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.
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| PLL Unlocked |
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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 .
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| 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 .
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| AF Board |
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| AF board removed |
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| CAR board removed |
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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.
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| Original Q1 - low gain |
After warming the faulty transistor the hfe came up slightly to 36
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| 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
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| Replacement Q1 |
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After re-installing the CAR and AF boards I powered on the set - the PLL locked immediately on all bands !
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| PLL Locked |
