Category Archives: HP 8645A Agile Signal Generator

8645A Agile Signal Generator: finally, all fixed

After quite a bit of work with the front panel (see earlier post, massive transport damage), and repair of its most beautiful and complicated inner workings, the remaining item to be fixed: the main output connector (N-type connector). The original one, damaged beyond repair, and the bracket holding it, severely bent, and a broken-off screw stuck in it.

Screening through my parts stock, found a rather old but still good panelmount N connector. It has a slightly different inner construction (the original 8645A has a quite long gold-plated coaxial air-line in the connector; replacement connector has no gold plating), but within reason, this should not matter.

This how it looked before:
8645a damaged connector

The bracket and assembly was fixed back home, in good old Germany, in the mechanical workshop, during xmas vaccation.

Now, looking at the result, quite pleased with the outcome of quite a few hours of work (and some expenses, for a replacement HP System 2 front panel frame, see earlier post):

8645a front fixed

8645a front

8645a n connector

Note that there are two handles now mounted to the unit, not so much for carrying it around (about 70 pounds!), but to protect the front panel.

8645A Agile Signal Generator: power-up, lots of mechanical repair – finally, calibration, and some first tests

The 8645A has been a real challenge. Not electronically, but with all the mechanical damage.

Most of the defective parts are now fixed or replaced, the last one missing:
8645a damaged connector assembly

The front connector and bracket, badly damaged – this will need to go back with me to Germany, don’t have the right tools here (anyone out there with a metal workshop, in the greated NYC area, let me know!). Will see if I can bend this back, or machine a replacement from aluminum alloy. Also, one of the screws is broken off and stuck in the thread – all pretty laborious to repair.

The temporary set-up – connected a SMA f/f adapter, to get the signal out.
8645a temporary connection

Electronically, major success! Not going into any details for now, explaining the inner workings of the 8645A would take hours. But, it is now back to a stage where power can be supplied to the circuits, and watch out, this is the first result:

8645a self cal

… after several minutes, still no change – checked the manual – and the self calibration can acutally take 5 minutes or more, so I am patiently waiting. Finally, this result:

8645a result code 0

Sounds good.

Went quickly through all of the basic functions, and the machine seems to put out almost +20 dBm over the full band, stays perfectly phase locked, and, as far as I can tell with the 8565A, it is pretty clean.

Machine under test –
8645a working

– showing a ~1.7 GHz signal, FM modulated, 30 kHz depth, 1.5 kHz audio.
8645a fm check

For now, repair-final check on hold, with more tests once the connector is back in place.

The many hours spent so far, certainly worthwhile,for a machine that cost about USD 39.5k in 1990, close to 90k nowadays… all its major capabilities, well described in the HP 1990 catalog.
8645a hp catalog 1990

HPAK 8645A Agile Signal Generator: pushing it back in shape, Chinese spares

The 8645A – see earlier post, reached my workshop in a really bad shape, electronically dead, and severely damaged. For any reasonable effort, damaged beyond repair.

However, I do this for a hobby and there is no rush, and I just can’t take this thing apart for scrap metal – the case suffered damage, but the inner parts, no major mechanical issue.

For most of their 1980-1995 equipment, HP used a so-called “System 2” modular case, mainly, die-cast aluminum alloy. The 8645A is not much different – the bottom and top lid are different (to provide better shielding than the regular System 2), but the front frame, it is the same.

Looking around at xbay – found a complete front system 2 front assembly, System 2, from a 8782B Vector Signal Generator. The Chinese vendor, he seems to make a cut by taking apart equipment, and selling it for parts – it wasn’t quite cheap, but still, for a 8645A, to get it back into service, it seems pretty much worthwhile.

It arrived already after about one week.
8645a new frame
8645a new frame 2

Not in best shape, but usable.

This assembly also has quite a few BNC connectors, and a precision N connector, all very handy to fix the replace the snapped-off parts.
As it turns out, the N connector is of a different length, but never mind, I have some more spares around.

The front panel assembly, it is apparently one of the weak points of the 864x series.
Two layers of rather soft aluminum, a ~0.8 m plastic sheet that also acts as filters for the fluorescent displays, and a thin outer layer with the labels.

This is quite easily pushed in, if force is put on the BNC connectors – it is always best to safely secure the instrument at the corners during transport, and to attach handles.

8645a front panel 1

The layers are held together by glue, and it seems, HP has used a very sticky, but not permanent type – with great care, it can be separated. Key thing is to keep the sticky surfaces clean, and they are best protected by a polyethylene foil.

To flatten things out, a carefully adjust iron is a handy tool – with the plastic sheets put in between two layers of non-stick paper.

8645a plastic sheet ironing a

8645a plastic sheet ironing

The more laborious part – the flattening of the aluminum panels, with a hammer, and a piece of wood, and about 2 hours of work.

The result – all things glued back together, pretty happy with the result.

8645a front panel repaired

8645a front panel repaired 2

Sure, there are some little blemishes, well, fair enough!

Now we just need to get the inner workings going…..

8645A Agile Signal Generator: when disaster strikes

For a long time I have been looking for a reasonably prices 8645A, and finally, I found one – with the note “doesn’t power up”. Well, most likely, a defective power supply. The 8645A and some related generators (not the 8643A) use linear supplies, because these are really low noise devices, and a switching supply just doesn’t seem to do the trick. Interesting, because for the very quiet 8662A/8663A signal generators, HP was relaying on switches supplies…. maybe they just could not fit anything else.

Back to the 8645A – this is no less than a miracle, a marvelous apparatus. No idea how many manhours (man-decades) of engineering went into it. It’s complexity, and subtle detail, nothing short of a moon landing vehicle, made for an electronic test lab. They way HP designed the shielding, and implemented a rigorous low leakage approach, this alone is worth special admiration.

Even better, the unit discussed here has option 1, which is an OCXO high stability reference, and it has a build-in doubler, extending the frequency range to above 2 GHz. About USD $50k in 1990, nowadays, nobody can afford such build quality anymore.

Well, all these are good reasons for looking forward to soon doing some repairs on such kind of unit, and make it “power up again”. Well, that was the thought.

This issue: while many hours of hard work went into fabricating this thing, not more than a few seconds were spent, to consider adequate packaging, to ship a box, 80 pounds.

8645a damage

8645a damage 2

8645a damage 3

8643a damage 4

Such kind of damage, not seen before. Except for the little bit of Instapak, no other protecting foam or anything – nothing to hold it in place in the box. The result – a badly damaged front panel, broken input connector, and even the front frame, damaged beyond repair.
Just the single front end connector (which has an internal airline, gold plated) – USD 200+; the machine, it seems beyond repair.

The only good news – the seller (who did not package it himself) seems to be a very resonable person, so we will work something out. To be continued.