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RE: Dynaco 400 amplifier problems,

Posted by GP49 (A) on February 21, 2012 at 08:54:58

In Reply to: Dynaco 400 amplifier problems, posted by daveyhockett on February 21, 2012 at 05:50:18:

You can, of course, assume nothing about an unmarked part that is just "hanging there." If you do some basic testing on the transistor and can verify that it is not failed and is a PNP type, your chances are better that it is the right one.

Q106 is part of the relay driver, half of a Darlington pair with Q105. In normal operation the relay driver grounds the low end of the relay RY301, allowing current to flow through the relay coil, and thus closes the relay to connect the power amplifier to the loudspeakers. If there is excessive DC sensed at the output of the power amplifier, the relay driver circuit senses this and shuts off, thus opening the relay and disconnecting the loudspeakers to prevent damage. The threshold for relay operation in early Dynaco Stereo 400 production was ±0.5 volt at the output of the power amplifier. Later it was increased to reduce premature triggering of the relay circuit at high output levels.

I would suggest checking the amplifier WITHOUT speakers connected, first by the Preliminary Tests on Page 27 of the Dynaco Stereo 400 manual. If it passes those tests, continue to "No Sound Output" on the same page. If you get it powered up, especially check the DC voltage at eyelets 10 and 11 on PC-29. These are the centerlines of the complementary-symmetry output stage (eyelet 10 right channel, eyelet 11 left channel). If both are not close to zero volts DC, you have another problem besides that "hanging" Q106, and it's probably in the power amplifier.

I am thinking that someone may have messed with that Q106 because the relay wasn't closing. But the ACTUAL reason for the relay not closing to begin with would more likely be an output stage failure.

GP49

Edits: 02/21/12 02/21/12 02/21/12 02/21/12 02/21/12

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