[ARC5] PS Hum in BC-453-B (solved!)

[Richard Knoppow]

      If you are running from DC then the hum has to be inductive or 
capacitor coupling .  It may help to put by-passs caps across the 
battery terminals at the receiver.  These should  be fairly large caps 
to short any AC to ground.


On 1/24/2015 6:15 PM, Bill Cromwell wrote:
> Hi Les,
>
> I have small traces of hum in the hi-Z cans when I have the volume 
> down low. It's barely there. I'm operating my BC-453 from brom 
> batteries on both the heater strings and the B+. I'm thinking the 
> exposed, randomly strung wires from the batteries are picking up a 
> small amount of hum from the nearby power lines and their wonderful 
> field. When I feel the urge I will replace those wires with shielded 
> cables (or at least neatly routed  and shielded) to see if more hum 
> goes away. I didn't notice the hum at all until I decided to run the 
> gain down low and then it was almost not there. Maybe I shouldn't 
> complain. The fellows on the regenrx list are whining about hum from 
> their AC supplies.
>
> 73,
>
> Bill  KU8H
>
> On 01/24/2015 08:49 PM, Leslie Smith wrote:
>>             Power Supply Hum - problem solved
>>
>> I would like to thank all the readers who helped me find (and remove)
>> hum in a BC-453.
>> In particular a single posting by Elden Meyer was most helpful.
>>
>> Detail.
>> The problem turned out to be obvious, once the proper observation was
>> made.
>> The common earth line carried BOTH 24V AC at 450mA (to filaments) & AF
>> (to the headphones).  Once I separated the current flow into two
>> separate conductors the hum dropped, (and even tolerable). There is
>> still a small amount - but it's now coming via the 12A6; it appears
>> about 15 sec after I turn the set on.
>>
>>
>> False leads.
>> (1) First, I run my set from a home-brew PS, where I double the 24V
>> filament line to generate +60V for the B+ line.
>>
>> (2) I understood the possibility of getting induced hum 'down' the
>> common earth line, and even provided a second common for the lower level
>> AF signal.  Hum wasn't a problem with a loudspeaker.  When I connected
>> modern 'phones, I continued to put the heater current & AF thru the same
>> common cable (external to the set, via J3 on the rear panel).  -
>> Stoopid.
>>
>> (3) I measured the B+ line with a 'garden variety' DMM.  B+ voltage was
>> 59V - marginally below the +60 I expected.  I believed hum was coming
>> down the B+ line - in other words the unfiltered supply fed to the zener
>> didn't have enough head-room.  Wrong.  When I used a Fluke meter I
>> discovered the B+ line was +60V, and clean.
>>
>> (4) I noticed the hum appeared the moment I turned on the power - before
>> the 12A6 heater could even warm a little.  Elden wrote:
>>
>>    "Since you notice hum before the tubes have warmed up,
>>      the 12A6 should not yet be drawing plate current. In that
>>      case there are other possible sources  [apart from
>>      a common conductor] for AC to get into the audio."
>>
>> And a little further on:
>>     "Also there can be voltage drop in a wire or ground
>>      connection common to power and audio circuits."
>>
>> It's obvious, now, isn't it?
>> Thank you EM!
>>
>> Les
>>
>> Now I must track down the fault in L.O.
>> It doesn't start easily.
>> Some have suggested 'tin wishkers' in the tuning gang.
>> I doubt this,
>>    (1) because I see the signal (on the plate) diminish from approx 20V
>>    to zip
>>        as I tune the set from 200 to 280kHz.
>>    (2) If the set doesn't oscillate (even at 200kHz) I can 'shock' it
>>    into starting by flicking the P.S. B+ line.
>>    (3) I replaced the 12K8.  Three times. (But that still doesn't prove
>>    anything).
>>    (4) The voltages around the 12K8 socket appear "about right".
>>          Pin #       Voltage/comment
>>           1          Metal shield - looks good.
>>           2          Heater - just under 12V, looks good.
>>           3          Plate - mixer.  +60V (approx) seems ok
>>           4          +60V.  G2+G4.  seems ok
>>           5          Oscillator grid.  Cant remember the voltage.
>>                      The grid lead has gone high - from 51k to 60k.
>>           6          Oscillator plate.  1/2 B+ (about right)
>>                        CRO shows LO signal at times.
>>                        Amplitude 16 to 20V AC, drops with incr.
>>                        frequency.
>>                        Above 300kHz, the oscillator starts for a moment.
>>                        So - get heaters warm, toggle B+, watch signal
>>                        appear.
>>                        Signal decr. over about 6 seconds - then zip, 
>> it's
>>                        gone.
>>           7          Filament voltage, 24V AC.
>>           8          Cathode, cant remember the voltage.
>>
>> Dave Stinson's hint:  Check for oxidation somewhere - esp in the tuning
>> "C".
>> My hint:  Use CRO carefully, remember Mr. Ohm, think carefully.
>>
>>
>> Les
>>
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL