• You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
  • A Winner has been selected for the 2025 Radioddity Cyber Monday giveaway! Click Here to see who won!

Cobra 142 gtl with a few problems

vswr

Disinterested observer
I Support WorldwideDX.com!
Aug 20, 2023
255
63
38
Bought this 142 that had a bunch of mods in it. I generally tolerate a clarifier mod but nothing more unless it is to unrestrict poor modulation. Anyhow, this 142 has a 10k frequency shift downwards. So for an example, channel 38 on this radio is on 39. Channel 1 is below 1. Channel 2 is the actual channel 1. Ive tried everything I can at my 1st andcsecond line technician level and cant figurenout why its dropping. The L13 cans slug starts working below the top of the can about 1/16 to 1/8th below the top. It does not tune in anything except the 10k drop on all freqs. Also from about channel 35, the clarifier unlocks the PLL at 9 o'clock and there is no tx or rx between 7 o'clock and 9 o'clock. Im stumped. I removed the channel mod and the clarifier mod. There is nothing I can do to get it it track correctly on each channel. Also, when the channel mod was done, the hack used very different tuning caps and removed CT3. I figured that CT3 was needed to tune in the USB so I place one of hisvtuning caps in its place. Both AM and LSB align perfectly. USB wont tune any where close to the correct alignment. The first steps from the sams for aligning the modes all tune in at 10k under what they should be. Im lost.
 

Ok so new findings. The truth table is correct on every channel minus the offsets caused by the alignment issue which I cannot do until I resolve the VCO. 5 volts is set as per the sams at tp9. So the multiplier is also correct at tp10. The 8719 checks out pin per pin as per truth table. Something is adding 10k to my channels. Maybe the crystal is off? Gonna check that tomorrow. I was in the basement all day messing with this and mybeyes need a break.
 
Some issue withbthe clarifier as well. At 9 o'clock it drops out. Even when aligned to the 5 volts I lose my transmit. If I field adjust L13 I can get Tx back but i losevtx on lower 5 or 6 channels. If I tweak it to get tx back, i lose the top few channels.
 
Is there a small RF choke added in line with the varactor diode? That trick works with similar radios that have three trimmer coils, one for each mode. This radio has a trimmer cap for USB, and a coil each for AM and LSB. That choke has to be installed differently for this radio, or better yet removed altogether. The dropout behavior suggests the clarifier circuit has been modified. Any chance a EXPO 100 channel kit was installed at some time in the past?

73
 
  • Like
Reactions: doffo
Is there a small RF choke added in line with the varactor diode? That trick works with similar radios that have three trimmer coils, one for each mode. This radio has a trimmer cap for USB, and a coil each for AM and LSB. That choke has to be installed differently for this radio, or better yet removed altogether. The dropout behavior suggests the clarifier circuit has been modified. Any chance a EXPO 100 channel kit was installed at some time in the past?

73
No expo kit. It had thec11.1125 crystal relocatedcalongvwith a 10.something for lowers and a 13.something for uppers. All tied in with 3 20pf caps and soldered to the spot that the 11.11125 was in. The clarifier was one side to ground and the other to a constant 8v source near the pll chip. No 3.3k resistor. D36 and r187 lifted. So, my clarifier I reinstalled it as per sams, and my TX and RX are no longer tied as one. I replaced the clarifier pot because at about 9 o'clock it seemed like a cut off or dead spot. You can actually hear it as you turn the knob. I have not noticed any extra chokes, caps, coils or varactors or superdiodes. No signs of choke removals either. Its becoming an enigma.

73s
 
  • Like
Reactions: doffo
If you connect the 11.1125 crystal directly to the two foil pads on the main pc board where it was originally installed, the clarifier glitch should go away. The added capacitors are causing the dropout symptom.

73
I did but the problem is still there.
 
I did but the problem is still there.
Other than ct3, there were only trimmers forvthebupper and lowers he carved up the noise blanker to isolate all 3. What a mess of such a great radio
 
  • Like
Reactions: doffo
That's odd. As a rule, adding that choke in line with the varactor reduces the feedback through the crystal causing it to stop running with the clarifier turned to the right. Adding capacitors can do the same thing for the same reason.

But I'm not used to seeing this fault with the crystal wired exactly as factory original with no added wires and such.

It would be easy to blame the crystal, but the only way to tell is to substitute one that's known to be good.

Quirky.

73
 
  • Like
Reactions: doffo and vswr
What other radios run this exact board? I have close to 400 radios here. Lots of 148gtl, galaxy 99v and many others with cybernet boards
 
That's odd. As a rule, adding that choke in line with the varactor reduces the feedback through the crystal causing it to stop running with the clarifier turned to the right. Adding capacitors can do the same thing for the same reason.

But I'm not used to seeing this fault with the crystal wired exactly as factory original with no added wires and such.

It would be easy to blame the crystal, but the only way to tell is to substitute one that's known to be good.

Quirky.

73
Sure is. Im pretty sure I can swap parts out of another radio. I have a lot to chose from . Just needcto know whats similar or exact.
 
What other radios run this exact board? I have close to 400 radios here. Lots of 148gtl, galaxy 99v and many others with cybernet boards
140 GTL mobile, Monkey Ward 719 (I think its 719), President Grant (Not the one with the power plugin that is vertical, but the one that is horizontal) President Washington base radio (The one with the 3 buttons on the bottom-right). Same goes with the Uniden label radios that look like the President branded ones.

****Anyone please correct me if I am wrong. I am doing my best pulling the rabbit out the hat with all that I can recall. :)
 
  • Like
Reactions: nomadradio
Oh oh!!! The Realistic TRC-450 and I think the base model was the TRC-459?

***I wonder if there are revision changes between them that would consider them not the same.
 

dxChat
Help Users
  • No one is chatting at the moment.