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Magnavox 9304 / Imperial Update

E

energumen

New Member
Thanks for all the advice in the other thread, last night I got the amp up and running, and it sounds great!

I replaced all the electrolytics with Panasonic caps in the amp, radio/preamp and mpx. Added a few terminal strips for them to sit on.

For safety, I replaced the filter cap that bridges the line in with an X1/Y2 cap, and added a 1.5 amp inline fuse and a new cord with a polarized plug.

Also upgraded the coupling caps in the amp from the stock .0015 / .047 to .01 and .1 per some advice elsewhere on this site, which gave the sound a big boost. Used IC film caps here. Maybe should have gone with polypropylene instead of polyester, but it is still pretty quiet.

Checked all the resistors and replaced anything 10%ish out of tolerance. I ended up using 1 watt carbon films that I got from a local part house.

When it all went back together, Channel 1 was really quiet, which I traced back to the 220k resistor on pin 6 of the 6EU7. It was giving the right ohm readings, but measuring the voltage on either channel showed 180vdc for channel 2 on pin 6 and only 150vdc on channel one. Replaced this resistor and the channels are now nicely balanced.

Some of the advice in the other thread suggesting bucking the power transformer, but I am getting 6.5V AC on the heaters, so I think I can leave well enough alone here.

All and all very happy with the results of my first restoration! If you guys have any advice on other things that could/should be done I'm all ears!

Now to get the turntable working....

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Thanks! it was a lot of work but the results are nice. There is still a noticeable amount of hum from the amp after it warms up, but I'm not sure what else I could do for that. I got the S600 turntable working again, the idler wheel arm was frozen and preventing the wheel from engaging the motor shaft. Some lithium grease solved that. Now if I get really excitable I may peak the radio and mpx. Not sure my little toy oscilloscope is up to the challenge though :)
 
try tweaking the hum pot, it may help. these amps are usually very quiet though, so it may be worth unhooking the rca jacks to see if the noise stops. if it does, the problem is not in the amp.
 
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