Magnavox 8803 Upgrade and Troubleshooting

punkzter

Brad
I purchased a Magnavox 8803 recently. I powered it up yesterday, and also hacked together the 390 ohm resistor update. It has almost no gain, and what I hear is distorted.

I've hooked it up to my signal generator and to my scope, but the best example I can give is that, hooked up to my phone, I have to turn the volume on the phone all the way up, and even then the volume is very low.

I did note that the voltage on pin 7 and 6 of the 6EU7 tubes is much lower than it should be. Around 105 volts instead of 150/170.

Could this be the culprit? And, if so, is the next step to replace the capacitors in the can?

Thanks!
 

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The plate load resistors on your 6EU7s have probably drifted with age and are way out of spec.
 
Both channels equally low? That would be a clue too. What is the sensitivity of the speakers you're testing with, this could be the result of lots of degraded components in the amp trying to feed a tough load. These little amps really work the best with very efficient speakers. Once I determined that mine had all good transformers, it didn't cost much more to just replace every cap and resister with modern equivalents. The Magnavox 8800 amps really sound sweet in my opinion, listening to mine as I type.
 
They are both equally low, but this isn't a "the volume is not as high as I'd hoped for" issue. I had my signal generator hooked up to the amp. I had to input a very large signal into the amp to get any response at all on my oscilloscope.

With my cell phone hooked up, I had to turn it up all the way to hear anything on the speakers, and even then the output was incredibly distorted.

And yes, it is on both channels.
 
What do you mean by "hacked together" the 390 ohm mod? If memory serves the symptom for when the mod or original tuner pre are missing (or not correctly hacked together, maybe) is low gain and would effect both channels.
 
What do you mean by "hacked together" the 390 ohm mod? If memory serves the symptom for when the mod or original tuner pre are missing (or not correctly hacked together, maybe) is low gain and would effect both channels.

Ha, yeah, by hacked together I meant that I didn't solder them in place, but used alligator clip cables to add them to the circuit. I'm assuming that this is not an awful idea. So, it wasn't pretty, but they were in the circuit.
 
By the way I always do this, remember to discharge the big filter caps before working in the unit once power is removed.
 
So in your research do recall that low gain was the symptom that led to adding the mod? it just strikes me that when two things are doing the same thing that the cause is most likely a common shared point. Akmens Razor.
 
So in your research do recall that low gain was the symptom that led to adding the mod? it just strikes me that when two things are doing the same thing that the cause is most likely a common shared point. Akmens Razor.

Yes, it was low gain that led to adding that mod. When I first fired up the amp, it had the low gain issue. Then I added the mod, and I still ad the low gain issue. Low gain and very distorted output.
 
I know that I should have soldered the 390 ohm resistors in place, but I wanted to wait to do that until I had some better quality resistors. So, speaking of that, and to the previous comment, for these magnavox amps, are people using Carbon Film in the signal path and metal film in the power section?

And, I apologize, but how do you tell what is in the signal path? On the attached pdf, are R23, R24, and R25 the power section resistors, and the rest in the audio path?
 

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Mod is to replace removed with tuner/ pre amp balance pot, and it simulates the wiper to ground when centered. Do you read 390 ohms to ground at the amp junction not your clips on both channels? I'm trying to look up the points can't really see your schematic.
 
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In the post above, I uploaded a pdf that also has the schematic. Is there a way to upload pictures in a higher quality? The original file is legible, but when uploaded, it is useless.

So, yes, the "mod" is to remove the balance pot. In this case, there is no balance pot on the amp chassis, just two wires that run to the molex plug.

In the picture below, I drew in how I setup the two 390 ohm resistors. One resistor is red, the other is blue. Both connect to the center "lug" to ground.

20170219_152147.jpg
 
I'm guessing when doing the mod you have somehow affected the feedback going to both channels. Did you verify they were 390 ohm resisters by measurement?
 
Ok, check what 6DZ7 suggested when you get home, hell he's named after a tube after all. As far as resisters, I just used all metal film resisters, should last longer. Only concession to audiophile dogma I made on mine was I did use Russian oil film coupling caps, replaced the orange drops I started with.
 
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