Cobra 25lTD no transmit/receive

[dss56]

I was given a cobra 25ltd nice clean condition but 1 problem.

There is no trans/receive.
Tried different mic
Led turns red when mic keyed
no meter movement.
Audio chip ok pa works.
Is there 1 part that would kill both trans/receive at the same time.
I have the voltage chart and gonna do some testing.

Did some voltage checks at the pll and all seem to be correct,

any help appreciated.

 

[NZ8N]

Pin 4 of the PLL is the Lock Detect pin, is it low or high (+8v)? High is locked, low is unlocked.

TP3 by the RX 1st mixer FET should have
16.270 in RX on CH 1, or
16.710 in RX on CH 40.

 

[nomadradio]

Um, doesn't the led stay green when you key the mike in this radio if it's unlocked?

Too lazy to consult the schemo, but a lot of radios won't turn the LED red unless the PLL is properly locked in to the channel.

(update)
Took a look at the diagram. Looks as is the 25LTD is different from the sideband radios. Looks as if the LED WILL in fact turn red just fine even if the PLL is out of lock.

Time to check pin 4 of the PLL chip. That LED is not a diagnostic element in this model, after all.

73

 

[NZ8N]

I looked, the Lock Detect goes nowhere near the TX switching area, diodes pull down the TX mixer, TX pre-driver bias and one of the last stages of the RX 2nd IF.

 

[nomadradio]

When pin 4 of the PLL goes low, it pulls pin 7 of IC2 the transmit mixer low through R70 and D12. Also shuts down the transmit-buffer transistor TR9 through R47, and the receiver's IF amplifier transistor TR2 through D18. That's what kills both receive and transmit in an unlocked radio. Just has no effect on the front-panel LED.

73

 

[dss56]

I did do some voltage checks on the pll and all seem to be there.
Also swapped out the pll with a radio that works and put the radio pll im working on in a good radio and pll works.

I also did a voltage check on almost all the transistors and the 2 FETs all voltages seem to be ok.

I had the radios side by side and both the same I was checking voltage in good radio then the bad one all check out.
I keyed up the good radio next to the bad radio and getting audio on the bad radio.
I key up the bad radio and nothing not even any small audio on the good radio.

I'm getting some small static/crackling whit noise on bad radio volume will lower and higher this noise as also the audio keyed up from the good radio.

Also checked voltages on the mixer IC 2 and all are present by the voltage chart.

I'm thinking a bad cap or a diode as mentioned above some place, I did try some near the audio chip but will try the rest tomorrow morning.

 

[NODENS]

As radio has no tx or rx at all and you have checked voltages and seems OK, I would center the effort to check if reference oscillator (10.240mghz usually) is present and if vco is generating the right frecuency. If pll is not generating its frecuency, it kills your tx and rx as band pass filters won't let pass RF in rx area or after mixer in tx.
I think it is a good starting point to check as vco frecuency is necesary for tx and rx.
Good luck mate

 

[dss56]

Well I think radio is fixed after doing all the voltages and VCO checked as you all suggested, I turned radio so component side facing me. I started doing more checks and I was checking near the FET2 area and the radio came back on. So i pressed on board both ways and it went out again. Cold solder joint? Well I checked that area closely with a microscope and did not see any. I retouched all solder joints in that area and flexed the board both ways and seems to stay on. I do have a good scope and could not see and cold solder joints but glad its fixed.

Thanks to all who made suggestions.

 

[RT431]

Glad you got it fixed.