Yaesu FRG-7700, Survival Manual

Page: 11 / 15

PAØPGA
11
A FRG-7700 adventure ( a resurrection story of a FRG-7700)
I had a recently a receiver with a bad displaydriver, and obtained another from Internet as spare, with a defect.
Well, as things go, this rx had also a display problem, the previous owner told me that he had used the receiving and PLL
board himself for a swap, because the powersupply of his had blown up, so I expected that I had just got a bunch of
spareparts for my other receivers.
After much searching, I found a source for the MSM-5524RS and repaired the original receiver, and after desoldering the
old IC, I put the new IC in a IC socket, because several times desoldering a 40-pin IC is not good for the quality of the
board and the IC…
In fact I use always a IC socket if I change IC’s. If they fail once there can be a reason they fail again in the future, and
putting in a socket is cheap and makes life easier when the need arise.
The spare receiver didn’t look too bad, so I decided to try how far I could get with the resurrection of this receiver, just for
the fun of it. ( After 46 working years I am retired now, so time is no issue).
First I had to construct a new powersupply, because the owner had trown away the burned board, but fortunately had still
the original transformer, complete with burn marks and burned wires.
First of all I tested the transformer, after cutting off the burned wires and repairing them with the use of crimping tube
and suitable wires. The transformer was ok, after a day under full load there was no trouble, so I decided to use him.
The transformer is a common type with a switched primary 110/117 and 220/240V, the secundary is 12V @ 2 Amps.
For the power supply board I had 2 options: copy the existing board from another receiver or construct one myself.
I wanted to use the original features, and designed a board, centered around a trustful LM-317 regulator IC and a NPN
power transistor. For the bridge I used 4x 1N5005 5A silicium diodes and the condenser is a 10000uF/50V type. I
mounted all on a piece of epoxy experiment board, see the pictures and schematic below.
The LM-317 makes the construction very easy, and the output to the rx is adjustable to exact 11 Volts.
I put in a couple of protection diodes over the LM-317, for safety reasons. ( The 2k2 resistor is a trimmer! )
The powersupply works fine, and stays much cooler than the original one. After a day on there is no warming-up, and
stability of this receiver is now the best of the lot.
After connection of the power supply, the receiver
seemed to work: it had a hissing sound, and after
much searching ( the display did’nt work, the
bandswitch was not in the right position and the
analog dial was off-scale, I found a station in the
13 Mhz commercial band, that I recognized, and
after confirming with another receiver, I was able
to “calibrate” the analog dial and put the
bandswitch knob at the right position. Well, that
was the only band with signals, but at least there
was hope for the better.
After a careful inspection of the PLL board, I saw
that the bandswitch had been tampered with, the
switch had been removed and resoldered, but
several print traces were pulled off, and repaired
The changes in the powersupply
(trimmer)