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orbanp1 · 6th Mar 2019, 2:07 pm

Hi Sebastien,

The last four transistors in that amp are DC-coupled, so a fault anywhere in that part of the circuit will result in that changed common emitter voltage.

Measuring the components when cold will not work as the radio works when cold.

Strategy is to replace components!
It is also a good idea to monitor/limit the total current uptake from the power supply so that you would not fry anything.
Freeze spray might work to find the faulty component.

Simulation has shown that a leaky C276 (or C212, depending which schematics you look at, the 1.8nF cap) would result in the increased common emitter voltage.
As Livewire mentioned, a leaky output capacitor (C277, or C213, 800uF) would not result in an increased common emitter voltage.

So do change C276, do change both Si driver transistors, V207, V207 (or V203, V204), you should also change that thick film resistor module, by making up a similar module from individual resistors.
I take the other components you have already changed in that part of the amp.
(My bet would be a flaky thick-film resistor module, though I am not a betting man ;-)

Also, here is a good article on repairing audio amplifiers:
http://sound.whsites.net/troubleshooting.htm

Good luck, Peter

6th Mar 2019, 2:07 pm	#24
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: '71 Blaupunkt E Hi Sebastien, The last four transistors in that amp are DC-coupled, so a fault anywhere in that part of the circuit will result in that changed common emitter voltage. Measuring the components when cold will not work as the radio works when cold. Strategy is to replace components! It is also a good idea to monitor/limit the total current uptake from the power supply so that you would not fry anything. Freeze spray might work to find the faulty component. Simulation has shown that a leaky C276 (or C212, depending which schematics you look at, the 1.8nF cap) would result in the increased common emitter voltage. As Livewire mentioned, a leaky output capacitor (C277, or C213, 800uF) would not result in an increased common emitter voltage. So do change C276, do change both Si driver transistors, V207, V207 (or V203, V204), you should also change that thick film resistor module, by making up a similar module from individual resistors. I take the other components you have already changed in that part of the amp. (My bet would be a flaky thick-film resistor module, though I am not a betting man ;-) Also, here is a good article on repairing audio amplifiers: http://sound.whsites.net/troubleshooting.htm Good luck, Peter Last edited by orbanp1; 6th Mar 2019 at 2:19 pm.