Fool for the 8656A Sig Gen

[Tony_G]

Interesting - I wonder if there was a more complex and tolerance dependent way they could have done it...

TonyG

[jrharley]

I know right? It's a pretty whacky gizmo.
I can't imagine what advantages this approach might have had, but who am I to second guess HP! It's a product of its time I suppose.

Anyway, close ups of an actuator fork are attached for those playin' along at home.........


JRH

[Tony_G]

Do they list the torque value anywhere? I don't see it in the manual.

[jrharley]

I can't remember where I saw that TG. Perhaps on the lazyelectrons site. Upon further reflection, it could also only apply to the later versions. I know I saw it somewhere though.

I suspect that ultimately though, the specification is going to be something like ....."just right". I don't have any way to torque a screw down to precise inch/lb values. Not likely to buy a device that could do that either. I'll tweak it under power till they work!

I've pulled all the covers off and removed all the forks. I'll be spending some meditation time cleaning up the cavities and rebuilding the intact sections while I figure out how to get some replacement parts. May have to park it for awhile, we'll see. O-rings are going to be a short term challenge. I have good dimensions, but need to source some new ones.

Thanks so much for your continued interest in this journey, we may just get it running again yet.

JRH

[Swainster]

I've not seen how they work but guestimating from your pics they look roughly the size of a 3mm cable grommet

[jrharley]

Not far off swainster, thanks!
It seems that the right size for replacement has an outside diameter of 5-6mm and an inside diameter of 2.5-3mm. The tricky part in this case is the cross section of the ring. Based on my measurements of the area at the tip of the fork that retains the ring, the 2 o-rings I'll be replacing the original with, should not be much more than 1mm thick. I've been experimenting with some rings from the stash that fit pretty well, but they're too thick to place two, side by side, at around 1.7mm. The original o-ring was a single ring, with a slot around the circumference. I think the actuator plate from the solenoid rode in this slot on the o-ring, and either pushed against one side, or pulled against the other, to slide the fork.

It'll take a little doing to find the right rings, but we'll get 'em. Could be time for an account at McMaster-Carr...... (The kits all over the interwebs aren't all that appealing so far).

JRH

[Tony_G]

+1 on McMaster-Carr - Have bought from them before and they've always delivered quality parts - This is something that I guess, my economic privilege, allows - Buy from professional providers and you'll get what you order - Buy from eBay or Ali and you'll get what you pay for, maybe...

TonyG

[jrharley]

HP 8656A drama continues...

McMaster - Carr is awesome. I could quickly order 3 different sizes of o-rings, 100 to a pack, for less than $20 including shipping, delivered next day! The "Goldilocks" ring was right where expected in the lot and fit good.

Still too thick though. Getting 2 of them to fit on the fork was not happenin'.

The original part is really best described as a "Grommet". But it's a pretty small one. Haven't seen anything that would be a direct "drop in". No worries though, I think. I found that I could shave the o-rings down to about 1/2 their width and get a pretty good fit. Things are seeming to fit together pretty well in the cavities.

Pics on the way.

JRH

[Tony_G]

I had no idea that there would be a store like this but maybe you can find something here

https://www.theoringstore.com/store/

TonyG

[jrharley]

Thanks Tony_G! I did come across that company as well. Could be an option. Their size classifications and descriptions are a little impenetrable, but they could ultimately be the right source, we'll see.

Meanwhile, I'm just about re-fitted with the existing assembly. I've figured out a way to use the o-rings I have, into at least a shot at getting something to work.

Update pics attached.

Thanks always for the feedback and interest!

JRH

[jrharley]

HP8656A update here ya'll,

I've got the existing components in the attenuator assembly re-installed with new "grommets". Cleaned the traces, and bolted everything back down. We'll see if things are back in place correctly, it was a little surprising how easily the assembly went back together. I still have the gaping maw of the missing section to deal with, but its siblings are at least now; all ready for a power up.

As for what to do about the missing parts, I've reconciled the available options for my situation I think. Tough choices no matter what. Balancing time, money and feasibility seemed to point to a parts unit as the best option overall. It should arrive Monday. As an interesting aside, I naturally did what anyone would do and said to myself, "how much is this thing worth versus just getting a modern one?" Pokin' around the web quickly affirmed the decision to spend just a little bit on this beast. Using bandwidth alone as a comparative criteria, even a modern unit from Asia is north of a $1000. I'll take the 8656's interface any day for that money.

Meanwhile, still rubbin' on the Delco......... I'll post up when the A9 board is re-populated and we're ready to see if we've got any "clackin'" going on.

Later,

JRH

[jrharley]

8656A update -

Well........ in a nutshell, everything has changed with the arrival of the parts unit. Much to my surprise, the parts unit appears to be functional. Since it doesn't make sense to destroy a functioning attenuator to fix a broken one, we are re-thinking everything at this point. It will be a little bit before I can figure out how to proceed, but in any case, the strategy is now to make the best unit I can out of the two units I have. I'll report back with progress when it's informative. Appreciate the interest in this project and all the helpful feedback!

Later,

JRH

[Tony_G]

It's always the way, you go from less than one working unit to more than 1 working unit

I would probably approach this by addressing anything majorly wrong with the working unit - By that I mean, are any chassis rails bent, how is the N type on the front, Fans not working, etc - Swapping good parts from the non-functional unit into the working one.

Then I'd address the cosmetic issues like case panels, front panels, buttons (if there are labels, wear, etc), and so on. The goal is to get the best looking and working unit and then you can either keep the parts mule as a backup or sell it (and clearly state what is missing, damaged, etc).

Personally, I tend to keep the parts mules because this stuff is old, and having it go tango uniform is not unexpected - When I end up with 2+ parts mules then consolidate and sell off or recycle so I have 1 parts mule.

Look forward to hearing what you end up doing - And the follow-on test/calibration cycle...

TonyG

[jrharley]

Of course Tony_G, all of that makes perfect sense in light of the new situation. It’s always very interesting to observe the differences between units, with each one having a different story to tell. Here’s what I’ve observed so far, with “unit A” being the original unit, and “unit B” being the new unit:

The two case designs are different, with unit B more closely corresponding to the ARTEK manual drawings. Unit A has better component cosmetics all around, so combining them is going to improve things.

Unit A’s filter caps are in pretty good shape, unit B’s, not so much. They seem to have seen some heat in the past, though they don’t seem to run hot with a quick test. The listed 13,000 uF cap in B has been replaced with a 24,000 uF unit, which doesn’t test great, …ok, but not great. My current thinking is to just swap cap sets.

The F1 fuse in unit B appears to be original, but is 5A @ 125V, not the listed 4A.

I’m sure that the attenuator grommets in unit B are wearing out, as they do, but I’m going to let that Dog sleep until I can’t.

I will be swapping out the faceplate/keyboard assembly from unit A onto unit B. along with feet and handle hardware.

Quick aside, and maybe I should post this on the HP forum, but does anyone know what the “warm gray” color numbers are for this era of HP gear? They used it a lot back in the day (80’s) and I assume it comes from the Government Standard 595 colors, but don’t know. I’d like to source some touch up paint in that color. The “Beige” color of the same era might also come in handy.

Anyway, that’s just a little “color” on what’s going on. Always appreciate any feedback.

Later,
JRH

[Tony_G]

There are a number of posts on the HPAK group about the grey color - Worth just doing a search.

TonyG

[jrharley]

Thanks for the link Tony_G !, I've joined the group and have started to browse around. Nothing "drop in" as a solution for that paint color yet, but there are some source references that could prove useful.

Anyway, posting up with an 8656A update here on a Sat-dee night!

Feelin' pretty good about where I'm at right now; a functioning sig-gen is nestled into its spot on the bench, awaiting some tests. And it's lookin' pretty snappy while doing so! The "swap about" went well and I've buttoned up the original unit for storage, complete with a status sheet to remind me of how it sits whenever I re-visit it for whatever reason. The swap was basically 3 stages: Filter cap swap, Faceplate/Keyboard assembly swap, cosmetic parts swap. The attached pics are about the first swap step. Next pics will be the keyboard swap.

Later,

JRH

[Tony_G]

Good to hear - Look forward to seeing shots of the finished work and then the results of testing...

They have several posts over at the HPAK group on that color but HP changed the color over time so you have to find the right era to ask. You could always try just taking your panel to a body shop and having them scan the color to give you some matched paint - Might not be the exact new color but would be the right color to blend in on the unit as is.

TonyG

[jrharley]

Yeah, we'll keep pondering the paint thing as we go along. Should be able to come up with something. If I get a good solution, I'll post it up for reference.

I'll be doing a basic start up check on the HP in the waning hours of tonight that I have left. Then I'll start to see what section V of the manual has in store for me for "maintenance adjustments" in the next couple days. So far, the beast seems to be pretty accurate. But then, I don't push it by any stretch. I mostly live with frequencies from the FM band down. A couple pics catches step 2 of the process.

As always, thanks for the feedback!

Later,

JRH

[Tony_G]

Looks good for its age - I have a bunch of spare feet if you need a couple (most of my gear is in a rack so I don't need them).

hoping for some good results from your tests.

TonyG

[ekoloski]

Looking great on the restore!

It makes sense to take the best parts from the two and get one unit back into full working order.

The question then becomes, what to do with the spare? Keep it for parts, or fix it up as a standby. From the sounds of it its just a few caps and the pesky attenuator that need to be repaired? I haven't seen the innards of the attenuator in this model, and I've only got a B to reference, which is entirely different. Could you post a picture of one of the forks?

Looking over the previous posts debating the feasibility of 3D printing a replacement I have to wonder if a FDM print using ABS or HIPS may be an option. They're more difficult to print with but may be better suited. Unfortunately do still absorb moisture to a degree, although less than PLA. Admittedly I am a newb in the 3D printing world, I'm sure someone else can chime in with their thoughts.

[jrharley]

Thanks for weighing in ekoloski!
I got a notification a few days back that showed you had commented, but I couldn't see the post for some reason, didn't seem to show up. Anyway, appreciate the comments and encouragement. While documenting and packing up the original unit for storage, it did occur to me how close the guy was to being up and running its own self! I'm confident it could still serve. Meanwhile, it'll be a good back-up for reference if nothing else. As for the forks, there's some pretty good pics of one right at the upper part of this page, reply #151 I think. Browse to either side of that for additional pics on the attenuator and parts. I'm not a 3-D printing ace either, but the more I examined the forks, the less confident I became that they could be successfully printed.

Thanks Tony_G, appreciate the thoughts and the offer! I never forget! 
 I'm going to take a brief side-track before getting into the maintenance tests on the HP though. I've got that A-300 tube amp to move to the next level. But for what it's worth, several spot checks on the output through my digital scope, showed the 8656A to be bang on with the frequency. I was able to calibrate up the little BK Precision Spectrum Analyzer, no problems. It involved switching back and forth between precise db levels. The HP is awesome for something like that. What a pleasure to use. It feels like my HP 15C calculator, which I still use daily!

Final cosmetic step pic attached.

Later

JRH

[Tony_G]

Looks nice, well done

[Tony_G]

I was taking a look at the performance tests and you're probably going to be a bit SOL on a lot of the tests seeing that you don't have the test gear.

That said, you should be able to do an OK job on the frequency and power output with that BK SA - I've never used one of those but you should be able to do the harmonics with it as well. Not sure how it will go when you start getting down to low-level signals.

The modulation stuff needs some form of modulation analyzer (the manual calls for an 8901A but if you have access to an 8902A then that unit is basically an 8901B with more features, mechanically very similar and I used one to fix up an 8902A I have) - A few tests call for a 11715A AM/FM source and these things are rare and costly to acquire (going for ~$4K on auction sites). I wouldn't bother with those tests.

I think you'll mostly be fine with power & frequency accuracy, the only thing you might need is a true frequency counter to get the 10MHz reference tuned correctly but you can actually work around that by using something like what Alan does here:



Look forward to seeing how you go.

TonyG

[jrharley]

Awesome Tony_G! Very interesting stuff.

A brief glance at the test procedures in the manual, did make my eyes roll back up into my head, but I haven't really had the time to focus on it yet. Thanks for the preview!

I've got to take a break from my passion for a few days to travel up to Georgia to help my Mom. Family stuff. Should be good to clear the mind.

Anyway, I've got a decent counter on the bench, but the video really has piqued my interest, so we'll see how she goes when I get back. I'm not going to be a metrology Nazi on the HP, it seems to be working great for my needs, but I do want to get it as ship shape as possible. I'll post up with anything of potential interest.

Have a great Easter break all......

Later,

JRH

[Tony_G]

Hope you had a good break and things with the family were good too.

If you look at the stats on the SigGen it really is only +/- 1dBm or so which might be within the specs of your SA.

Modulation is really the only thing that you can't specifically measure but you can work around that to get an idea of the output by modulating a signal and then either analyzing the audio output (which is basically what the 8901 does when it demodulates) with a PC audio card - This won't be accurate but really for non-lab work it is close enough.

Look forward to hearing from you with pic's of the cleaned up working unit

TonyG