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Cobra 142 GTL
- Thread starter johne2hotty
- Start date
Time for me to put in my 2¢...........
Try replacing L24 to see if your 10K problem goes away, after all it is part of the clarifier circuit.
Are you also using an Oscilloscope as called for in the alignment section for L14 & L18? (This can't be done by ear.)
Have you verified that the setting for ct1 & ct2 are correct even though you say you haven't adjusted them?
Also need pictures (of the bottom side of the board) of the VCO and clarifier circuit area, also to see what that cap between crystals is all about too.
Good Luck and 73. . . . .
Try replacing L24 to see if your 10K problem goes away, after all it is part of the clarifier circuit.
Are you also using an Oscilloscope as called for in the alignment section for L14 & L18? (This can't be done by ear.)
Have you verified that the setting for ct1 & ct2 are correct even though you say you haven't adjusted them?
Also need pictures (of the bottom side of the board) of the VCO and clarifier circuit area, also to see what that cap between crystals is all about too.
Good Luck and 73. . . . .
I will look into L24. Thanks. Im using a 100mhz cathode ray tube type scope. The i have avdigital scope also but prefer thhe crt. The C1 and C2 i went nowhere near them as Im still atuck on the first adjustments because my only service info was a sams set with misleading info that has thrown me off. This radio was a modified hack that Im trying to put back to factory. That lump of crap between the 2 crystals is some sort of goo that they used that causes corrosion. Its a diode or a choke. I'll replace it. I will get that pick for you asap. Thanks for your help.
73s
What is throwing me off is that capacitor soldered on the pack of crystals onto the resistor? I don't see other 142 GTLs with that.
I 2nd clearing up that corrosive glue. That is where my 142 GTL went bad since it did a number on the diode there. (I forget offhand what replacement diode was used, but it was more readily available vs what was used.)
Its been too many years, but there was a person in the CB Radio Show and Tell group on facebook who was a "military radio technician" and said that the glue is not corrosive... Justin Shultz I think was his name. Glad to of left Facebook back in 2022.
I 2nd clearing up that corrosive glue. That is where my 142 GTL went bad since it did a number on the diode there. (I forget offhand what replacement diode was used, but it was more readily available vs what was used.)
Its been too many years, but there was a person in the CB Radio Show and Tell group on facebook who was a "military radio technician" and said that the glue is not corrosive... Justin Shultz I think was his name. Glad to of left Facebook back in 2022.
Yeah. I took off 1 lead andnit changed nothing. Im gonna getbthat pic of the cap under the board also...
Yes. It had a ground strap on it and on the other 2 upper and lower crystals. Did I mention that thiscwas a ebay hack? Full of issues. Im trying to put it back stock.
73s
Here's a better picture of the clarifier circuit to help with troubleshooting.
Transmit and receive both form their own voltage divider to supply the varactor with tuning voltage.
The reason clarifier mods always require realignment afterwards is because the 8v (always on) supply is a slightly different voltage than the RX supply, and this affects the voltage at the wiper shifting it slightly off center. If the supply voltages were precisely matched, realignment would not be necessary.
Clarifier unlocks eliminate the TX side entirely (usually by clipping D36) and tie the RX voltage divider to the 8v (always on) rail like this.
You could probably repurpose VR5 into the other side to adjust center clarifier back to center after unlock, but I haven't seen anyone try it.
If you want more slide without the clarifier being completely unbalanced, unfortunately, it isn't going to happen. It would seem like all this parallel combination crap could be gutted in the name of simplifying things, but the problem is that the varactor isn't linear. That nonlinearity is why the RX divider isn't just a pot to ground, it needs the parallel resistors.
For example, if I assume the 8v regulator is a clean 8v, the pot at 99% provides 1.209v, the center position is 3.264v, and the pot at 1% is 7.224v. On the SVC251 datasheet (no pF curve on the 1588 datasheet), this corresponds to 33pF, 23pF and 13pF respectively. So, even though the center wiper voltage is not "center rail", it balances out the non-linearity of the varactor.
From the varactor datasheet, it is apparent that higher voltage won't really get us much lower pF's (in fact, we are running out of voltage anyhow), but there is plenty of varactor swing hiding under 1.2v. This is why all the clarifier mods seem to cause an uneven change, little in one direction, a lot in the other. And there is nothing you can do about it without varactor/cap mods.
Transmit and receive both form their own voltage divider to supply the varactor with tuning voltage.
The reason clarifier mods always require realignment afterwards is because the 8v (always on) supply is a slightly different voltage than the RX supply, and this affects the voltage at the wiper shifting it slightly off center. If the supply voltages were precisely matched, realignment would not be necessary.
Clarifier unlocks eliminate the TX side entirely (usually by clipping D36) and tie the RX voltage divider to the 8v (always on) rail like this.
You could probably repurpose VR5 into the other side to adjust center clarifier back to center after unlock, but I haven't seen anyone try it.
If you want more slide without the clarifier being completely unbalanced, unfortunately, it isn't going to happen. It would seem like all this parallel combination crap could be gutted in the name of simplifying things, but the problem is that the varactor isn't linear. That nonlinearity is why the RX divider isn't just a pot to ground, it needs the parallel resistors.
For example, if I assume the 8v regulator is a clean 8v, the pot at 99% provides 1.209v, the center position is 3.264v, and the pot at 1% is 7.224v. On the SVC251 datasheet (no pF curve on the 1588 datasheet), this corresponds to 33pF, 23pF and 13pF respectively. So, even though the center wiper voltage is not "center rail", it balances out the non-linearity of the varactor.
From the varactor datasheet, it is apparent that higher voltage won't really get us much lower pF's (in fact, we are running out of voltage anyhow), but there is plenty of varactor swing hiding under 1.2v. This is why all the clarifier mods seem to cause an uneven change, little in one direction, a lot in the other. And there is nothing you can do about it without varactor/cap mods.
Great level of instruction there. I appreciate it. My plans though, were to remove all the mods. Technically, I couldnt explain it the way you did, but I knew hiw the circuits work interdependentl and dependently. A good refresher so thanks for that. . Im not having luck sending pics due to my crappy internet issues but I'll keep trying. Heres what I did find after starting to realign all over again. My sweet spot on channel 19 is 34.775 not 34.875. I simply cannot get there at all. Its just not an option considering the issue. I replaced ct3 twice. Once with a 60pf then with a 20pf. The 11.1125 crystal was severely heated by the person who hacked the box. I can however get my 34.875 if I heatbup the crystal. So my channel issue has to be becausevthe crystal is bad. I might have one in another radio. Theboddcthingnis, my transmitter frequency is spot on. Only the receive is off by 10k. Its very strange. There does not appear to have any mods around the PLL. There is a cut in the run between the 2 crystals near ct1 and ct2 and a small disc cap placed between the 2. I bypassed it and all it did was shift the frequency slightly. Im confused. I'll try sending those pics again.
A
As soon as I can get my proper L13 frequency I'll know more. I do have a spare but I dont think its the problem because the truth table checks out.
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The red white wire is goingbto a 3.3k resistor. I could have used a better color but its temporary until I sort out all then issues. Here is that disc cap that joins the cut run. As you can see bybthe solder, its a aftermarket hack. Again, it just alters the frequency very slightly and just shifts the clarifier by a notch to the right.
