When entering the are of old tube receiver repairment, I found that a signal tracer device is crucial. With this device, you can trace a signal in the stages of the radio device and identify the stage that is not working anymore, so it is a great tool to troubleshoot radio receivers.
Signal tracers are not produced anymore, I think they passed when receivers and amplifiers were designed with ICs, where you have multiple or all stages of an electronic circuit in a single chip. Then a signal tracer is if no help anymore. So, usually a used a signal tracer usually must be found.
ebay is your friend here and after some days checking, I got the Grundig SV41. Grundig is a famous name in germany, but today chinese-owned as far as I remember. The signal tracer is from the 70ies I guess.
It can be operated from battery or external power supply at 9V=. My device showed dead and leaked batteries in the battery pack section. The external power supply connector was broken, maybe during delivery. I replaced the external power supply and cleaned the battery pack.
The device offers a 1 Khz signal generator with attenuation -10 .. -60 dB, an amplifier, a speaker and a panel meter. It has connectors for generator output and signal input.
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.
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.
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…
All tubes were glowing. So their filament at least works.
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…
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 🙂
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.