15-09-2017, 04:49 PM
(This post was last modified: 15-09-2017, 04:55 PM by Geordie McBoyne.)
Repairing the Little Maestro. The HT 16 + 16mfd double ended can reservoir and smoothing capacitor was found to be useless so for the time being I've tacked in two wire ended capacitors, I'll order a proper smoothing capacitor. Couldn't find a 25A6G output valve so a 25L6GT will be employed until the correct valve is found. All the usual waxies have been replaced including the one they call on another Forum as "that capacitor". In the original circuit only the neutral side of the mains is switched, not nice, but fortunately the on-off switch is a two pole type so now both sides of the mains are switched. I've opted for a capacitor heater supply dropper for this set as there no space inside the cabinet for a transformer and a resistor is out of the question as it will dissipate over 40watts! Four 1microfarad capacitors connected in parallel will serve as the heater dropper, at least for the time being. The correct value should be 4.19mfd for a 76 volt heater chain. Trevor checked out the calcs on the UKVRRR website. Also a class X component should be used.
Time to test the set. Connect up to the variac and bring the mains supply up slowly, the 25Z6GT gave up in a shower of sparks, haven't a replacement so an old Mazda U291 (PY32) has been pressed into service. Clipped pin 1 off because it might be an internal connection. Variac finally turned up to max volts.
HT at the anode of the 25L6GT is correct at 220volts and the earlier stages receive 165 volts though a 5Kohm resistor. No results so far although the volume control crackles when turned. Huge 18 volts negative present on the grid of the 6Q7G amplifier, the negative volts vary when the grid connection of the 6K7G IF amplifier is touched, that's got be some kind of instability. Quick fix, a 0.22mfd decoupling capacitor had been clipped out and had not been replaced, mea culpa! Capacitor duly fitted and I'm greeted with noises coming out of the 'speaker. The set works, good results on medium waves and hopeless on the long waveband.
The attachment shows a messy repair job, rest assured this will be tidied up later when all the correct parts are found.
Geordie McBoyne.
Time to test the set. Connect up to the variac and bring the mains supply up slowly, the 25Z6GT gave up in a shower of sparks, haven't a replacement so an old Mazda U291 (PY32) has been pressed into service. Clipped pin 1 off because it might be an internal connection. Variac finally turned up to max volts.
HT at the anode of the 25L6GT is correct at 220volts and the earlier stages receive 165 volts though a 5Kohm resistor. No results so far although the volume control crackles when turned. Huge 18 volts negative present on the grid of the 6Q7G amplifier, the negative volts vary when the grid connection of the 6K7G IF amplifier is touched, that's got be some kind of instability. Quick fix, a 0.22mfd decoupling capacitor had been clipped out and had not been replaced, mea culpa! Capacitor duly fitted and I'm greeted with noises coming out of the 'speaker. The set works, good results on medium waves and hopeless on the long waveband.
The attachment shows a messy repair job, rest assured this will be tidied up later when all the correct parts are found.
Geordie McBoyne.
