HP8640B: work in progress

[Nymrod121]

Another of these wonderful old workhorses is with me (thank you, 'Dickie' )

This evening I've had the covers off for a look inside:

The A9 unit gears are definitely cracked but at least the A9 assembly itself is the later p.c.b. type rather than the wafer switch version which is a lot more difficult to work on. I'll be getting in touch with Paul Koretko over in USA via his eBay site re. brass replacement gears as Emanuele Girlando (Turin, Italy) no longer makes the resin ones;

The output hybrid is not in place in the A26 assembly box but I have a spares mule 8640B which has both hybrids present so hopefully that'll be a quick (and cheap!!) fix;

One side handle had lost its spring rod - that's now been sorted.

Onwards & upwards ...

[Radio Wrangler]

If you're going to order a set of brass gears and want to share the overheads, Guy, I now have an 8640A but the deviation and band switch knobs have got that feeling which gives away the gears' condition. So I need a set too. Mine's on the round tuit pile so there isn't any rush.

I have the 3580A 50khz low noise spectrum analyser and a few mixers, which makes up a fairly good phase noise measurement setup.

David

[Nymrod121]

Yes, David - I'd be very happy to share costs; I'll be ordering two sets by way of this repair plus a 'passve reserve' in the event of any failure in my resin-gear-equipped unit.
I'll PM you later today.
Best wishes
Guy

[Nymrod121]

Originally Posted by Nymrod121

"One side handle had lost its spring rod - that's now been sorted"

Should've signed that post off as Springrod121 ...

[Diabolical Artificer]

You could try Olly - sales@petersenengineering.co.uk who made me some bespoke brass knobs for a pre amp I built Guy. He did a really good job for a good price. It may work out cheaper than ordering parts from the USA.

Hope you get it fixed, Andy.

[ex 2 Base]

Not being familiar with the gears you require, but have you tried Davall Group In Hatfield or HPC in Chesterfield, both have a well illustrated catalogues full of technical details. Ted

[Radio Wrangler]

THere's also Reliance Engineering, Penistone Road, Kirkburton, Huddersfield who do precision gears in stainless. but the standard gear firms normally do finer pitch and at staggering prices. The HP originals were fairly coarse, intended for plastic. If I'd known, I used to have access to all the engineering drawings. But the guy in the states is affordable and it's known that his parts fit. To control the MF deviation switch, a differential gearbox is used to subtract the position of the bandswitch from the front panel deviation range control. The 8640 works by having a series of octave dividers down from a base 256-512MHz cavity oscillator, so successive ranges also halve the deviation.

Plan B would be to have relay attenuators and a bit of fancy logic from some plain switches.

David

[Alan Bain]

I seem to remember they were fairly standard 48dp 14.5PA gears as I cut some replacements for my 8640B mostly in brass, but the one which had to be isolated (on the modulation generator capacitor) in delrin from a repurposed 1MOD plastic gear with brass hub which I turned down to size. The number of teeth was sane on all of these - unlike the 61 tooth gear in the 5254 plugin for the 5245L!

[factory]

These are what I bought for the 8640B I have, I've still not got round to fitting them.


I wasn't aware that the gears were being made in the USA too, mine were bought from India.

Last year I acquired a 8640A with options 001 & 002 (2130U01091), I'm probably going to need another set of gears at some point.


David

[Nymrod121]

Thanks everyone. The ones currently available online (faint creak of eggshells being walked on at this point) are indeed from India and not USA. I have trawled through my previous email correspondence and the last message from Paul Koretko indicated prices that were considerably higher than those from India. I'll try sending P.K. an email just in case he's still doing them and if so, at what price.
Best wishes
Guy

[Nymrod121]

Update: I've been in touch with an ex-Holme Moss chum who has an HP8640B (512kHz - 512MHz) that was working when last tested ... and he's offered it to me as he feels it's unlikely that he'll get round to using it in the foreseeable future. Just like buses ... you wait for ages then two come along at once

[Malcolm G6ANZ]

While following your thread I found this article that may help you:
https://www.radiomuseum.co.uk/HP8640B.html

Malcolm

[Nymrod121]

Very informative! Thank you.

[factory]

Originally Posted by Malcolm G6ANZ

While following your thread I found this article that may help you:
https://www.radiomuseum.co.uk/HP8640B.html

Malcolm

That's Allan's website, his HP 8640B was also the subject of this thread; https://www.vintage-radio.net/forum/...d.php?t=166293

David

[Andrewausfa]

Glad to see you're getting somewhere with it and most of all that I didn't have to break it for spares.

It obviously blew the line filter as soon as I applied power and I replaced a number of the diodes on the supply board with 1N5408, from memory. After that it started 'working'. I was sure I left the other hybrid in the RF box. Anyway, it was a rattler so the IC had come unstuck from the substrate and was no good anyway. Unfortunately from then on it seemed like a money pit in the offing and having a nice Marconi 2018 and a Systron 1702 already, this had to go.

Andrew

[Dickie]

Hi,

You did leave the other hybrid inside the case to rattle around! Out of curiousity I sawed the top off it and yes the substrate was loose and broken away from all its bond wires. Unrepairable by an amateur, though in a previous life I would have snuck it into work and put it back together with a ball-bonder just to see if it was OK. Anyway, the instrument seems to have a future at last.

[Andrewausfa]

Hi Richard,

Thanks for confirming my memory, it'd been under my dining room table for months. I was going to stick the hybrid on my lathe and take the top off, I just never got around to it but anyway, strongly suspected it was irreparable.

Andrew

[Nymrod121]

That's all very helpful follow-up information - thanks everyone

A query: is the output hybrid device broadband i.e. is the same part used on 512 and 1024MHz options?

Best wishes
Guy

[factory]

The 1024MHz option is an internal doubler.
There are different part numbers for the hybrid, depending on the age if I remember correctly.

David

[Nymrod121]

Thanks, David
Best wishes
Guy