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Dynaco 400 issue popped up last night..

Delta17

Delta17

Super Member
Last night the Dynaco 400 gave up the right side, and this morning the left side is gone to.

Swapped out with NAD 2200 and the signal is going to the speakers.(though thin and yucky...I digress.)

The fuses appear fine, same cables, is this a known issue?

I am not a tech. The Amp was gone over a year or so ago. I do use it a lot.

Might heat be an issue over time? It has room for circulation, though no fan.

I do have the giant colored diagram. I can locate various components.

Been wanting to add a faceplate with meters...May be my excuse to learn more and do that. Bt that would be another thread.

I am at a loss here. Any info is greatly appreciated.
 
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Every 400 I've seen which failed had one of a small number of modes.

Check the driver bias resistors R228-233; if one or more of them open due to heat cycling, the channel will go dead. I've seen them fail in such a manner that you can't even see a problem but when you wiggle them, they're split in half.

Also check R235-238, the driver emitter resistors. Not as likely to die, but any one of them could silence the channel. R301-302, the output emitter resistors are even less likely but possible.

A quick first check is to remove the 4 power supply fuses and measure from the amp side of the fuse blocks to ground. It should be almost exactly 3000 ohms for each of the 4 test points. If any of them are far off, it's likely something like resistors and transistors in the output stages; otherwise, something further up the chain and harder to isolate.

Page 27 of the manual gives more detailed tests but they're primarily intended for initial testing after building the kit. They may be helpful if none of the above yield any results.
 
If it helps anyone out my problem was shorted q7 and open r24 on pc28. This is a 10 ohm resistor. These are the driver power resistors where the b+ comes in and if the power supply has a shorted cap or diode like mine did it will cause too much voltage to the driver board making these components fail.
 
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