Repair of tone control circuit in old transistor radio

6 Nov 2017

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I have an old radio ITT Schaub-Lorenz Touring International 103 which had small issue with bass control circuit. Issue was caused by "cold joint" around the potentiometer used for bass regulation.

Here are steps of the radio disassembly:

1. Remove the knobs from top panel.

2. Unscrew all screws from rear panel.

3. Unscrew all screws from top panel.

4. Remove rear panel.

5. Unscrew two screws located near the handle.

6. Remove front panel. Be careful because speakers are connected with wires to main board.

Debugging part.

After short analysis of part of schematic related to tone control circuit and quick measurements of continuity I have found that issue was caused by "cold joint". There was no connection between of R334 potentiometer and C388 capacitor. The "cold joint" was marked by red square on one of following pictures.

After re-soldering of this joint, the issue was fixed. Here is short video of disassembled radio.

Here is short video after issue fix.

Top Comments

Parents

shabaz over 7 years ago

Awesome repair log. Thanks for showing the board detail too, it was nice to see the little touches, the designers had fun : )
I have a (far less exotic) pocket radio to repair sometime, it has some noisy slider volume controls : (
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e14phil over 7 years ago in reply to shabaz

What a beautiful PCB, is anyone aware if the 3 spirals on the left have any function other than artistic flair?
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shabaz over 7 years ago in reply to e14phil

Hi Phil,

I'm only guessing, but probably it is partial artistic flair : ) and maybe partially acting like an inductor, it was common to put a very tiny ferrite bead on a transistor leg, if there was a risk of it oscillating. I can't tell if this is on a transistor leg, but it is possible the designers saved the ferrite and made the pattern instead. Since there are three of them, it could be a high gain RF amplifier portion maybe.
I quite like the octopus above it : ) and the inverted earth symbol or Xmas tree : )
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shabaz over 7 years ago in reply to e14phil

Hi Phil,

I'm only guessing, but probably it is partial artistic flair : ) and maybe partially acting like an inductor, it was common to put a very tiny ferrite bead on a transistor leg, if there was a risk of it oscillating. I can't tell if this is on a transistor leg, but it is possible the designers saved the ferrite and made the pattern instead. Since there are three of them, it could be a high gain RF amplifier portion maybe.
I quite like the octopus above it : ) and the inverted earth symbol or Xmas tree : )
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