-
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.
Cobra 29LTD no TX
- Thread starter Robalo
- Start date
Just some questions, so I can firure out what's been done, then work on the next section(s) 
What ever happened to the Original 2816 PLL?
You had "noise" from L23,
Also ...
Ok, your voltage shift on Channel 1 and Channel 40 are too great, so that POSSIBLY means either L19, or L21 are giving your radio the hiccups.
You would not get even a frequency for RX if L22 wasn't working.
So you might want to continue tweaking L22 and L23 to peak them out, and look at TP3 to max out L18 ACROSS THE SPREAD OF CHANNEL 1 to CHANNEL 40 - make sure the "RX" volume pops up, and in lock - Thru TP3.
L22 is one of the weaker links in the loop to the VCO (IC2) - uses the VCO's Pin 4 "Tap" so if you can scope the pin, try to maximize L22 so it can tune thru Channel 1 thru 40 without any glitch.
TP1? Continue to monitor the voltage shift - this problem SEEMS to be from the differences between L18 and L19 / L21 peaking - so expect that L19 and or L21 might need to be investigated - pulled - or replaced. L18 seems to work and the RX does seem to be present.
LOCK pin that checks TX output, the output of L21 is "detected" and sent to PLL's pin 15, so L21 should also be checked and investigated - checked for cracked slugs or open winds...if L21 is bad, you can replace IC3 every time, but L21 role - is the mixer, peaking coil - that IC3 needs to send out TX transmit.
IC3 - the 2 IF's it gets. 1 from the 10.240MHz, the 2nd from the VCO - IC2 - thru L19 - mix and peak in L21 - which that Diode D14 - sends, as a small voltage signal to the PLL LD pin - Pin 15.
If one IF signal exists, but the other is not there, L21 works as a BAND PASS - it can't generate enough carrier to tell the PLL the IF's are combining. The IF combined are not strong enough to make the 27MHz signal Pin 15 needs which keeps the PLL in LOCKOUT - TR21 never keys up.
So it's important to Peak L21 so it can tell the PLL TX is working and TR21 can then work.
What ever happened to the Original 2816 PLL?
You had "noise" from L23,
Also ...
Ok, your voltage shift on Channel 1 and Channel 40 are too great, so that POSSIBLY means either L19, or L21 are giving your radio the hiccups.
You would not get even a frequency for RX if L22 wasn't working.
So you might want to continue tweaking L22 and L23 to peak them out, and look at TP3 to max out L18 ACROSS THE SPREAD OF CHANNEL 1 to CHANNEL 40 - make sure the "RX" volume pops up, and in lock - Thru TP3.
L22 is one of the weaker links in the loop to the VCO (IC2) - uses the VCO's Pin 4 "Tap" so if you can scope the pin, try to maximize L22 so it can tune thru Channel 1 thru 40 without any glitch.
TP1? Continue to monitor the voltage shift - this problem SEEMS to be from the differences between L18 and L19 / L21 peaking - so expect that L19 and or L21 might need to be investigated - pulled - or replaced. L18 seems to work and the RX does seem to be present.
LOCK pin that checks TX output, the output of L21 is "detected" and sent to PLL's pin 15, so L21 should also be checked and investigated - checked for cracked slugs or open winds...if L21 is bad, you can replace IC3 every time, but L21 role - is the mixer, peaking coil - that IC3 needs to send out TX transmit.
IC3 - the 2 IF's it gets. 1 from the 10.240MHz, the 2nd from the VCO - IC2 - thru L19 - mix and peak in L21 - which that Diode D14 - sends, as a small voltage signal to the PLL LD pin - Pin 15.
If one IF signal exists, but the other is not there, L21 works as a BAND PASS - it can't generate enough carrier to tell the PLL the IF's are combining. The IF combined are not strong enough to make the 27MHz signal Pin 15 needs which keeps the PLL in LOCKOUT - TR21 never keys up.
So it's important to Peak L21 so it can tell the PLL TX is working and TR21 can then work.
as you see in the picture it has a lot corrosive glue
not any more.
i dont thing this radio is going work again i dont see hard beeps
L19 works L18 works,L 24 .L22 AND L 23 DOES NOT WORK
Last edited:
L24, L23 part of the IF strip, L24 Mixes DOWN while L23 supplies Mixing UP for TX.
These 3 work with PLL - but need 10.240MHz Crystal.
L23 is straight up 10.240MHz.
L22 is 1/2 of this - 5.12MHz
IF you don't see these colors on your TV set...
In order to even get anything on L18/L19 something needs to appear from PLL.
So if Corrosive Glue killed the PLL, the Radio Star is now you...
Got a spare 10.240MHz Xtal and also verify the 10.240 Oscillator section is even firing up.
These 3 work with PLL - but need 10.240MHz Crystal.
L23 is straight up 10.240MHz.
L22 is 1/2 of this - 5.12MHz
IF you don't see these colors on your TV set...
In order to even get anything on L18/L19 something needs to appear from PLL.
So if Corrosive Glue killed the PLL, the Radio Star is now you...
Got a spare 10.240MHz Xtal and also verify the 10.240 Oscillator section is even firing up.
ic1 pin 13 10.240 mhz
ic 1 pin 10 5.12 mhz c94 15.36mhz c87 15.36 mhz on both size
ic 2 pin 4 15.36mhz mmm
now after replaced tr20,L23,C130,C87,C94,C106 I have rx green led and tx red led.
now channel 40 i have 16.710 mhz and channel 1 16.269757 mhz
on RX I dont have white noise and on TX red LED but no power output.
I was enjoying this old thread but it seemed to end abruptly.