This is the first valve
radio overhaul I've done for many years and, as I approach retirement,
I'll definitely find more time for this activity.
Work has been tailing
off the last week or so... maybe my prices have gone up too much
and put people off?
This particular model of the
AR77, which is a superhet designed primarily for the amateur
radio enthusiast, was first sold in 1939 and has a different
front panel to later models. It's been in the hands of a radio
ham or shortwave listener for most of its life as the 40 &
80 metre bands are pencilled on the dials. Bearing in mind the
receiver is 75 years old it will probably need a collection of
new parts. I'm not a purist so replacements will be sourced from
what's readily available. Some people like finding replacement
parts that are the right vintage, or dig out the innards of old
condensers and fit new capacitors inside the old cases. When
it comes to resistors most will be of the carbon rod variety.
The smaller type nearly always drift up in value because the
connections between the wire ends and the carbon deteriorate
and, as new old stock parts also degrade in this way, it's best
to use modern reliable components.
Some people completely strip
a chassis and clean or replace the parts with new. I'm afraid
I don't go that far as I like to see an equipment look its age.
A bit like patina on an antique.
When tackling a radio overhaul
such as this, getting organised is vital.
Besides getting hold of information
on the set and its valves, you must also prepare yourself mentally.
There will be high voltages present, unlike most modern equipment,
so the utmost care must be taken in working on a powered chassis.
Remember that modern test equipment is designed for use on equipment
where the highest voltage present is no more than 30 volts or
so. Valve sets will employ an HT line of anything up to 300 volts
and more so check the rating of scope input sockets and in particular
the rating of a spectrum analyser input.
There are many sets that don't
use a mains transformer, instead being wired directly to the
mains. These are "live chassis" sets and include a
host of Eddystone receivers. Before AC mains became standard,
lots of towns had DC mains and set manufacturers catered for
these by making "universal" sets... meaning powered
from AC or DC mains. No mains transformer means that the chassis
connects directly to the mains and, as many sets had a two pin
mains plug and had a two pin plug on the back, it's a 50% chance
that live mains rather than neutral connects to exposed metalwork
once the case has been removed. Ideally use an isolating transformer
as you can then work without the live chassis risk and, more
importantly, don't blow up your scope by connecting its case
to the chassis of the set. In fact I'd say do not proceed to
dismantle a live chassis set until you've got yourself an isolation
transformer and be aware that some transformers are not isolation
transformers, but are "auto-transformers" which do
not isolate their input common connection from their output.
Something else needs to be pointed
out here in the UK and Europe. Most receivers made in the USA
are designed for 115 volt mains and need to be operated from
a step-down transformer.
The first step in a potential
overhaul of an old set is to gauge whether or not the exercise
will be worthwhile. I mean by this that there must not be anything
insurmountable. To this end check some fundamental things like
the mains transformer primary, see that the various mechanisms
are intact and complete, look for missing valves in case there's
one that's hard to get and see if there's anything vital that's
broken beyond repair. As a rule of thumb, an old receiver that
hasn't been used for decades might have been put away because
something really nasty happened to it. Something either very
expensive to have repaired or an impossible to get part had failed
and you are now retracing the steps of an enthusiast 40 or 50
If everything seems reasonable
get hold of a circuit diagram and/or an operating manual and
make some notes. For example, here's some useful information
regarding the AR77.
The mechanical layout
of the AR77 is quite complicated so I made a sketch of the underside
and the top of the chassis showing the valve types and transformer
I also familiarised myself with
the valve connections. This was useful because some had totally
different pinning to common valves. For example the 6SK7 and
6SJ7 valves have their anode brought out to pin 8 rather than
the more usual pin 3.
Once I'd got enough information
to make sense of things I did some basic tests and found it was
safe to plug the set into UK mains. There are several communications
receivers around that plug into 115 volt US mains but this AR77E
is an export model and thankfully has a 240 volt mains transformer.
Powering an old set with an
unknown history can be a risky business but I have a large variac
so I wheeled this out and connected it up. My variac is an early
Air Ministry version in an open frame so I built it into an old
computer case with a couple of meters showing me the output voltage
and the current consumption of the load. Cranking up the output
voltage made the pilot lamps come on so things were looking good
but I left the output at around 200 volts or so briefly and made
some measurements. The HT was present and there were no burning
smells so I increased the voltage to 230 and waited. The set
was upside down in its outer case with the baseplate removed
so I could see into the chassis. After a few minutes there was
a burning smell so I switched off.
Using my hand as a temperature
sensor I quickly found an 8uF condenser that felt exceedingly
warm so I removed it and noticed the pitch at one end was recently
melted. I should point out that the condenser might have been
saved if I'd applied a lower voltage for a longer time. This
might have have restored the condenser rather than wrecking it.
This condenser was in parallel
with a section of the original smoothing condenser. I say "original"
but in fact it looks like a 1960s type with "Made in England"
on the side. This replacement includes smoothing with a reservoir
condenser and the can was cold to touch. I disconnected both
wires to the condenser live tags and measured both halves with
an ESR meter. Both were roughly correct and had resistances of
less than half an ohm. Oddly the hot 8uF condenser measured 10uF
and a third of an ohm but it clearly wasn't up to the job so
I didn't reconnect it, just the ones in the metal can.
Powering the set a second time
proved OK. No nasty smells at least, and the voltages at the
two condensers measured about right according to the charts in
the user manual.
At this point, although the
set had a number of obvious problems, I connected the aerial
input to a signal generator set to 455KHz, the supposed IF of
the AR77. Despite an input of 100mV I heard nothing so I commenced
measurements at the valve pins and quickly discovered that a
couple of valves were missing their anode voltages. This was
not wholly unexpected because at the front of the AR77 is a Transmit/Receive
rotary switch and mine was missing its spindle so most likely
the set was operating in transmit mode with a number of valves
Access via the underneath inspection
panel is limited so the next step was to remove the receiver
from its outer case. Removing the chassis was straightforward
as the knobs came off easily which isn't always the case.
On the left isn't a quarter
inch jack socket; it's the Transmit/Receive switch missing its
Note that the waveband windows
are in the wrong positions.
The twin dials are in
excellent condition and the front is very clean compared with
the rear and top of the chassis.
The next picture shows the front
end RF components. plus the rectifier and stabiliser at the back.
The large central box holds
the main and bandspread tuning capacitors and at the rear you
can see the aerial trimmer capacitor.
The final IF transformer, detectors
and audio output valve are at the far side with the mains transformer,
and you'll note the aerial input socket is mounted on a small
piece of black crackle finished metal. A modification carried
out by a previous owner.
I don't know what the
recommended workshop procedure is for handling a chassis like
that of the AR77, but I usually make up a set of wooden legs
which I screw to the sides of a chassis. These are long enough
to prevent the dial and valves etc from grounding on the bench
when the set is upside down and I needed to drill a couple of
extra holes in the chassis to hold the legs in place. Fixing
screws are countersunk so the set can be turned onto either end
The wooden legs enable
the set to be turned over without damaging anything. This picture
shows the assortment of knobs fitted by the last owner, although
thankfully the three centre knobs are correct.
Once the set was upside
down I could see the T/R switch was missing its wiper as well
as its spindle. In fact the wiper was wedged inside the wiring
at the opposite end of the chassis. Finding a replacement was
a little tricky and the immediate options were to use a two pole
toggle switch which would mean drilling out the fixing hole (and
it wouldn't look good when the set was back in the case), or
to use a three way switch in place of the two-way switch (my
junk box was devoid of a suitable 2-way switch).
As my three-way switch was electrically
suitable I fitted this and soldered up the wires to the appropriate
tags so the positions were Transmit-blank-Receive and I used
a stand-off to remove strain from the new switch solder tag because
I had to solder all the connections to the new switch before
wiggling it into position and securing it.
Once I'd fitted the new
switch and powered the set for the third time I was rewarded
with loud crackles when the wavechange switch was rotated and,
switching on the signal generator, gave me a good response at
455KHz. I turned down the output to around 1mV and tweaked the
IF transformers. All needed tweaking slightly and there was no
problem peaking the signal. I used to use an old Avometer set
to AC volts to monitor the audio output but I picked up an audio
wattmeter on Ebay many years ago and I now use that.
Once I'd verified the IF strip
was generally OK I removed the signal generator, plugged in a
long wire aerial and turned the wavechange to medium waves. Clearly
the set was working well as all the wavebands were incredibly
noisy and I was able to tune into a strong station playing pop
songs from the 50s. Checking the frequency proved the set was
tuned to about 825KHz. This turned out to be Classic Gold 828
Unfortunately there are now
a large number of problems still to sort out, but having established
that the set is working well I don't mind tackling them.
The following list will probably
not be exhaustive as new problems may show up as work proceeds.
The last owner used loose parcel string on the main tuning
dial and it slipped badly.
The next picture shows the main
tuning drive with parcel string and with a second cord wrongly
located and connected to the wavechange switch. The parcel string
wasn't wholly to blame for the slippage because the wavechange
cord above the chassis was sharing the main pulley and causing
drag which made the loops round the drive spindle slip. Once
the latter was fixed (see 3 below) I was going to leave the parcel
string in place but I decided against this when I found that
tuning was too sloppy. Replacing a dial cord is a messy business
but eventually I got it done.
This picture shows the
bandspead tuning mechanism with decent drive cord and again with
wavechange cord wrongly located.
(2) There are no adjusting screws in the
tops of the IF transformer cans. Should there be?
This was a real puzzle until
I checked the circuit diagram and compared it with the chassis
drawings. There are five IF-style cans. T5 is the BFO coil and
this is adjustable. T1 is the mixer output transformer with a
single tuning slug; T2 is the IF amplifier transformer with two
slugs (top & bottom); T3 and T4 are shown as a single double-tuned
transformer but is in fact split betweeen two cans, one either
side of the chassis tuned from the underside of the chassis.
(3) The wavechange switch
has more parcel string coupling it to two metal shutters with
holes for viewing the correct scales on the two dials and the
windows are out of sync with the scales
After puzzling over the routing
of the wavechange cord and peering at the mechanism, I noticed
a pair of one inch idler pulleys mounted near the chassis. These
were clearly designed to be used for routing the wavechange drive
cord and, judging by their condition, hadn't been used for 50
years or more, so I removed the old cords which were completely
wrong and fitted a pair of new lengths, securing one end with
a knot around each screw head shown below. I then wound a couple
of turns around each drive pulley and passed the ends up through
the elongated holes in the chassis. Where the cords had been
wrongly routed over the shafts the metal was highly polished
so clearly the old receiver had seldom showed the correct band
and had probably suffered from tuning slippage. No doubt a source
of extreme annoyance to its pevious user!
Above the chassis each cord
passes over its corresponding one inch idler pulley and is secured
at an adjusting screw at the top of the waveband indicator.The
adjusters needed sorting out as bits were missing. Each needs
three nuts, two for securing the adjusting screw to the top bracket
and the third for holding a small plastic bush in place. The
plastic bush is used to secure the end of the drive cord and
prevent chaffing. Moving the adjusting screw downwards lowers
the indicator window and vice versa.
Fitting the cords has to be
done with the indicators at the top of their travel with the
wavechange switch positioned at the highest band. Then, as the
wavechange switch is turned to the lower frequency bands, cord
is wrapped around the drive pulleys and the indicators step downwards
revealing the tuning scales for the selected band.
Each of the indicators (main
tuning and bandspread) has to be carefully positioned by setting
the adjusting screw. Above the scales for the highest bands are
the logging scales which are only visible at the verniers. Note
that the drive cords have to be always in a state of tension
otherwise they will drop off the idler pulley and the indicators
will be out of adjustment.
I added a spot of superglue
to each knot in the cord to prevent it slipping. The old cords
which I removed had very complicated knots. Maybe the last owner
was a boy scout?
The next day I decided to fire
up the receiver and check it still worked before tackling the
next job on the list.
I plugged in the mains lead
and the loudspeaker then the aerial and twiddled a few controls
until I could hear noises. I then set the wavechange switch to
Range 1 and found the medium waveband to be fairly quiet with
just a few stations, and these seeming to be in Arabic. Strange..
so I set the dial to 1MHz and plugged in the signal generator.
Nothing much to be heard so I changed the frequency of the generator
until I could hear louder and louder clicks. Digital generators
are OK but not easy to swing up and down a band. Having found
the clicks to be loudest I altered the next digit (the 0.1MHz
digit) until at 8MHz a big signal appeared. At this point the
penny hadn't quite dropped as I'd been thinking the medium waveband
had been modified to say Top Band, then I realised what was going
on. Tuning the generator to 24MHz an enormous response appeared.
I wasn't listening to the medium waveband on Range 1, but Range
6 and the set really was tuned to 24MHz.
The reason was that the waveband
windows were moving the wrong way because the cords were winding
the wrong way, despite having made a sketch of the original parcel
string. I'm now wondering if the receiver was ditched because
of this problem or maybe I'm just getting senile? I had to go
through the whole process of fixing the cords yet again.
(4) The BFO
switch has been replaced, but only part wired and that not soldered!
The other end of the Radiospares capacitor is just floating in
Easily sorted out...
(5) The BFO
can is very loose and needs to be investigated and refitted
I looked at the BFO can and
found it's only secured by a single nut. Because the whole weight
of the set was resting on the can at some stage in the past the
aluminium had become distorted and could no longer be held in
place by its single nut. I straightened out the can and refitted
it. The parts inside looked OK.
(6) I'll need to replace
any duff decoupling capacitors as all are original wax covered
types, also I'll change the output valve grid coupling capacitor
If there's even the tiniest
leak this will forward bias the 6F6, making it very hot and reducing
The AVC or AGC circuitry is
also highly dependent on the quality of its decoupling capacitors
because of the high impedances found in the circuitry.
(7) All the resistors
need checking as some will be miles away from their marked values
I measured all the resistors
and found only one that had failed completely. This was the BFO
anode load resistor marked 100Kohm and it was open circuit.
The 6F6 cathode resistor measured
about 800ohms and should be 470ohms so I'll fit a new one which
will increase the audio output.
Nearly all the remaining resistors
were reasonable apart from one 470kohm that read around 700kohm.
(8) All the wavebands
will need aligning with scale markings and tuning up
to see progress
Below is a picture of the tuning
capacitors revealed when the metal lid is removed. You can see
that things are not straightforward because the main tuning capacitors
(on the right of the picture) not only have three sections, but
each section has two sets of stators & rotors.
This technique allowed the designers
to vary the width of each waveband without having to resort to
high accuracy capacitors in series with the tuning capacitors.
The highest waveband will use the smallest sections, the short
wavebands the larger and the lower frequency bands both sections
On the left is the bandspread
tuning capacitor. This is even more complicated as each of the
bandspread sections of the main bands is allocated its own tuning
Hopefully, although the circuitry
is quite complicated, the end result is simple as all the wiring
is carried out in the wavechange switch.
Two technical aspects
are involved in aligning the receiver; resonating coils at the
dial markings and ensuring the local oscillator coils track with
their associated input frequency coils.
These two tasks are referred
to as tuning and padding. The latter is often a laborious process
and sometimes (if someone has resorted to random twiddling or
a critical component needs replacing) requires a knowledge of
the basic design of the particular set.
Because the receiver is a superhet
you need to know whether the oscillator should be 455KHz above
or 455KHz below the RF input.
I'll go into more detail once
I've tackled this task.
Below is a picture of the aerial
tuning capacitor located at the back of the chassis, connected
by a long shaft passing between the two main tuning capacitors
to a knob on the front panel. I'd noticed a crackle when adjusting
it and the reason is obvious. It's had a bang and the end stator
vane was bent so that it rubbed against the rotor as this was
turned. It straightened when eased back with pliers, curing the
9 The front escutcheon
plate is a gaudy red and I'll repaint it in a different colour
As the case is painted a warm
coloured black crackle and I had a cheap spray paint called Japlac
metallic lilac bought at our local surplus shop, I used that.
Above is the finished plate with the RCA badge refitted.
(10) The signal
strength meter is missing
I found a 1mA Ferrograph meter
of the correct diameter for the fixing bracket and as the parts
list does not specify a sensitivity I'll have to try it and see
if it's suitable. The new meter is a backwards reading type calibrated
1 to 10 which turned out to be perfect for the AR77.
During the alignment of the
receiver I recorded the following results on range 1 (medium
Above is the new meter
installed. The manual is silent on the type of meter but this
old Ferrograph meter is perfect. It reads back-to-front and has
a 1mA movement and seems to behave just right. I can set the
zero reading using the rear mounted potentiometer to "0"
and maximum strength signals push the reading to a little over
I removed the old brittle yellow
plastic sheet behind the escutcheon and fitted a new sheet of
(11) Some of the dial
lamps need replacing
Three new 6.5v bayonet lamps
(12) There's a loudspeaker
inside the case but not wired up
I removed the extra loudspeaker
because I don't like the sound from case-mounted loudspeakers.
(13) I'll need to tidy
up the outer case and find some decent knobs to replace some
Below is just about the only
colour picture of an AR77 I could find on the Net. This one has
a different hue to mine but seems to have the correct knobs.
Above is the case as it
was when I started.
Below is how it looked after
repainting the escutcheon and rubbing some black paint into the
crackle finish. This usually seems to work and just revitalises
the original colour and keeps the well used look.
I'll need to find some
correct knobs which apart from the central four are the old style
of pointer knob which are currently available from my two main
suppliers for about 50p each.
Now, a few more pictures showing
how I tidied up the case.
First, the newly painted escutcheon
Below... parts of
the case were badly distorted. The case with new perspex and
inverted on a workmate.
Gently easing out the
Now fairly straight
After reassembly being
tested on 40 metres. There are a few outstanding things to sort
The noise limiter doesn't work
and the crystal filter needs tweaking, but performance generally
is really good..