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RE: Dynaco 400 amplifier problems,

Posted by GP49 (A) on February 23, 2012 at 09:18:06

In Reply to: RE: Dynaco 400 amplifier problems, posted by macs348hemi@yahoo.com on February 23, 2012 at 08:59:28:

NOTE that I only bypassed the AUDIO section of PC-29: the filters and the two-transistor stage that serves to raise the input impedance of the Stereo 400. If you do this, the Stereo 400 will no longer be usable with the Dynaco PAS preamps; its input impedance will be too low.

I disconnected the twisted-pair wiring at eyelets 19, 20 and 22 of PC-29 to eyelets 1 and 2 of each PC-28; and ran an extra twisted pair, one signal and one ground, from the input jacks way down on the heatsink, to eyelets 1 (signal) and 2 (ground) of each PC-28.

You could also run that extra twisted pair from eyelets 21, 23 (signal) and 22 (ground) of PC-29 to the corresponding eyelets on each PC-28. Much shorter wiring and easier to do.

This change duplicates the operation of the bypass switch on the Stereo 416, in its bypass position. It leaves Dynaguard in operation; since Dynaguard is out of circuit until actually triggered, disconnecting Dynaguard is not worth the effort if it's working properly. Leaving the inputs connected at PC-29 will provide Dynaguard with a signal to work with.

You still need to fix the protection and relay driver circuits on PC-29, so this is not an easy way around diagnosing and fixing the Q105, Q106 problem.. . unless you want to bypass the relay and depend on fuses to protect your speakers (Dynaco did that on the Stereo 410). I don't recommend it because fuses depend on excess current to blow, and by then your speakers could be fried.

GP49

Edits: 02/23/12 02/23/12

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