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In Reply to: RE: Magnavox 9302-00 Schematic posted by Eli Duttman on December 12, 2014 at 11:40:48
Here are a few pictures. As you can see, it is a project waiting to be done. Resistors hanging loose, etc.Thank you,
Jim
Edits: 12/13/14 12/13/14 12/13/14 12/13/14 12/13/14Follow Ups:
First it is obvious that someone tried to upgrade it albeit rather sloppily. The first thing I would do is convert the grounding system to a star ground system using a solid, tinned copper buss bar wire of about 12 awg. This buss bar would be connected to the chassis at only one point (preferably the large can filter cap where you see the splash of solder to the chassis). This is the star ground. The 110V AC green ground lead, the CT of the power transformer, the speaker common of the output transformer all connect to this point. The buss bar would travel from the input jack to the large can filter capacitor which would be the only ONE ground point on the chassis. All grounded resistors and capacitors would connect to this buss wire throughout the amp instead of the chassis here and there. I would insulate the input jacks using Switchcraft 3505F or similar input jacks and a nylon shoulder washer on one side and a flat washer on the other side of the input jacks. The ground stud would connect to the buss bar wire. This would make the chassis a SHIELD not a conducting ground. I would install a Cinch 141Y bakelite connector as speaker output connector. M.B.
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
Hello all,
I am trying to get a feel for all the great tips you gentlemen have provided and determine how to go about implementing them. Seems like it will be a challenge but I hope to learn a lot in the process. Thank you.
Jim
The remarks M/B made about grounding and shielding are spot on. "Single" point grounding (SPG) goes a long way in reducing residual hum. Two proven methods of achieving SPG are "star" and buss. Either way, the idea is to keep the large currents close to the unitary point of chassis contact. Sensitive, "low" level, circuitry is kept away from the hum sources.
Eli D.
Jim,
That is a ROYAL mess! It is painfully obvious that the unit has been gone through, at least once, and that the workmanship is/was poor. ;> (
The need to (IMO/IME) completely rebuild is an opportunity. ;> ) We can employ better circuitry than that used by Magnavox. Several possibilities come to mind. The bulk of my thinking revolves around SS rectified B+ and combination biased O/P tubes. Combination bias derives the total grid to cathode negative potential from a RC bias network and a C- supply. SS rectifying the B+ frees up the 5 VAC filament winding, which will be voltage multiplied to generate the C- rail.
Are you willing to work with me and other "veteran inmates"?
BTW, the O/P transformers are on the small side, which hampers obtaining full bass extension. That can be dealt with, by high pass filtering at the inputs to the circuitry. I don't think you will have problems listening to a double bass. OTOH, don't try recordings of 32 foot organ stops.
Eli D.
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