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Old 6th Feb 2018, 1:27 pm   #1
DigitalNoMore
Pentode
 
Join Date: Jan 2013
Location: Nottingham, Notts. UK.
Posts: 228
Default Pilot Little Maestro T105

I acquired this set about a year ago and thought it was about time to see if I could get it going. The case was in very good condition but the electronics a complete unknown.

A prod around with my DVM showed that all the valve heaters were okay but the pilot bulb (10V, 0.2A), connected in series with the heaters, was open circuit. The smoothing electrolytics and the output valve cathode bypass electrolytic were all leaky. In addition, six of the resistors had gone high and out of spec. Most notably, the mains dropper wirewound resistor was way out of spec.

First of all I decided to try and reform the two 47uF smoothing capacitors in situ after first disconnecting them. I picked up the mains supply in the radio and fed it to each capacitor in turn via a 1N4007 diode in series with a 10k current limiting resistor. I fed the mains supply to the radio via a variac which I gradually ramped up to give the max working voltage on the capacitors. Fortunately the leakage current to each smoother dropped to about 0.1mA after around 8 hours so reforming was deemed to be a success.

Next job was the dropper resistor. I removed this and completely dismantled it. The taps of the winding are made by means of copper collars which loop round the coil and are then bolted in place. I cleaned up the winding and copper collars (all very grimy) and then replaced the collars on the winding in order that the correct resistance values were obtained on the tappings.

The output valve cathode bypass electrolytic and all the faulty resistors were then replaced. The pilot bulb was replaced with a 12V 0.2A which was the nearest I could find. For good measure, a squirt of Servisol was applied to the wavechange switch and volume pot.

I then took the plunge and powered up the radio through my variac and lamp limiter and monitored the current with a current clamp.
Thankfully all seemed okay and the voltages around the valves looked good. Stations on both wavebands could be received with reasonable volume too.

The only problem was a fairly loud hum even with the volume control down at zero. This caused a nasty modulation with the volume turned up.
One resistor I hadn't replaced was the grid leak bias to the audio preamp valve. This had read 13M as against the 10M it should have but I'd thought it was probably okay.
Shorting it to ground though pretty much got rid of the hum so I decided to connect a 1.5M resistor across the 10M. This did the trick and brought the hum down to a reasonable level as well as getting rid of the modulation. It was also high enough to not roll off the low frequencies significantly.

Finally I did a quick alignment on both wavebands and that was it finished. Quite a nice sounding little radio considering it was a budget offering when it came out. A time consuming repair but very satisfying.
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