Ei RR 220-T
Nis, Serbia, the town I live in, used to be well known for electronic and machine industry. “Elektronska industrija” (Ei) used to employ tens of thousands of people and produced everything from vacuum tubes to tank and plane parts. Unfortunately, it’s all gone now, Ei is bust, nothing is made anymore. On my quest to experiment with a wide range of vintage germanium radios and convert them to guitar amplifiers, I stumbled upon a broken old leather radio made here and bought it cheaply. Model name is RR 220-T. The only schematic I found references Telefunken OC604 and OC604 Spez transistors in the audio amplifier stage. This one had Ei-made AC542 (gain and driver stages) and AC550 (output stage) transistors, also made by Ei. My best guess is that it’s built between 1955 and 1960.
I originally planned to gut it out and use components, maybe (part of the) PCB, but when I got it I felt it would be better to try to restore it and keep the vintage radio look. Nothing was broken, but it was very dirty. I took it apart, scrubbed all plastic parts with soap and water and cleaned the leather with leather cleaning wet wipes. The metal “Tranzistor” logo was very dirty and corroded. I removed the corrosion by dipping it in the vinegar/salt mix followed by baking soda diluted in water to prevent the acid from corroding it further. Then I polished it with very fine sandpaper to bring back brassy shine. Altogether, it got a cool retro, relic look.
Circuit Restoration & Changes
Most radios from this era are rats’ nests as far as component layout and wiring goes, but this one is reasonably well made. The circuit board is mounted on the steel chassis that carries most of the heavy components (only driver transformer is board mounted). The whole assembly can be taken out of the enclosure relatively easily, after few wires are cut (took a photo before so I can go back and verify which wire goes where).
As expected, majority of electrolytic capacitors haven’t survived after more than half of the century and had to be replaced. I put the same capacitance for the most part, but in some cases replaced with the closest available value. I used a mix of Nichicon FW, Panasonic and Vishay BC, whatever I had in my parts bin, but all the good stuff. All the replaced capacitors are highlighted in blue on the schematic. Actual value for C34 in my unit was 10uF, not 100uF as shown in the photo. I replaced it with a 15uF cap I had.
With all other radios I’d just cut the circuit in half and use the amplifier section of the radio. In this case I wanted to keep and restore as much of the original functionality of the radio while still being able to plug in the guitar and to take the output to guitar cab instead of internal speaker if I want to. To accomplish those two tasks, I used switching jacks that break the circuit when plugged in, as shown below.
I split the circuit downstream from the 10uF capacitor that goes from the volume pot (green highlight), but it left me with DC on input lead causing crackle when guitar volume pot is used, so I added another 3uF poly film capacitor in series and placed the input jack between the two capacitors. I could’ve just moved the jack between the volume pot and 10uF capacitor for a simpler solution, but both work.
As a simple solution for mounting the jacks, I widened three of the venting holes in the leather back and installed jacks through them. One of the jacks is for DC power, so I can use my variable regulated power supply.
Another cool thing about this radio is the tone switch (marked “Ton”) which doubles the shunt capacitor on the first audio stage and cuts some more treble. It’s highlighted orangey-red in the schematic.
The next photo shows how everything’s wired. I used a shielded wire (extra from a 4-conductor guitar pickup) for input to try to reduce the noise.
As a guitar amp, it’s a pretty cool sounding little thing. It’s surprisingly loud both through the internal oval speaker and especially through a proper guitar cab, louder than normal TV level. There’s some mild creamy crunch without sounding fuzzy, reminiscent of vintage tube amps.