Mystery fault in IF stage - any ideas? - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

kalee20 · 18th Jul 2009, 7:06 pm

As per anthonys radios post above, if you return your AVC decoupling capacitor direct to the cathode pin of the valveholder, you will have the best possible return.

I can't remember if the EF89 has two cathode pins or not (the EF80 definitely does). If so, connect the AVC decoupling capacitor to one cathode pin, and all other connections (cathode resistor, screen and anode decoupler) to the other cathode pin.

Doing this will give maximum separation between the input-related circuitry and the output-related circuitry.

MichaelR · 18th Jul 2009, 11:20 pm

Quote:

Originally Posted by kestrelmusic

Well the set is now working! I finished the aerial and oscillator stages this morning and powered up, and after a little tweaking of the coil cores it came through loud and strong!

.

This is great news , it would be interesting if you get the time to test the AGC to see how effective it is.

Mike

kestrelmusic · 19th Jul 2009, 6:48 am

Quote:

Originally Posted by MichaelR

This is great news , it would be interesting if you get the time to test the AGC to see how effective it is.
Mike

Using the 'scope as a voltmeter, it seems to be developing around -5V with a strong signal.

MichaelR · 19th Jul 2009, 7:12 am

Ideally it would be nice to inject a modulated AM signal into the front end and then vary the signal level a known amount, monitoring the AGC action to see how effective it is for a range of signal variation.

Mike

18th Jul 2009, 7:06 pm	#41
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: Mystery fault in IF stage - any ideas? As per anthonys radios post above, if you return your AVC decoupling capacitor direct to the cathode pin of the valveholder, you will have the best possible return. I can't remember if the EF89 has two cathode pins or not (the EF80 definitely does). If so, connect the AVC decoupling capacitor to one cathode pin, and all other connections (cathode resistor, screen and anode decoupler) to the other cathode pin. Doing this will give maximum separation between the input-related circuitry and the output-related circuitry.

19th Jul 2009, 7:12 am	#44
MichaelR Retired Dormant Member Join Date: Mar 2006 Location: Sheffield, South Yorkshire, UK. Posts: 1,587	Re: Mystery fault in IF stage - any ideas? Ideally it would be nice to inject a modulated AM signal into the front end and then vary the signal level a known amount, monitoring the AGC action to see how effective it is for a range of signal variation. Mike