Cobra 148 GTL no output

[Unit 75]

I purchased a Cobra 148 GTL made in Malaysia. The radio did not transmit. I re-capped the radio (all electrolytics) and still no tx. The rx/tx LEDs work when you key the mic. PA has output. The synthesizer has the correct frequencies and will align. PLL has correct voltages. L20 has been peaked. MB3756 has correct voltages on all pins. IC5 has correct voltages. TR39 has correct voltages. Driver and final are good and I have 13vdc on collectors dropping to 6.7vdc on AM tx. I have 13vdc on bias wires. D49 and D50 are good. When I hook my DC milliamp meter to set the bias (VR9 and VR8) ....I have nothing. I checked the variable resistors (VR9 and VR8 and they are good) L42 and L39 are good, no high resistances.
I then replaced all the ceramic caps in the RF section...still nothing. I did a follow thru to check for any cold solder joints etc...all is good. So why isn't this radio transmitting??? I'm pulling my hair out...lol.

 

[kaos513]

Do you have 8volt rail ?

 

[Unit 75]

yes

 

[kaos513]

Oh ok great, you probably will have to look at schematic maybe check oscillator section .

 

[kaos513]

What is the model number of the board ?

 

[kaos513]

Check mode switch should always be 8vdc.

 

[Unit 75]

I have my 13vdc at the mode switch. The model number of the board is EPT041810Z. As mentioned above, the oscillator section is functional. I have all frequencies and L23, (34.765MHz) L22, (34.76350MHz) L59 (34.76650MHz)are all adjusted with correct frequencies... PLL is good. PA mode works great with good strong audio. Oscillator section is fine. All voltages on MB3756 (regulator) are correct. IC5 (SO42P) mixer has correct voltages and is mixing the local frequency generated by the PLL (local oscillator) and carrier oscillator. L46 (27MHz) and L45 (27MHz) look good on the scope. TR39 (buffer pre-driver) has 0vdc on base in rx mode and .70vdc in tx mode. 0vdc on the emitter (ground) , 13.26vdc on collector in rx mode with 5.5vdc in tx AM mode and 13.6vdc in tx SSB mode. Thanks

 

[kaos513]

Did you check the audio chip?

 

[Unit 75]

yep. One problem is I have nothing at VR9 and VR8...so I cannot adjust bias. I do have 13v at TP7 and TP8 as mentioned in my initial post.

 

[Unit 75]

OMG! pushing on the PC board, while adjusting the driver bias, I found that I have a intermittent contact. I now have bias control via VR8. (hair line crack on trace) Had to get my magnifing glass out. Still no RF out..but should be an easy fix now. Will update findings after I troubleshoot some more. Thanks for your input, kaos513

 

[kaos513]

Nice find ! Hope it comes back to life again

 

[kaos513]

My last post was going to be check for any bad solder joints guess you nailed that.

 

[Unit 75]

Man...I tell you what. I always seem to get the real hard intermittent problem radio's for some reason. lol.

 

[NODENS]

Did you check signal path from final to antenna connector? Check if RF is present at finals, if yes, check the path to antenna connector, starting with coils in that path. I have seen several times the first coil after final transistor open. Check it.

 

[nomadradio]

If VR8 and VR9 both set to the correct DC current, the final and driver are almost-certainly okay.

If either one fails, this should screw up the bias adjustment.

73