Cabra 148GTL Malaysia USB alignment issue

[Lkaskel]

Good Day Men,
I have a 148 from Malaysia (1992) on the bench for aligment. I have removed the glue and I will say, this one had more corrosion than I am used to. I had to replace a few resistors, caps and 1 diode becasue they were so corroded. The radio was way off frequency when I started. Going through the PLL alignment AM and LSB are aligning fine. USB has an issue though. When aligning L59 for 34.7665 the best I can get is 34.766410 with L59 bottomed out. L22 and 23 are about centered to achieve their AM and LSB points. Kinda stumped here.

Thanks for any thoughts!!!

 

[brandon7861]

Are there any changes in the clarifier circuit? Is it unlocked? What diode was swapped?

Thats only 90Hz low, which is only 30Hz low at the oscillator. It wouldn't take much to move that can out of tuning range if the varactor was getting the wrong voltage.

If the voltage on the varactor goes down, its capacitance goes up (reactance goes down). This has the effect of lowering the frequency of oscillation. If a mod moved that voltage down a little, maybe L59 was the only can that didn't have the range to compensate.

Other than finding physical faults like a corroded can or maybe a bad electrolytic at the varactor (C231) pulling it down, there are a couple ways to try to force it back.

One method is adding capacitance to the USB path while minding the DC switching. The other is making a couple resistor changes to shift the entire clarifier up a little and realigning all three cans to the new clarifier centered voltage.

EDIT: It appears that you are aligning your radio on ch1. Although irrelevant to the current issue, it can lead to misaligned IF cans. Using ch 19 or 20 when peaking the cans puts the cans in the center of the band. Peaking everything on ch1 may degrade signals at the other end of the dial.

 

[Lkaskel]

Are there any changes in the clarifier circuit? Is it unlocked? What diode was swapped?

Thats only 90Hz low, which is only 30Hz low at the oscillator. It wouldn't take much to move that can out of tuning range if the varactor was getting the wrong voltage.

If the voltage on the varactor goes down, its capacitance goes up (reactance goes down). This has the effect of lowering the frequency of oscillation. If a mod moved that voltage down a little, maybe L59 was the only can that didn't have the range to compensate.

Other than finding physical faults like a corroded can or maybe a bad electrolytic at the varactor (C231) pulling it down, there are a couple ways to try to force it back.

One method is adding capacitance to the USB path while minding the DC switching. The other is making a couple resistor changes to shift the entire clarifier up a little and realigning all three cans to the new clarifier centered voltage.

EDIT: It appears that you are aligning your radio on ch1. Although irrelevant to the current issue, it can lead to misaligned IF cans. Using ch 19 or 20 when peaking the cans puts the cans in the center of the band. Peaking everything on ch1 may degrade signals at the other end of the dial.

I'm going to look at this tomoorow but you know.... I am using the alignment guide for that version 148 and I noticed that the alignment guide for the more recent (China) versions have you align the oscillator on 19.