This is only a preview of the August 1990 issue of Silicon Chip. You can view 57 of the 112 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Digital Sine/Square Wave Generator; Pt.2":
Articles in this series:
Articles in this series:
Articles in this series:
|
VINTAGE RADIO
By JOHN HILL
•
Tuned radio frequency receivers
Because of their basic simplicity, TRF receivers
are usually fairly easy to restore. These
receivers are usually worth collecting although
their performance is not up to superhet
standards.
TRF [tuned radio frequency)
receivers were popular during the
early years of radio, particularly in
the first decade of broadcasting. In
the 1920s, they took the form of
multi-dial sets with two or three
tuning dials, while in the early
1930s they had the more conventional single knob tuning of that era.
These latter types look a bit like
early superhets but a close examination soon reveals that they
are not.
The TRF receiver had a band of
devoted followers that kept it in
production for a longer period than
it deserved. A c·o mparison between
a TRF and a superheterodyne
receiver would soon prove that the
superhet is the better of the two.
But despite this, many listeners
preferred the TRF because of its
straightforward design and simple
alignment procedures.
A TRF receiver can have from
one to three stages of tuned radio
frequency amplification prior to the
detector stage. It may or may not
have reaction but if it has the set
will give a better account of itself.
A good TRF will also have a couple
of audio stages as well.
In other words, a TRF receiver
has no frequency converter valve
Early TRF receivers had individually controlled tuning stages. This particular
set has two stages of radio frequency amplification ahead of the detector.
6
SILICON CHIP
or intermediate frequency amplification stages such as a superhet
has. The TRF simply amplifies the
radio frequency input from the
aerial, detects that signal and further amplifies it in the audio
section.
Perhaps the worst aspect of a
TRF receiver is its poor selectivity.
Nearby radio stations will occupy a
considerable portion of the dial
compared to a superhet. This is
particularly evident with smaller
TRFs having only one stage of RF
amplification. Such sets have very
poor selectivity and there could be
positions on the dial where two or
three programs can be heard
simultaneously.
TRFs with two or three RF stages
are much more selective but still do
not approach the selectivity of a
superhet.
Most TRF receivers have two or
three aerial terminals and using
these to advantage will make the
receiver more selective. So too will
increasing the rear.tion control if
the set has reaction. As the reaction is increased to almost the point
of oscillation, selectivity improves
greatly.
However, although a reaction
control gives a considerable boost
to volume and can improve selectivity, it also gives rise to distortion
when used at maximum levels.
TRF receivers are comparatively
rare and to find such a set today is
indeed a lucky break. TRFs are very
collectible items, even if they do not
perform as well as a superhet.
Most TRFs, particularly the ACoperated receivers from the early
1930s, are easily restored due to
their relatively simple construction,
but one can always strike unexpected problems.
The 5-vafoe TRF shown in the ac-
The chassis of the old 5-valve TRF receiver scrubbed up really well. A TRF
receiver has no frequency converter or intermediate frequency amplification
as in superheterodyne models.
This beautiful TRF console model
should be sufficient proof that TRFs
are very collectible. It is the 5-valve
unit mentioned in the story and was
recently restored by the author for a
friend.
This close-up view shows a typical TRF layout. The valve, coil and tuning gang
section of each RF stage are in line. The three grid-cap RF valves are type
24As.
companying photographs gave
more than its fair share of trouble.
The speaker was a total write off
with a busted cone and open circuit
field coil and speaker transformer.
The set had only one valve and the
dial was jammed up solid. The
power cord had been cut and much
of the wiring needed replacing, not
to mention numerous other incidental repairs. What's more, as the set
belonged to a friend, it would have
to be a job at minimum cost.
A few preliminary checks are
always in order before starting a
restoration job and the power
transformer was checked out to see
if it was still operative. Both the
high and low tension windings were
OK. Likewise the radio frequency
coils. At that stage of the proceedings the set appeared to be
restorable.
When repairing an old receiver
for someone else, it is important to
find out what type of restoration
the owner wants. In this case, the
owner was only interested in getting the set working again and
whether it was original or not
didn't really matter.
That suited me because of the
speaker problem. I haven't many
spare electrodynamic speakers and
really didn't want to part with one.
Therefore, a relatively modern permag speaker was used as a
replacement.
Now that type of repair will make
some readers shudder with disgust
and as the replacement speaker
was made in Brazil, it makes the
originality aspect even worse.
However, I was only doing what the
owner wanted and what was practical. What's more, the end result
sounded very good indeed.
A little experience always helps
when selecting unknown valves for
an ancient receiver. A 4-pin rectifier socket suggested that an 80
would do nicely in that particular
location. A 5-pin output socket indicated that an old 47 should work
OK and two 24As should restore
the radio frequency stages to working order.
The owner was given the phone
number of a valve supplier and I
suggested that he obtain an additional valve of each type for spares.
The seven valves cost $90 which
really isn't expensive considering
their age.
Nearly all TRF receivers were
made prior to 1935 and, as a result,
are usually fitted with 2.5V valves.
Many of these old valve types are
AUGUST
1990
7
The high-tension dropping resistor in the 5-valve chassis was open circuit in
one of its sections (near crosses). This problem was overcome by bypassing
this section with a wirewound resistor.
This old chassis shows an early attempt to gang individual tuning capacitors.
They are rack and pinion driven from a single central dial. Note the small
trimmers at the front of each tuner to correct misalignment.
fairly rare and one major problem
regarding the restoration of a TRF
receiver could well be obtaining
suitable valves. In some instances,
substitutions may have to be made.
Scraping plates
Restoring an old valve radio is
simply a matter of repairing or
replacing worn and defective components. On this particular
receiver, one thing that really needed repairing was the tuning
capacitor. The tuner was a 3-gang
type with a nasty problem - all the
plates of all three gangs were
scraping. The tuner was a plain
8
SILICON CHIP
bearing type with riveted end
plates.
In this instance, the small brass
rivets had come loose, allowing the
main shaft to move forward under
the pressure of a strong spring that
took up the end float in the shaft.
Trying to re-rivet the end plates
was impossible because there was
no room to fit anything inside that
was strong enough to hammer on.
One can only rivet if the rivet head
is backed up by something solid.
A different approach was tried.
The loose end plates were pulled into position using G clamps; then
Superglue was run into the joints to
secure the endplates in position.
This was followed by a couple of
dabs of solder to ensure that the
end plates were electrically connected to the main body of the
tuner.
The Superglue repair was completely successful and holds the end
plates far better than the original
brass tubular rivets ever did.
Finally, the capacitance of the
three gangs was checked with a
digital multimeter and corrective
adjustments were made to the outside movable plates. The tuner
tracked very well for two thirds of
its travel, but one gang wandered
off a bit over the rest of the movement. Nothing much could be done
to correct this error.
Removing the tuning capacitor
from the chassis makes cleaning up
much easier and the chassis was
wire brushed and repainted. By this
stage of the proceedings, the old
TRF looked a good deal better than
before.
The previously mentioned tight
dial mechanism was mainly due to
the problem with the scraping tuner
plates and the gear driven dial
mechanism required no further
maintenance other than a thorough
clean and a smear of grease. The
dial lamp holder, however, required attention as it had come
apart.
Like the tuning capacitor, the
dial lamp holder was of riveted construction and had come apart due
to poor assembly. The repair required a couple of fibre washers for
insulation and a few drops of
Superglue to hold everything
together. Handy stuff, Superglue!
It is jobs such as the dial
mechanism and the lamp holder
repair that come under the heading
of " incidentals". While nothing
much in themselves, they collectively add up to quite a lot of time at the
end of any restoration project.
The next item to check out was
the 10k0 wirewound volume control. A multimeter check indicated
serious internal problems and it
was replaced with a modern
counterpart. As mentioned in
previous stories, modern wirewound potentiometers have very
short control shafts and so an extension shaft was fitted to overcome this problem.
This 4-valve TRF radio has only one
RF stage and no reaction. It is a very
poor performer indeed. The styling is
hardly a thing of beauty either.
As the high tension supply was
still connected to the original electrolytics, it seemed like a good idea
to replace them. Likewise with half
a dozen paper capacitors. In keeping with many early AC receivers,
the high tension was exceedingly
high and 600 volt electrolytics were
used on both the input and the output sides of the HT filter.
Originally, the speaker field coil
was part of the HT filter, but as the
electrodynamic speaker was to be
replaced with a permanent magnet
speaker, a substitute field coil was
used.
Unfortunately, the original loudspeaker was in such a poor condition that the field winding
resistance was illegible. When this
is the case, a suitable resistance
must be built up so that the HT
filter output voltage is approximately 250 volts when the set is
operating.
As the set was not operative at
this stage, a 2k0 20W resistor was
wired into the circuit in place of the
speaker field coil. If its value needed to be increased or decreased, it
could be sorted out later on.
The speaker plug and socket then
had to be rewired to suit the new
arrangement. The speaker transformer also had to be wired in
somewhere and this was mounted
under the chassis for convenience.
It's not a bad idea to do this
becaus'e it keeps all the high voltage
components out of the way. The
worst thing that could happen to
probing fingers in the back of the
set is a mild burn from a hot valve.
A few more incidentals needed
attention. The frayed grid cap connections were tidied up. A few carbon resistors were also checked
and although they had gone high,
they were still close enough to their
original 20% tolerance. Waggling a
couple of suspicious looking solder
joints revealed two exceedingly bad
connections which were carefully
resoldered. A couple of mica
capacitors were also checked and
found to be OK.
One component that would have
caused trouble if it had not been inspected was the high tension dropping resistor. In this case the
resistance winding between two
taps was open. Rather than replace
the whole unit, the faulty section
was bridged across with a wire
wound resistor. The value of this
resistor was estimated by measuring the resistance of a similar
length of the dropping resistor in an
unbroken section.
Silence is not golden .
It was time to see if my many
hours of work had restored this old
TRF to working order. The valves
were inserted, the speaker plugged
in, the aerial and earth connected,
and the set switched on - nothing
but silence. Total silence!
The rectifier did not light up. A
quick check showed that green corrosion on one of the socket contacts
was possibly the cause of the trouble. With no valves in the sockets
for so many years, it was not
surprising.
Try again. Success - all valves
light up and music fills the room.
The final job was to adjust the
trimmers on the tuning capacitor
and the task was completed.
This 5-valve TRF receiver was
the second radio of that type I had
worked on in less than a fortnight.
The 5-valve model was a much better performer than the 4-valve one,
but neither can match a superhet.~
Resurreetion
Radio
The original vintage
wireless specialists
Our skilled technicians offer QUALITY
repairs & restoration .
We have a large stock of bakelite and
timber radios fully restored and for sale.
Valves, high voltage capacitors and
other specialised parts available.
ADDITIONAL SERVICES
COPIES OF:
CIRCUIT DIAGRAMS $5 .00
SERVICE MANUALS $10.00
AVAILABLE FOR MOST MAJOR AUSl.
MAKES FROM 1934 ONWARD.
WANTED - valves, radios , etc.,
purchased for cash .
Call in to our showroom at:
51 Chapel Street, Windsor, Vic 3181
PO Box 1116
Telephone: (03) 529 5639
SOUND
AUSTRALIA
can offer you
semi conductors
passive components
tools - hardware
Hy-0 coloured mic cable
neutrik-cannon connectors
public address systems
project cases
+ many more!
10% OFF
EVERYTHING
AUGUST ONLY
ph (03) 791 1622
28 Walker St,
DANDENONG 3175
AUGUST 1990
9
|