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T.amp 2400 bias setting

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Frost View Drop Down
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    Posted: 04 February 2022 at 8:08am

I have a T.Amp 2400 where one channel occasionally goes into oscillation upon power on (the two 10 ohm 3 watt parallel resistors in the snubber circuit close to the output relay get hot). Especially if the room, where the amp is located/stored, is a bit to the cold side. Recycling the power switch a couple of times normally get the amp back into expected working condition. Reading elsewhere on this forum suggest that it is a known failure which often is caused by a too low bias setting for the troubled channel and an adjustment to the same value as the healthy channel is the recommended action.

However. Measuring the voltage drop across the 12 ohm emitter resistor on either of the driver transistors for the output power transistors shows 118 mVolt on the healthy channel but 142 mVolt on the bad channel which is kind of the opposite that one would expect. Neither of the channels shows any kind of crossover distortion nor have they any DC offset on the speaker terminals. So does anyone know the recommended bias voltage?

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Frost
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kedwardsleisure View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 04 February 2022 at 12:17pm
it's always possible you have another fault, are the rail switches working? Also check the polyester capacitors in the R-C networks, as they can go leaky. Check the amp 0V rail is intact, it can blow open circuit under certain circumstances and leave the mid point of the amp floating.
Kevin

North Staffordshire

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Frost Quote  Post ReplyReply Direct Link To This Post Posted: 08 February 2022 at 9:17am

Kevin Thanks for the information. I measured the 0 volt line throughout the board and it seems to be ok. But increasing the input signal to a level where the higher voltage rails should kick in revealed that things are not behaving as expected. And the behavior seems to act symmetrically. It seems that if the amplifier - when started - wants to go into oscillation, it happens when the input signal is raised close to the level where the high voltage rails are about to kick in.

If I get the amplifier to a point where it seems stable and I then raise the signal level on the input, it looks like that the rails are trying to open, (roughly 100 v peak to peak - no speaker load attached) but it kind of look like the output signal collapses to approximately to 70 v peak to peak and afterwards increases up towards the level where the rails should kick in but just to collapse again. Those measurements are done at the speaker output.

I use a tone generator for input and the time interval between the collapses varies. The higher the frequency the shorter the time interval.

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Frost

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kedwardsleisure View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 08 February 2022 at 10:55am
the rails should switch in and out whenever the error signal crosses the low threshold and vice-versa. If scoping the output, the switching is normally invisible (some amps do oscillate a little, but AFAIK the t-amp/cmark/audiohead do not and are usually well behaved around the switch point).
You can measure the rails at the fuse holders WRT ground with a simple multimeter which you should see rise and fall as the signal level (continuous sine wave test) is brought up and down.
At all times the rails and output should be symmetrical otherwise you risk triggering the DC protection at the very least and burning the speakers at worst.
I've had the rail switch snubbers (resistor & capacitor) fail on these, the little brown caps go leaky and stress the resistors, although the factory fitted ones can look like they've got rather hot even on a working unit.
Kevin

North Staffordshire

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