Cobra 148 GTL DX MK1 - SC106 Help

[Otter]

Sorry Ade . I'm losing it a bit here .

Can you just confirm your COARSE centre frequency on MID on both US and UK (no trimmer)

Also . A 5K is fine .

The coarse contacts are the set closest to the *front* of the radio. The set closest to the rear of the radio are the fine contacts.

[cb4ever104]

The coarse contacts are the set closest to the *front* of the radio. The set closest to the rear of the radio are the fine contacts.

Not sure why you're telling me this Tim ?

[Otter]

I'm not telling you, I'm telling the OP. He may be thinking that the contacts at the back are the coarse, as the knob at the back is the coarse. If you haven't worked on radios much it would be possible to make that mistake and think you were doing it right.

[cb4ever104]

Mid Band set, US/UK switch on UK, (wiper CONNECTED a), Ch 20 (centre: 27.7875), FM mode, Coarse Tune centred: 27.7805 - 27.7899 TRIMMER SWING

Sorry, I added this to my previous message just after your last message.

Ade . Coarse always centred . Are you saying that you have 27.795 on CH20 with the trimmer disconnected . But when you connect it the trimmer range on CH20 is 27.7805 - 27.7899 ?

[cb4ever104]

I'm not telling you, I'm telling the OP. He may be thinking that the contacts at the back are the coarse, as the knob at the back is the coarse. If you haven't worked on radios much it would be possible to make that mistake and think you were doing it right.

OK . I get that . It was just that you quoted me , and not him , so I thought it was directed at me . Not to worry .

[Phuture]

Hi both,

Yes, Sammy, that's correct for the UK band with the coarse tune at the front of the rig centred (all my "coarse tune" references refer to coarse tune at front of rig) and the connection to the trimmer disconnected. So just to clarify:

Mid Band set, US/UK switch on UK, (trimmer disconnected), Ch20, Coarse Tune centred = 27.795 COARSE TUNE SWING = 27.7876 - 27.8011

Mid Band set, US/UK switch on US, (trimmer disconnected), Ch20, Coarse Tune centred = 27.205 COARSE TUNE SWING = 27.1976 - 27.2111

[cb4ever104]

OK . So can you just confirm again .

Mid Band set, US/UK switch on UK (trimmer connected) : Coarse Tune centred : Frequency range clockwise and counterclockwise .

Also . can you repeat the test with a 10K trimmer as well and post those results too .

[Phuture]

Sorry, I was about to answer but can you just say if you want the coarse or the trimmer swing for your answers?

[Phuture]

I assumed the trimmer swing, so here goes...

Mid Band set, US/UK switch on UK, (trimmer connected), Ch20: Coarse Tune and Trimmer centred = 27.7875, Freq Range (endstops) of Trimmer Swing: CCW 27.7805 - CW 27.7902

Is this what you were after or did you want the coarse tune swing only for the above?

Now with a 10K trimmer:

Mid Band set, US/UK switch on UK, (trimmer connected), Ch20: Coarse Tune and Trimmer centred = 27.790, Freq Range (endstops) of Trimmer Swing: CCW 27.7785 - CW 27.792

[cb4ever104]

Hi Ade. Well leave the 5k in and set it to 27.79125 and that's you done .

[Phuture]

Sorry, I have corrected my previous results for the 5K trimmer - someone was mooching around here looking for things to fiddle with while my back was turned! The updated swing is in fact what the range is. So it would appear I need a 10K to be able to reach the UK frequencies, which is what I will swap the 5K out with.

Thank you very much Sammy and Otter for your invaluable help, advice and time on this! It always helps when you slip into brain fog on these things.

So after all this, are you a little surprised that a 5K (4k7 specified) was supplied with the SC106 kit or am I just unlucky that my Cobra didn't have the expected swing, which a 4k7 trimmer would normally have comfortably been compatible with?

[Phuture]

I spoke to soon - this is something that happened originally. I get up to channel 30 UK then the frequency gets stuck at 27.8970 - 27.8980. If I turn the VCO a quarter of a turn how much does that upset anything else?

[Phuture]

Okay, I turned the VCO a quarter-ish of a turn and I get all the channels now. Is there a knock on effect elsewhere?

[cb4ever104]

There shouldn't be. Just check CH1 on the LOW band to make sure that it's locked down there too.

[cb4ever104]

So after all this, are you a little surprised that a 5K (4k7 specified) was supplied with the SC106 kit or am I just unlucky that my Cobra didn't have the expected swing, which a 4k7 trimmer would normally have comfortably been compatible with?

Putting in a 10k will give you a bit more voltage "to play with" on that line . The voltage is key , as it gets fed to a varactor diode on the other end which ultimately controls the capacitance across your crystals . I made a little circuit of it .

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According to my circuit (Coarse in centre) , a 5K trimmer will give you about 0V to 1.4V on that line . A 10k will give you 0V to 2V . So about 0.5V more .

S.