Waldorp 120 (1940/41)

Somewhere at the start of 2006 I saw an advertisement for this radio. I found it interesting because it was a battery-operated radio. I was able to buy it at a reasonable price and started to venture into the repair.

The Waldorp model 120 has MW, LW and SW wave bands. It is a rather common superhet with an IF of 473 kHz. It runs on batteries:

• A 2 V (storage battery) or 1.4 V (dry) battery (a resistor is added in the filament circuit if a 2 V battery is used as the filaments only need 1.4 V)
• A 90 V battery for the plate voltage.

The output transformer is remarkably large for the 170 mW a DL21 output penthode can deliver. Maybe this is to improve efficiency.

An interesting feature is the chassis that is completely made of Pertinax (phenolic board). I don't know if this is for reasons of wartime shortages of metal or for a different reason. All the valves are "red" Philips types on Octal base:

• DK21 octode as frequency changer
• DF21 penthode as IF amplifier
• DAC21 as detector and AF triode amplifier
• DL21 as output amplifier (well, this one isn't red)

Waldorp was a smaller radio brand, settled in the city of The Hague. They did not have much electronic development of their own but they used radio designs by others. Because Philips owned most of the important radio patents in The Netherlands, Waldorp, like many other smaller Dutch radio manufacturers, was very much under the power of Philips. The Waldorp 120 has two "Philips License" stickers on the back. It is almost identical to the Philips 614B. I happened to have a circuit diagram of that model, which helped a lot.

The radio looked like in reasonable shape when I got it. I hoped the valves would still be OK. But when I connected 2V and 90V power supplies, there was no sound at all. There were currents flowing rom the power supplies, although a bit too low. I took the chassis out of the case and examined the circuits. Everything seemed fine. However, when I measured the filament voltages, they were too low. I followed the filament circuit and found out that there was a 0.8 V voltage drop across the on/off switch. I cleaned the switch and got slightly better results. I tried the radio with slightly raised filament voltage and finally I could receive some stations. Apparently, the oscillator circuit wouldn't function if the filament of the DK21 was too cold. I checked the components in the oscillator, but still couldn't find why it wasn't working. The DK21 needed 1.5 V to oscillate while 1.4 V is specified. I suspected that it had bad emission so I tried a different one.

After this, the radio worked OK. I put it on a shelf and admired it from time to time. Using it wasn`t very practical.

In 2012, I traded it on the NVHR radio swapmeet.