10 February 2018

First MW reception on the RD-160

So, with the lamp limiter just built I could dare to power the Radiomarelli RD-160 medium wave receiver.

I scrounged home for filament light bulbs and I located 40W, 60W and 100W.

I stared with 40W, that lit up and nothing came out of the receiver. Maybe I wasn't patient enough, or its rating is just too low.

One step further to 60W and ... after the first bright flash, the lamp dim dim dim .... until some noise comes out of the receiver! The residual light is very warm and pleasant too! It works!! It receives RAI1 broadcast on 999 kHz and nothing else.

Problems noted:
  1. When the tuning capacitor blades go towards full mesh the local oscillator stops: there's a short.
  2. The volume control crackles
  3. Knobs don't hold tight on the shaft because their plastic is cracked near the screw
Not bad at all! It is electronically fine! Now I will replace capacitors.

06 February 2018

My poor man's AC limiter

Reading around the Net, everyone mentions that powering old valve circuits under restoration should be done gradually by applying a lower voltage and observing what happens. That's until all capacitors are confirmed to be working (or replaced) and transformers are not shorted. This is accomplished with a tapped or variable autotransformer, that I don't have.

One solution would be to wire back-to-back two step-down transformers, like 220-12 and 220-24 that should provide 110V if connected 220-12-24-220. Of course, I don't have such a selection of old-school transformers.

The "poor man's AC voltage transformer" trick comes to the rescue: connect a filament light bulb in series with the D.U.T. and observe what goes on. It will not allow to control the actual voltage, nor to change it on the fly, but it will limit the current through D.U.T. thus reduce the risk of smoke/fire/damage. Lower wattage bulbs provide lower voltage to the D.U.T. and let less current pass through, therefore less damage is done in case of troubles.

As I understand it, the principle is that a fault in the D.U.T causing excess current draw will be tamed by the light bulb which in turn will light up and provide visual feedback of a problem. Begin with lower wattage and replace with higher, step by step, as the home stock of old filament light bulb allows.

In order to build a "safe" transformer device I decided to modify a power strip: it has a power cord, outlets and an embedded switch. The mod is to wire in series two outlets, the first being used to plug the light bulb (installed in an existing lamp, for example), the second for the D.U.T. A third socket, left in parallel with the second, can be used to measure AC voltage across D.U.T.
The light bulb limiter, finished.

Things to do and remember:
  • AC mains is lethal;
  • add visual signs to the power strip that it is NOT a power strip anymore;
  • mark sockets with their intended use;
  • filament light bulbs get hot, so do not hurry to change it for the next!
If all goes well, the first troubleshooting session will not trip my home's AC protection.