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Cobra 29 ltd receive issue

Spector1975

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Dec 4, 2020
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Hello, I have a cobra 29Ltd classic 2014 model. I have a weird receive issue, with rf generator hooked up can hear no tone until I was using a multi meter and probed pin 2 of ic2 to check voltage and tone from rf generator was heard. No apparent damage to parts, vco and transmit good, pull appears good as well (24 pin pll). Radio does have transmit and modulation. Pa works as well as external speaker. Any help would be great, I am schooling my self on how to repair these things. Thanks in advance
 

brandon7861

Well-Known Member
Nov 28, 2018
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Northern MN
My guess, something gave the front end a bad headache... Check TR7, the RF amp for RX. That feeds a jfet mixer with a signal from IC2, so if your probe lead is picking up the RF signal and adding it to the IF frequency at IC2, both go into the JFET mixer on the same trace doing essentially the same thing in a much crappier way.
 

nomadradio

Analog Retentive
Apr 3, 2005
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Pin 2 of IC2 is the emitter of the VCO oscillator transistor. Touching a meter probe to it would have the effect of pulling the VCO frequency downwards, due to the capacitance of the wire leading from the probe tip.

Makes it sound more like the radio's PLL is not locked on frequency. Like maybe someone tweaked the slug in the VCO coil. Pretty sure it's L19. If the wax looks disturbed on that one, this is probably what's gone wrong.

The DC tuning voltage on R88 is the key to this problem. L19 gets set so the radio stays locked on frequency for both receive and transmit from channel 1 to 40.

73
 

Spector1975

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Dec 4, 2020
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Thanks for all the tips and tricks, y’all are a big help appreciate it very much. As far as Lou Franklin I wish I could find his books, can never locate them. Thanks again gents
 

Spector1975

Member
Dec 4, 2020
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If dc voltage on pll can’t be adjust what could be the cause, I have seen that before where it doesn’t go under 4.5 volts
 

nomadradio

Analog Retentive
Apr 3, 2005
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I have seen that before where it doesn’t go under 4.5 volts
Any time the tuning voltage won't behave, it's the fault of one or the other of two inputs to the PLL chip. Either a problem with the 10.24 MHz input, or with the downmixed VCO signal input.

Or a bad PLL chip, but that's not very common.

The tuning voltage is the symptom of more than one possible fault. Not the cause of a problem.

73
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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Hmmm...

1655853388767.png


Best to start simple.

Not the best idea to prod, probe or even touch a working PLL IC unless you have buffered (read: High Impedance input) tools - else the coupling of signal and stray errant currents of static noise - can and will damage IC pins to the point their irreparable and you'll need a new PLL IC - with these things are rare in supply as now, probing that thing while it's live would be a bad move.

Use the graphic above as a guide - we're here to help.
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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To get receive working there are other things you can check - especially around FET1 TP3 is R17 - that open bare lead. Connect your test lead there.

IT reflects the PLL's tracking - so the IC will shift in RX from the TX - you should see that on any simple frequency counter. TX and RX "out" reflects the status of the PLL thru this R17 leg.

Check when you RX to TX - the front panel RX / TX light should change from Green in RX to Red in TX (use a dummy load on the antenna jack)

The lack of RF tone can be many things - but the basic premise is to get 27MHz in there - mix it with 17MHz from the PLL - as a SUBTRACTION to get 10.7MHz IF than the 10.240MHz main Xtal to parse it down (another Subtraction) to 455kHz - hence the "shift" in IF back at 17MHz (TP3) of 455kHz in RX mode to TX mode then back again in RX mode.
 
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Spector1975

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Once again thanks for the info, it will be well used. I have learned a lot over the past year and still learning. Thanks again
 

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