Cobra 142 GTL

[brandon7861]

The 10kHz never leaves the PLL.I don't think thats whats going on. I think you should check the other oscillators and see where they are at.

I do remember seeing a mod about a 10kHz shift, not sure if it is relevant. Here it is:

The Defpoms radio add-on sales Pages

A page which lists items that I am able to sell, such as kits, add-on boards, circuit or block diagrams and service manuals.

web.archive.org

The Defpoms MB8719 add-on Manual Page

The online manual for the MB8719 EPROM frequency convertion board, allowing more bands including UK, NZ and a 10KHz shift, as well as another band above and below existing channels.

web.archive.org

 

[brandon7861]

what 11MHz crystal does it have? (I assume the 11.1125MHz crystal).

 

[vswr]

The 10kHz never leaves the PLL.I don't think thats whats going on. I think you should check the other oscillators and see where they are at.

I do remember seeing a mod about a 10kHz shift, not sure if it is relevant. Here it is:

The Defpoms radio add-on sales Pages

A page which lists items that I am able to sell, such as kits, add-on boards, circuit or block diagrams and service manuals.

web.archive.org

The Defpoms MB8719 add-on Manual Page

The online manual for the MB8719 EPROM frequency convertion board, allowing more bands including UK, NZ and a 10KHz shift, as well as another band above and below existing channels.

web.archive.org

Thats cool but the 10k shift throws some of my channels into the A channel area. I did think about doing that mod but when I got into testing transmit and receive, I saw that I couldnt communicate between the walkie and the 142 due to the A channel skips.

 

[vswr]

what 11MHz crystal does it have? (I assume the 11.1125MHz crystal).

Yes. The 1125

 

[brandon7861]

If someone played with L20 and got the 11MHz crystal to go down 3.333kHz, that would be 10kHz at the tripler. You should see if that oscillator is running at 11.111666MHz or 11.108333MHz.

 

[vswr]

If someone played with L20 and got the 11MHz crystal to go down 3.333kHz, that would be 10kHz at the tripler. You should see if that oscillator is running at 11.111666MHz or 11.108333MHz.

I'll check that. The L20 is the AM? I can tune the 2 cans but like I said im up by 10khz on each channel. Would you happen to know the value of ct3? The sams only gives me a part number. Mine was swapped out. I tried a few, 20nf, even a 70nf. Nothing changed.

 

[brandon7861]

CT3 is a 20pF trimmer cap.

 

[brandon7861]

I'll check that. The L20 is the AM? I can tune the 2 cans but like I said im up by 10khz on each channel. Would you happen to know the value of ct3? The sams only gives me a part number. Mine was swapped out. I tried a few, 20nf, even a 70nf. Nothing changed.

Unless that's a typo, nF values are a big problem there. Having something in the nF's there is like putting the crystal to ground. It will lower the frequency. That would kill the clarifier and TX adjust too. Thats like shorting across the varactor.

Was that a typo or did you really mean theres nF's in CT3's spot?

 

[vswr]

Pf pico

 

[vswr]

CT3 is a 20pF trimmer cap.

Ok. I'll replace it in the morning.

 

[vswr]

Unless that's a typo, nF values are a big problem there. Having something in the nF's there is like putting the crystal to ground. It will lower the frequency. That would kill the clarifier and TX adjust too. Thats like shorting across the varactor.

Was that a typo or did you really mean theres nF's in CT3's spot?

Lol. Im full of typos. Especially uding the phones keyboard. I always hit c v b x and n instead ofvthe spacebar

 

[vswr]

Ok. Time to check the crystals. The 11.11125 checks out at 11.11136 clarifier to extreme right, and 111110716 to the left. The 7.802 is 7.80238. The 7.7975 is 7.798338.
The 10.240 is 10.24002. The one up by L13 is glued over the numbers but has 3 legs so its not really an oscillator crystal... A filter....I think. Then there is that box of crystals there is 6 in there so its a filter. L13 in lsb is 34.75417 clarifier left, 34.76883 right, am its 34.75588 left, 34.7705 right, and Usb its 34.76115 left, and 34.77438 right.
All chrcks were done on channel 1 which is channel 2 on this box, hence the problem...

 

[brandon7861]

As you mentioned, the PLL data checks out on all channels. Verifying the math: On ch1, N=143, and since 10kHz*143=1.43MHz, your measured VCO frequency "should be" 11.11166666*3+1.43=34.765MHz on AM. With your 11MHz crystal off 306Hz like that, the VCO would come out to 34.76408MHz. That's 1kHz off, not 10kHz, so that's not the only problem. Moving on.

Your clarifier varies the VCO from 34.75588 (left) to 34.7705 (right), and our target is in between those two values, so that's working fine. That clarifier range seems a bit wide for use in a SSB radio though, but the correct frequency is within that range (even if its not at the center). Side note, you don't want that much slide on a sideband radio, it makes setting it right really hard in SSB as mentioned in the first few replies to this thread. Still, not the only problem, moving on.

Since the 11MHz error only accounts for 306Hz*3=918Hz total, we still need to explain away the other 9kHz worth of error. The only explanation would be at the 7.8MHz oscillator. You didn't measure that in AM, only in LSB and USB. Is there any reason you didn't measure it in AM? Hint: L17 is only switched in during TX, so you gotta key up for that one (and set it for 7.800).

Can you share your alignment procedure? I don't have the sams for this and I just want to see if there is something in there that might be the cause, like something tuned in the wrong order.

The fact that the clarifier makes the oscillator drop out is also a hint that something is wrong there. Did you put the correct trimmer back in? Is C152 missing? When the varactor voltage goes up, its capacitance drops. If there is no parallel capacitance, it could very well cause the oscillator to drop out from insufficient capacitance (too much reactance in series with the crystal). I suggest putting it all back to bone stock until you get it working. Then modify.

Go check out the 7.8MHz signal in AM TX and let me know what you get.

 

[vswr]

As you mentioned, the PLL data checks out on all channels. Verifying the math: On ch1, N=143, and since 10kHz*143=1.43MHz, your measured VCO frequency "should be" 11.11166666*3+1.43=34.765MHz on AM. With your 11MHz crystal off 306Hz like that, the VCO would come out to 34.76408MHz. That's 1kHz off, not 10kHz, so that's not the only problem. Moving on.

Your clarifier varies the VCO from 34.75588 (left) to 34.7705 (right), and our target is in between those two values, so that's working fine. That clarifier range seems a bit wide for use in a SSB radio though, but the correct frequency is within that range (even if its not at the center). Side note, you don't want that much slide on a sideband radio, it makes setting it right really hard in SSB as mentioned in the first few replies to this thread. Still, not the only problem, moving on.

Since the 11MHz error only accounts for 306Hz*3=918Hz total, we still need to explain away the other 9kHz worth of error. The only explanation would be at the 7.8MHz oscillator. You didn't measure that in AM, only in LSB and USB. Is there any reason you didn't measure it in AM? Hint: L17 is only switched in during TX, so you gotta key up for that one (and set it for 7.800).

Can you share your alignment procedure? I don't have the sams for this and I just want to see if there is something in there that might be the cause, like something tuned in the wrong order.

The fact that the clarifier makes the oscillator drop out is also a hint that something is wrong there. Did you put the correct trimmer back in? Is C152 missing? When the varactor voltage goes up, its capacitance drops. If there is no parallel capacitance, it could very well cause the oscillator to drop out from insufficient capacitance (too much reactance in series with the crystal). I suggest putting it all back to bone stock until you get it working. Then modify.

Go check out the 7.8MHz signal in AM TX and let me know what you get.

The plan all along was to put it back straight stock. I got this hacked radio on ebay. I started removing the channel mods then the clarifier mod and I replaced the 20pf trimmer on the 11.11125 crystal as wevdiscussed yesterday. I need to check it to see if any parallel disc caps were removed because I did read that in the mod "how to" and generally thats how I reverse the hacks by reverse engineering them. I was on a GE superbase all day that has intermittent problems and only got to the C 142 at the end of the day. The 7.802mhz freq was the same on all modes thats why I didnt mention the usb and lsb for it. Yes it slides way too much for my liking. I can slide to the right and pickup another channel. Its a mess. I put that back stock a couple days ago. I can control the TX separately now and AM and LSB tune in fine but USB is 3k off. Im still off by 10kHz (normal channel separation) so all channels are off. My channel 1 is 26.955, my 40 is 27.395. Im on estern time so I will get you more info tomorrow. I will also get you the sams for it. BTW, You're an absolute wizard and I sooo appreciate this. How you know such detail blows my mind. Its great to know that you are understanding whats going on with it.

So a quick recap...clarifier is too wide which cuts out at 9 o'clock, channels all drop by 10k, and it wont go into transmit mode on about 10 to 15 channels if I slign the L13 to 5V as per sams. If I do a static alignment, I can get all 40 channels to transmit in all 3 modes and the L13 voltage is around 2.7V. A far cry from the 5V listed in the sams. The rest of the problems are minor things like replacing pots, caps, etc.

 

[brandon7861]

5v might be too high. Let's use ch20 as an example. If the VCO needs to be at 35.005MHz, the VCO's varactor needs enough range to make it go down to 34.765 ( for ch1) and up to 35.205MHz (for ch40). For this reason, the radio is set to ch20 and L13 is adjusted so that 35.005 happens at the middle of the varactors tuning range. When you get the upper channels to drop out, it is because the voltage is already too high, and by the time you get to the higher channels, it falls out of lock because the PD output is already maxed out and can't go any further.

Edit: now that I think about it, this also explains the clarifier causing it to drop out. Its moving the 11MHz, which moves the loop mix, which moves the VCO. When the VCO runs out of voltage to follow, it falls out of lock.

Here is the tuning chart from the datasheet for a varactor (SVC251S, couldn't find chart for the actual one, but they all behave similarly):
This chart may lead you to believe that 5v is in the center, but the pF scale on the left is logarithmic. Going from .5v to 5v changes the capacitance from about 40pF to about 12pF. Going up to 10v from there only changes that another 5pF. The most valuable part of the varactor is the first 5v. If you already have 5v at ch1, that's no good. I would recommend setting it for about 3v at ch20 and see how it behaves.