[ARC5] BC-454-B Dymamotor Question

[Mike Hanz]

Thanks for the scope pictures, Mark. In your case,  what I hear you 
saying is that Dynamotor 1 has a spike roughly every 6 milliseconds, 
which would certainly produce what I would agree is a "buzz" frequency 
spectrum.  It also works out to 10,200 rpm, if only a single  stuck 
brush on either LV or HV commutator was involved, which is about twice 
the normal operating speed of the dynamotor.  Note that every other 
pulse has a different shape from the alternate pulse.  The pulse width 
is slightly less than a millisecond at the half power points.  Now, 
there is a count of 13 opposing commutator segment "pairs" on the LV 
side versus 19 pairs on the HV side.  The commutation frequency per 
revolution on the HV side is therefore higher and perturbations are 
faster within a revolution.  The longer pulse width suggests to me that 
it's on the LV side - it's about equal to 1/13th of the total single 
revolution time of 12 milliseconds. Ken and John have quite correctly 
suggested some possible physical causes, but if those appear to lead 
nowhere, then a shorted or open winding between one of the opposing 
commutator pairs could also cause the problem.  The manual suggests 
removing the brushes and checking the continuity between pairs all the 
way around the commutator.  The correct resistance for opposing 
commutator segments on the LV side is 0.4 ohms, and for the HV side, 25 
ohms.  It would also be interesting to see scope pictures of the LV 
input voltage and HV output voltage, and even more interesting to see 
the current waveforms.  The HV output waveform should be roughly the 
same as your audio out waveform.

I don't know what I would do without an oscilloscope.  I grab it more 
quickly than I do a meter these days - it just makes everything a lot 
faster.

There is another source of noise that is often overlooked , and that is 
caused by poor bonding of the dynamotor frame to the frame of the 
receiver.  Postwar ARC receivers had a couple of flexible straps 
installed under the small dynamotor shock mounts and over the snap slide 
posts to provide a more solid path for dynamic ground currents that 
might overwhelm the small #20 ground wire that provides the three pin 
connector ground.  One can check that by manually jumpering the dyno 
mounting plate to a screw on the side of the receiver.  All it needs is 
a temporary touch to determine if it will help.  They didn't install the 
flexible ground straps on the later receivers because it was a fun thing 
to do.

In that same vein, the command sets seem particularly prone to 
electrical noise because of the box count and cable lengths for the 
whole system.  A common ground plane works best for them, the same 
environment they had in aircraft.  AB5S uses aluminum tape, but 6" wide 
aluminum flashing is good, too.  I prefer  mounting a sheet of .020" 
aluminum from the home improvement stores to a sheet of plywood, but 
they all work fairly well.

Good luck, and let us know how it turns out,
  - Mike  KC4TOS

On 1/17/2013 8:56 PM, Mark K3MSB wrote:
> Thanks Gents;  I have not taken apart either dynamotor, but will do so this
> weekend.
>
> All caps in the receiver are replaced.
>
> When I'm not powering the dynamotor,  I'm using the same 28V supply to
> provide 28V directly to the radio along with 250V from an HP-23A.
>
> Here's the waveforms by the way:
>
> http://www.k3msb.com/temp/dynamotor_1.jpg
> 170 Hz waveform.
>
> http://www.k3msb.com/temp/dynamotor_2.jpg
> 1250 Hz waveform (hard to tell)
>
> 73 Mark K3MSb
>
> On Thu, Jan 17, 2013 at 8:01 PM, David Stinson <arc5 at ix.netcom.com> wrote:
>
>> ----- Original Message ----- From: "Mark K3MSB" <mark.k3msb at gmail.com>
>>
>>
>>   Is what I'm hearing with the dynamotors normal?
>> Not at all.
>> Here are common causes of dyno noise:
>>
>> *  RF bypass caps in the ends of the dynamotors    directly across the
>> brushes bad.
>> *  Bad 5 mFd B+ buss filter.
>> *  Bad .22 uFd bypasses (the big bathtub cap).
>> *  Bad brushes.
>> *  Worn or dirty communtator.  Consult    Mike Hanz's work before
>> attempting    to clean or burnish commutators.
>> *  Bad or dry dynamotor bearings.
>>
>> Check these and we'll go from there.
>>
>> 73 DE Dave AB5S
>>
>>
>>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>