R Telefunken Sonata U1825

This is the first part of the repair of a Spanish Telefunken Sonata U1825 radio. This is a miniature series universal radio with tubes. Initially, the on/off switch contact was bypassed so that it remained permanently on. After doing this, when the radio was switched on, there was no sound from the speaker. It is quite common that even with the filaments functioning correctly, the 35W4 rectifier remains isolated, meaning it cannot transfer electrons from the cathode to the plate. The rectifier was checked and found to be faulty; it was replaced with a new one, the electrolytic capacitors were changed, and the radio was restored to working order. It was observed that while the radio was operating, the volume gradually decreased, and it seemed to revive when the phono stage was touched. Upon inspecting the condition of a 220K capacitor in the speaker-volume potentiometer feedback circuit, it was decided to replace it with another 220K capacitor rated for over 1000V. It was also deemed necessary to replace the coupling capacitor between the plate and grid of the output lamp, which was 10K and was replaced with a 9.2K capacitor rated for over 1000V. With all these changes, the volume seemed stable and no longer degraded. The dial wire was replaced, which proved to be a very simple operation since the old one was already in place, albeit badly degraded. When the keys were reattached, the radio went silent, and despite adjusting the tuning switch and all the tubes, the radio did not come back to life. Analysis suggests there is a problem in the radio frequency amplification stage, as the beep of a 462 KHz modulated signal sounds when connected to the detector board but does not sound when connected to the grid of the radio frequency amplifier lamp.