Cobra 148 GTL DX MK1 - SC106 Help

[Otter]

I think that one of the lines marked 32 on the drawing is connected to a positive voltage rather than where it's supposed to go. Or shorted to a +ve voltage. It's always high and it shouldn't be.

[Phuture]

Hi Otter,

On my drawing and in the radio, the holes into which the channel selector's ribbon cable go are into the series of six holes beside the UK hole on the SC106. Then, the first three holes from the left of the radio's case, i.e. binary 32, 16 and 14 on the main PCB, are wired to the other side of the SC106 and to the corresponding holes. The same goes for the next three holes, so binary 2, 8 and 1 on the main PCB, except that you twist the 3-wire ribbon cable once to flip the two outer holes to their corresponding holes on the left side of the SC106, so 1, 8 and 2. I know I have the wires coloured red on the drawing as that happened to be the colour strip available on the ribbon cable I used, but can you clarify where you mean the binary 32 connection is connected to a positive voltage or potential short? Thanks.

[cb4ever104]

I've only had a minute to look at it , but it looks as if your "32" line is wrong . It's HIGH when it shouldn't be .

Capture.JPG

The "32" line is pin 11 on the 8719 .

S.

[Otter]

The frequency issues you describe are consistent with the line marked 32 on the drawing going to the the PLL being high when it shouldn't be.

[cb4ever104]

32 on the drawing going to the the PLL being high when it shouldn't be.

Sorry Tim . Beat you by 60 seconds

[Otter]

Sorry Tim . Beat you by 60 seconds

Not really. Scroll up a few comments...

I think that one of the lines marked 32 on the drawing is connected to a positive voltage rather than where it's supposed to go. Or shorted to a +ve voltage. It's always high and it shouldn't be.

[cb4ever104]

Not really. Scroll up a few comments...

Well . Let's not split hairs . We can call it a joint reply

[Phuture]

Taken a look. I think I may have found the fault, but it may be additional to the original problem. Anyway, more of that later if this repair works. Discovered after moving the Cobra's transparent ribbon cable around so I could actually see the binary 32 wire coming out of the channel selector board that it's all frayed where it enters the hole. Possibly done with all the to-ing and fro-ing of the SC106 trying to work out the original problem. Back when I've done the repair...

[163TM847]

I know this doesn't solve your issue, But i just wanted to add that the diode in the offset image isn't critical as Sammy said, I only put that there because i wanted to use one pole of the switch for UK and offset, And the other pole to switch the band switch off [So when switching to UK it doesn't have to be on a specific band] Hope that makes sense, But when you do switch the UK and offset even though both use Ground you must use both sides of the switch, Don't connect UK and offset on the same pole unless you use the diode, And if using a diode then a larger value VR will be needed, 10k is usually enough.

[cb4ever104]

Cobra's transparent ribbon cable

I'd replace that ribbon cable anyway while you're doing the work . Either use new ribbon cable or just use wire .

[Phuture]

Okay, back on track, I have 27.605 on the High band, UK channel 1, Coarse Tune wiper disconnected from US/UK switch and centred, and switch in UK position. Thank you both for analysing and isolating the frequency fault.

Okay, looked back over posts and I think you meant the swing with the US/UK switch on UK, but I have provided the High and Mid band swing as well just in case.

Mid Band set, US/UK switch on UK, (wiper disconnected), Ch 20 (centre: 27.7950), FM mode: 27.7876 - 27.8011 swing

High Band set, US/UK switch on US, Ch20 (27.655), FM Mode: 27.6473 - 27.6613 swing
Mid Band set, US/UK switch on US, Ch20 (27.205), FM Mode: 27.1976 - 27.2111 swing

And lastly with the coarse tune wiper connected and the supplied 5K trimmer about centred:

Mid Band set, US/UK switch on UK, (wiper CONNECTED), Ch 20 (centre: 27.7875), FM mode: 27.7836 - 27.7995 swing
Mid Band set, US/UK switch on UK, (wiper CONNECTED), Ch 20 (centre: 27.7875), FM mode, Coarse Tune centred: 27.7805 - 27.7899 TRIMMER SWING

[cb4ever104]

Mid Band set, US/UK switch on US, Ch20 (27.205), FM Mode: 27.1976 - 27.2111 swing

Good to see you got the frequency issue fixed Ade .

OK . So that's about 7khz down and 6khz up . Great .

Now connect the 4,7K VR wiper (leave Coarse in the middle) between GND and the centre pin . What's the complete frequency range with the wiper clockwise and anti-clockwise ?

[Phuture]

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.

[Phuture]

By the way, it is a 5K trimmer that I have installed and was a component of the original kit. Do I need a 4k7? I don't have one but there has been a suggestion to try a 10K, which I do have.

[cb4ever104]

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 .