HI to all,

I'm restoring a Marconi #74

I did replace many paper caps (big ones) & all the electyrolytics.
But i did not replace all the small paper caps.
Tubes are tested good.
The radio had no speaker, but fortunately it has it's audio transformer.
So i installed a 820ohms/10W résistor to replace the field coil. (i didn't increase the value of electrolytics, audio has no hum at all)

After the first test i noticed this:
B+ is a bit too high compare to the Marconi specs.
310Vdc instead of 240Vdc. Should i increase the value of the resistor ?

Audio amplifier circuit works very well.

I installed the other tubes and prepared for alignment.
At power Up, i was able to tune to some stations, but after a minute, the radio became suddenly quiet.

If i try to inject a strong signal of 462Kc at the antenna, i hear nothing at all.

If i inject the signal on the cap of the 6K7 RF stage, i hear nothing at all.

If i inject the signal on the cap of the 6A8 osc, i hear a strong signal. So i suppose i might have an open coil somewhere in the RF stage or a bad cap ?

I tried to proceed with alignment , tuning the first IF has no effect at all, tuning the second IF has effect only on the secondary side, just before the detector.

I see that the 6K7 between IF is overheating.
On my other sets, i never noticed that before.
The tube is too hot for the fingers !!!!!

i have these reading on that 6K7:(compare to the specs)

Cap:0v
Pin2: 0v
Pin3: 310v instead of 240v
Pin4: 153v instead of 95v
Pin5: 6.7v instead of 5v
Pin6: -
Pin7: 0v
Pin8: 6.7v instead of 5v

Should i check plate current drain ?
What should be the current drain on that 6K7 between IF ?

Thanks
Vianney

The original field coil says 1300 ohms. Your B+ is high because more resistance is needed. The small paper capacitors are just as important as the big ones. Replace all of those too. The tube getting hot may be due it’s working into a leaky capacitor you did not change. The high B+ probably did aid to the failure of the old capacitors. After you replace the rest of the capacitors. Try swapping the RF tube with the IF one. See if the tube still gets way to hot.

Ok thanks Warren,

i will replace the small paper caps too.

For now, i checked all the coils winding dc resistance. Everything is normal. No traces of oxidation on windings.

I found an open resistance R10 which is a 100K !! But no change, 6K7 still overheating.

I go for the paper caps replacement.