QUESTIONS & ANSWERS
can’t find a tube checker, just ohm
the filaments; if the filament is good,
it should do something. You did not
mention a filament supply; these are
probably 1.5 volt tubes (1R5, 1S4,
etc.) but the filaments are probably
connected in series; you will have to
check that. There will be a high
voltage supply producing 400 to 500
volts DC for the GM tube; if that is
working and introducing a small
signal to the amplifier produces an
output but no clicking from the GM
tube, then the GM tube is bad and
you may as well start from scratch
with a solid-state design and a new
GM tube. This URL has some good
information, including a schematic:
www.circuitsarchive.org/index.phh/
file:geiger-counter.png. NV
Revisited, March ‘ 10, page 31:
In looking at your code from
March and researching the forum,
try adding these parameters in
your code before “start.”
#Picaxe 28X1
SetFreq EM4 (or whatever
speed you have selected)
Other SetFreq options: internal
k31, k125, k250, k500, m1, m2, m4,
m8, and internal em4, em8, em10,
em16, em20.
I do not know if this will solve
the problem or not, as I have never
used an external crystal. Within a
forum thread, this statement
prompted the idea to use SetFreq
to solve the matter: “Attaching a
resonator won’t automatically
cause the 28X1 to use the
resonator, you need to tell it to use
that with the SETFREQ command.
As you are using a 4 MHz
resonator, you would use a
‘SETFREQ EM4’ command.”
— Terry Meyer
Response: Thanks for your input,
Terry, I think you have found the
answer. I will put that in my
memory bank.
Dear Russell:
Re: Phono Preamp, April ‘ 10,
page 22:
I enjoyed reading your April
column as usual. I do have a
comment on the phono preamp
circuit. You say this has a flat
response; surely it should be
designed to implement the RIAA
equalization curve?
— Robert Atkinson G8RPI
Response: You are right, but not
being an audiophile, I didn’t think
of that. Also, I didn’t realize that
the wide line parameter had
changed which caused the lines
not to reproduce in printing.
Consequently, I have re-done the
schematic with compensation
(Figure A). The response is not
exactly RIAA, but close.
June 2010 25
