HP8640B Signal Generator
 

I acquired a very rough example of an HP8640B for £45 hoping to use it. Aluminium corrosion has cleaned up except for the attenuator knob. Both that and the main tuning knob need a proper Allen Key as none of mine fit. It seems to be something like 1.7mm across the flats? I managed to remove all the other knobs (with difficulty) using a key approx 1.27mm AF.
There are several faults, most concerned with stiffness of switches etc but one obvious problem is the main tuning control only rotates a revolution or so. One end appears to lock but the other is squishy. Is this a common fault? I notice a few early threads so hopefully someone can point to an easy fix? Possibly someone turned the knob too far?
I also need to fix a broken catch on the AM/FM switch which looks removeable and a seized RF on/off switch?
What's the best way of restoring a smooth finish to the exposed alumium parts.. emery cloth and oil?
The main question though... No sign anywhere of the mains voltage requirement. Is it 230/240 volts ?? And.. it was sold as dead but the mains fuse is OK and I see about 20 ohms into the mains terminals when the on/off switch is pressed.
Allan G3PIY

Re: HP8640B Signal Generator
 

That looks (and sounds) like a robust project (!)

I'll PM you my phone number as I've done a fair bit with these :) so lots of info to share with you.

Best wishes

Guy

Re: HP8640B Signal Generator
 

DO NOT as in NEVER EVER try forcing a knob on an 8640. The plastic of the knobs and the gears behind them are polycarbonate and have gone brittle.

Dismantle the thing so you can get in, clean and lubricate.

The tuning knob oterates a special screw. The nut is a nylon piece made to have no backlash by a spring pressing the segments of the nut onto the screw. This drives a piston to and fro in a stainless steel cylinder.

The end stops for the screw shaft are done with a stack of plastic discs on the shaft with overlapping protruding segments. At each end of travel the protruding segments hit each other and stop the shaft turning. Turn in the other direction and you get about 300 degrees rotation per disc before the segments hit in the reverse order, forming the other end stop. There's a conical lump of diecasting supporting a reamed bush around this mechanism. Gears off of the main tuning shaft turn a pot. The pot triggers switching of the filters that give a sinewave out of the divider chain. The basic oscillator runs 256 to 512 MHz. A long chain of divide-by-two stages creates the lower bands. Each divided band has to be split in two for a pair of filters in order to get harmonic removal at the band ends.

There is a plastic differential gearbox driving the deviation range attenuator, by subtracting the angle of the freq range switch (which sets the divider factor) from the angle of the deviation knob on the front panel.

Very nice generators once going. Keep the room warm, too. Beware of knocking the counter reference int/ext switch on the back panel.

David

Re: HP8640B Signal Generator
 

Should say that if its output amplifier microcircuit is a goer, it's worth more than you paid. Plenty of idiots transmit up them and fry them and the output attenuator.

David

Re: HP8640B Signal Generator
 

Is the grub screw socket hex? The multiturn knob for a 10 turn pot I bought a few of years ago had a 0-80 unified grub screw with a Torx socket.

Re: HP8640B Signal Generator
 

Some were Bristol Spline

David

Re: HP8640B Signal Generator
 

I'm sure all of the grub screws ( and everything else ) is good old imperial. So your 1.27mm is 50 thou.
I've got 2 of these and the knobs and plastic gears are a real weak point at the age they are. Also I read somewhere if you store them the wrong way up (on they're side?) the grease in the main oscillator cavity goes in all the wrong places.

Ging

Re: HP8640B Signal Generator
 

About 1.7mm could be 1/16" perhaps?

Re: HP8640B Signal Generator
 

Most likely a Bristol spline or SAE size, not metric.
Sometimes, I say sometimes, you can get by using a Torx bit to loose up a Bristol screw and vice versa.
Even here in the States, Bristol wrenches are hard to find. Vaco & Xcelite do/ did make sets though.
I think you can do like I have and carefully grind down the next size up Allen wrench to get a good tight fit.

Re: HP8640B Signal Generator
 

Mains voltage selection is made inside the mains input socket module. Remove 'plug', look for "Fuse Pull" lever, remove fuse then rotate lever and withdraw voltage selector card.

Re: HP8640B Signal Generator
 

The main tuning and level knob screws are hex 1.25 and 1.5mm

Re: HP8640B Signal Generator
 

HP used UNC grub screws for the knobs, below are the commonly found sizes & appropriate Allen keys, the larger size is sometimes found on the older valve based test equipment.

#8-32, 5/64"
#6-32, 1/16"
#4-40, 0.05" (50 thou)

David

Re: HP8640B Signal Generator
 

Oh no, now you're telling me after all those years on its side? ???

Re: HP8640B Signal Generator
 

I thought I read it was a problem if they were left with the front facing up, mine is on it's side at the moment due to space problems.

David

Re: HP8640B Signal Generator
 

Phew, I hope it's the front not the side, but mine's on its feet from now on!

Agreed, 1/16 is a better fit (1.58mm). The 1.25 feels like a perfect fit but I'll look for a 0.05"

Re: HP8640B Signal Generator
 

A quick search found the above quote here:
https://vintage-radio.net/forum/show...05&postcount=9

Maybe it was sold as dead, as nothing lit up when they tried it, it could well have a secondary power supply fault, or problems with those aluminium rivets HP liked using as ground points (not sure if the 8640 uses them as I've not looked in mine for a long time), I did have a pulse gen that used them & many had corroded through.

David

Re: HP8640B Signal Generator
 

"A thing of beauty is a joy forever" :)

The 8640B (shown 'locked') and the 9025 are both referenced to an external GPS-disciplined oscillator whereas the Meteor drifts from +4Hz to +28Hz error when the display is active (presumably from the non-TC internal HC6U crystal warming up as more power is consumed)

(re. 9025 ... Mark Hennessy, take a bow :thumbsup: )

Re: HP8640B Signal Generator
 

Wow.. the HP8640B is very popular and no shortage of experience.
I'm writing it up as I go but there is a spot of gardening and a few (very few) household chores so refurb may be a bit sporadic.
Latest find 5 of 20 bridge diodes duff. 3 open circuit and 2 high Vr so 3 of 5 power supply rails will be struggling.
C6 went phut!!
Loads of gears need some TLC (much like I do after spending £45)
http://www.radiomuseum.co.uk/HP8640B.html
Allan G3PIY

Re: HP8640B Signal Generator
 

Really lovely generators. It is just a shame about that plastic which goes brittle.

Has anyone come up with 3D printed replacement parts yet?

Re: HP8640B Signal Generator
 

I repaired a CNY2 receiver some time back and had to source some new gears.
It seems there's a lot available quite cheaply once you work out how they're specified, I imagine the gears in the 8640 have standard parameters.. maybe not though?
Allan