155
Item nr.
The Shortwave listener's pride of the turn of the century.
Production | The Netherlands, 1949.
Price was 385 guilders (later 360 guilders). |
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Bands | LW (715-2000m) MW (185-580m) 4xSW (32-50.5m, 21.5-32m, 17-26m, 13.5-20m). |
Tubes | ECH21 EAF42 EAF42 EBL21 AZ1 EM34. |
Cabinet | Wood. Size 50x34x24 cm. Weight 10,5 kg. |
Power | AC 50W. |
The radios offer the same amount of air wave surfing. Wow, to sweep the dial with the flywheel tuning knob! The nice deep sound with these basses! The precision of tuning achieved with the bandspread!
Obtained | 4/2000 from Marcel Sidler. |
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Condition | 9. |
Disposed | Sold 10/2000. |
There was motorboating and the B+ voltage was way too low, so I suspected the filter cap of leaking (I had just experienced this fault a few days before in a Perfecta tube portable) and replaced it, but no result. A spare AZ1 rectifier brought the B+ back to the normal 240V, but the motorboating and RF oscillations remained. Also replacement of several decoupling caps in the HF part did not solve the problem.
I posted a question on the Dutch Forum voor Oude Radio's and got an excellent explanation from Nico den Haak. Philips used in this radio a somewhat rare way to obtain negative bias for the control grids, namely over a resistor in the B- supply. Indeed, the center tap of the power transformer is not grounded directly, but through a resistor which drops about 6 Volts, smoothed by a 100uF capacitor C3. The bias for the HF tubes (mixer and IF stage) is also taken over this resistor. When C3 (an electrolytic) dries out and looses capacity, the HF current through the HF tubes generates an HF voltage over the dropper and this is fed back to their input, causing oscillation. Replacement of C3 cured all my oscillations. Thanks Nico! (If you ever read this.)