OBSERVATIONS FROM SCOTLAND                          25 May 2008                               GM1SXX

Repairing the Yaesu FT102 Transceiver.

Last week, I bought a faulty FT102 at the magnum Radio Rally. Over the past week, I discovered that the 300V DC supply was unserviceable so I set about purchasing the parts I needed to repair the rig.  The 300V supply provides power for the 12BY7A valve driver stage, so without that, TX was disabled, although I copy copy the leakage from the preceeding stage on my Racal RA1772 with ease.

The photograph below shows the power supply PCB that carries the 300V supply after repairs. The supply is very simple consisting of a silicon diode in series with a pair of 10uF@450V electrolytics. A 330R takes the smoothed DC to another pair of 10uF@450V electrolytics with a 470K bleeder resistor to ground.  The two electrolytics on the right of the PCB are the replacement 10uF ones. There should also be another pair next to them but here I used a single 33uF@450V part instead.  Hidden from view are 33R, 330R and 470K resistors plus the IN4007 diode I fitted to replace the faulty diode. The yellow wire below the electrolytics is the AC feed and the red wire is the 300V DC out to the driver stage. The other parts on the PCB supply PA and driver voltages. Since these were healthy, I left well alone.

Next photo shows the underside of the FT102. The board carrying the large black electrolytic to the right of the transformer cover (black object) is the 900V and 6V DC supply for the VFO. Note how untidy the wiring is around it. Not good. I'll tidy it with Ty-wraps to keep all those low voltage wires well away from the bare PCB tags carrying 900V DC.. The green PCB  (bottom right) carries the PA stage valveholders (3) for the 6146B PA bottles.

These old radios need to be treated with great respect. They use voltages that at best could spoil your whole day, and at worst can kill you.  Never work on such radios without discharging the major HT power supplies. I use an old meter probe with a series resistor (10K is good) in a heatshrink sleeve and TWO crocodile clips that go to ground.  Why two?  Well it's just a safety thing.  I also check using a meter that the HT lines are actually discharged.

This radio had open circuit bleed resistors on the 900V HT line. Had I not discharged the HT voltage manually, I certainly would have received a 900V DC shock..... and that was HOURS after I switched off the radio.  You can never be too careful when working on valve / hybrid gear.  I also strongly recommend that you employ the 'one hand in the pocket' rule when working on high voltage equipment. It could just save your life.

The FT102, driver stage working and on bench soak test.

Next on the agenda... smoke testing the PA.

73 AL.
GM1SXX

 73 AL.
GM1SXX