[ARC5] BC-453 B dynamotor hum

Tue Nov 3 11:10:45 EST 2015

I also suspect a stuck or badly worn commutator brush.  On accession I have had the CW signal if my command set transmitter setup go from a normal clean note to one with a buzz saw character.  A sharp "mechanical agitation" (thump) to the dynamotor on the BC-456 modulator cleans things up.  Carbon/copper dust buildup in the gap between the brushes and their holder causes binding and resultant poor contact between the brushes and commutator.  Do you have another dyno to swap in to see if the problem follows the dyno?  

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> On Nov 3, 2015, at 10:42, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:
> 
> The output from the dyno is approx 187 HZ and looks somewhat like a half wave rectified sine wave with the flat part on top.LV DC is clean except for about 400 mv of hash from the dyno with random very fast rise time spikes of about 1.5 volts.
> I cleaned and lubed the bearings a while ago but didn't clean the commutators, they looked OK- but obviously not the case.How does one go about cleaning them?
> 
> 
> 
>     On Tuesday, November 3, 2015 10:05 AM, Michael Hanz <aaf-radio-1 at aafradio.org> wrote:
> 
> 
> Typical ripple for a clean dynamotor is usually *less* than one volt per 
> hundred volts of output, Jim.
> 
> 1)  What's the frequency of the ripple?
> 
> 2)  Is it sinusoidal or pulse-based?
> 
> 3)  How clean is your low voltage DC supply?  Put the scope on it while 
> running.
> 
> 4)  Have you checked the brushes while running for unusual arcing? 
> (Helps to turn the lights off.)  When was the last time you cleaned the 
> commutators?
> 
>> On 11/3/2015 9:44 AM, J Mcvey via ARC5 wrote:
>> OK, it's confirmed, it's a stupid problem!!!!  I THOUGHT that I scoped the DC, but when checked again, It has deep ripples, more like alternating DC than DC!I jumped a 0.5 uf to ground at the B+ terminal of the output and the hum volume  dropped dramatically, but there was still an unacceptable 80V ripple.
>> 
>> Question: What is the the ripple normally seen at the dyno output?                Is there possibly a defect in the dyanamotor where there is a missing winding in the armature causing the ripple?
>>   
>> 
>> 
>>       On Monday, November 2, 2015 11:41 PM, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:
>>     
>> 
>>   I found an exact replacement audio output transformer in an "extreme restoration challenge " receiver that was way on the back burner.No joy., the problem still persists. I even replaced the 2 meg grid resistor because it seemed unstable and it read 2.85 meg.
>> This makes no sense, so its going to be something really stupid!
>>   
>> 
>> 
>>       On Sunday, November 1, 2015 6:57 PM, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:
>>     
>> 
>>>   On 1 Nov 2015 at 18:42, WA5CAB--- via ARC5 wrote:
>>> 
>>> It would need to be a plate to 70.7 volt line.  Your garden variety 70.7V
>>> and 25.? V line transformers are line to voice coil.  A 70.7 volt line
>>> happens to be 500/600 ohm line (it's a long story).
>> Well, yes, but it also depends on the exact type of 70.7 V transformer. Those
>> which are labeled or specified for varoius "wattages" output have
>> impedances up to something like 5K ohms. I have forgotten the exact values,
>> but can dig out the info.
>> 
>> As I remember it, using the .625 watt output as the primary matches the
>> output of the 12A6 stage almost exactly.
>> 
>> I'll dig out the definitive info and post it here asap.
>> 
>> In any case, secondaries are 4 and 8 ohm, and in some rarer cases also 16
>> ohms.
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> 
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