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Radio Receiver Valves “REMA 2072” From The 50’S Past Century
About a month ago a friend and client of mine came to my workshop with an old REMA 2072 valve, German stereo and manufactured by 10 valves, EZ81, 2xEL84, 2xECC83, 2xEM84, EAF801, ECH81 and ECC85. Already on the bench for the first test, only the filaments remained warm, but no sound. The 30-year-old equipment stopped in a corner of the house was so dusty that it did not recognize the components on the board.
Receiver seen from behind
I proceeded with the cleaning work, verifying that some tracks of the printed circuit of the amplification section and the intermediate frequency were spent and interrupted. I plugged in again and continued speechless. I noticed that the voltage of approximately 350VDC at the “cathode” output of the EZ81 rectifier valve was present.
The cause was the R159 divider/reducer resistor of 250Ω, 1Watt, next to the main 50mF filter capacitor that was open and responsible for supplying the 230 Volts, 45mA voltage to the auxiliary grids and plates of all valves except the of the EL84 that receive slightly higher voltage.
Once these defects have been corrected and the R119 of 1M polarization of the g1 (control grid) of the ECC83 driver and the 270Ω R136 in parallel with the C151 of 100mF of the cathode of the right channel EL84 output valve, responsible for audio response curve, the amplifier returned to work, but no radio signals.
With the correction done in the printed circuit and the exchange of some components in the steps of F.I., I made the realignment of this with the RF Generator calibrated in 10.7 Mhz and 460 Khz, modulated at 1,000 Hz, respectively.
In post-F.I. is 455 Khz.
With this procedure the F.I. worked and the signals on the two channel speakers were satisfactory. The signal level was evaluated through the “magic eye” of the EM84 valve, in addition to an analog multimeter connected in parallel with one of the loudspeakers.
I captured some stations in SW and MW, after having also corrected the interruption in the coils of the resonant circuits of the antenna input with the completion of the readjustment of the pre-tuning capacitors.
Finally, I installed a new quadrant wire for the Short Wave bands because the former was fragmented.
I added a 0.047mF / 400 Volt polyester capacitor in parallel with the main power switch in order to reduce the “tic-tac” noise caused by opening and closing it.
I added two more polyester capacitors, one of 2,200 pF and another of 4,700pF, both of 400 VAC in parallel with the filter network of the rectifier valve EZ81 for the decoupling of unwanted signals such as noise and other interferences that may arise from abroad.
The “magic eye” signal indication on the left side of the receiver to work capturing an FM station
And so I finished this technical task that came to my workshop on May 26 and came out on June 11 of the current.
This article was prepared for you by Manuel Loto, native of the city of Praia, capital of the Cape Verde Islands, West Africa. Electronics Technician with more than 30 years of experience. Worked with FM and AM transmitters, communications equipment and air navigation. Specializing in Digital Electronics and he also restore old, vintage, transistorized radios, valve radios and power amplifiers.
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