Cobra 148GTL (Malaysia) low voltage on TP9 PLL

[Lkaskel]

Hi Everyone,
So, I am finishing up this 148GTL and had an issue with TX after the alignment. I put it on air and after a few minutes it quit switching into TX mode (no TX light). I took it back to the bench and figured out that it was getting 1.6vdc at TP9 (PLL voltage) instead of 3 or more so the PLL was not locked. I shut off the radio, turned it back on and the voltage was back to 3.2 on TP9 (CH1). This was manufactured in 1992 so I was curious if the PLL regulator is an issue like in service bulletin 1226 or should I look for other issues?

Thanks as always!!

 

[nomadradio]

I'd be more concerned with the signal level reaching pin 17, the 8719's input. The signal level from L21 is critical. Much below 1 Volt peak-to-peak and the chip can get twitchy. Easy way to peak L21 is to watch pin 17 with a 'scope and peak L21 for max signal level.

Might or might not be the culprit, but it's a place to look first. And if L21 appears to peak with the slug dead even with the rim of the hole, the capacitor inside the 10mm can has gone bad.

73

 

[Lkaskel]

I'd be more concerned with the signal level reaching pin 17, the 8719's input. The signal level from L21 is critical. Much below 1 Volt peak-to-peak and the chip can get twitchy. Easy way to peak L21 is to watch pin 17 with a 'scope and peak L21 for max signal level.

Might or might not be the culprit, but it's a place to look first. And if L21 appears to peak with the slug dead even with the rim of the hole, the capacitor inside the 10mm can has gone bad.

73

I did re-check L21 and measured it from pin 17 instead of TP10. It was spot on. I checked the regulator (IC4) and it had no heat sink compound. It did have a clear insulator in between it and the chassis. I applied compound and also replaced the 3 caps (C133, C134 and C135) and it still was not always going into TX mode. I lastly replaced C90 (pin 5 of IC2). It did go into TX about 10 times after that. I am unsure if that could have been it though. I hate to get this back to the op and have it happen again.....