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QCX finals blown?

Ross Keatinge
 

Hi all

After about six months of happy but infrequent operating, my 40m QCX no longer transmits. I think the finals are blown.

I just dug out my old oscilloscope that I haven't touched for about ten years. Fortunately it still works.

Symptoms:
At 15v the QCX draws about 350ma on receive.
Finals are too hot to touch after about 30 seconds.
Voltages are IC3 seem correct except pin 3 which has about 6v on receive.
On transmit, IC3 pin 3 is modulated with about a 1v p-p square wave centered around 6v.
I can hear clean transmitted CW signal on a local receiver so most of it is working.
Receive and display seem to be working fine.

The fact that I see a signal on TC3 pin 3 makes me hopeful that IC3 is okay and it's just the finals which are easier to replace. Does that seem like a reasonable conclusion? Anything else I should check before I order some BS170s?

Do the finals typically all blow at the same time? I'm not aware of any "event" such as high SWR happening that might have caused this.

Thanks, 73
Ross KT1F

Alan G4ZFQ
 

The fact that I see a signal on TC3 pin 3 makes me hopeful that IC3 is okay
Ross,

Switch off and remove the finals before you blow IC3.
6 volts on pin 3 is being fed from a damaged BS170, it can damage IC3.

Do the finals typically all blow at the same time? I'm not aware of any "event" such as high SWR happening that might have caused this.
Best replace them all for safety. And Q6 is blown, that's where the 6 volts is coming from.

73 Alan G4ZFQ

Ross Keatinge
 

Thanks Alan.

I've removed the finals and I'm getting a nice almost 5v p-p on IC3 pin 3 now so I seem to have avoided more damage. I'll replace Q6 too.

Ross