Yaesu FRG-7700 Survival Manual

Also see for FRG-7700: Instruction manual Maintenance service manual Service manual Instruction manual

Contents

12PAØPGAwith some wires, so I measured the connections and found a broken trace. To repair the board, it was necessary to takethe PLL board out of the receiver. While I was at it I measured all diodes and inspected the rest of the board verycarefully. All seemed ok.After studying the schematic, I suspected a partially defect MC14504. It is a level translator between the BCDbandswitch, memory unit and the MB-8718 programmable divider, so I soldered the IC out and put in a IC socket. If I gota 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 outputof 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 discoveredthat pin 15 had no true 0 or 1, but had always a voltage of 3.15V on it. Whentouched with the multimeter probe, the divider switched to the right frequency,so my suspection of a bad MC14504 proved correct and I ordered a newMC14504. 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 righttuning voltage.I could see that here had been a very fustrated amateur at work, who hadturned on every coil and trimmer, in the process making more trouble thanhe started with.After re-alignment, the PLL board was working again, but now the VFOwas not working well.When I tested the output with the counter, I found that the output wire inthe connector was broken, so I had to repair the connectorNow I had signals on every band, but had another curious problem: OnUSB or LSB I had signals, but also a steady tone of around 3khz,independent of the incoming signal. It seemed to me that both sidebandoscillators were running at the same time, but I could not see how,because the sideband switch was working correctly. After pulling the boardout, I discovered that there was a solder connection between the powerpins of the two oscillators, so they worked at the same time. Maybe a previousowner didn’t like to switch between USB and LSB.While the board was out, I desoldered the MSM-5424 and the 74LS196 IC andput them in a socket. Close inspection discovered some more shorts betweensolder joints. The OM who worked on this receiver was clearly not qualified towork at this kind of equipment, and has a BIG soldering iron…After reinserting the board, I had now signals at all bands. The remainingproblem was now the counter and the clock. The counter displayed only arighthand 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 wasworking. My first impression was that the clock/counter IC was defective.I tested the counter amplifier and discovered that the 74LS196 dit not get aproper signal. The first 2 transistors of the amplifier were working, but then things went wrong, so I took the board outagain to change the transistors, but after close inspection I found that the BIG soldering iron has been there also, andmade a short between collector and base of the third amplifier transistor. After correction of this I had a signal at the74LS196 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 truezero. 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 ona 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 aburned – out Led, they seem to have a ethernal life!

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