Cobra 148 MK2 10Khz off All Bands

[currentstatus]

Put it on FM, CH1, low, set KC/Clarifier to centre, measure the voltages on PLL pin 17-9, measure the frequency on PLL Pin 2.

Repeat for CH1 Mid and Hi.

Post the results here.

Hi.. PLL measurements.

pin 17 - Low 0v / Mid 8.4v / High 0v
pin 9 - Low 0v / Mid 0v / High 0v
pin 2 - Low 820.053 KHz / Mid 1.27008 MHz / High 1.72011 MHz

Thanks!

[currentstatus]

Something I noticed.. the coarse clarifier is giving me +/- 3KHz .. in the spec it shows +/- 7KHz

[cb4ever104]

He was asking for all pins from 17 through 9 .

[currentstatus]

Got it.. thanks!

pin 17 - Low 0v / Mid 8.4v / High 0v
pin 16 - Low 8.4v / Mid 8.4v / High 0v
pin 15 - Low 0v / Mid 8.4v / High 8.4v
pin 14 - Low 0v / Mid 8.4v / High 8.4v
pin 13 - Low 8.4v / Mid 8.4v / High 0v
pin 12 - Low 0v / Mid 8.4v / High 8.4v
pin 11 - Low 8.4v / Mid 8.4v / High 0v
pin 10 - Low 0v / Mid 0v / High 8.4v
pin 9 - Low 0v / Mid 0v / High 0v

pin 2 - Low 820.053 KHz / Mid 1.27008 MHz / High 1.72011 MHz

[Kaliphan]

I actually asked for pins 17 down to pin 9 but I wasn't very clear, sorry.

Something makes no sense here from what I am reading but let's try anyway.

if the VCO is locked the PLL should have a frequency on pin 2 that's 10KHz x the N-Code on pins 17 to 9.

0.82MHz says the N-Code should be 82

1.27MHz means 127 and you can probably work out the one for Hi?

That means the adders and PLL chip should be OK.

Can you check the PLL pis 17 to 9 are what I (and the service manual) predict? (8.4 is approximate)

Low Mid Hi
Pin 17 0 8.4 0
Pin 16 8.4 8.4 0
Pin 15 0 8.4 8.4
Pin 14 0 8.4 8.4
Pin 13 8.4 8.4 0
Pin 12 0 8.4 8.4
Pin 11 8.4 8.4 0
Pin 10 0 0 8.4
Pin 9 0 0 0

The Cobra works by mixing the VCO frequency with the 15MHz band crystal and feeding the difference frequency to the PLL on pin 2.

The PLL then divides that input signal by the N Code and compares it to a 10KHz signal thats been created by dividing the 10.240MHz signal by 1024, when they match the PLL is locked.

If they don't match, the PLL chip outputs pulses which are 'filtered' (integrated) to make an error voltage that drives the VCO up or down until they do match.

So, for CH1, Lo, the VCO should be 15.82MHz, Mid should be 16.27MHz and Hi 16.72MHz, please confirm that.

If the VCO is correct, please also confirm the voltages on 17 to 9 and VCO frequencies don't change on TX (pull out the bias board near the finals)

[cb4ever104]

I actually asked for pins 17 down to pin 9 but I wasn't very clear, sorry.

Something makes no sense here from what I am reading but let's try anyway.

if the VCO is locked the PLL should have a frequency on pin 2 that's 10KHz x the N-Code on pins 17 to 9.

0.82MHz says the N-Code should be 82

1.27MHz means 127 and you can probably work out the one for Hi?

That means the adders and PLL chip should be OK.

Can you check the PLL pis 17 to 9 are what I (and the service manual) predict? (8.4 is approximate)

Low Mid Hi
Pin 17 0 8.4 0
Pin 16 8.4 8.4 0
Pin 15 0 8.4 8.4
Pin 14 0 8.4 8.4
Pin 13 8.4 8.4 0
Pin 12 0 8.4 8.4
Pin 11 8.4 8.4 0
Pin 10 0 0 8.4
Pin 9 0 0 0

The Cobra works by mixing the VCO frequency with the 15MHz band crystal and feeding the difference frequency to the PLL on pin 2.

The PLL then divides that input signal by the N Code and compares it to a 10KHz signal thats been created by dividing the 10.240MHz signal by 1024, when they match the PLL is locked.

If they don't match, the PLL chip outputs pulses which are 'filtered' (integrated) to make an error voltage that drives the VCO up or down until they do match.

So, for CH1, Lo, the VCO should be 15.82MHz, Mid should be 16.27MHz and Hi 16.72MHz, please confirm that.

If the VCO is correct, please also confirm the voltages on 17 to 9 and VCO frequencies don't change on TX (pull out the bias board near the finals)

Did you read the guys first post ?

Also re-aligned the PLL TP-3 16.49000 mid / TP-2 5.4v

He's already got 16.490 on TP3 on CH19 MID band AM. So I doubt it's the PLL , VCO or Loop oscillator stage , as it's not "+10"ing there .

Good luck with it though .

[currentstatus]

He's already got 16.490 on TP3 on CH19 MID band AM. So I doubt it's the PLL , VCO or Loop oscillator stage , as it's not "+10"ing there .

No Sammy.. ch19 mid is 16.5000 on TP-3 (it just won't go to 16.4900 unless I go to ch18)

Sorry it's confusing testing and trying to align on ch19.. but the rig is actually on 27.195 (not 27.185). It's becoming more apparent now that the adders & PLL are probably OK.. and the fault is beyond that section. I've been speaking to Nick (great guy btw) and we've tested a couple of things but to no avail as yet.

I'll post those other measurements also tomorrow.

[Kaliphan]

He's already got 16.490 on TP3 on CH19 MID band AM. So I doubt it's the PLL , VCO or Loop oscillator stage , as it's not "+10"ing there .

I read all the posts and watched as everyone got really confused which is why I went right back to basics so we've got a clear view of it all.

No Sammy.. ch19 mid is 16.5000 on TP-3 (it just won't go to 16.4900 unless I go to ch18)

Sorry it's confusing testing and trying to align on ch19.. but the rig is actually on 27.195 (not 27.185). It's becoming more apparent now that the adders & PLL are probably OK.. and the fault is beyond that section. I've been speaking to Nick (great guy btw) and we've tested a couple of things but to no avail as yet.

I'll post those other measurements also tomorrow.

So the VCO/PLL is locked, 16.5MHz is the right VCO frequency for 27.195MHz.

There are only two things I can think of that would cause it to be 10KHz up if it's locked.

Either your N-Code is wrong or the 15MHz is wrong.

Pin 9 (carry in) Pin 6 or Pin 7 on IC 6 changing would add 1 to the N code and cause exactly that problem, let's keep it on 19, check the voltages on IC6 pin 9 7 and 6 and also IC5 pin 17 on RX and TX please.

The 15MHz being 'off' by 10KHz would cause it too (and 10KHz off is a lot for a crystal so the KC slide would be reduced)

It's not impossible that something is changing on TX, either because it's been deliberately modded or because of a fault/half undone mod.

Did you check if it worked before you removed the EPROM board?

[currentstatus]

So the VCO/PLL is locked, 16.5MHz is the right VCO frequency for 27.195MHz.

There are only two things I can think of that would cause it to be 10KHz up if it's locked.

Either your N-Code is wrong or the 15MHz is wrong.

Pin 9 (carry in) Pin 6 or Pin 7 on IC 6 changing would add 1 to the N code and cause exactly that problem, let's keep it on 19, check the voltages on IC6 pin 9 7 and 6 and also IC5 pin 17 on RX and TX please.

The 15MHz being 'off' by 10KHz would cause it too (and 10KHz off is a lot for a crystal so the KC slide would be reduced)

It's not impossible that something is changing on TX, either because it's been deliberately modded or because of a fault/half undone mod.

Did you check if it worked before you removed the EPROM board?

Yes the PLL is locked.. 16.5000 on ch19 (it should be 16.49000) so yes the whole rig is basically 10KHz off on all bands TX & RX.

ch19, mid, AM (27.195):
IC6 pin 9 - 0v TX & RX
IC6 pin 7 - 8.4v TX & RX
IC6 pin 6 - 0v TX & RX

IC5 pin 17 - 8.4v TX & RX

Crystal.. after a chat with Nick the 15MHz was swapped out for another I had here (from an old spare rig).. but the issue remained nothing changed. TBH I'm not 100% confident in that other 15meg although two Xtals off by exact freq would be.. impossible?

Agree with the half undone mod.. but I don't see any cut or joined tracks solder side.. and there are no joining wires or wires hanging off. The EPROM was half fitted.. as if someone had tried but gave up because the wires were hanging off an not all connected. What I should have done is tried to work out what the mod was and how it was supposed to be wired.. rather than desoldering and trying to restore everything by chance.. my bad!

I've just swapped out c138 (47uf elec).. no change.

[Kaliphan]

So the adders aren't flipping on TX and adding a 1 to the N-COde, the VCO is locked but to something 10KC up, the channel switch changes pin 17 of the PLL and the channels go up in 10KC steps.

The only thing left is that 15MHz crystal, but you've swapped it and rightly suspect it'd be unlikely to have two with the same fault and I agree.

But, if we assume the crystals are OK the only thing left is their tuning, have you actually checked/tried adjusting the frequency because I might have missed it but I can't see anywhere in this thread where you say you have?

And it would explain why you can't 'swing' it far with the KC.

[163TM847]

Something I noticed.. the coarse clarifier is giving me +/- 3KHz .. in the spec it shows +/- 7KHz

This is a clear indication the tuning coils for the 15.000 are tuned too high, The further down the slugs are the less coarse slide.

[Kaliphan]

Something I noticed.. the coarse clarifier is giving me +/- 3KHz .. in the spec it shows +/- 7KHz

This is a clear indication the tuning coils for the 15.000 are tuned too high, The further down the slugs are the less coarse slide.

It absolutely is and there's nothing else left to check now.

[currentstatus]

Guys thanks !!.. I'm not in front of the radio right now but I will be tomorrow.. bare with me.

And hopefully it is the tuning.. because I think i've tested every component in the rig by now!

[currentstatus]

Yes agree the slugs are a bit low see pic..

IMG_5789.JPG

So going through the PLL alignment.. I'm up to the TP-3 stage with freq counter.

ch19, AM, mid: Adjust L21 for 16.4900 (no.. will not lock).
ch19, USB, mid: Adjust L22 for 16.4925 (no.. will not lock).
ch19, LSB, mid: Adjust L23 for 16.4875 (no.. will not lock).

This is exactly what I've been getting on ch19.. the ONLY way to get it to lock on those 16MHz frequencies is to go ch18, and the slugs are then low. Now today I've started to recap with Rubycon, Panasonic FR electrolytics, no faults found on older caps but required doing. I came across an NEC transistor "TR31" that had been swapped out "reversed".. changed that to original 2sc945.

Also.. I got a 15MHz, 10.240, 10.6975 crystals from Farnell today.. swapped them out.. EXACTLY the same!

I'll carry on with the cap change.. most of it is now the RX & Audio section.

One thing I've noticed.. I'm not getting that much of a signal on TP-4.. procedure says to expect (0.9v p-p) getting about 80mv.
Shall continue..

[Kaliphan]

Please, measure the 15MHz signal, I think you should be able to at the junction of C94/R131.

KC at 12, FM.

Once we know what that's doing we can give you more advice