Fool for the 8656A Sig Gen

[Brumby]

The other day I tried to list all the projects and repair candidates on my to-do list ...

I'm too scared to even consider that.   

[jrharley]

Thanks swainster! I would never have thunk someone would use a device on a DC load, that wasn't rated for DC loads! 
The kits generally do seem to use those devices though.
How do you tell on the spec sheets whether they will have a DC operating zone?

Thanks again for all the comments

JRH

[Swainster]

Hi JRH,

For DC operation I just quickly scan through the datasheet looking for the graph with a 'house' shape - typically titled something like "Safe Operating Area". If it doesn't have a DC line/contour then it pretty much means that the manufacturer has optimised the silicon for switching performance so much so that the DC performance is not worth even worth specifying. Not all switching MOSFETs are this highly optimised so its worthing checking for this graph even if the MOSFET has "switching" in the description.

Note that SOA is not just a MOSFET thing - it is fully applicable to power BJTs as well.

At the opposite end of the scale are so-called linear MOSFETs. As linear operation is not so popular, these tend to be rather pricey. I've attached a quick comparison of the 110N8F6 with the same package similar voltage ixys linear MOSFET to show the difference.

Just to be clear, you can use non DC specified MOSFETs in a DC application, but if you do, then it is up to you to characterise the safe operating conditions (and they may not have much relationship to the datasheet numbers)

[jrharley]

Great info swainster, thanks!

I had a few minutes to study the charts and it's much clearer what you mean now.

Tony_G is right about the philosophy of the journey for sure, I'll have to start planning the next one! Even though like you, brumby, and I suspect everyone else, there's no shortage of projects on the spreadsheet!

Hope to put in a new 5V regulator in the 8656A tomorrow night, so more to come.


JRH

[Tony_G]

Looking forward to what you find when you swap out the regulator.

TonyG

[jrharley]

SA-WEET EVER LOVIN' LIZARD !.....Look what we have....................................


JRH

[Tony_G]

Outstanding - Congrats - Now you can get to work on that output attenuator...

Or you can intercept the feed into A9 and measure there (which might be non-trivial as the schematic seems a little vague on whether there is soldered hardline or connectors).

Either way, great work.

TonyG

[Brumby]

   That's what my 8656B looks like after power up as well!

I love seeing old HP kit up and running!

In fact, I have another (incomplete) 8656B that will likely end up as a parts unit - so if you are in need of anything desperately enough to cover shipping* from Oz, let me know and I will see if I have it.

* If it's only little, I could handle that.

[jrharley]

Thanks Tony_G, brumby! And everyone else, jeez, what a fascinating Rabbit Hole! All the input has just been invaluable!

I've had a chance to re-fit the faceplate and stabilize things enough to flip it over into a proper operating posture, so I can play with it a bit.

It SEEMS as if the keyboard is functional. I'm able to set and adjust frequency, amplitude, and modulation like you would expect. I'm thinking this bodes well for control overall.

I've attached a couple pics of the regulator mount just prior to power up with the new 5V regulator.

More to come I'm sure,


JRH

[Tony_G]

Looks nice - It cleaned up well.

Just a reminder, if you can get me a model I should be able to get some of those actuators 3D printed for you.

Also, those keys should be the classic Bill West design so if now you're pressing them and you find they stick then CuriousMarc did a video on cleaning them on his YT channel.

TonyG

[Swainster]

Great news indeed! Interesting that the screw retains the cover on the 8656a. On my b version, there is just a clip on plastic cover for each package. Also, I don't recall the paint being removed where the regulators are mounted, but I haven't needed to do any work in that area.

Hi Brumby, spares unit you say?... Actually my 'B came with a spares unit which is only missing its power transformer (and a burnt out inlet filter). I understand that the previous owner of the 2 units 'repurposed' the transformer for a DIY project. I'd be interested in buying a replacement transformer to complete the unit. That said, posting a heavy item like that to Singapore is probably pretty painful on the wallet.

[jrharley]

Thanks again swainster, Tony_G, et al, it's a good place to pause I think.

I'll spend a little time cleaning up the scene of the crime, documenting the project file a bit and getting the shop set up for the next challenge on the 8656A. I want to run it through the primary checks for instance, to see what I can learn. Then of course I can start to look at the attenuator. Appreciate the offer Tony_G, we'll see how things go.

In the meantime I'm going to see if I can cal up the little BK Precision spectrum analyzer. I got it for a song on the bay recently. It looks like it had been mishandled by impatient hands, but after some work on things like shafts to potentiometers, it seems to be functioning on a basic level. The "calibration" procedure looks to be pretty straight forward and done from the front panel.

Report back on the 8656A in a bit.......

JRH

[Tony_G]

Fingers crossed for your SigGen workout.

I couldn't tell from the photos or the schematic, do those missing actuators disconnect the N-Type on the front from the rest of the generator or are you just missing the ability to add in attenuation?

TonyG

[jrharley]

Appreciate it TG. Here's a couple pics of the attenuator cavities on first open for reference. We'll be gettin' the sleeves rolled up for this area come this weekend. (I'm not sure I'm understanding your question just yet).

Had to LOL at myself upon diving in to the Spectrum Analyzer calibration. The irony is, that I really need something like the 8656A to do said calibration! We'll get back to that later....

Meanwhile, I'm lettin' off steam by rubbing on the cabinet of a 1948 Delco R-1233, all American five radio. Hoping to get it ready for car wax before weeks end. There's an astonishing amount of tar on it

Later......

JRH

[Tony_G]

I was wondering how the actuators work to switch in and out the various resistors for the attenuator segments - It appears that they slide back and forth rather than lift.

With those missing parts then you won't actually get a signal out of the N-Type connector on the front of the unit (ie while the unit may "work" there will be no signal).

Sorry, I can't help with removing that cover.

TonyG

[Swainster]

Haha, I was thinking that the spectrum analyzer would be useful to cal the sig gen, but seems like you have a chicken and egg situation.

Its always interesting to see how HP and others combined electrical and mechanical engineering, especially in older instruments. My Fluke AC calibrator has a front panel with belts and cogs and so on, all working together like clockwork, and of course there is the famous HP 8640 sig gen (I saw one languishing in a local surplus dealers office, on the random kit shelf).

Regarding your missing plastic parts, from the look of them I'd guess that they are made of injection molded POM (delrin). Sourcing salvaged parts maybe the best option, but if I was forced to tackle their replacement myself, I'd be concerned about using a MSLA resin 3D print as I find that it has poor long term stability (though that could be just be due to my lack of skill and using cheap resin). I'd rather try using an FDM (filament) printer, but I'm not sure if it could manage the small details. The advantage is that, although rare and difficult to print with, it is possible to get POM printer filament. Nylon of any sort may not be suitable for this application (due to being too flexible and also absorbing water may affect the attenuator performance), but it's a relatively popular 3D printing material so probably worth a try. A custom machined POM part would be great, though likely rather costly. It would also be difficult to exactly match the injection molded part as molded parts are "negatives" based on a machined tool i.e. it's difficult (expensive) to get a sharp radiused inside corner on a milled part but easy to get a sharp external corner. The opposite is true for a molded part.

[jrharley]

Always great comments guys, thanks!
Looking forward to diving back into the beast this weekend. It'll be mostly eval and dis-assembly.

Yes Tony_G, those actuator forks slide back and forth. They make a very charming "clack" when they engage. At the same time, it's "a bit how ya' doin'" by modern standards. PS - I can confirm that there is currently NO output signal.

Wow swainster, thanks for sharing the fabrication and materials knowledge! I'm following you 100% on that. These are going to be tough parts to acquire, no matter what the route is. Should be interesting.

I have discussed the problem with one local outfit that seemed to be a good fit for the task..... Shared pics, drawings etc. with them.

They ran away, screaming.  No worries, there are others. Haven't even scoped out the cost scenarios yet.

As always many thanks for the input(s)!

JRH

[Tony_G]

Seeing that they slide back and forth they shouldn't be under that much mechanical stress (famous last words) and as long as you use a material that doesn't impact the RF in the cavity you should be able to make them "fairly easily" - The ends being the most important part I think.

[jrharley]

Yeah, I suppose an "exact replica" isn't necessary TG. Good points, we'll just have to see how this roles.

I believe swainster hit it on the head with the Delrin, injection molded supposition, I think that's exactly what the parts are made of. But to your point TG, Nylon should work.

A model of the parts is going to be the first step on the fabrication route, no matter what, it seems to me. It also seems to me that the cost threshold for fabricating the parts is about $300. Beyond that, I should be able to get a parts unit for in that neighborhood. The trouble with having to go the parts mule route, is that the attenuator design has changed over the production life of the 8656A. Whether or not these parts were kept consistent in form or not, remains to be discovered.

Anyway, I continue to ponder the HP as I keep rubbin' on the Delco.......................................

A Hell of a lot of rubbin'......

JRH

[Brumby]

Hi Brumby, spares unit you say?... Actually my 'B came with a spares unit which is only missing its power transformer (and a burnt out inlet filter). I understand that the previous owner of the 2 units 'repurposed' the transformer for a DIY project. I'd be interested in buying a replacement transformer to complete the unit. That said, posting a heavy item like that to Singapore is probably pretty painful on the wallet.

Partially pulled out the transformer.  Pictures below.  The cut wires were already that way.  Weight is over 2kg which puts it into the range of AUD$62 - AUD$68 for basic shipping.  (Actual cost will depend on packed weight.)  I can include the cover and mounting hardware.  I would be inclined to supply as shown up to the DB15, cutting the 4 smaller wires (long enough to be usable).

If you are interested in proceeding, let me know and I'll do some electrical checks.

[jrharley]

HP 8656A update here, thanks again for all the feedback and support from everyone!

I've been diving in to the attenuator assembly, trying to understand what makes it "tick". I've made some progress, learned how to take the actuator forks out and witnessed first hand the o-ring failure that has been well documented by lazyelectrons. (www.lazyelectrons.com).

All of the remaining actuator forks in my unit have cracking o-rings. We're going to be re-building the entire attenuator assembly, there's just no escaping that. I've been working on how to replace my missing forks, we'll see how that goes. There may be a solution developing as I write this. "I get by with a little help from my friends"....... We'll see!

So, for anyone interested, I'll try to document the re-build here, starting with the attached pics.

JRH

[Swainster]

Hi Brumby, spares unit you say?... Actually my 'B came with a spares unit which is only missing its power transformer (and a burnt out inlet filter). I understand that the previous owner of the 2 units 'repurposed' the transformer for a DIY project. I'd be interested in buying a replacement transformer to complete the unit. That said, posting a heavy item like that to Singapore is probably pretty painful on the wallet.

Partially pulled out the transformer.  Pictures below.  The cut wires were already that way.  Weight is over 2kg which puts it into the range of AUD$62 - AUD$68 for basic shipping.  (Actual cost will depend on packed weight.)  I can include the cover and mounting hardware.  I would be inclined to supply as shown up to the DB15, cutting the 4 smaller wires (long enough to be usable).

If you are interested in proceeding, let me know and I'll do some electrical checks.

Hi Brumby,

Yes, I'm definitely interested but so as not to distract from the restoration of the 'A' I'll drop you a private message. Or at least, I'll give the PM system a try as I've never used it before.

[Swainster]

Hi JRH,

I was just thinking about how I'd have tackled this a few years back, before I picked up CAD. When I was back in the UK I would have just popped out to the shed and grabbed the nearest piece of scrap plastic (maybe a plastic ice-cream tub lid or an old chopping board) and started carving away with whatever tools were laying in the bench. Seems like the fork at least might lend itself to this method of improvisation. The cover might be easier as a 3D print

[Tony_G]

I'm wondering if you could, as a test, not long term, get away with just pushing a cam into some modeling clay and then filling the resulting void with epoxy - Little trimming and sanding like Swainster said and you might have something that would at least give you the ability to do some testing while you look for a full long term replacement.

TonyG

[jrharley]

I hear you guys, and I've been mulling over DIY fabrication options as well. Anything from direct milling out of plastic stock to resin casting are options.

In studying the assembly though, such an approach would be a heavy lift in order to achieve the precision needed I think. Not impossible, but one would spend a fair amount of time doing 2 forks and a cover, (which holds everything in place very precariously).

I've attached a sketch of how this crazy contraption switches. It relies on an actual deformation of the little gold contacts, to make or break the connection. Pretty wild.

As always, I really appreciate the interest and the feedback!

JRH