Electronics > Repair

Tranciever Repair question?

(1/4) > >>

Crambone:


On page 58 - 59 I highlighted part of the schematic.
I’m not seeing the 7.8MHz at pins 7&8 of IC5 (SO42P) all the voltages are correct. I tried to trace back with a scope looking for a 7.8MHz signal and do see it at TP62 & 63 but not at TP61 & 64??? How is this possible? All the components marked green have been either removed and checked.
Am I following the circuit correctly looking for the lost 7.8MHz.
The issue I’m having is the radio won’t develop/key without the 7.8MHz and 35MHz present. I have modulation when sniffed with a second radio but no power out. All other functions seem to be working properly.

Link to service manual and schematics

https://cbtricks.org/radios/cobra/2000gtl/graphics/cobra_2000gtl_sm.pdf

Wallace Gasiewicz:
Suggestions
First of all does the radio RX at all???What are you using to check the signals? A scope may load down the circuit at some points and even eliminate the signal
Check at pre driver for the 27 MH signal first.If it is there tune it to max with L47 and L48
If this is not there check at TP 14 for carrier 7;8 MHz, this should change with SSB and AM , no signal with SBB without audio input
Let me know

Crambone:
Thank you, I have great receive in all modes. I also have modulation on all bands & channels using a handheld set to the channel I key on I can hear it. I will check what you suggested.

Crambone:
Well I went to test TP14 using an Oscilloscope and I was seeing the 7.8 MHz but it slowly drift out so I checked voltages on IC3 and all the voltages low. I decided to check a bunch of other voltage points and everyone I tested was around 1/2 and would drift down. I was getting ready to test anther point and POP went the AC filter before the transformer. The 2.2m resistor let the smoke out. I checked the transformer, rectifier, regulator and found nothing. I replaced the resistor and put on Variac and all seemed good.
So I decided to change out all the electrolytic capacitors in the power section before testing again.
Update to follow.

floobydust:
Well that 2.2MEG R415 is the "death resistor" needs to be at least 1/2W and I would consider also replacing the 4 Y-caps 0.001uF C412-C415 as they can also be a safety concern and modern parts are tougher for taking mains overvoltages. Usually the antenna is the chassis earth-ground but... hey where is the mains fuse? I would have at least ~1A SB in there with a polarized line cord.

I would carefully check the many power supply regulators are OK first. I will desolder and lift the final's collector lead, TR36 2SC1969-B to disable the transmitter PA until I get more signs of life. Make a fake mic, the aviation connector with a toggle switch for RX or TX to leave it keyed up.
I think you can keep troubleshooting by following the PLL signals for 7.8MHz and 34MHz. But so many diodes are switching the 7.8MHz on/off to various stages, so I would instead look if the RX/TX transistor switches are working so the mixer IC5 is getting the right signals.

Check the 8.2V regulator IC4 is good, it has 3 (switched) outputs and pin 5 is the TX/RX control. Also at node #12 on the Sams schematic.
I would make sure the TX switch is working starting from the mic, I check there because people will short a mic or wire it wrong and damage IC4.

Navigation

[0] Message Index

[#] Next page