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The turn-on issue was caused by a severely worn-down power switch. The contact surfaces had literally vaporised and disappeared from switching that 1,3 kVA transformer for thirty years. At least not without giving it a helping hand. A helping hand in the form of a V relay. I even went the extra step and added a capacitor across the coil in order to make the life of the switch easier.
Testing the relay circuit animated GIF — clickit! The hot-glued, PVC-taped relay board is simply placed loosely behind the front panel. This is in order to reduce the start-up current somewhat. Starting with a re-cap. The cause for that was two-fold and somewhat unexpected. Short-circuiting C is equivalent to turning the bias down to zero. The affected channel had been serviced before and a lot has been replaced. The power supply rails in the P add up to V; V transistors have no place in such an amplifier.
It may work, but any safety margins intended by the designer are lost. For shame. Anyhow, with that bitter-sweet story out of the way, only the cleaning-up remains.
The only things remaining to do is to give the incompetently serviced channel another overview in the future, replacing driver transistors and of course outputs. One of the thermal indication thermostats has also failed, so I might replace that some time in the future. Thank you for visiting, I hope you enjoyed reading about this project as much as I enjoy listening to it!
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