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VINTAGE RADIO
By JOHN HILL
New life for an old Kriesler
A vintage radio receiver must be correctly
aligned if it is to function correctly. However,
many vintage radio enthusiasts neglect this
important procedure.
There are two types of vintage radio
collectors - those who do not do their
own repairs and those who do. Alf
started out being one of the former
but over a period of time has become
one of the latter. He has read up on
the subject, asked lots of questions
and is now doing reasonable repairs.
But Alf had a problem. He had
restored a 5-valve Kriesler mantel
receiver but it didn’t work very well
at the high frequency end of the dial,
although it functioned reasonably
well at the low frequency end.
Receiver alignment was a particular
problem for Alf. Despite the fact that
he had a very good signal generator
and various alignment instructions
to follow, he was unable to get on
with the job because he didn’t really
understand the instructions he had.
I can sympathise with anyone in this
situation because I have been there
myself.
My first receiver alignments were
total disasters due to not having the
right equipment on one hand and not
knowing what to do on the other.
The vague instructions I had at the
time summed up alignment by saying:
“adjust the iron cores or slugs at the
low frequency end of the dial and the
trimmers at the high frequency end.”
As the set being aligned didn’t have
iron cores in the aerial or oscillator
coils, that presented a problem. But
there were slugs in the intermediate
frequency (IF) transformers – and
one slug is as good as another when
an overconfident mug like me has
absolutely no idea of what he is doing.
So the IF transformer slugs were
twiddled at one end of the dial and
the trimmers twiddled the other. To
make matters worse, these adjustments were made at the wrong ends
of the dial.
This dial error was possibly caused
by my father who dabbled in radio in
his younger days. Dad always referred
to the low frequency end as the “top”
end of the dial – which it is if you
happen to be thinking wavelengths
in metres and not frequency in kHz.
Whatever the cause, my early attempts
at receiver alignment were not what
could be described as good and I was
guilty of totally misaligning a number
of receivers; that is, until I learned
how to do it correctly.
My turn to instruct
The Kriesler’s IF transformers were out of adjustment, to the extent that they
were double peaking. IF alignment is an important aspect of any receiver tune
up to ensure that the set performs correctly.
84 Silicon Chip
It’s all very well for those who have
been properly trained to be critical
but when it comes to alignment, it
is difficult for novices to find understandable instructions for receivers
that became obsolete half a century
ago. I blindly blundered on until a
kindly old bloke took pity on me and
showed me how it was done. Being
shown and being told are two different
things and the former is much easier
to comprehend.
So it looked as though it was my
turn to pass on the favour and show
Alf how to align his Kriesler mantel
set.
One problem with receiver align-
This Kriesler mantel model from the mid-1960s is a commonly encountered
valve radio. Although this unit was fully restored, it lacked performance until
it was correctly aligned. It also required a valve replacement.
ment is that it varies from set to set
because the components themselves
changed as radio developed over the
years. Early superhets have air-cored
aerial and oscillator coils and may
also have a bandpass filter or a radio
frequency stage. In addition, the IF
transformers are tuned by adjusting
the variable capacitors that are placed
across each of the two transformer
windings.
If we go forward a little in time we
find that receivers no longer have
bandpass filters, while the IF transformers are adjusted with iron cores
and tuned to higher frequencies.
Howev
er, the aerial and oscillator
coils may still be air-cored.
Other varieties have iron cores in
the aerial and oscillator coils or, in
some instances, the oscillator coil
only. There are also dual-wave receivers of various types to worry about.
Confused? I know I was! Learning
vintage radio repairs from scratch
isn’t easy.
primary and secondary windings and,
instead of using a slug, the coil is
tuned by sliding the winding along the
rod. The oscillator coil is slug-tuned
with an iron core, as are the two IF
transformers.
With the 5-valve Kriesler on the
workbench, an aerial was connected
to the set so that the problem could
be assessed. It was as Alf claimed and
performed poorly at the high frequency end of the tuning range.
A few preliminaries had to be taken
care of before commencing the alignment. First, some frequency checks
were made on Alf’s signal generator
using a modern receiver with a digital
readout. These checks indicated that
the little “Palace” transistorised signal
generator was quite accurate and that
it was well within the usual 2-3%
Alf’s Kriesler
Alf’s Kriesler was from the mid1960s and it had a built-in ferrite rod
aerial. The coil consists of the usual
The oscillator coil was adjusted at the low frequency end of the tuning range. In
this instance, it needed little alteration.
August 1997 85
This photo shows the ferrite rod antenna fitted to the old Kriesler. Moving the
coil position can sometimes improve the reception but, in this case, it worked
best in its original location.
The 6N8 valve was extremely sick and was one of the reasons for the set’s poor
performance.
tolerance these instruments have.
Next, an output meter was improvised by connecting a .047µF 630V
capacitor in series with a multimeter
lead. The meter was then set to AC
volts and connected between chassis
and the output valve plate. The capacitor blocks the DC plate voltage and
passes only the audio signal, which is
shown on the meter. An output meter
has much greater sensitivity to level
changes than the human ear.
While reasonable alignments can
be done without instruments (a
signal generator and output meter),
these accessories make the job so
much easier. So if you are thinking
of taking the plunge and doing your
86 Silicon Chip
own alignments, now may be a good
time to consider buying the necessary
equipment.
With everything in place, a modulated signal of 455kHz (the receiver’s
IF) was fed into the control grid of the
converter valve. If the IF transformers
are correctly aligned, there will be
a single peak which will be heard
through the receiver’s loudspeaker
and seen on the output meter when
the signal generator is swept slowly
through 455kHz. In this case, there
were two peaks which were quite
some distance apart – not the ideal
situation!
When adjusting the first IF transformer, it was noted that the iron
slugs had been screwed well in and
were touching each other. IF slugs
will usually peak in two places: either
screwed in or screwed out. The outer
position is correct. The second IF
transformer also had its slugs badly
adjusted.
(Editorial comment: many IF transformers, fitted with iron cores, could
produce a false peak. When the cores
were screwed in too far, the false peak
resulted from unwanted coupling between the windings, rather than peaks
in the winding inductance. It was a
well known trap in the early days of
iron-cored IFs.)
After the transformers were correctly tuned, there was only a single peak
when the generator was swept across
the IF. So far so good! It was now time
to align the aerial and oscillator cir
cuits and so the signal generator leads
were moved to the receiver’s aerial
and earth connections.
Most Melbourne radio stations line
up very well on old dials, as their frequencies have changed little over the
years. As the worst one is only 4kHz
out, it is possible to do a reasonable
alignment to station callsigns rather
than to the frequency scale on the
dial, if it has one. The Kriesler has
no frequency scale to align to but the
stations lined up quite well with their
dial markings, even before the alignment was commenced. Apparently,
that part of the receiver had not been
tampered with as had the IF transformers.
There was a line on the dial marked
PS (pointer start). With the tuning
gang closed, the pointer came to rest
on the mark. With the signal generator
set to 621kHz (3AR) and the receiver
tuned to that frequency, the oscillator
coil slug was adjusted until the output
meter indicated maximum deflection.
Strictly speaking, both the oscillator and aerial coils should be peaked
at this stage but the coil assembly on
the ferrite rod was securely taped in
place, indicating that it had never been
moved. As a result, it seemed logical
to leave it where it was and to simply adjust the oscillator circuit. This
was done by rocking the dial setting
across the generator signal and simultaneously adjusting the oscillator coil
until maximum signal (on the output
meter) was achieved. Actually, the
original setting was not far out and
these adjust
ments put the pointer
right on 3AR.
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This “Palace” brand signal generator is a compact transistorised unit and is
powered by a standard 9V battery. Frequency checks proved the generator to be
quite accurate.
A frequency of 1422kHz (3XY) was
then selected for the high frequency
adjustments. Once again the dial
pointer was spot on. If the pointer
had not been accurately positioned it
could have been corrected by adjusting the oscillator trimmer (this trimmer controls the dial pointer position
at the high frequency end of the dial).
All that remained to do at this stage
was to adjust the aerial trimmer for
maximum output meter deflection
while tuned to 1422kHz. We found
that the aerial coil trimmer was out
of adjustment but not badly so.
It’s still crook
After disconnecting the generator
leads and attaching an aerial, we
found that the receiver still performed
poorly at the high frequency end.
There had been an improvement but
not as much as had been hoped for.
So in spite of all the previous adjustments and the observation that the
aerial coil had never been adjusted,
it was felt that this was now worth
checking, just to make sure. Unfor
tunately, removing the tape and sliding the former back and forth around
did nothing to boost the performance
and so it was eventually returned to
its original position.
It was time to check a few valves.
Alf’s valve tester cannot test 9-pin
valves because of a broken socket. On
my tester, all the valves except one
checked out OK, the exception being
the 6N8 IF amplifier. This valve was
very weak and the meter needle struggled to rise to the halfway position on
the “bad” scale.
Replacing the 6N8 made a noticeable difference to the set’s performance
at the high frequency end of the dial.
However, it still seemed to be lacking somewhat and the final solution
was to connect an earth lead to the
receiver. This increased the volume
noticeably and considerably reduced
interference hash from a 22,000V
power line in the street outside.
It should be noted that most valve
receivers work better with an earth.
It not only helps regarding reception
but also eliminates or reduces a lot of
interference. Most valve radios have
an earth connection for good reason
–they work better with one!
Receiver alignment is an important aspect of restoring an old radio.
There is not much point in replacing
all those age-damaged components
if the alignment is not restored as
well. Only then will it perform as it
SC
should.
Price: $A11.95 plus $3 p&p each
(NZ $6 p&p).
Just fill in & mail the handy order form
in this issue; or fax (02) 9979 6503;
or ring (02) 9979 5644 & quote your
credit card number.
Coming
Next Month*
Capacitor Discharge
Ignition System
Capacitor Discharge Ignition systems are particularly suitable for use
with 2-stroke engines, older 4-stroke
engines and some high-performance
engines. This design operates from
reluctor, points or Hall effect signals
and features multiple spark output to
improve fuel burning in the cylinders.
Addressable Card For
Driving Two Stepper Motors
Continuing our series on stepper
motor controllers, this latest card is
similar to this month's design but is
capable of controlling two stepper
motors. We give the full circuit and
construction details.
On sale 27th August Australia-wide
*Note: the preparation of these articles is well
advanced but circumstances may change the
final content.
August 1997 87
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