Sony CRF-320 Repair
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Not much wrong with this
very heavy portable radio sold between 1976 and 1980 and which
is now much sought after even though its price is boardering
on the ridiculous I even spotted a secondhand back cover being
offered for £200.
The customer reported the main
tuning dial was out of kilter and new LED lamps were no longer
working. |
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Above front view and below
with the front cover detached. |
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I removed the front and
panels to access the circuitry. |
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I powered the radio from the
mains supply and checked the various observatios made by the
owner. Noticeable was missing illumination for the clock,
meter and FM dial. The LW/MW dial and the SW dials could both
be lit when these modes were selected and were very bright. Radio
4 tuned in at something like 190KHz instead of 198KHz but MW
and, in particular short wave, dials seemed OK. I did notice
however that the SW dial has a cursor adjustment and if this
is offset it would give out of sync readings between the digital
readout and the dial calibration. Sensitivity on LW/MW wasn't
particularly good no doubt because of component aging. |
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The first step was to
align the LW/MW dial. Using my Tiny SA as a signal generator
revealed the lowest frequency I could tune was about 160KHz against
a dial reading below 150KHz. The receiver has a standard analogue
design in its long and medium reception modes and tweaking the
oscillator brought in 150KHz and further adjustments corrected
tracking. Alignment is straightforward once one is familiar with
the layout of the coils and trimmers. The manual is strangely
pretty unhelpful because it shows two drawings, one each for
long and medium wave adjustments with neither showing all coils
and trimmers, and it has the positions of the medium wave oscillator
trimmer and coil reversed which was unhelpful to say the least!
To aid alignment I've shown above the locations of the complete
set of coils and trimmers.
The standard method of alignment
is used ie. trimmers at HF and coil slugs at LF. The dial markings
are very vague and Sony didn't bother to indicate alignment points.
The FM range seemed to be OK
as was the shortwave range which employs a digital synthesiser
and a red digital readout. Tuning upwards by a single step from
say 3.999 gives a display of 3.--- rather than 4.000 which is
a trifle annoying but which I assume to be normal. Tuning to
a frequency standard proved the dial read perfectly.
Next I need to figure out the
problem with the LEDs which I understand replaced small filament
lamps. |
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This is the connection
panel for the various lamps used in the receiver.. being Clock,
Meter, FM dial, LW/MW dial and SW dial with common black and
five red each feeding a ballast resistor for dropping the LED
voltage chosen to suit its forward voltage and supply.
The customer reported that the
LEDs had failed one by one which suggests poor reliability rather
tan a broken wire or bad solder joint. |
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LEDs are pretty well anonymous,
as they are rarely marked with any identification, so you need
to check things like the package style, lead mechanical shape
and internal structure to identify the manufacturer and type.
The blue types fitted in the Sony were installed in 2018 and
appear to be similar to a 5mm Kingbright range in a package called
"T one and three-quarters" and, looking in a supplier
catalogue for this period, suggests the following options.
The supply voltage measured
as 14.75 volts. VF = forward voltage drop, IF=forward current,
P= LED dissipation, Lv-mcd= a measure of LED brightness.
|
Make |
Type |
VF volts |
Lv-mcd |
Supplier Code |
IF mA max |
IF mA nominal |
P mW max |
Ballast resistor |
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Kingbright |
L7113PBC-A |
3.2-4.0 |
280-1000 |
SC08060 |
30 |
20 |
120 |
560 |
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Multicomp |
OVL-5523 |
3.2-4.0 |
1950-4300 |
SC08062 |
30 |
20 |
120 |
560 |
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Kingbright |
L7113QBC-D |
3.3-4.0 |
1700-2700 |
466-3548 |
30 |
20 |
120 |
560 |
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I removed one of the open
circuit LEDs and found it was fitted with a 180 ohm ballast resistor.
This would drive the LED current to something like 60mA with
a dissipation of close to double the maximum. This being so explains
the poor reliability of the existing LEDs if indeed they have
the spec given above. Clearly the remedy is to replace the LEDs
with a set having the best rating for the job. I selected a high
brightness white LED made by Nichia, type NSPW510DS which has
a forward drop of 3.2V at 20mA having a luminous intensity of
6.1cd and a viewing angle of 45 degrees.
The table below gives the original
LED operating conditions, with basically any type having the
same actual over-dissipation, together with the Nichia used to
replace the originals. |
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Make |
Type |
VF volts |
Lv-mcd |
Supplier Code |
IF mA max |
IF mA nominal |
P mW max |
Ballast resistor |
IF actual |
P mW actual |
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Kingbright |
L7113PBC-A |
3.2-4.0 |
280-1000 |
SC08060 |
30 |
20 |
120 |
180 |
64 |
211 |
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Multicomp |
OVL-5523 |
3.2-4.0 |
1950-4300 |
SC08062 |
30 |
20 |
120 |
180 |
64 |
211 |
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Kingbright |
L7113QBC-D |
3.3-4.0 |
1700-2700 |
466-3548 |
30 |
20 |
120 |
180 |
63 |
214 |
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Nichia |
NSPW510DS |
3.2-3.5 |
6100 |
713-3996 |
30 |
20 |
105 |
510 |
22 |
70 |
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I chose a ballast resistor
of 510 ohms which will result in a current of about 22mA (dissipation
etc about 70mW) and fitted these. Apart from the clock LED, the
others are glued into rubber rings which fit snugly into apertures
in the different assemblies. When I checked I found the LEDs
produced a pleasant greenish-hued illumination due to characteristics
of the plastics used in the receiver.
With the panels refitted and
the battery leads plugged back (only the clock D-cell is used)
I noticed the clock LED was pulsing at around a one second interval. |
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