[ARC5] Converters for Q5-ers (PS hum)

[Leslie Smith]

#2 Power supply hum in my set.

Hello Bill,

I'm diverging from the points you raised about the M.F. converter. The
converter is still the topic of interest. It's working again - I found a
transformer to replace the burnt out 24V AC job; the intermittent fault
is still there, but not evident at the moment. The current interruption
involves sound quality.


(1) I think I can describe the set-up adequately with out a schematic.

(2) The set is running according to my usual practice.

Filaments - 24V AC. B+ 60V, zener regulated, supplied by doubler
from 24V AC.

I take the AF from the rear connector - J3, pin #2. This is (I
believe) 500 ohms.

The set is BC-453-B, not ARA. That's why I think - 600 ohms.

(3) Until recently I fed the AF into a power transformer, 110V :
    11V. Small.

With this the AF lacks bass, and in the "middle" of the tuning is
muffled.

Remember, I'm using an HF converter to listen to the BC stations on the
'range' rx.

If I tune the '453 across the station I hear 'tinny' high pitched sound

(like you might expect from a cheap transistor radio)

After that the signal becomes 'wooly' lacking highs.

Tuning further gives a second region of 'tinny' sound; the set is
clearly

mis-tuned, and on the edge of the pass-band.

(4) The result is quite different when I use (modern) headphones.

The sound quality is quite good (except for the point below)
considering the

narrow I.F. (The I.F. 'slugs' are pushed 'in'.)

(5) Problem. Heaps of hum. 50 or 100Hz hum (I can't tell the
    difference).

The bottom plate from the set is missing.

My desk/work-bench has a steel frame that is 'infected' with hum.

The set has previously been 'hacked' and I have no idea (at this moment)

where the under-chassis A.F. cables 'go'. (Actually they 'go' from

the detector to the 12A6, but that's not what I mean.)

My question: Can anyone on this list comment on the following.


DC filament supply v AC filament supply. Does AC to the filaments
matter?

Hi-Z signal path between the (diode) detector & the 12A6 grid.
How much hum

is picked up here.

B+ supply, from voltage doubler. 1/2 wave doubler, followed by x5 12V

zeners in parallel with 470uF filter cap. (always thought 470uF would

take out a lot of hum!)

Where should I start looking for the hum?


First thought: Get a 24V battery and build a 'clever' voltage
doubling circuit

to supply 50V B+. That way I can cart the whole she-bang into the middle
of

a paddock and see if the hum is still there ....

All comments received with thanks! Les


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