Products > Test Equipment
Fool for the 8656A Sig Gen
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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'...... :phew:

JRH
Brumby:

--- Quote from: Swainster on March 05, 2023, 03:50:45 am ---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.

--- End quote ---

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.
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