Panasonic R-12

I stumbled across this cool little radio and bought it for around 10 euros shipped. Designed like a retro speedometer, it’s probably the most interesting looking radio I’ve got so far. The one I got was in decent condition, showing little wear here and there, but nothing was broken or missing.

The chassis and layout design is clever and neat. The whole circuit is squeezed onto a circular PCB together with the scale and control pots making it easy to disassemble and service. There aren’t too many wires flying around and PCB layout is good. Only one underside component and no jumper leads. The whole amplifier part of the circuit is arranged on one half of the board. One thing I found odd is that they put input lead right next to the output lead, probably to make wiring neater. Usually I try to avoid this, but in this case it didn’t introduce noise or feedback.

It features a 6 transistor circuit with only 3 transistors in the audio amplifier section, which means that it has one gain stage that amplifies input signal and drives the interstage transformer. That shouldn’t be a problem, as we can always hit it with higher signal level from a booster or overdrive pedal. Looking at the schematic, it’s a pretty standard topology for that era. Two transformer push-pull design, temperature controlled bias for output transformers, 2SB175 and 2SB176 transistors. All standard. I was puzzled by the way grounding was done. They put a 220ohm (R15) resistor between the output stage and driver stage, but they put it in the ground path (red line in the schematic), not in the negative voltage rail (green). That means that input ground and output ground are not on the same potential, but all negative voltage points are. So they are effectively making a negative ground circuit using positive ground transistors. Usually it’s the other way round.

Without even trying it I went ahead and replaced the four electrolytic capacitors in the amplifier section of the circuit. Input capacitor is 1uF electrolytic. I usually replace them with a nice Wima red box poly film capacitor, but this layout is so tight, there was only room for a compact electrolytic capacitor, so I put a Sprague 1uF cap I had in the bin. It’s axial, but I twisted one of the leads to make it fit. The remaining three 33uF capacitors are replaced by Nichicon FW 47uF.

I removed the tiny 2.5″ speaker and drilled two holes in the back for output jack and DC jack. There was a jack on the side for headphone. I removed it, widened the hole and installed the input jack there. Volume pot is not in the circuit, but I left the switch part of the pot in, so I can use it to turn the amp on and off.

Leave A Comment

  • About

    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.