Each band will produce
two main responses from a signal generator.
For example (line 5 above) at
a dial setting of 10.2MHz you will hear 10.200MHz AND 11.110MHz.
Setting the dial to 11.110MHz
and tuning the signal generator to 11.110MHz at a level of say
1mVolt will produce a huge response.
Retuning the dial to 10.2MHz
will produce another huge response.
Switching to Range 6 and tuning
to 22.220MHz you'll find another sizeable signal and another
at 23.130MHz
In fact, because the IF is 455KHz
the images are always roughly a meghertz higher, or more precisely
0.91MHz higher.
Finding an unknown IF or coil
values and padding capacitor
As an aside; many years ago I was aligning
an old Hallicrafters set whose dial markings on its highest range
had no connection with received signals because a long wave range
had been substituted for it and after hours of twiddling decided
to use a spreadsheet to help sort out the problem of restoring
things to their original state. At that time, before Excel, I
used Lotus 123 which, like Excel, lets you write equations into
cells and produce tables. I investigated the short waveband which
had been removed. By measuring the maximum and minimum values
of the tuning capacitor a listing of frequencies can be produced
for various tuning capacitor settings in a column given a particular
coil value. For example at 100pF you would tune a signal of 18MHz
with the short-wave coil. These frequencies can be compared with
the dial markings until a perfect match is obtained.
Next a similar calculation is performed
to determine the value of the new oscillator coil. Its inductance
is estimated by trial and error and after measuring the tuning
capacitor max/min values and using the known IF comparisons are
made with marked dial readings. This time an important component
must be taken into account. This is the padding capacitor which
effectively modifies the action of the tuning capacitor. Depending
on the precise circuit position of the padder the oscillator
frequency equation needs to be modified. The oscillator and RF
columns in the spreadsheet can then be compared.
As the exercise had been primarily to
determine the new coil values and the padder value these items
were kept outside the table and used as presettable constants.
Keyboard "F4" was used in Lotus and it was also adopted
in Excel for this purpose. The padder was appended with the label
F4 and by changing the value from say 100pF to 200pF one could
see its overall effect on the main tuning and the oscillator
tuning at the same time.
At this point I'd got things working
to the level where some fine tuning on the spreadsheet was feasible.
I added in trimmer capacitors, again as F4 values, and set these
as reasonable values, say 10pF. By typing in numbers, progessively
using more significant figures, things resolved themselves and
I was rewarded with new coil values and a padder capacitor value.
Work proceeds 5th March 2014
I decided to sort out some of the really
bad resistors and also swap some of the old capacitors.
I changed a 470kohm resistor reading
675kohm for a new part and then replaced an open circuit 100kohm
resistor. The latter I discovered later resulted in the BFO starting
to sort-of work.
I suspect this 100kohm resistor failure
stopped the last "restorer" in his tracks as a new
switch dealing with BFO selection was wired in place but no solder
had been used and the connections were very loose.
I also fitted about a half dozen new
capacitors in places where high value resistors were associated
with them. This would remove any significant current leaks as
old condensers tend to develop resistance in parallel with, and
very often also in series with, their capacitance making the
efficiency of the condenser pretty poor.
At this point I decided to retest operation
and turned to the lowest range, setting the dial to exactly 0.54MHz
where I was rewarded by a signal from my generator slightly off
tune.
The correct method for tracking is to
adjust the oscillator coil at the low end of the band, then retune
the receiver to the top end, 1.3MHz and trim out any frequency
error with the preset capacitor.
Having set the peak exactly on 0.54MHz,
I then tuned the set and generator to 1.3MHz. By increasing the
generator output level I could just hear a signal off tune so
I twiddled the oscillator preset capacitor and peaked the output.
Turning back to the low end the error was now very small, and
by twiddling the coil and trimmer a few times the dial settings
corresponded perfectly.
Next was to repeat the alignment but
this time on the two RF amplifier coils. Both responded well
to tuning and tracking and I found I could reduce the generator
output level to 0.2 microvolts at both ends of the band and still
just hear a signal.
During this process I had to unplug
my network camera which generates a tremendous broadband mush
and found I could hear broadcast signals even with the generator
plugged in because the underside of the AR77 was unscreened.
I plugged in a long wire aerial and there were masses of powerful
stations right across the band, although mostly distorted.
So far so good. Presumably there are
no serious problems with the set that will affect alignment of
the other five wavebands.
Before I proceed further I must fix
a couple of glaring faults. First there is no automatic volume
control operation and to hear decent audio I have to reduce the
RF gain control. I would have expected at least one setting of
the various knobs to provide AVC. Second, and this may be related
to the lack of AVC is to solder the new switch connections and
find out if all the wiring is in place.
Fixing the AVC switch
The first job was to remove the "new"
old AVC switch which had six positions and find a replacement
with only three positions which is correct. The junk box turned
up a four pole three way switch and after sawing off half an
inch of spindle I fitted this. I removed a capacitor and three
wires from the old switch and then decided to draw a circuit
for the switch and trace the wires to see how they corresponded.
During this process I found that the circuit diagram I have and
the actual AVC circuitry didn't quite match, but the two versions
were close enough for me to re-establish the wiring to the new
switch.
There are three sets of wires.
The AVC circuitry which operates at
position 1 and 2 and connects the AVC detector diode to the RF
amplifier and first IF stage grid bias feed. My set uses only
one of the two diodes in the 6SQ7 valve and incorporates a 1
megohm resistor whereas the circuit diagram shows both diodes
in parallel and a 220kohm series resistor.
Negative feedback selection at position
1 links the audio output via a 5.6kohm resistor to the earthy
end of the volume control. A new wire had been added grounding
the earthy end of the volume control so I cut this off, letting
the new switch place the ground connection in positions 2 and
3.
The third set of wires are for position
3 switching HT to the anode of the BFO and simultaneously grounding
the AVC line.
Once the new wiring was in position
I'd discovered the previous restorer had disconnected the negative
feedback feature, failed to discover an open circuit anode feed
resistor to the BFO and disconnected the AVC line. This sort
of supports my theory about why some apparently good receivers
are put on one side for ever, until sold or scrapped. When I
turned on the set to check it was still working it had been totally
transformed. Tuning across the medium waveband revealed it was
full of intelligible stations. Before, the RF gain control needed
to be carefully adjusted in order to resolve anything, but now
with the gain at maximum everything was really clear and as a
bonus the S-meter was working properly.
Alignment again
I returned to alignment and found range
1 was almost perfect and after a few adjustments I was able to
see a small S-meter change when tuning across a 2 microvolt signal.
I switched to range 2 and tweaked this and found it was also
perfect. Range 3 proved to be a problem. No amount of twiddling
could line up the dial markings with the signal generator so
back to thinking about potential problems and wondering whether
my spectrum analyser could help as it had with aligning my Racal
RA17. A spectrum analyser enables one to see the local oscillator
moving up and down as one tunes a receiver both in terms of frequency
and amplitude. In the case of the RA17 the VFO signal had suddenly
vanished mid-scale whilst tuning the receiver because of a bad
EF91.
7th March 2014: Still tackling the problem
of dial alignment on the higher frequency bands, I traced the
circuit to discover exactly how the various ranges are configured.
The table below lists the various
coils and capacitors involved in tuning |
