A SWEET OLD RADIO!
(2020)
KLIK HIER VOOR DE NEDERLANDSE VERSIE
A sweet old radio!
A sweet old radio!
A sweet old radio with tubes. That radio needs to be repaired!
I immediately thought of Alex, who I knew when I was a student. Alex loved radio tubes! It was very unusual that such a young person liked those antique radio tubes so much! I wanted to shoot those tubes with an air rifle... Alex taught me a lot about radios with tubes. How to fix it with a simple voltage tester screwdriver with neon light! I thought that you needed expensive measuring equipment for such a repair! And a poor student has absolutely no money for expensive measuring equipment! But even a poor student could buy such a cheap, simple voltage tester screwdriver with a neon light! Awesome! It was now possible to make a lot of money with repairing tube radio's! But they weren't that many radio's with tubes anymore, those repairs didn't make me rich...
Usually you can fix a tube radio with a simple voltage tester-screwdriver with neon light!
Simplicity is often better!
Can I fix this radio just like Alex used to do? Very sober just barefoot with just a simple voltage tester screwdriver with neon light? That works much faster and also much easier than expensive measuring equipment! This allows you to quickly check whether there is a high voltage in the radio. The brighter the neon lamp lights up, the higher the voltage! And when you touch a grid with it, the radio should "hum". This also works with the grids of the tubes in the high-frequency and medium-frequency part!
Alex always walked very sober and simple barefoot! Just like me, Alex loved pure simplicity. Why should you wear shoes when you can walk barefoot! Walking barefoot feels better, is healthier and cheaper! So as a student I often walked barefoot! And barefoot won't damage expensive computers by static electricity! Indeed, simplicity is often better!
What an exciting challenge! Very sober barefoot on the ice cold stone floor of my
radio lab using only the simple voltage tester screwdriver as a measuring instrument!
The radio is a Philips Iberica BE-341-A
The radio is a Philips Iberica BE-341-A with medium wave and two short wave bands. Two shortwave bands? The first covers the bands 16 meters - 50 meters. And the second a small part that can also be found on the first shortwave band, the 20 and 25 meter band. I have no idea why... Maybe that was done to achieve a better band spread and easier tuning of the 20 and 25 meter band.
There are five tubes in it: ECH81 mixer and oscillator, EBF80 middle frequency of 452 kHz of which the diodes are not used, EBC41 rectifier and low-frequency preamplifier, EL84 as power amplifier and an EZ80 as rectifier tube for the high voltage. Why such a "40" tube between those more modern "80" tubes? I really do not know...
And it must have gone wrong here. It is not for a 220 volt mains supply, but 125 volts! It was probably found by someone in the attic and connected to 230 volts instead of 125 volts...
First clean and see if anything is damaged. It was not very dirty, cleaning was done in a day. And there was nothing special to see, everything seemed fine.
What an exciting challenge! To repair that radio, very simple with only the simple voltage tester screwdriver as a measuring instrument!
There was nothing special about the radio, everything seemed fine.
There was nothing special about the radio, everything seemed fine.
For such a repair, You have to take your time and do it carefully to avoid unnecessary damage! First take a good look if there is anything unusual to see. There was nothing special about the radio, everything seemed fine. Exciting! But those black capacitors remain suspect, and also the electrolytic capacitors. Usually there is something wrong with them.
A light bulb in series with the mains connection limits the current when there is a short circuit in the radio!
Current protection
That simple voltage tester screwdriver, what a beautiful, handy thing! I could also use it to remove the back and bottom of the radio! And remove dirt! And to clean my toe nails!
You can't just switch on such an old radio. I have made a circuit with which a light bulb of 40 or 60 watts is connected in series with the mains connection. When there is a short circuit in the device, the current is limited by the lamp. You can see if there is a short circuit, then the lamp lights up brightly. When all is well, the lamp lights dimly. And then you can turn off the lamp with the switch.
The EZ80 rectifier tube was replaced with two diodes and a 100 ohm resistor
Broken EZ80!
Exciting! The radio switched on and... The lamp lighted dimly, so there was no short circuit. But it didn't work! The test with the simple voltage tester screwdriver with neon light showed that there was no high voltage! First take a look and the problem was clearly visible. That's why tubes are so beautiful, the filament of the EZ80 was broken, it gave no light. A new EZ80 was expensive, but there was a cheaper, excellent solution. The tube can be replaced by two 1000 volt diodes and a 100 ohm series resistor. I am using two BY228 diodes here. An EZ80 has an internal resistance, which is simulated by that 100 ohm resistor. The parts can be soldered to the back of the tube base and the original EZ80 can just stay on its place! But... unfortunately the radio didn't work yet! And... the series lamp for the current protection is now lighting very brightly, so there is a short circuit in the radio!
The capacitors were defective and caused a short circuit. But there was space enough for two new capacitors.
Faulty capacitors!
What causes the short circuit? The wire after the smoothing capacitor was disconnected, so only the rectifier and the smoothing capacitor were still connected. The smoothing capacitors were the problem! They were broken and caused the short circuit. Would the radio once have been connected to 230 volts instead of 125 volts? And would the capacitors therefore be defective and cause the short circuit? I will never know. But there was still space for new capacitors, the modern capacitors are much smaller and could be glued to the bottom of the chassis between the other parts. There were still a few capacitors among the old stuff I once had gotten. The original capacitors remain in place, so there is nothing to be seen of this repair at the top.
The lamp is still lightning brightly, there is still a short circuit!
What now...
Again the lamp lights brightly... What was the defect that caused the short circuit?
It was a simple problem but not that easy to find. Where is the short circuit? It could be anywhere. Is it a tube? Is it a faulty capacitor? Many components were disconnected, but the lamp continued to light brightly, it was not a capacitor or tube. I was desperate, it was impossible to find the fault... I had been busy for a few evenings... All wires were disconnected and... finally I found it! In an invisible place, the insulation of a wire was damaged and this caused a short circuit. But not always. It was hard to find this simple fault! It was a manufacturing defect! And now that the wiring was old and less flexible, it became a problem. Who expects to find a factory defect after 50 years!
A factory defect! The insulation of a wire at the arrow was damaged and caused a short circuit.
The invisible killer of the final tube!
The radio is now playing fine and you would think everything is okay now! But there is always that invisible fault. That's the invisible final tube killer! It is the coupling capacitor between the low-frequency amplifier stage (usually a triode) and the output stage. This capacitor must have an extremely high leakage resistance, so it has to be of a very good quality. And you can imagine they couldn't make it up as well in the past as they are now. And that such a capacitor has a much lower leakage resistance after so many years. Even a very high leakage resistance of 50 Mohm can cause too much current through the final tube by 20 ma, it will become too hot and shortening its life enormously! We're going to short the grid to ground. Of course with the simple voltage tester screwdriver, that's the only tool we can use for this challenge. A loud click could be heard from the loudspeaker. So that grid has too much positive voltage due to a too low leakage resistance of the coupling capacitor!
An additional 1Mohm resistor and a 0.1uF capacitor. Why? You can read that below!
I measured that with this radio there was already 3 volts positive voltage on the grid, caused by the leakage resistance of the bad coupling capacitor. Indeed, I used a simple, cheap universal meter, bought at the supermarket! But that was allowed, it was not for the repair. For this article I also wanted to note the real value.
But the solution was simple. After the black coupling capacitor, an extra capacitor of 0.1uF to the grid of the power tube and a resistor of 1Mohm to ground. No more positive voltage on the grid, problem solved. For example, if the voltage is reduced 100x with 1 RC combination, then that is 10,000x with 2 RC combinations of the same, not too good quality. Then you have no problem, even with outdated, bad coupling capacitors! They should have put these extra capacitors in all those tube radio's! Because all those old coupling capacitors have this problem! But that costs a little more money and money was also very important in the past...
An additional 1M ohm resistor and an additional 0.1uF capacitor reduce the effect of the leakage current
from a bad coupling capacitor and unwanted positive grid voltage caused by it!
Now the radio was back to 100%. The reception is very good on medium wave and short wave bands. The sound also sounds excellent. The radio could go back to the owner. And he uses it very often! Because that medium wave sound still sounds very special, especially very nostalgic! And that radio is so sweet and beautiful, because it is so ugly!!!
It worked!
Very sober with just a simple voltage tester screwdriver with neon light it was possible to repair this radio! Even faster and easier than with expensive measuring equipment!
And Alex is right! Walking barefoot is healthier, cheaper and feels better! Even on the ice cold stone floor of my radio lab! But I certainly had to get used to that!
Index PA2OHH