Fool for the 8656A Sig Gen

[jrharley]

Hey Tony_G, thanks for that. They're as good as they can be I suppose. Anyway, appreciate the thoughts on the testing as well.

The BK 2615A Spec An is a really basic unit. I got it for less than a Franklin, shipped. BK Precision seemed to have a bit of a niche back in the day supplying schools and training programs with reliable, rugged equipment that handled the basics well, were easy to use, but dispensed with the bells and whistles. This seems to be one of those. No tracking generator, no start/stop, just span. It's perfect for me to get my feet wet with this kind of analysis. I think the resolution is +/- 2 dB. (on a good day  ) As a side note, my analog scope is a BK precision 30 MHZ, 2-channel, of the same ilk, and I've loved using it. I got it as NOS at a Hamfest for $30 a few years ago. It still had the screen protect film on it. No complaints from this bodger on the BK gear. It should get me in the ballpark no?

As for the 8656A, I'm probably a week away from looking at the performance. Right now, I'm elbow deep into a tube amp chasing down some missing voltages. Why does that sound familiar? 

Thanks for the interest and I'll post up soon on the test set-up.

Later,


JRH

[jrharley]

The Fool for the 8656A  Sig Gen Declares “Victory”–

Four score and seven months ago……..  I started trying to get an old EBay Sig Gen back into service. While I wouldn’t ever claim that it’s ready for a real Lab at this point, I’m pleased to report that for the time being, this thing is doing what I need it to do. That’s victory to me! As is often the case with projects, the journey never goes exactly as one might expect. Such was definitely the case with this one.

Thanks so much to all of you folks who weighed in on this effort! -bdunham7, swainster, et al. It’s great to know that there’s a community of like minded brothers and sisters out there who appreciate the efforts to keep this old stuff alive. Special shout out to Tony_G for his particularly comprehensive and consistent feedback on the project. The journey benefited from his input immeasurably. I encourage all following this thread to check out Tony_G’s work on You Tube. He has a number of thoughtful and knowledgeable videos worth watching. - https://www.youtube.com/@tgsoapbox/videos

The first attached photo shows the 8656A outputting a 90 MHZ, -20dBm signal into my 100 MHZ frequency counter. Second shot shows the counter display. I think I’m good with this for now. I tested the output down into the audio range, which is where I’ll most likely be using the thing honestly. This is, of course, well below its specified bandwidth. I’m very pleased to say that it was very stable and accurate down into the lower ranges as well.

As for the Calibration on it, I could do a few of the maintenance adjustments with the gear I have, but not all of them. At the moment, I’m going to move on to my bloated project queue, before I worry about trying to eek another digit out of the HP. Simple priorities at work there. That said, I welcome any inquiries about my 8656A experience, if I can help anyone out with their own efforts by way of photos or observations. etc.

On to the next boat anchor........

Later,

JRH

[Tony_G]

This is great to see - The journey is worth even more when the destination turns out to be what you wanted - Happy to help out however I could on this - Congrats.

You can now get your full member badge for the TEA thread by acquiring the gear needed to "calibrate" this and then the gear to fix and calibrate that gear and, in short order, you'll have a bunch of other jobs written up on the whiteboard for you to do when you have "free time"

Look forward to seeing your next project post...

TonyG

[Swainster]

Well done JRH, it's always satisfying to finish a long running project (or even a short one, so I'm told )

Just in case anyone is interested in more HP 8656x action, I've started a thread about my own signal generators: https://www.eevblog.com/forum/repair/hp-8656b-repair-attempt-ongoing/. It wont be as interesting as this thread as I don't think there is much mechanical stuff on the 'B, but we will see how it goes.

[VK5RC]

Thanks to jrharley for some personal messages helping me fix my old HP 8656A with a dodgy attenuator. This post and LazyElectrons' YouTube were very helpful. I thought I could add some other issues that came up in my repair.
Disassembly went pretty straight forward until I was trying some different o ring sizes when I broke the attenuator fork! Plastic doesn't glue well so I had to embark on a learning curve of casting and moulding - guided by my close friend Richard - a retired dentist.
Repair was good enough to hold together for a 2 part silicone mould. See picture 1 below.
First efforts were porous (bubbles) and weak so de-gassing in a vacuum pot, and then reinforcing the shaft with some kevlar - pinched out of some ultra light antenna wire. See photo 4. Results seemed ok. See photo 1 of following post for old and new cast fork.

The epoxy was JB Weld, a filled epoxy. The mould was pre-sprayed with a releasing agent. The mould had 3 vents to allow bubbles and over flow to escape.
I found Robert Tolone's youtube channel good for teaching about moulding and casting.

The actuator arms/forks are compressed by a cover (with screws torqued to a very low level) so the height between the top of the ball bearing - that slots in a groove in the top of the fork and the lug underneath the fork that engages the gold plated switch underneath is critical.
Measurement by micrometer of the cast models were pretty close mostly within 0.05mm of the original. (4.43mm). The ball bearings are 2.5mm I am pretty sure - they measure 2.498mm.
More stuff to come...

[VK5RC]

I used a pair of metric o-rings to replace the original hp grommets, I used ID 3mm, 2mm thick o-rings but sanded them down to about 1.2mm using the same technique as jrharley - stuck the o-ring to aluminium tape and used a disk sander. Lost a couple in the shed!
See photo 1 in above post old grommet on LHS new o-ring type on RHS.
Reassembly was a bit tricky - my actuator cover - when it's screws are loosened - doesn't lift up much but by prying it gently up with a screw driver and angling the fork in - see photo 3 below - and with a little encouragement from a long fine flat blade screw driver advancing the grommet part of the fork into the actuator mechanism.
You can confirm that it is lodged by pulling and pushing the fork and you get a reassuring clunk of the actuator solenoid.
Final reassembly was pre-dated by cleaning in IPA and a bit of de-oxit D5 on the contact surfaces.
Testing so far looks good 👍
I have a few spare forks and are happy to forward to fellow HP 8656A restorers.
Again thanks to jrharley for his PMs and this post
Rob
VK5RC

[jrharley]

Great work Robert! Thanks for adding to the information on this notoriously problematic component of the 8656A. The information on the tolerances is good to know.

I think I've come to the conclusion that, even though the attenuator approach seems pretty whacky within the context of more modern equipment, those actuator forks and the solenoid assembly, seem to be pretty tolerant of small dimensional variations mechanically. Nice to see someone go the distance on molding replacement parts as proof of concept.

Well done!

jrh

[Tony_G]

That's outstanding guys - Well done.

When you created the mold, did you 3D print the piece originally? I'm just wondering if you might be able to get away with just printing using something like PLA-CF?

TonyG

[VK5RC]

@TonyG - I used the original glued (epoxied) back together fork  for the original in the moulding process.
I don't think the resolution of a 3D print - unless an exceptional printer - would be accurate enough - and the strength would also be a bit questionable - the 'neck' of the fork - just before the base that holds the grommet is only 2-3mm thick - my repair deliberately made it a bit thicker there - maybe 3-4mm and that is where the kevlar is in the epoxy. Getting that shape into your fav 3D program would be a right PITA as well.

Talking to my dental mate - some dental practices have 3D laser scanners attached to CNC mills that can scan then make your next large filling or section of tooth from ceramic blanks. They have to be super careful as the forces that go through a tooth when you bite down are huge.
Dentists are a fantastic resource about making small things and their materials / bonding knowledge is amazing 👍