Yaesu FRG-7700, Survival Manual

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12
PAØPGA
with some wires, so I measured the connections and found a broken trace. To repair the board, it was necessary to take
the PLL board out of the receiver. While I was at it I measured all diodes and inspected the rest of the board very
carefully. All seemed ok.
After studying the schematic, I suspected a partially defect MC14504. It is a level translator between the BCD
bandswitch, memory unit and the MB-8718 programmable divider, so I soldered the IC out and put in a IC socket. If I got
a spare, changing the IC was done in no time.
After inserting the board, I checked the bands, and now I had several more bands with signals.
Several bands were there, but on the wrong position of the switch, 7.000 –
8.000Mhz was now 12-13Mhz, and other bands had also weird switch positions.
Because the display didn’t worked either, I put a counter to the counter output
of the PLL, to have at least a indication of the frequency. (the counter frequency
–455khz is the receiving frequency).
I made a truth table of the 6 input pins of the MB-8718, and soon discovered
that pin 15 had no true 0 or 1, but had always a voltage of 3.15V on it. When
touched with the multimeter probe, the divider switched to the right frequency,
so my suspection of a bad MC14504 proved correct and I ordered a new
MC14504. After putting it in the socket, all bands were now in the correct order.
I checked and adjusted the two PLL circuits, because they were way off the right
tuning voltage.
I could see that here had been a very fustrated amateur at work, who had
turned on every coil and trimmer, in the process making more trouble than
he started with.
After re-alignment, the PLL board was working again, but now the VFO
was not working well.
When I tested the output with the counter, I found that the output wire in
the connector was broken, so I had to repair the connector
Now I had signals on every band, but had another curious problem: On
USB or LSB I had signals, but also a steady tone of around 3khz,
independent of the incoming signal. It seemed to me that both sideband
oscillators were running at the same time, but I could not see how,
because the sideband switch was working correctly. After pulling the board
out, I discovered that there was a solder connection between the power
pins of the two oscillators, so they worked at the same time. Maybe a previous
owner didn’t like to switch between USB and LSB.
While the board was out, I desoldered the MSM-5424 and the 74LS196 IC and
put them in a socket. Close inspection discovered some more shorts between
solder joints. The OM who worked on this receiver was clearly not qualified to
work at this kind of equipment, and has a BIG soldering iron…
After reinserting the board, I had now signals at all bands. The remaining
problem was now the counter and the clock. The counter displayed only a
righthand zero, the clock was partially working, the AM Led was on all the time,
the ON and OFF timer had the same time, only the countdown timer was
working. My first impression was that the clock/counter IC was defective.
I tested the counter amplifier and discovered that the 74LS196 dit not get a
proper signal. The first 2 transistors of the amplifier were working, but then things went wrong, so I took the board out
again to change the transistors, but after close inspection I found that the BIG soldering iron has been there also, and
made a short between collector and base of the third amplifier transistor. After correction of this I had a signal at the
74LS196 IC and surprise!, had now also a 3 digit frequency display. The displayed frequency was the counter input,
without the IF offset of 455khz, and divided by a factor 10. (I had the counter still connected).
So, it seemed that the counter IC did at least something. The Xtal oscillator was working and at the good frequency.
Mode switching is done at pins 7, 8 and 9, so I checked the in and outputs of the IC and found that pin 8 was no true
zero. I checked again the mode switch, which was ok, and the diodes D42/D43/D44, who are used for switching.
Bingo.. D43 was defect and displayed a resistance of 5 K-ohm in both directions.
The diode wires were very corroded, so I changed all three diodes. I found out afterwarts that this receiver was used on
a sea-going yacht, and some spots were corroded from the salt water.
After changing the diode the frequency display was working correctly, and the problem with the clock was solved also.
The AM Led was checked, and was changed because it seemed at the end of his lifespan. It was the first time I saw a
burned – out Led, they seem to have a ethernal life!