Arkona radio repair and Bluetooth integration

I have a deep affection for Western European tube radios from the 1960s; they always remind me of the one I had as a child. I recently came across this beautiful Arkona Unitra, made in Poland and distributed in Canada by Stertone-Toronto. This unit is a classic 1960s design, featuring an FM section built around an ECC85 triode and an ECL82 output tube. Despite a few hiccups, I’ve restored it to its former glory and even added a modern Bluetooth module. After posting it on Facebook Marketplace, it found a new home in just a few days.

The restoration process

As is standard practice, I replaced all the electrolytic and wax-paper capacitors. However, when I powered the radio up, I noticed an alarmingly high current running through the output pentode. I tried the usual fixes—increasing the cathode resistor and lowering the control grid leak resistor—but nothing worked.

Fortunately, a quick search suggested a potential socket issue. It turned out the socket was a bit loose; after tightening the terminals and reseating the tube, the current stabilized. While the best practice is to replace the socket entirely, I didn't have a spare on hand, so left it as is.

Even with the radio running well, I did a final check of all the control points. I discovered the ECH81 mixer was drifting out of its operating point because the 33K screen grid resistor was dropping twice the expected voltage. The culprit? A leaky ceramic disc capacitor from the grid to ground. These are usually reliable, but this one showed leakage even on a digital ohmmeter. With that final component replaced, the restoration was complete.

Bluetooth integration

With the wide variety of ready-made Bluetooth modules available today, this task initially seemed straightforward. However, a few challenges quickly surfaced:

• Low signal voltage: The output from generic Bluetooth modules is significantly lower than the levels required by a tube radio’s preamp input.
• Ground loop hum: Noise from the Bluetooth module was bleeding into the signal circuit through the common ground.

Luckily, Mr. Caldeira from Portugal came to the rescue with his excellent, detailed guide on how to Add Bluetooth to a Tube Radio with no Noise The build he suggests is based on the MH-M28 module. To solve the signal issue, the circuit includes a TL072 IC configured as a 3x amplifier to boost the signal to the required level. To eliminate noise, a B0505S DC-DC isolator fed by an LM7805 regulator provides complete ground isolation. Mounted on a prototype fiberboard, the module looks great and works like a charm! The three wires going from the module back to the radio are heater 6.3V AC, ground, and shielded output. The output is connected to AUX jack so Bluetooth is engaged by PU pushbutton.