Trio TS-120S PLL unlock fault

I recently acquired a Trio TS-120s HF transceiver with an intermittent fault - when powered up the PLL would fail to lock , resulting in no TR or RX and no digital display - just two dots.

PLL Unlocked

Once the set warmed up the PLL would eventually lock , first on 14MHz then  on the other bands.  Once the set had been switched off for a while the PLL would return to the unlocked state on power up .


I checked the PLL board for cold solder joints but nothing obvious was found.






Tapping on the boards when the PLL was unlocked made no difference , so I tried gently heating the boards using a hairdryer , this helped pinpoint the problem - the PLL came in to lock whenever the area near the back right hand side of the AF board was warmed up .
I didn't expect that a fault on the AF board would prevent the PLL from locking , but underneath the AF board is the carrier board which produces a ~8MHz signal which is fed to the PLL board .

Testing J19 CAR signal

I attached a frequency counter to Pin2 of J19 on the PLL board and a volt meter to TP1 on the PLL board .  When the PLL was unlocked there was no 8MHz signal reaching the PLL board . When the CAR board was gently heated  the 8MHz signal appeared and the PLL locked .

An article on mods.dk indicated that there was a common fault where the  carrier board would fail to oscillate due to the Q1 transistor going low gain, so I removed the AF board to gain access to the CAR board to replace Q1 .

AF Board

AF board removed 

CAR board removed

I removed Q1 ( 2SC460)  and tested it on a transistor tester - the device was ok , but the gain (hfe) was very low at around 29 . It should be about 100 for this device.

Original Q1 - low gain

After warming the faulty transistor the hfe came up slightly to 36

Original Q1 warmed up

This additional gain was probably enough to get the oscillator to start up once the transistor warmed up.  I tested the new replacement transistor prior to installing it - it had a hfe of 107

Replacement Q1 

After re-installing the CAR and AF boards I powered on the set - the PLL locked immediately on all bands !

PLL Locked