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Cobra 142GTL My next project, Unable to get CT3 into spec.

S

slingslingbinks

Member
Aug 31, 2023
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I have a customer with two 142GTL's and i managed to bring one back to life, but the 2nd one is kicking my rear.
It has the clarifier unlocked and added channel's using the 11.325 Xtal and cutting the trace at pin 10 of the 8719 pll. It has the PC-385AG board in it.
While performing the alignment i got to the step in Sam's Vol. 219 I couldn't get the CT3 adjustment in spec,
I thought the clarifier mod was fubar when i seen the R-187 still in the board, but the previous tech cut the trace leading back to the control, I reconnected the trace and removed R-187. The D-36 was removed and i also noticed the previous tech grounded the wire from the clarifier to chassis ground and i reconnected it to a PC board ground and connected the other wire from the clarifier control from the pll area to IC4, Pin 3.
I basically wanted to make the clarifier mod as it's shown in this article, https://cbworldinformer.org/cb_world_961201_6.htm
But here is a few thing confusing about this posted clarifier mod. I never seen it posted to jump D-36 and put the choke on D-20. What i was reading was to put the choke on D-37 to get more clarifier travel,
But in all this mess it appears the clarifier mod is working, SO with that i decided to recheck the alignment. I still can't get CT3 in spec.
I pulled the 11.325 Xtal and checked it and it was off a bit. I looked through my extra xtals and i didn't have one, I contacted 6 star on e-bay about his xtals and he has some 11.3258 xtal's so i got them and installed it in the X4 spot, I still can't get CT3 in spec,
I did have this same problem with this other 142GTL but i cured it by pulling the 20PF CT3 and installing a 40PF part, But this 142 still won't get the CT3 into spec.
So i researched and found some new Varactor diodes for D-37 and it still won't go into spec. This board doesn't have C-132 in it as it's optional and i could try putting one in the board. I see no need to add a chole to D-37 to make CT3 in spec.
It seam's like i always get the tough one's to work on here, I need to get CT3 into speck before i go any further,
Not trying to give out any headache's
Any thought's on my new problem.? TIA
 

I forget off hand, but a few here can chime in about the 142 GTL. Isn't CT3 affected overall with the LSB adjustment? Or maybe it was the AM adjustment.. I thought with one out, it will make the CT3 adjustment off etc...
With the clarifier unlocked, just make sure the knob is centered up, and use a 2nd radio to listen on it and just make your adjustment that way to see if it ever comes close to transmitting on frequency.
 
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While i'm not 100% positive that the clarifier mod is correct since all that ties into the CT3 pot adjustment, One thing that was different between the working 142 and this one was a choke on the end of D-20. like it was mentioned on the link i posted from CB world, But i swapped the chole -diode over to this 142 and i still couldn't get CT3 in spec. I'm not going for the extra slid until i get CT3 in spec.
 
So, which direction is it out of spec? Too high and won't come down low enough? Or too low and won't come up high enough? That's what we encounter when a choke gets clipped in series with D37, the varactor diode. The choke pulls the crystal down too far for CT3 to reach the channel center.

But knowing which direction your error lies, above or below is a serious data point.

73
 
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Hey Nomad.
I was looking for you last April and i was in your area, Thank's for the get back,
I can not get up to the 34.9875Mhz. on CT3, Closest i can get it to is around 34. 9750 or so.
Now the D-37 P/N listed is a 1S2687D, This crosses to the NTE612 and the speck is 12PF, Another number listed for D-37 is the 1S2687 and it also crosses to the NTE-612,
The 3rd number for the D-37 is 1S2687 and it crosses to the NTE-614 and it's a 33PF.
I did find a seller and i got the NTE-612 and NTE-614. These diodes have no anode or cathode marking's on them so i didn't use them.
I did find some 1S1658 Varactor diodes and there spec is 32Pf and are marked anode and cathode so i ordered them and these are what i replaced D-37 with.
I did measure the original D-37 with my e-bay cheap tester and it tested at 39Pf,
The new NTE-612's are rated at 12Pf but test at 22Pf,
The new NTE-614's are rated at 33Pf but test at 24Pf,
The new 1S1657's are rated at 32Pf but test at 24Pf.
Perhaps the cheap tester isn't the correct way to test a varactor diode and i did research for a way to test these diodes and found this article on the www
https://www.qsl.net/k8gd/varactor
It's something to build and i never found anything prebuilt to test them,
This is like the el-cheapo tester i was using.
www.ebay.com

Component Transistor Tester Diode Triode Capacitance ESR LCR Meter M328 | eBay

For example. You pick up a transistor, you do not consider the order of 123 BCE, just insert. Then press the test button. Basically you just need to plug in and press the button on it. Transistor Range: 2 or less (including 2) PN junction consisting of transistors do not have other internal...
www.ebay.com
 
Varactor diode specifications can be slippery.

My idiot light just came on. You said 11.325 MHz crystal. This radio is built with a 11.1125 crystal. I have made this substitution to add upper channels, but it leaves me wondering how much the crystal is contributing to this mystery. If you unsolder one end of C132 does this get you any closer? This was a strategy to counteract the side effect of putting a RF choke in the circuit to stretch the clarifier's range suggested in the EXPO 100 channel kit instructions. Might be worth seeing how it affects this radio.

73
 

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