Pilot Little Maestro.

[Geordie McBoyne]

Recently bought this slightly tatty Pilot Little Maestro on Ebay.   Seems nobody was interested in it possibly because of the condition of the cabinet.
Anyway, the cabinet was an easy fix so now it's the electronics that need attending to.  I believe this is a series 2 receiver. The valve lineup is:  6K8G, 6K7G, 6Q7G and 25A6G.  The rectifier is a 25Z5G.  The latter two are broken but I have a 25A6GT and a 25Z6GT which should serve as replacements.
The line cord has been cut-off so some other means of supplying the 0.3amp heater chain will have to be considered, a capacitor perhaps?

Geordie McBoyne.

[Geordie McBoyne]

Circuit diagram of the first version of the Pilot Little Maestro, the pre-war model. The second version is similar except that the mains line cord has no dial lamp tapping and supplies the valve heater chain only. Separate series resistor to the anodes of the 25Z6G rectifier.  Frequency changer valve changed from 6A8G to 6K8G.
Later version 2 models have a PM loudspeaker and the field winding is replaced by a 1.5Kohm resistor.

Geordie McBoyne.

[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.

[boater sam]

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.

Is it correct to use class X or indeed Y as a capacitor dropper? I have doubts. Could someone who knows more comment please?

Sam.

[ppppenguin]

Class X is fine for a capacitor dropper. Class Y is also OK but you're unlikely to find them in the values you need. Motor run capacitors and power factor correction caps (often used in fluorescents) are also Class X and can be a good choice. beware of motor start capacitors which are often only rated for short duty cycles.

[Murphyv310]

Nice start on the set David and a good result so far. Regarding run caps the majority now are dual purpose as a lot of motors use only one cap constantly connected. Some older motors had centrifugal switch that disconnected the start cap once the motor was up to speed, this method now has fallen out of favour. You normally can tell if a motor has one or two caps. During my time in industry we always ordered start/run caps as they were suitable for both situations, they may have been pennies more expensive though.

[Geordie McBoyne]

Circuit diagram of the version 2 of the Pilot Little Maestro. Later V2 sets have a PM loudspeaker. L3 replaced by a 1.5Kohm resistor.

Geordie McBoyne.

[Geordie McBoyne]

Found a bracket to mount the heater dropper capacitor. Just need a capacitor clip to finish the job.

Geordie McBoyne.

[Stitch1]

Hi, I have what I assume is the pre-war version, I've never restored it because of the dropper cord issue but I really should spend some time working out what I need to do, this thread is very helpful. 

John

[Nick]

Found a bracket to mount the heater dropper capacitor. Just need a capacitor clip to finish the job.

Could you not hide it under the chassis and out of sight?