[ARC5] PS Hum in BC-453-B (solved!)
Dennis Monticelli
dennis.monticelli at gmail.com
Thu Jan 29 17:24:55 EST 2015
I'm going to take wild guess here, Bill.
Unless you are very close to a source of 60Hz field, it takes a long wire
to pick this energy up. I suspect your antenna (which is probably a long
wire with some degree of coupling to house mains wiring) may be depending
up the receiver's common to supply the return path to earth and that may be
an ill-defined path that passes through sensitive circuitry. If this is
your situation you may with to earth-return the antenna right at the case
of the receiver.
If that doesn't do it for you then you must have a source of 60Hz very
nearby giving you grief. The BC-453 is well shielded for
capacitive coupling but the thin non-ferrous Al case is almost worthless
for stopping a nearby 60Hz magnetic field from inducing currents within.
Maybe your receiver is sitting on top of another box positioned right over
its power transformer! BTW, oscilloscopes have this problem and resort to
using a high mu metal shroud around the CRT. Maybe someone on the list can
fashion an ARC-5 case out of mu-metal :-)
Dennis AE6C
On Sat, Jan 24, 2015 at 6:15 PM, Bill Cromwell <wrcromwell at gmail.com> 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
>>
>>
> ______________________________________________________________
> 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
>
More information about the ARC5
mailing list