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Old 29th Mar 2006, 2:11 am   #41
Top Cap
Octode
 
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Location: Watford, Hertfordshire, UK.
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Default Re: Motorboating

Hi Neil, I have been comparing your circuit with that of a radio made by Champion in 1953 namely model 820. It uses the ECL80 and confirms that the output transformer primary is around 10k Ohms. Big noticeable differences are:-
1) Grid leak resistor on pin 9 is 680k, yours is 330k.
2) Cathode resistors are 150 Ohms each with the bypass capacitor at 470uF, much higher than your present 47uF.
3) The 1M resisistor between pin 2 and the cathode centre tap is in fact a 1M volume control, the wiper is connected to pin 2, one end to the cathode centre tap, the other end is the signal input via a 10nF capacitor.
4) Your triode anode load resistor of 47k+3k3+22uF Capacitor is 220k on the Champion circuit, quite a difference and there is no 22uF de-coupling capacitor except for a 100pF mica to earth which may prevent the RF oscillation as previously mentioned.
5) Also as mentioned above, there is a 3M3 resistor connected between both anodes for stability.
Not sure how to get photos on here but I will try pasting one in and hope it works.
Good luck with the project, getting these things sorted out is part of the fun.
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Old 29th Mar 2006, 2:17 am   #42
Top Cap
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Default Re: Motorboating

BIG P.S. Can you spot the mistake? They have pins 7 & 8 incorrectly labelled. The scren grid is pin 8 not pin 7, their mistake not mine. I had spotted it earlier but forgot to warn you in my previous. Your pin outs are correct!
All the best Neil.
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Old 29th Mar 2006, 6:31 pm   #43
Nico
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Default Re: Motorboating

Quote:
Originally Posted by PJL
Nico, makes more sense than my answer...I assumed motorboating would describe a low frequency instablility. I did say novice didn't I.
I just had a similar problem in an old 1935 Burndept as well. Therefore I was triggered. In the Burndept the IF started to oscillate, did buid up negative, stopped etc. Nice motorboating. There it was the metal paint at the IF tube that didn´t make contact with the ground of the tube. It did set me at the wrong leg first as well, thinking of feedback via the powersupply. Fun these old radio´s don´t you agree?
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