Cobra 148 GTL-DX Frequency Problem

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Animal
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Radio Make and Model: Cobra 148 GTL-DX
PCB Number: PB 010AA
PLL Number: MC145106
Test gear available: Stand Alone Frequency Counter, Digital Volt Meter, Power Meter

Symptoms: TX & RX are miles off frequency

What is working: Everything - just not on the correct frequency.

What has been tried: Checked TP3 R84 (Ch19 on Mid Band & AM mode) for frequency and it's reading 17.90002, then checked TP6 R60 (same conditions but with TX) and I read 28.59503

Did it suddenly happen ?: Was working fine.

Anything else that may be of help?: Low Low block now no RX or TX, all other blocks TX/RX

Many thanks in anticipation.

Merry Christmas & a Happy New Year to all.
Last edited by Animal on Tuesday 13th Jan 2009, 22:40, edited 1 time in total.
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Anyone? :?
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Cobra 148 GTL-DX Frequency Problem

Post by timrim »

What are the VCO voltages on channel 1 and 40?

Usually the first thing I do when I suspect a PLL/VCO problem is go through the complete
alignment procedure, making notes of any incorrect readings. CB tricks will have all the info you need for that radio. That will point you in the right direction.
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Thanks for the reply Tim :D

I'll check them out this weekend and will post the findings.


Steve
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Okay - here goes!

Started realignment (as best I can with what I've got):

DC Voltmeter to TP2 (R126) & set it at 5.4V (it's stable)

Set to Mid Band, Ch19, AM Mode & checked TP4 (R124) with counter and it reads 17.08055 with the slug almost to the top of L21 (& 17.92005 when the slug has bottomed out in L21) Therefore I can't set it at the required 16.490MHz

Set as above but counter on TP6 (R60) & TX mode, the counter reads 28.61547

Anyone offer any suggestions as to what is causing this???

Many thanks.
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Cobra 148 GTL-DX Frequency Problem

Post by zodiac »

Animal,
Put the counters probe to TP3 R84 and adjust L21 for 16.490 on channel 19 Mid band in AM or FM in RX mode.
Let us know if it does adjust to the correct frequency.
You haven't said if it's off on all modes or just AM/FM.

Dave.
How far is it.
Twice it's length from halfway.
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Re: Cobra 148 GTL-DX Frequency Problem

Post by Animal »

zodiac wrote:Animal,
Put the counters probe to TP3 R84 and adjust L21 for 16.490 on channel 19 Mid band in AM or FM in RX mode.
Let us know if it does adjust to the correct frequency.
You haven't said if it's off on all modes or just AM/FM.

Dave.
Sorry Dave, my last post should have read TP3 R84 (not TP4)

Anyway, readings are as I've stated (had my counter calibrated this week) - it's off on all modes & I can't get it to read 16.490 (without the slug falling out of L21, lol)

Probe to TP3 (Ch19, Mid Band, AM RX) it reads 17.92019

Probe to TP3 (Ch19 Mid Band USB RX) it reads 17.91275

Probe to TP3 (Ch19 Mid Band LSB RX) it reads 17.90817

Any thoughts?
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zodiac
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Cobra 148 GTL-DX Frequency Problem

Post by zodiac »

Well it looks like the adjuster of L21 is working as you said it is adjusting from 17.08 to 17.92.
Check the binary pins of the PLL on channel 19 mid band, you should have pins 10,13,15 and 17 at about 8v, that's a count of 149 in binary, plus the 10.695 crystal plus the 15.000Mhz crystal giving you a total of 27.185 (channel 19 mid band).
If there not high at 8v then the fault could be with one or both of the binary adders.
The high reading your gtting at TP3 of 17.???MHz added to the 10,695 will give you the frequency the radio will TX on and that's why you cannot get the L. L. band, the radio has moved up in it's binary count for some reason.
Dave.
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Twice it's length from halfway.
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Okay Dave,

Since I've adjusted L21 LL band has become operational again?

Anyway, Here are the voltages from the PLL on Mid Band, Ch19

From left to right (starting with the little dimple):

Pin 1 - 8.54
Pin 2 - 3.90
Pin 3 - 3.90
Pin 4 - 4.39
Pin 5 - 2.81
Pin 6 - 7.80
Pin 7 - 4.90
Pin 8 - 8.21
Pin 9 - 8.50

From left to right (Side Opposite dimple)

Pin 18 - 0.00
Pin 17 - 8.50
Pin 16 - 8.50
Pin 15 - 0.00
Pin 14 - 0.00
Pin 13 - 0.00
Pin 12 - 8.50
Pin 11 - 0.00
Pin 10 - 0.00

Now counter to TP6 shows 27.17100 (fluctuating to 27.17120) in TX on LL band & TP3 shows 16.48342 in RX

Hope this gives you more of a clue?

Thanks for your input.

Steve
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Cobra 148 GTL-DX Frequency Problem

Post by zodiac »

The codes you have at the PLL binary pins 9 to 17 are incorrect but the rest of the pin voltages look OK so you can eliminate the PLL at this stage and the rest of the tuning circuit untill you have the correct codes at the PLL's binary pins.
I would replace the binary adders IC6/7 (MC14008) because if one is faulty then the other one will be just as old but replace IC7 first, then re check pins 10,13. 15, and 17 of the PLL to see if they are at 8v.

Dave.
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Twice it's length from halfway.
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Thanks Dave,

I take it these are what I need:

http://www.electronicprojectsonline.co. ... 08B&cat=51

Description is a 4 bit full adder DIL-16

Cheers
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zodiac
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Cobra 148 GTL-DX Frequency Problem

Post by zodiac »

Yep they are the ones, let us now how you get on.


Dave.
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Twice it's length from halfway.
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Will do Dave.

Do I need to take any anti-static/heat precautions with these adders???
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Cobra 148 GTL-DX Frequency Problem

Post by zodiac »

Yes but I've never had any problems with them.
Dave.
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Twice it's length from halfway.
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Just an update Dave,

I got hold of some adders advertised as 'new old stock'

I know, I know........................ I should know better :roll:

Anyway, when they arrived, the pins were so rusty they just snapped off when I handled them :doh:

The remainder, which initially stayed intact, then blew away in a red-oxide cloud as I opened the fridge door for a Stella! :wink:

New ones should arrive before the weekend.

There's a moral to a story here........ :-k
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Changed IC7 and checked voltages on PLL

They're just as they were - no change at all. :-k

TP3 still reads 17.920 (should be 16.490) :-s

I'll change the other adder (IC6) & report findings later.
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Changed IC6 - both adders now replaced.

Voltages at PLL still the same :doh:

Any ideas where to go next???
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Cobra 148 GTL-DX Frequency Problem

Post by zodiac »

Check the voltage on diodes 54,55,and 56, you should have 8v both sides of the diodes with the band switch set at Mid band.
How far is it.
Twice it's length from halfway.
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Is there an easy way to get to the diodes Dave???

They're right underneath the channel selector mechanism/ribbon cable & it looks like the front has to come off???

Many thanks,

Steve
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Okay, I managed to measure voltages either side of the diodes:

They're all approx. 8V

Cheers,

Steve
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Cobra 148 GTL-DX Frequency Problem

Post by zodiac »

I see your asklng Rick for help on his euroradioco forum without any luck and from what's been said, it's looking like it needs to be taken to someone like Rick who knows what they are doing.
The radio has been moded for extra bands but you didn't mention that. What I'm getting at is, it must have extra diodes fitted to get the other bands and maybe one or more of them could be causing the problem. They will all have to be checked out to see if there's a shorted one or more.
I did try to post a pic of the board with the diodes to make it easy for you to find them in my last post but it didn't work for some reason.
This is it, it worked this time.
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How far is it.
Twice it's length from halfway.
zodiac
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Cobra 148 GTL-DX Frequency Problem

Post by zodiac »

Animal,
Did you find any faulty diodes ?.
How far is it.
Twice it's length from halfway.
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Sorry for the late update Dave,

I did sort it eventually by removing the mod for the extra bands (it's back to standard three blocks of 40)

So I guess you were correct about the diodes - thank you for your patience.

Unfortunately, I've uncovered another problem which may or may not be related.

When I'm on Low band LSB & USB is off on TX/RX by 2Khz

E.G. Ch19 Low USB = 26.943 TX/RX

I don't know whether this is of any consequence but I noticed the 10Khz shift is taken from JP54 (jumper broken & taken to Ch9 switch with no resistor & 8v from board)

Next time I'm over your way I'll drop this pile off - in a drive-by stylee from a moving car! :twisted:

:wink:
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Cobra 148 GTL-DX Frequency Problem

Post by zodiac »

JP54 is the 0v connection for Pin 8 of IC6 and must be connected.
It is not for the alphas.
How far is it.
Twice it's length from halfway.
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Cobra 148 GTL-DX Frequency Problem

Post by Animal »

Okay - thanks.
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