisions ahead of me. I've attached a photo of the plundered attenuator cavity.

Thanks so much for weighing in. -JRH

[jrharley]

Greetings all,

Hope the summer was a good one for everyone.

I'm back to the HP8656A for more punishment, studying the circuits and theory of operation. I'm having difficulty getting any datasheet, pin-out, or other technical info on the microprocessor, U13. Does anyone have a data sheet for the Motorola 1820-2099? I'd like to do a little probing on this guy but don't feel great about doing it blind. Any resource suggestions for such info would also be appreciated.

To briefly recap, the units power supply rails all seem to be solid. Unit does not default to the standard display settings on power up. All keys are unresponsive, but all display segments and LED indicators appear to illuminate. I discovered where the previous bodger had removed a wire from connector J2 and when I jump this back into circuit, the display goes blank except for a single "1" in the amplitude domain window.

At any rate, any assistance in getting a datasheet for the Motorola would be much appreciated!

Best,
JRH

[bdunham7]

I believe that is a Motorola MC6802P plastic PDIP-40.

[bingo600]

I believe that is a Motorola MC6802P plastic PDIP-40.

The X-Ref i have says that too
1820-2099   IC   MC6802

/Bingo

[jrharley]

Thanks guys, you are both spot on from the looks of it.
For some reason, I didn't get notifications of replies to this post, so I've been busy on my own trying to find it and doing some careful examination of the boards in the meantime.

The MC6802 datasheet however, is easy to get and fills the bill. Really appreciate it!

Is there a reason that any of you can think of, why I shouldn't just put a new processor in the socket and see what happens? Yeah, I'm still tracin' and reckonin', but I'm really only a hobbyist in the end, let's face it, this beast is over my head. This is no old Heathkit signal Generator!

Right now though, I don't recognize anything in the circuit that could ruin my day by doing that. Fuse F1 had been jumped with a solid wire when I found it. I tacked a 2A cartridge fuse into that spot for testing, the original is listed as 4A.

Anyway, thanks a lot for the help and the info! I'll post up with any relevant updates moving forward.

JRH

[Tony_G]

Welcome to the world of HP Signal Generators...

Just a couple of suggestions:

• If you haven't already then I recommend that you get the Artek Manuals scan for the unit. They're way better than the scans that appear on the Keysight website - http://artekmanuals.com/manuals/hp-manuals/
• The manuals for this era of units are generally excellent. Section 8 of the manual will take you through the troubleshooting process in pretty decent detail. I'd start working through their steps rather than just trying to address issues as you find them.

Good luck with the unit, look forward to seeing how it turns out.

TonyG

[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

[Tony_G]

Replying to my own post to make sure you see this (not sure if you get re-notified if I modify the post after you've been notified originally).

I created a searchable version of the 8657B service manual for you (the one on the Keysight website isn't searchable) - You can grab it here:

https://1drv.ms/b/s!Amqar8_XQ9Uzk48LDIJuGPej5NWrWA?e=RXh0me

TonyG

[cncjerry]

where are you located?  I have a signature meter.  I don't know the 8656 but here's my quick scan of the thread:

1) does it have option 1, 01, 001 listed on a badge in the back? That is usually the high stability time base, basically an HP ovenized oscillator on a board you plug in. Without it, you usually need the bnc strap between the posts on the rear that would be labeled like 'ref out' to 'ref in".  Sometimes you need to strap them if you have the oscillator.  Many pieces of gear are scavenged for the oscillators.  I pulled them out of selective level generators that were shot.
 
2) if it is halting on post, it could be a rom chip if it has them or something hanging the signal lines. 

3) Your symptoms look like they are related to a power problem though.  I've had a few generators that when your probe the power near the PS board, they look great but you need to check at some of the chips.  sometimes the cables running from the power supply go bad, I have a couple of older generators that need cabling.

4) someone else mentioned about the shafts and I agree, that is the least of you worriers.  I have a 3d printer and CNC gear but do you have the complete parts to copy?  They would have to be drawn as I doubt scanning would give you what you want dimensionally.

I would go back and probe the power pins around some of the ICs.  If the display is getting anything on it then it looks like it is running but hanging on code someplace, possibly from a bad rom chip?  I don't know if I would purchase a signature meter to fix one piece of equipment.  I got mine cheap and I think I've used it twice in 10yrs or so.

Jerry

[jrharley]

Wow guys, thanks for posting up on this!
The effort continues. I'm starting to learn the language for navigating the HP manual schematics and info. Lot's of study going on. Not any soldering.

TonyG- Thanks for the link! Good to have. Not sure if there's a test switch in the system as you describe, but I'll keep an eye out. "Thou shalt test voltages" right? So yeah, I'm probing wherever I can and pouring all over this thing. Some interesting results and on going discovery. I'm starting to learn how to run things down in the info efficiently so, stand by for more updates.

cncjerry - Appreciate you weighing in! I'm in Florida, Tampa Bay area. Also appreciate the thought on loaning the test gear, but I'm realistically a ways away from any signature analysis level troubleshooting. I tend to share your instinct that this is a power related issue and so I'm covering the basics first. I'm currently in the "thorough visual inspection" phase of this project, along with basic probing. The manual has a pretty simple series of tests using a scope to evaluate the signals from the keyboard to the rest of the circuitry. I'm focusing on that thread for the moment.

Thanks again all, I'll keep posting anything relevant.

Best,

JRH

[Tony_G]

Just thoughts I'd check in and see if you've had the chance to get any more data etc

[jrharley]

Hey tony G, thanks for reaching out.
Not much data yet, I've taken a sidetrack to clean keyboard switches and get to know that section. Repaired a missing stand-off assembly on the keyboard stack that was allowing too much flex when some keys were pressed, but I got that stabilized. The more I look, the more evidence of rough handling I discover. It's hard to understand why equipment like this gets abused, but it does.

I've attached a photo of a connector that I found disconnected when I opened it up initially. I've been studying the circuit associated with it and have started doing some simple tests on the keyboard per the manual procedure. The good news is that I do get a 5V DC offset when I press any key. So far though, I'm not getting any evidence of data packets coming out of the keyboard, but I'm still working that, I could have my triggering set incorrectly on the scope. Anyway, comments are always appreciated and I'll keep posting any relevant discoveries.

Thanks for the interest!

JRH

[Tony_G]

I can see how that would be a problem...

It would be interesting to see if you were getting clock into the keyboard shift registers (I think they generate the pulse train but I'd need get the manual again to confirm).

TonyG

[Tony_G]

Long time, no hear (actually I was on the road in MO and UT and then out at the cabin here in WA so I was pretty much out of contact and thus my fault).

Thought I'd just reply to see how you're going? My 8657B is probably my fav of the HP sig gens I have so I'm interested in seeing you get this one going again.

Look forward to hearing from you.

TonyG

[jrharley]

Hey Tony G, no worries, thanks for checking in.
Although I've had to take a few unrelated sidetracks recently, I have discovered something that I didn't notice before. Not sure how I could miss it. At any rate, I've discovered that when I jumper pin #2 into the circuit, (see the photo I posted a few comments ago for the connector), The 5.4 V rail indicator LED goes out! Or at least very dim. Something is pulling that rail down when pin 2 is connected. I simply didn't notice that before. So, I've been walking the dog along the schematic path looking for potential culprits. This means that every surface in my shop is covered in schematic diagrams! I can't use a lack of documentation as an excuse, that's for sure! Anyway, pins 1,2 and 3 on this connector are all originate from the 5.4 V rail, but only #2 causes this. The next stop on the path for pin #2's signal appears to be an IC labeled "DEMUX" on the schematic, which I guess is a de-modulator. There's also a diode straight to circuit ground in the path too, and I'm currently hunting that one down to test it since it just seems more unlikely that it would be an IC problem. Then again, I'm really green when it comes to the digital world.

So, that's where I am. I'll locate and test the low hanging fruit on this signal path and report back with any news. Thanks for your interest! I'll be struttin' around like Richard Pryor and Gene Wilder if I do get this beast working, and I really appreciate everyone weighing in. Best regards,

JRH

[Tony_G]

I decided that I couldn't help anymore without getting the manual so I just bought the Artek copy - Where are you looking at the supplies from J2 (so I don't need to go find it all myself )?

[jrharley]

Thanks TG, I really appreciate your interest in this misadventure!

I'm in the middle of getting my bearings on exactly what you're requesting, so, standby for that info.

At the moment though, I'm replacing the battery on a laptop that was clearly designed to discourage anyone from doing just that, including service techs. Gotta' get this out of the way before diving back in to the HP.

For what it's worth in the meantime, I started with the power supply schematic, sheet 22, page 8-127, 128 I think. J2 is shown in the lower right corner of the diagram and for pin 2 you follow to the "H" in other section schematics for where pin 2's path goes. Yeah, I'm still trying to learn how to follow the bread crumbs......

More info soon,

JRH