Pilot Little Maestro distorted

[Malcolm G6ANZ]

I have got my Pilot 'little maestro' up and running with a 4uF capacitor and a few resistors instead of the line dropper. The voltages measured are close to those specified on the trader sheet. Although it receives stations it is quite insensitive, even with a long wire aerial, and the audio on medium and long wave is distorted. Feeding signal into the audio amplifier gives a reasonable sine wave on the scoped audio transformer primary, so i am starting to suspect the speaker. It is difficult to substitute the speaker as it has a number of windings on it and its magnet is the HT smoothing choke. I replaced the HT caps when I fitted the dropper capacitor, along with the wax ones and out of spec resistors. The HT has about 2v of ripple on it.The cone moves freely with no scratching sounds. It has been repaired with some tissue paper and glue, presumably where it got damaged at some point.
The IF and tuning trimmers are untouched.
Could I put a replacement output TX and ordinary speaker in place of the original one as a test?

[kestrelmusic]

No reason why not, just unsolder the anode lead from the existing Tx and use croc leads to connect a replacement temporarily. However, before doing so it might be worth while just to check the speaker by itself. These little speakers were pretty reliable, but it will be eighty years old by now.

Either way, be very careful! There will be anything up to 200 volts D.C. relative to earth on the output transformer.

[davebms]

hello all, I have been running my maestro for about 3 weeks I had
distorted sound, it seemed to get better as a ran it for longer.
I have also, disconnected c11, v4 cathode bypass capacitor, (trader sheet
444) and changed c12 tone corrector, to .03 for a small speaker set ,its quite good,, good luck with yours

dave

[Malcolm G6ANZ]

I've been looking further into the distortion on this set. I have concluded that its not in the audio stages but somewhere in the RF/IF area. Reason, it it 'motor-boating' and with the volume at zero it doesn't sound therefor its not the AF amp taking off. By scoping the anode of the IF amp it is normally a good sine wave however when it starts to 'motorboat' this turns into a series of pulses at a low frequency with the IF frequency inside the pulses. It's the same both Long and Medium waveband. It will oscillate with or without an aerial connected. The AVC line changes value depending on the strength of the signal received as it should.
Its difficult to trace the exact stage which is oscillating as putting a scope probe on useful points usually stops the oscillation.
I have double/triple checked the replacement caps and they are all in the correct place and the correct value. I have cleaned and retensioned the valve bases.
Two things I have changed;
The rectifier was low emission and this has been changed to a 1N4007 + resistor. Secondly, Cap C2 (anti resonance) was originally connected directly to the aerial and not after C1 the aerial isolating capacitor. I have rewired C2 to be in the correct position. I cant put up a diagram as my copy is as a PDF and I cant copy bits of it
I think if the motor-boating was cured the distortion would disappear. So how can I stop it?

Malcolm

[Malcolm G6ANZ]

I managed to copy part of the aerial circuit showing C1 and C2

[jonnybear]

Check C6 .05uf decoupling 6k7g grid 2.
John