telefunken: 2284; Concerto

I recently purchased a 2284 concert telefunkem an antique shop, did a little restoration and is already working. But there's a problem, which is as follows. In order to be working some minutes, maybe 5 to 10 minutes, the audio quality will be lost, and begins to distort as much as we increase the volume and the lower it is without quality. It replaces each capacitor in the circuit that is one of the valves ecl86 because they were damaged as a resistance. Do the valves are tired? This symptom is noted in both FM and AM. I ask for help for this machine to work 100%.

greetings

Nuno António

To thank the Author because you find the post helpful or well done.

Dear António,

first of all you should be sure of stability in supply and bias voltages during the warm-up.

You should check both the B+ supply voltage and the grid biasing of the output tubes when the radio starts to operate and then repeat the measure after the warm-up. Unfortunately the readability of the schematic diagram for this radio is very poor. Anyway you should read approximately the values given below.

- B+ voltage, measured on the aluminum capacitor in the bottom-left side of the diagram, should be around 230 volts.

- To measure the grid bias voltage you should use a multimeter with high input impedance, say 10 Mohms. You should read zero against chassis ground on the pins 8, pentode control grid, and some +6.2 volts on the pins 7, cathode, of both ECL86 output tubes.

In normal conditions the above voltages should not be significatively influenced by warm-up. Use the greatest care when measuring the B+ voltage.

Regards, Emilio

To thank the Author because you find the post helpful or well done.