Yaesu FRG-7700, Survival Manual

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6
PAØPGA
Be Careful: Remove the powercord first!!
The lamp of the VFO is mounted in a small reflector that is very
close to the soldering side of the receiving board and this lamp is always “glued” to the grommet from the heat of the
lamp, so changing is very difficult. To get access, you have to disassemble the front panel and the analog dial. During the
change it is easy to bend the reflector somewhat, and make a short to the board , destroying the counter/clock ic in the
process, because the IC (and part of the powersupply) is On all the time. It is really necessary to remove the power cord,
otherwise you do not remove all the power, with disastrous results.
Displaydriver IC MSM-5524RS:
If the display is not working, there can be several problems, but the most common is the failure of the big driver IC the
OKI MSM-5524RS or the surrounding switching circuit.
The MSM-5524RS is a 40-pin LSI chip, with a clock, dual timer (on/off time), a countdown timer for 60 minutes, and a 5-
digit frequency counter, which is used in this receiver for the display of the received frequency.
The MSM-5524RS drives a 5 digit fluoricent (green tube!!!) multiplexed digital display for clock and frequency. A mode
switch changes the display for frequency, clock, on-time, off-time and sleep function.
If the receiver is switched off, the display is off, however power to the IC is always on. The display stays on when the
mode switch is in one of the clock modes and then displays the time even when the powerswitch is off.
The MSM-5524RS IC is the same as used in the Kenwood R-1000 receiver, and you can find some info of the chip and
circuit diagrams in the R-1000 manual, which I reproduce at the next pages.
There are some things to know about the frequency display:
The displayed frequency is the PLL frequency ( 0.455 – 30.455Mhz) offset bij the IF frequency (455khz), resulting in a
frequency display of 0.000 to 30.000Mhz. There is just one offset possible, so in SSB mode there is always a frequency
difference between the carrier frequency and the displayed frequency of about 1.5 khz because the carrier of a SSB signal
is at the lower or upper side of the signal. The highest resolution of the display is 1 khz, so there is always a frequency
difference of 1-2 khz between the true and the displayed frequency.
This explains the difference in frequency as you are listening to a ham net on a given frequency.
For AM however, the displayed frequency is correct, if the received station is tuned in the center of the filter.
The MSM-5524RS IC is very dependable, and, when normally used, there is seldom a failure.
You see seldom a R-1000 with a display failure, because they use a different board layout.
Problems with this IC are almost allways man-made, like a short during the change of the light bulb in the VFO, (see
above) or poking around in the set with the powercord attached.
If this IC fails, you are in big trouble, because the manufacturer stopped the production some 15 years ago, and supply is
very scarce if found at all.
I saw one (used!!!) on e-bay, for a rediculous price (around $ 60.-), so be prepared for a long search.
Maybe they have been used in some clock radio’s from the eighties, but I did not find any yet. Your best bet is to obtain a
spare set with a defect display or otherwise, and use the good IC.
Kenwood used the same IC in their R-1000 receiver, so that can be a source as well.
Causes of trouble for this IC are the switch and the switching diodes which choose the mode of operation.
One of my sets had a bad mode switch. Using the clock was no problem, but the switch could not change to the
frequency mode, it still displayed the time. The reasons where bad switching contacts in the mode switch.
I took the switch out and removed the back plate. If this is carefully done, you can put the switch back together again
without problem. After a squirt of contactspray and working the switch a couple of times, everything worked nice again.
In another case I found diode D43 that was no diode anymore but a resistor of 5k-ohm, the on and off time of the display
was the same, and there was a curious frequency display. The frequency displayed was the receiving frequency /10, plus
455, so 7455 displayed as 791, so apparently the 455 khz offset was not switched on.
I changed all three diodes for 1N4448 types, because they had a bad oxydation of their leads. Later I found out that this
particular receiver has been used in a yacht, and salt water spray left its corroding mark…
If you have some weird display problems, check those diodes first, or the attached switches and transistors.
Display:
The display type is FIP 5A8B, a 5-digit green fluoricent tube, of the kind that is much used in home audio equipment and
in practically every receiver and transceiver of the eighties. Yeasu uses a amber filter, to change the color in the style of
all their equipment of the time (FT101ZD, FT901)
This tube has a filament, and has in the long run the same problem as all tubes: the filament is coated with emitting
material, but after long use, this material is exhausted and the display is getting dim.