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In Reply to: RE: Magnavox 9302-00 Schematic posted by jec@afo.net on December 11, 2014 at 20:09:14
Some fast remarks. Details tomorrow. Photos of the amp will be helpful.
Do you have the tuner/preamp that was associated with the power amp? If you don't, we have to build something. Notice that no listening level controls, whatsoever, are to be found in the schematic. What's labeled Bal. is a trim pot., not a control.
BTW, a "quick and dirty" solution to the volume control issue is to replace the R107/R207 fixed resistors with 500 Kohm log. taper controls and connect the pot. wipers to the grid circuitry.
Eli D.
Follow Ups:
Yes, dual log-taper pot will be needed - 500K is probably best, as the phono stage has a high output impedance. If the 750 Ohm balance control R117 is installed, leave it as-is - if not, you need to add two 390 ohm resistors in its place to use the amp stand-alone. If you have the Molex harness from the console, it can supply the power to the preamp as Eli described - if not, ask around the forums - most haven't used it, and eliminated the connector on the amp. A 5V4 can be used in place of the 5U4, giving slow warm-up and less load on the transformer heater winding.
Edits: 12/12/14
nt
"...the fool doth think he is wise but the wise man knows himself to be nothing but a fool." Will Shakespeare
I'm not sanguine about using a 5V4. Check the data sheet out. Current capability is 175 mA., which should be enough for the power amp, but may be problematic if ancillary equipment's B+ is to be provided. Another thing to watch out for is the small 1st filter cap. value limit.
A 5AR4, like the 5V4, draws only 2 A. of heater current, but like the 5U4, the type is good for 250 mA. Also, the 1st filter cap. limit is greater than that of a 5U4. Therefore, the 5AR4 is the vacuum rectifier type I'd look at, IF a change was to be made.
If either rectifier with a cathode sleeve is installed, B+ has to be taken from pin 8, not pin 2. Both the 5V4 and the 5AR4 will raise the B+ rail voltage, with the 5AR4 raising it more.
Eli D.
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