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

[Radio Scotland]

Originally Posted by ms660

I've no idea, DMM gizmo's were after my time, 'twas an Avo and spit back in my day.

Lawrence.

Sounds good, Might give it a try


Jay

[Radio Scotland]

Soon be switch on time

Can i expect the correct voltage from the rectifier without a load connected to it ?

[Simon Gittins]

Good luck!
The datasheet in Post 81 recommends heat sink compound under the HT resistor.

Can i expect the correct voltage from the rectifier without a load connected to it ?

No, the voltage will be higher than normal (not really a good idea). When you say "without a load", do you mean with the new electrolytics and the field coil connected? You won't get DC without them of course.
I suggest getting the heater chain working properly first (without a supply to the rectifier). Then try it with it all connected using your lamp limiter at first.

[ms660]

The no load DC voltage across the reservoir capacitor will be the same as the peak of the mains voltage, eg: for 240 Volt mains then it's 240 multiplied by 1.414 which is 339 Volts.

Lawrence.

[Radio Scotland]

Yeah thats what i thought would happen without a load connected, Currently the only thing connected to the rectifier is one smoothing cap, Speaker and field coil / Transformer are disconnected.

I know i'd be better with heatsink compound on the resistor, But i'll try it as is and go from there.


Simons calculations from earlier in the thread state that the original line cord HT Resistance was 115 ohms and dropped 42 volts, New 470 ohm resistor will drop around 30 volts,

I Thought the higher the resistance was the more voltage that was dropped ?


Cheers all

Jay

[Herald1360]

Yes, but remember the HT ballast section of the line cord also carries the heater current. The new 470R HT ballast doesn't.

[Radio Scotland]

Originally Posted by Herald1360

Yes, but remember the HT ballast section of the line cord also carries the heater current. The new 470R HT ballast doesn't.

Cheers chris, Had forgotten about that.

Jay

[Radio Scotland]

Well my lamp limiter will be a floor standing uplighter Because i only have one incandescent light bulb and its a screw in type, And my uplighter has a screw in socket,

So it'll be a shango066 effort with wires hanging out plugs with the odd choccy block connector here & there, And maybe even some paper masking tape




Jay

[Radio Scotland]

Lamp limiter cobbled together and tested, Still waiting on the other capacitor for the heater dropper (hopefully tomorrow) Obviously first test will be via the lamp limiter, What voltage can i expect from the rectifier taking into account the voltage drop across the lamp limiter ?

[Simon Gittins]

The lamp limiter page is here:
https://www.vintage-radio.com/projec...p-limiter.html
If all goes well, the lamp will only glow slightly and the radio should show signs of working. The voltages shouldn't be much below normal with the lamp in circuit and when you are convinced that nothing nasty is about to happen you can switch to full mains.

[Radio Scotland]

Originally Posted by Simon Gittins

The lamp limiter page is here:
https://www.vintage-radio.com/projec...p-limiter.html
If all goes well, the lamp will only glow slightly and the radio should show signs of working. The voltages shouldn't be much below normal with the lamp in circuit and when you are convinced that nothing nasty is about to happen you can switch to full mains.

Cheers simon, My next post should be yippee it works or I'll report back when she's been tested.


Jay

[Radio Scotland]

New dropper capacitor soldered in, Dropper Capacitance, , overall dropper voltage . Dial lamp voltage. results pictured below,

[Radio Scotland]

6Q7G (V3) Voltage

[Sideband]

Spot on if that's the heater voltage.

[Radio Scotland]

Originally Posted by Sideband

Spot on if that's the heater voltage.

Dropper output 67 = Volts
Dial lamp = 5 Volts
V3 = 6 Volts

[Radio Scotland]

The capacitor i needed to top-up the dropper capacitance is under the rectifier socket, Was easier to put it there,

After fitting it and turning the radio on i got a big fat zero I've since learned it helps to put a fuse in the plug


The big switch on might need to be delayed, The wax bomb on the volume pot broke (5nf) i stuck in a 6.5nf polyester cap i had laid around, But whilst i was working on the radio a leg snapped off, And i don't have a replacement


But getting very near the grand turn on. But i don't wanna rush the last mile and make a silly mistake and blow out the field coil, Voice coil or audio transformer, Speaker currently disconnected as it needs work on the cone, I'm in no mad rush..

Last night i totally stripped the on/off volume pot as the on/off mechanism was sticking, All done and reattached,

This is how she looks now.


Jay

[Simon Gittins]

That's the heater supply sorted then!
Good progress, you are quite right not to rush things.

[Radio Scotland]

Originally Posted by Simon Gittins

That's the heater supply sorted then!
Good progress, you are quite right not to rush things.

Aye happy with my progress so far, But can't afford to hook the field / voice coil / transformer up wrong, Be tough to get a replacement.


Ordered some capacitors to replace the remaining wax bombs, Just not keen on having them in there..Weak spot might be a valve or two, And maybe a dodgy coil. Time will tell.


Cheers
Jay

[Radio Scotland]

The 50 ohm, 5 watt surge limiter in line with the heater chain doesn't half heat up after about a minute, Need to find something a bit chunkier

10 watt @ 33 ohms ?

[Simon Gittins]

Yes, with 50 ohms that would be about 4.5W of heat, close to the limit. It looks as though the mains lead is close to a wirewound resistor (photos can be deceptive though).
Changing to 33 Ohms would increase the total heater chain by about 5 volts which would bring it to 72 volts which is very close to the ideal 75 volts and a 10W resistor would give a better margin as long as there is room for it away from the mains lead.