Constructed with standard parts Alba made a truly small radio set.
It's a tiny set but it has a proper coil pack
The valve line-up is: UCH21 frequency changer. UCH21 heptode section IF amplifier and triode section as audio amplifier. UBL21 double-diode-pentode detector, AVC and output. UY1N rectifier. Total heater chain voltage is 145 which is also the mains voltage requirement for this set. So for 240 volt mains we have to get rid of 95volts somehow.
In my set the HT smoothing capacitor needs replacement. 16 + 16 + 8 microfarads in a single can, but that's the last of my concerns. The resistive mains lead is open circuit and the simple answer is to use a 680 ohm resistor which will dissipate 15 watts but this is not so easy to do because of the lack of space inside the cabinet. How about a capacitive dropper? 2.2microfarads seems to be the correct value for this set. Whatever method is used 95 volts @ 150mA has to be dissipated by some means or other.
Till Eulenspiegel.
Lovely little radio, always fancied one! I have the slightly smaller C116LW, quite a good performer too. Mine got smashed in the post, got my money back on it, and was told to keep it or throw it away, so I spent a good few hours working out where all the bits went, then stuck it back together!
Regards
Lloyd
How about a capacitive dropper? 2.2microfarads seems to be the correct value for this set. Whatever method is used 95 volts @ 150mA has to be dissipated by some means or other.
For the benefit of other readers, I trust you took into consideration when calculating the value of your capacitor dropper that it's a reactive load, not resistive therefore voltage and current will not be in phase.
Again, for the benefit of newer readers - would you "show your working", as maths exam papers used to say?
That's right is not just simply finding out what the value of a capacitor is for XC 680 ohms @ 50hz
Till Eulenspiegel.
As an aside, is it still possible to obtain resistive mains dropper leads?
To understand the black art of electronics is to understand witchcraft. Andrew.
Nice explanation of dropper calculations from Paul Stenning, resistive, capacitive and diode.
https://www.vintage-radio.com/repair-restore-information/valve_dropper-calcs.html
Is a diode dropper worth considering? I have not looked at the maths, hands full at the moment back in Suffolk on childminding duties.
I think the asbestos part of “line cord” would rule out any new products.
Frank
Hi Frank, there was a discussion about resistive mains cords in the vintage-radio forum. The C112 has no mains adjustment and when the set was supplied direct from the variac I found it would operate from a supply as low as 100 volts. So I guess that if a silicon diode was fitted in series with the mains lead the set would work from the half waves. For protection from the series diode going short circuit a reverse connected diode can be fitted inside the set. A fuse will be inserted in the live mains for extra protection.
Till Eulenspiegel.
Tested the C112 today with a 2mfd capacitor in the mains neutral return on the on-off switch. The set works albeit with hum from the loudspeaker because the 16 + 16 + 8mfd smoothing capacitor needs replacing.
A 1 ohm test resistor is connected in series with the dropper capacitors. A two channel oscilloscope is connected across the resistor. The other 'scope channel is connected to heater chain side of the capacitor. The 90 degree phase difference between the current sensing resistor and heater dropper capacitor is clearly shown on the 'scope traces.
Till Eulenspiegel.
Due the compact dimensions of the cabinet fitting voltage dropping resistors anywhere inside the receiver is completely out of the question.
To keep things cool, the line cord resistor has been replace by a 2.7uF capacitor. Actually 2.2uF and 0.68uF capacitors connected in parallel.
The total heater chain voltage is 155V which is also the voltage supplied to the anode of the UY1N HT rectifier.
Waveform across the 1ohm resistor inserted in series with the mains neutral lead.
Till Eulenspiegel.
