Pilot Little Maestro pre war AC/DC 1939 restoration project.

[davebms]

Originally Posted by Radio Scotland

Replacement aerial wire connected to the band switch ??

I have photos of most of my set but today they do not want to up load. The aerial does go to the band switch via a ceramic cap (I think).

[Radio Scotland]

Originally Posted by davebms

Cheers Dave another small part of the puzzle solved, Yes according to the schematic it's coupled via a small value cap.

Cheers, Jay.

[Simon Gittins]

The maths puzzled me back in 1983. The important thing is that when it's all working properly, the valve heaters will have their correct voltage which can be measured with a meter.

There is no particular reason to measure the voltage across the dropper capacitor, but if you do it's a lot higher than the difference between the heater chain and the supply. I learned the reason shortly afterwards; it's all explained on that page of course.

[Radio Scotland]

Originally Posted by Simon Gittins

The maths puzzled me back in 1983. The important thing is that when it's all working properly, the valve heaters will have their correct voltage which can be measured with a meter.

There is no particular reason to measure the voltage across the dropper capacitor, but if you do it's a lot higher than the difference between the heater chain and the supply. I learned the reason shortly afterwards; it's all explained on that page of course.

Am i right in thinking the dropper voltage will float up when its unloaded ? As in not connected to the heater chain ?


Cheers
Jay

[Simon Gittins]

Yes, your meter would show the mains voltage without the heater chain connected.

[Radio Scotland]

Originally Posted by Simon Gittins

Yes, your meter would show the mains voltage without the heater chain connected.

Cheers, Whats the best way to measure the output of the dropper ? Positive DMM Lead on the output of the dropper and the DMM Negative on the chassis ?



Jay

[superjust]

Hi all, I restored my Pilot 'Little Maestro' about two years ago using a 'motor run' capacitor (not a motor start) of value 4 microfarads and a few other components. I know it should be about 4.2 microfarads but when I measured the heater voltage it was about 67V and so I left it as it was. At the moment the capacitor can be bought from CPC for £3.10. The picture shows my arrangement with components soldered onto stripboard.

[Radio Scotland]

Originally Posted by superjust

Hi all, I restored my Pilot 'Little Maestro' about two years ago using a 'motor run' capacitor (not a motor start) of value 4 microfarads and a few other components. I know it should be about 4.2 microfarads but when I measured the heater voltage it was about 67V and so I left it as it was. At the moment the capacitor can be bought from CPC for £3.10. The picture shows my arrangement with components soldered onto stripboard.

Thanks for your input, Really appreciate it. Was thinking of using X2 safety Caps, But the budget is tight and the cost is mounting by the minute. I wonder if this type would do ? Can't see why not.


Jay

[Simon Gittins]

Cheers, Whats the best way to measure the output of the dropper ? Positive DMM Lead on the output of the dropper and the DMM Negative on the chassis ?

With the DMM set to measure mains AC volts, yes, if you want to do that but provided it's a good capacitor you will just measure the mains voltage without the load of the heater chain present.

With the heater chain present, you should see around 70V including the pilot light and its shunt resistor and that is the best way to check that it's working correctly with the DMM negative on the chassis and the DMM positive at the junction of the dropper capacitor and the top of the heater chain.

The one pictured looks ok to me as the description is "motor run".

[Sideband]

Originally Posted by Radio Scotland

Thanks for your input, Really appreciate it. Was thinking of using X2 safety Caps, But the budget is tight and the cost is mounting by the minute. I wonder if this type would do ? Can't see why not.

That type will work ok. I've used a couple of those in my Pye TV to replace the heater dropper section. So far they have been fine. I had to use two to get the right value.

I think you are right about them being motor run types...the pair that I used came out of a fan. They are well rated at 400V so should be just 'ticking over' as an ordinary dropper.

Talking of 'Motor run' caps....these are OK to use as a dropper capacitor. Beware of 'Motor start' capacitors. These are not really suitable since they are not rated for continuous use whereas 'motor run' types are.

[Radio Scotland]

Thanks for the info regarding motor run capacitors and how to measure their output, Think i'll stick with the safety caps as two of them (Total 4.4uf) is cheaper than a motor run cap and theres the added bonus of them not shorting.



Jay

[Radio Scotland]

Originally Posted by agardiner

If one of the valves has an open heater, then there is a good chance that someone powered up the set without any kind of series resistance, ie replaced mains cord without realising that it was a dropper. Seen this a few times!

I have seen these sets with both Z4 and Z6 valves. I would start with cleaning, re-capping and installing your capacitor dropper.

Turns out i was measuring the wrong pins on the 6A8GT, Measuring the correct pins i'm getting 3 ohms.

Valves are new technology to me

[Radio Scotland]

Ok, Iv'e looked at various configurations of the dropper circuit all have various resistor values, Some with the lamp shunt some without, Heres what i'm thinking.

Two 2.2uf safety caps in parallel. The original line cord has a 15 ohm tap for the lamp shunt. And a 370 ohm tap for the HT Supply,

So i'm gonna go with those values,

HT - 370 ohm 10 watt cement resistor.
Lamp shunt - 15 ohm 2.5 watt resistor..

Not sure whether to include the 33 ohm surge limit resistor from my original dropper schematic

[Sideband]

No harm including the surge limiter but in practice, I've not found it necessary. About 10 years ago I used a cap dropper in my Little Maestro Model 10 and found, without a surge limiter, there was no inrush as such as the pilot lamp lit at normal brightness and remained so during valve warm-up. Previously with the dropper in place, there was a surge at switch-on and the pilot lamp was brighter at first and then dimmed down as the heaters warmed up. Still going strong after 10 years.

[Herald1360]

A capacitor dropper with a fairly low voltage heater chain tends to act as a constant current source, hence the lack of brightness surge in the panel lamp at switch on. There may still be a fast current spike as the capacitor initially charges if switch on is near peak mains, but the energy in this is small compared with the thermal inertia of the heater/panel light system.

[Simon Gittins]

For the value of the HT resistor I just selected one that gave about the right HT value which the Trader sheet 444 gives as 160V at V4 screen and 202V at the rectifier cathode.

The total line cord resistance is given as 500 ohms, tapped at 15 ohms and 370 ohms from V5 heater. I think that means the section between the mains and the HT tap is 115 ohms, however, both the HT current and the heater current goes through it so a higher value is needed. The circuit in Post 1 uses 120 ohms and the one in Post 12 uses 470 ohms which I think is more likely to give the correct HT.

[Radio Scotland]

Cheers for the input folks. Forgot about the 100k bleed resistor in my above dropper circuit parts list, May need to double check all those values before ordering the parts up. So far the only parts iv'e ordered are 2 x 22uf 450v smoothing caps, Original canned one was 2 x 20uf, The replacements which are currently in it are 16uf.


Tests iv'e done today.

Field coil 684 ohms (Good news)
Audio transformer primary 270 ohms
Voice coil with transformer secondary in parallel with it 0.5 ohms ??

Oscillator section of the tuning cap seems to have a DC Short, Unless i'm riding the short bus as shang0 would say

Power switch seems to have a short across it (Carbon tracking ?)


Can only work on it for short periods due to back pain kicking in


Jay

[Simon Gittins]

Voice coil with transformer secondary in parallel with it 0.5 ohms ??

Transformer secondary is given as approx 0.3 ohms, I don't think there's likely to be a problem here.

Oscillator section of the tuning cap seems to have a DC Short

Is this on both positions of the waveband switch? If so, there could be a problem, especially as you mentioned in Post 16 that the tuning shaft is bent.

Power switch seems to have a short across it

The contacts can become stuck together, you may be able to open it up or fit a modern one with a double pole switch.

[Radio Scotland]

Originally Posted by Simon Gittins

Transformer secondary is given as approx 0.3 ohms, I don't think there's likely to be a problem here.

Is this on both positions of the waveband switch? If so, there could be a problem, especially as you mentioned in Post 16 that the tuning shaft is bent.

The contacts can become stuck together, you may be able to open it up or fit a modern one with a double pole switch.

Hi simon, I don't know what position the band switch was in but it shouldn't be hard to track down the fault.

Yeah theres a few ways to fix the power switch.

I noticed this odd 50 ohm 5 watt ''made in uk'' resistor which seems to be taking the full mains input on one side and the other side of it is hooked to the valve next to the rectifier. No sign of it in the schematic or parts list... And the pilot lamp seems to be in parallel with one of the heaters


Cheers
Jay

[ms660]

Originally Posted by Radio Scotland

I noticed this odd 50 ohm 5 watt ''made in uk'' resistor which seems to be taking the full mains input on one side and the other side of it is hooked to the valve next to the rectifier.

The output valve socket tag 6 that the 50 Ohm is connected to doesn't connect to the valve itself according to the valve data (6 = no pin) If the 50 Ohm is in circuit trace where in the circuit it's connected to in the schematic.

Lawrence.