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Old 25th Sep 2020, 12:17 am   #1
Skywave
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Default HP 8640B: no F.M.

This, at time of writing, is a brief introductory post to record the above fault - which is currently W-I-P - and to request any relevant input from knowledgeable members.
I will be adding to this thread as-and-when.

Al. (Sept. 25)
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Old 25th Sep 2020, 8:47 am   #2
Radio Wrangler
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Default Re: HP 8640B: no F.M.

As you haven't said 'No AM', then I assume the internal modulation oscillator is running and AM is available.

Where the FM path starts to differ from the AM path is that the deviation is controlled not only by the deviation range switch, but also by the band switch. The band switch in applying progressive divide-by-two stages to the main oscillator also divides the deviation, so that (insert rude adjective here) differential gear arrangement shifts the FM attenuator switch to increase or decrease the FM baseband signal going to the FM varactors inside the cavity oscillator. There is also, I think, some control of FM sensitivity by the geared pot on the main tuning shaft.

Does the meter with the FM button pressed read anything?

David
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Old 25th Sep 2020, 9:39 am   #3
Jon_G4MDC
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Default Re: HP 8640B: no F.M.

On mine this is the spring contact behind the Mod slide switch.
It's probably in the bottom of the box...
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Old 25th Sep 2020, 4:20 pm   #4
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Default Re: HP 8640B: no F.M.

Al, when I first got my 8640B it had no FM output. I eventually traced the problem to an O/C resistor, possibly on the A5 board. This resistor was dissipating its full rating of 0.5W when you looked at the circuit diagram and I found that later versions of the board changed it to a 2W version. I think it was a dropper from the main 44V rail and located near the exposed end of the board (as opposed to the PCB connector end). I cannot find the details at the moment and the 8640B is not readily accessible. Sorry this is all a bit fuzzy but you might just find the change note where a 0.5W resistor is changed to a 2W one of the same value.
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Old 25th Sep 2020, 5:27 pm   #5
Skywave
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Arrow Re: HP 8640B: no F.M.

Quote:
Originally Posted by Radio Wrangler View Post
As you haven't said 'No AM', then I assume the internal modulation oscillator is running and AM is available.

Where the FM path starts to differ from the AM path is that the deviation is controlled not only by the deviation range switch, but also by the band switch - etc.

Does the meter with the FM button pressed read anything?
Para. 1: A.M. - and all auxiliary functions associated with that mode - are fully functional.
Para. 2: I am aware of all that, but thanks anyway. There is no FM on any freq. band and with any chosen deviation. Plus . . . .
Para. 3: No illumination of the scale indicators; no meter needle movement, either.

I suspect that the problem could be caused by the concentric rings rotary switch plate (the one with two contacts) for the FM deviation in the A9 module. (I'm unable to 'decode' the presentation of the relevant cct. diags. in the manual). And that's where things get a bit 'iffy'. . . .

I've owned this S.G. for about 15 years and during that time I've used the FM facility very infrequently: maybe I recall some success. For this fault, initially, I discovered that both contacts for that rotary switch plate were missing: I suspect that they were so upon my acquisition of this S.G. I tried various suggestions to fix that - as found on the 'Net, but suitable parts thus specified for replacements were - and are - not at hand. So I've tried alternatives, the effectiveness of which I am not sure about, but I have my doubts.
So, the current plan - 'divide the problem in two' - determine if that rotary plate is the cause of the problem - or not. And if it isn't, then where and why? But I do have a tactic which should be indicative - and if successful, will explain later.

Lots to do - must get on with it.
TTFN,

Al.
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Old 25th Sep 2020, 5:31 pm   #6
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Arrow Re: HP 8640B: no F.M.

Humber 888 & Jon_G4MDC: thanks. Will bear both ideas in mind and investigate accordingly.

Al.
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Old 25th Sep 2020, 8:04 pm   #7
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Arrow Re: HP 8640B: no F.M.

Humber 888: I've looked at the A5 board and identified a 100 ohm resistor which matches your location description. It looks like it's at least 1-watt rating; it measures 100 ohms. It has no signs of over-heating.

Al.
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Old 28th Sep 2020, 11:28 pm   #8
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Arrow Re: HP 8640B: no F.M.

Update: 30-9-2020. Problem now fixed!
The cause was the 4-track annular disc associated with the switchable F.M. deviation control. (It had to be that, didn't it? )

As I reported, the two wipers were missing. Each wiper connects to a pair of adjacent tracks. There are several entries on the 'Net where others have dealt with this problem (q.v.), but I did not have the parts that they were using, so I tried to fashion my own - unsuccessfully, after many attempts. (I'm talking well over 10 attempts here!). The essence of fixing it that way is that the size, shape and location of such home-made D-I-Y wipers on the circular plastic disc must be very exact - to within less that 1 mm. And the nature of the material used - its 'springiness' - is also critical. Eventually, (almost in desperation) I took a 'stand-back and look' approach at the problem: what, in principle, do I have here? Answer: it's a two pole rotary switch with 11 ways per pole: now that may be an unusual configuration for a rotary switch . . . but not an impossible arrangement.

A prod around in my box of assorted rotary switches produced a 6-gang, 1-pole per gang, 11-way rotary switch: it was dismantled and two wafers were set aside. Each wafer had an uncharacteristically small outside diameter - smaller than a traditional Yaxley type, but the hole for the shaft was not so reduced. A quick check revealed that the shaft for the 8640B dev'n. control was an exact fit to that centre hole in each wafer. Hence, idea: simply replace that dev'n. rotary switch with a traditional rotary switch using these two wafers.

Now there is little space available on that dev'n. control shaft to fit those wafers, but these wafers being of a small outside diameter, there was room - just. Using 'superglue', I secured each wafer to each side of the pcb (making sure that the registration was correct) where the original circular tracks of the original rotary switch was and using Karno wire, connected the contacts from each wafer to the appropriate pins on the out-going 16-way D-I-L connector or to the appropriate points on the deviation fixed resistor potentiometer chain.
All that is much easier to understand when the cct. diag. is examined.

To do all of the above, much analysis of the appropriate circuitry was required - including detail that is not presented in the HP manual. However, all of my researches, diagrams and notes, etc., have been carefully documented: all far too much to present here as attachments. But if any member wishes to do what I have done - or simply wants to read that documentation - simply P.M. me. (The drawings etc. say much what I have written above, but as is always the case, in a much easier way to comprehend.)

Finally, I was unable to take photographs ; my Olympus camera is playing up (again). However, a replacement combination brass gear is on its way - and when that has arrived and is fitted, perhaps some photos will then be taken - and presented here.

Al.
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Old 28th Sep 2020, 11:42 pm   #9
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Arrow Re: HP 8640B: no F.M.

Quote:
Originally Posted by Skywave View Post
I've owned this S.G. for about 15 years and during that time I've used the FM facility very infrequently: maybe I recall some success.
Al.
Whilst trying to fix this problem, I suddenly remembered that many years ago when I was employed as a technician by a firm who manufactured equipment for FM Broadcasting, on my bench I regularly used an HP 8640B - which, therefore, was often used in F.M. mode. Hence, my confusion / puzzlement.
Ho-Hum.

Al.
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