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

[Bill Cromwell]

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
>