[Boatanchors] [Hallicrafters] Fluke 6060a/an or HP 8640b ?

[Jim Wiley]

Michael et al -


At least in my 8640B, the symptoms of broken gears primarily show up 
when using/generating FM signals.  The way the 8640B works, it has to 
increase the modulation/deviation of the oscillator to maintain the 
correct output signal as the frequency is lowered from the "basic" range 
of 256-512 MHz.  If the gears are broken, the front panel modulation 
meter will indicate (using the 128-256 range as an example) 5 kHz 
deviation but the generator is only actually producing 2.5 kHz 
deviation.  The modulation of the basic oscillator must be increased by 
a factor of 2 each time the output frequency range is halved.


The purpose of the gears is to operate an internal control that boosts 
or reduces the deviation of the main oscillator in step with the 
band-switch so that the output signal is what the user wants.  There may 
be other uses as well, but that is certainly one of them.


A generator with a broken gear set will still work fine for CW and 
possibly AM signals, bit not for FM signals.  The carrier frequency and 
output level attenuator will be fine, and sometimes that's all a person 
needs, but to perform alignments of FM receivers, and for certain other 
uses, such as using the 8640B for slow to medium frequency sweeps, as 
for example when sweeping an IF stage, having the FM section working 
correctly is necessary. 


BTW, that last technique works well when the 8640B modulator is driven 
from a function generator with a sawtooth wave, and synchronized to a 
scope and RF detector for an output display.  The 8640B modulator stage 
will respond from several hundred kHz down to DC.  The combination  
makes an excellent sweep generator for aligning IF stages, clear down to 
500 kHz or so (some 8640B's will under-range to 455 kHz, some will not.  
To do the latter properly, one should have the low-frequency 
converter.   The converter is HP part number 11255 or something close to 
that, and gives you the full performance of the 8540B down to 20 kHz or so.


And, oddly enough, visual inspection probably won't be too revealing.   
The action of the gear set is complex, and not easily determined by 
eye.  I stared at mine for some time and still couldn't figure out what 
was wrong, but it sure shows up when using the unit.  To see the gears, 
which are behind the front panel, you need to remove the bottom covers.  
The gears are small white plastic gear sets that are clustered around 
the band-switch control shaft. 


- Jim, KL7CC


Michael OBrien wrote:
> Hi All
> How can you tell if your gears are broken while using it and how far do you have to open it up to see it
>
> Mike
>
>   
>
>