Radio Loewe Opta Komet 53

I bought this radio as defect, does not play for some euros. It had no power plug anymore, and my initial idea was to tear it apart to salvage the ancient parts used in the 50ies.

Radio after minor repair work

The wooden case was in quite good condition and all switches and knobs were present. So I decided to check for the issues in the radio and maybe repair it.

Look inside the case. At top left, we can see the socket of the magic eye, an EM4. Top right is the speaker, with some fabric around it. At the bottom left, we see the HF input tube, an EF80, with antenna connector in. 2nd tube from the left is ECH81, then 2 times EF41 and the tube at the right is an power pentode EL41 to deliver the 2.5 watts or so. The arrangement right to the EL41 is the power supply transformator. The big knob in the middle is a connector for a remote control (wired of course). The big wheel is driving the variable cap to select radio stations.

For first test, I hooked up the radio to my isolating adjustable transformer where I can start with 0 volts up to 250 volts for power supply of the radio. If there would be a short or so, I could stop at any time before some serious would happen…

Shortwave fine tuning

All tubes were glowing. So their filament at least works.

Chassis pulled out.

This radio has a turntable input, so I used my mobile phone output to check that the NF amplifier works. It did, and I could hear the music quite loud from the tube amplifier part.

When removing the bottom carton, I found there the schematics. What a nice idea from the old engineers…

Komet 53 schematics. It already contains some kind of modernity compared to pre-war receivers, because it uses a semiconductor rectifier – a SSF B 250/85

The schematics told me that the EL41 is working. So far I had no radio sound and no magic eye. It stays 100% black.

I decided to buy an HF generator for further testing. The device, a WF PG1, is described here. It arrived some days later and need also to be repaired 🙂 . After repair, I could continue with the Komet 53…

When creating a 90 Mhz FM modulated signal at antenna, I checked the anode of EF80 with an oscilloscope. Some signal was there. I repeated that with ECH81 (ok) and first EF41 – no signal there. When switching the two EF41, the signal was there. So I derived from that that the first EF41 was broken. I ordered that tube as NOS from a german electronics seller for about 6 euros, plugged it in, and: sound was there!

The sound was not perfect, I guess the radio was working the first time for 30 or 40 years. But I was proud anyway 🙂

HF/IF section. The tube EF41 at the right was found to be broken

Next thing was to check the magic eye. It does emit nearly no light. This is a very early version, the EM4. Magic eyes do age, and loose their brightness with time.

Magic eye

How a magic eye works is described here. The EM4 is well described here. And there is also described how an aged magic eye still can be used.

So I plugged out the magic eye to check if the tube is working at all.

The first image below shows what can be seen if the tube is attached to the radio set and radio gets 200 volts AC.

A very little brightness can be seen. Only if room is dark.

Next I have connected the tube to my NEVA power supply to test it.

The next image shows the tube when connected to 225 volts and 2 resistors with 1 Megaohms are used.

Tube a t 225 volts plate voltage

And the next image shows the tube at 364 volts plate voltage.

Tube at 364 volts plate voltage.

So the tube is not broken, but has aged.

Here, the filament glowing can be seen

Next I tried to vary the gate voltage.

High signal, lower quarter is 100% and upper quarter is already lets say 20% filled. in this and following image, tube connectors 1-4 are on top
Low signal, lower and upper quarter are not 100% filled

This all looks ok, so I think the tube is a) undervoltaged and b) does not get proper gate voltage on the radio.

WF PG1 Prüfgenerator

East german product. With tube technology. From 50ies. Obsolete, Outdated. Unhip?

The root of all this came from Corona agony. I thought: Why not to look again to tube technology?

Device after cleanup and fixing

Last year a friend gave me an old tube radio as a gift. I love the sound of this small Lowe Opta Bella Luxus and handled it like a religious device, e.g. the Bundeslade.

Time passed and I thought: why not getting another tube radio.
I bought a non working Lowe Opta Meteor 53 from 1953. 15€. An old lady had this thing for ~40 years, all time in defect state.

Looking inside, it was all dust and dirt. It uses an ordinary, poor EF80 in UKW tuner. Wow.

After checking optical tubes (all are glowing) I understood that I really need more support that thing, To do real radio testing, its obvious 🙂 to have these tools in lab:

HF generator, able capable to create AM/FM modulated signal.

An Signal tracer, if possible for HF and NF signals.

Some days later I got the WF PG1 Prüfgenerator from an ebay auction. It is east german technology from early 50ies. 5-235Mhz, AM and FM modulation.

WF means „VEB Werk für Fernmeldewesen“ in Berlin. Gerät must be from around 1952.

Despite many west german people consider east german technology as being poor, aged and poverty-driven, this device may even service as how to do it in the best way. After opening the device and checking it, it is obvious that the engineers were trying to do the best possible with devices and technology from the 50ies, at no cost. Everything inside the PG1 is build like being part of a Panzer. I mean a rugged military device. Build to last as long as possible, maybe forever,

I found the device to be operating inside its original specs after 70 years. I could only test it up to 160 Mhz, because my Rigol 1054 only triggers up to this frequency. But I assume it will work up until original guaranteed 250 Mhz.

After opening case for first time

Only thing that did not work was FM-Modulation. Modulation is done by some EF80. Because I have bought a 140 tube convolut :), I was able to replace the old tube with a new (NOS) EF80. And: FM-modulation was back.

(*NOS=New Old Stock, tubes build dozens of years in the past, so „old“, but never used before, so „new“)

Scheme from manual

A short look at the interesting (at least for me 🙂 ) scheme:

In the upper left, the oscillator can be seen, based on an ECC81 double triode. 12 different frequency ranges can be selected with rotary switch S1. This switch changes the value of a resistor. Inside a selected range, frequency adjustment is done with variable cap C5. There we can read „5 .. 235 Mhz“.
For FM modulation, the input signal comes via W6 (resistor 6) and germanium diodes Gr1-I/Gr1-II.

The oscillators output signal then is an input to the modulator section (upper right). Here we have an ECF82 Pentode/Triode. AM-Modulation comes in via coils Dr1/Dr7 (Dr means „Drossel“, which means coil). I assume the oscillator signal is not only AM-modulated, but also improved/amplified by modulator section.
Because output signal strength varies by frequency range, we have an „Eichen“ (calibrate) pot named RW1. The output signal strength is displayed with instrument J1 and can be calibrated with RW1.

The generator can have an external signal („Fremd-Modulation“) or an internal 1 KHz signal generated by the section in the lower right with the EF80. This is both for AM and FM modulation.

Final section is lower left, power supply, called „Netzgerät NG1“. This is all semiconductor-less. Wow. For all tubes, the heater AC is provided by line transformer in different coils to prevent any interference – nice detail. Plate DC voltage comes from a EZ80 rectifier. The DC is cleaned from ripple with coil Dr1, caps C2 and C3. Tube STR 150/30 regulates the plate voltage to 150 volts (and I suppose 30 mA). I never was aware before that there were voltage regulating tubes.

Power on gas lamp was not working. I replaced this with a nowadays red LED.

Cleaning up the whole device was next. After this it looks quite beautiful again.

After some cleanup
Modulator block at the left, power supply in the middle and at the right

Based on this device, I found that the HF-amplifiying EF41 in my Meteor 54 radio did not work anymore.

I ordered that tube (as NOS) for ~6 € from, replaced it and the radio gives a good sound again -after at least 40 years or so of being defect!

Modulation switch (externally modulated FM, AM(Ton), AM(Video) and 1Khz modulated FM and AM. The instrument is used together with the „Eichen“ knob to somehow calibrate the device
This chain has the size of a bike chain and is used to transfer rotation from hand crank-wheel to the big variable cap used frequency adjustment