One day, I got a request from someone who had this old radio that he would like to work again. It had belonged to his uncle who had kept the radio during the war in spite of German orders to turn in all radio receivers. He had listened to it right under the nose of a German soldier quartered in their house. When AM radio became obsolete, it had been stored in the attick. It still worked, but the sound was bad and the tuning mechanism was broken. He knew about my other 667A restoration, so he asked me.
I checked the usual suspects. The coupling capacitor to the output valve had not been replaced. I replaced it by a polyesther capacitor, discreetly wrapped in black tape to make it les conspicuous. The power supply electrolytics had been replaced some time ago. The replacements seemed to date from 1959. I checked the current drawn from the plate voltage when the radio was cold. This was 2 mA. I realised this wasn't necessarily bad because the 667A has a voltage divider that draws some current. I checked the electrolytics and they were OK. I removed the two rf filter capacitors at the rectifier. These capacitors have to endure 350V AC, which often too much for old and leaky paper capacitors, making them heat up and burn.
I plugged in an external speaker and antenna and switched on the radio. First, there was silence. When I fumbled with the band switch, I suddenly got noise. I turned the tuning capacitor and hooray, I received some stations. The sound was muffled, the tone control didn't work except at the end of the turn of the pot, where the high notes came back with a crack. The owner, who had watched me up to here, was very pleased to see these quick results. I told him I would have to do some further work on the tone pot, the band switch, the rf filter capacitors and the tuning cord and more, but that things looked good for his radio.
The next day, I mounted the chassis and the tuning dial to my service brackets. That way, it is much easier to work under the chassis and damage to the tuning dial is prevented. I dabbed the contacts of the band switch with a cotton tip soaked in contact cleaner. I turned the switch to and fro for a number of times. Then I cleaned the switch again with a cotton tip with alcohol. Let it dry and lubricated the contacts with contact vaseline. The switch was much better now. I unmounted the tone control pot and pried it open. Then I cleaned the carbon surface and the wiper, added a bit of graphite spray, lubricated the wiper and mounted the pot again. The result was fine: excellent tone control and no cracking. I had to remove the volume control shaft to unmount the tine pot. While I was here, I also resoldered the shielding plate around the on-off switch and replaced a few missing M3 screws to fix it to the chassis. These seemed to have been forgotten by a previous repairman.
I decided to replace the uncoupling electrolytic of the cathode resistor of the output stage. This is a cylinder of some kind of phenolic material, capped at both ends with tar. These are often dried out, resulting in poor amplification or an unstable output stage. This one did not look very bad, only a single crack in one of the caps. Unsoldered it and removed the caps with a screwdriver. Pushed the inside out (aluminium foil and paper drenched in electrolytic). Actually, it hadn't dried out yet as the electrolytic was still damp. I put a modern electrolytic inside and melted the tar caps again to close the cylinder. Resoldered it. This did not make much difference.
Then I removed the broken tuning cord, lied the two halves on the table and measured its total length. Made a new tuning cord having the right length. Looped the cord round the shaft of the tuning knob and strung it to the large bakelite wheel driving the tuning capacitor. The tuning pointer moved neatly when I turned the knob. I also soldered in two new rf filter capacitors on the socket of the rectifier valve. I used 1000V types suited for mains filters. Wrapped some black tape round the capacitors to make them less conspicuously new.
I replaced two paper capacitors in the tuning circuits. Finally, I realigned the radio. I followed the instructions in the service manual to first align the IF bandfilters. Then I used the 15° gauge to set the 1442 kHz on the 15° position of the tuning capacitor and peaked the input band filter. Then I set the LW alignment. After this, I went to the reiterative process of aligning the pointer to the right frequency on MW band over the whole scale. This involves shifting and turning the steel plate on which the bakelite and brass gears are mounted that drive the tuning capacitor. As I said in the description of the restoration of my second 667A, this is a quite unique mechanical device to align the pointer position over the entire range.
After this, I tried the radio again and was pleasantly surprised by the improved reception quality. I invited the owner to collect the radio and we put the chassis back in its case together.