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Magnavox A9304 amplifier project

trainbuftony said:
Honestly computers frustrate me to the point I won't even play with them.
If I need a schematic of a something as simple as a 9304 I just draw it myself on graph paper to save time. Its generally more accurate too because its exactly what's in front of me. If I spend more than an hour trying to get the computer to spit it out, the pencil method ends up being faster.
It can be very relaxing to doodle your own schematics...
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Drawing by hand would have been way easier... My problem is once I get started on something like the printer I have to get it fixed... Can't be quitting projects over simple computer stuff that I already know well...

Stupid obstacles to progress...
 
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gadget73 said:
Printers are concentrated evil. I don't even own one. If I really need to print something I use one at work. They have an entire IT staff thats paid to deal with those miserable pieces of junk.
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I'm glad to just get the printer replaced with something simpler and move on to more important things like drilling holes for speaker binding posts and making my world a better place musically...

A trip to HF for some new hex step drill bits and cut off wheels and I'm all set for now...

It seems like my patience is just not there for computers anymore... Computers rank about like a trip to the dentist for a root canal...
 
gadget73 said:
Printers are concentrated evil. I don't even own one. If I really need to print something I use one at work. They have an entire IT staff thats paid to deal with those miserable pieces of junk.
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I ditched ink jet printers sometime back. Laser printers are less hassle. No dried out clogged ink jets and no cartriddges having to be exchanged a lot. I've had zero problems with the laser printer. Simple B&W laser printers are the that expensive and the toner cartridges last along time.
 
FlaCharlie said:
Wait! . . . you have a job? :yikes: Since you seem to post on numerous AK forums, and at all hours of the day and night, I always assumed you were retired.
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weird sleep habits and my job has lots of parts of the day where I'm waiting on other stuff and have nothing else going on. I've been forbidden from reading, listening to music, or working on my own projects so I waste my idle time on here.
 
I am just working my way through the circuit checking everything... So far I've found a single missing filter cap...

IMG_20220531_093751~2.jpg

Dave said that there was one capacitor missing on his build I'm wondering if their the same one... This filter cap was deliberately cut out...

Everything is in really awesome condition it has me debating on just replacing the coupling/filter capacitors and the few out of spec resistors and calling it done...

I probably won't be satisfied with anything but a 100% complete rebuild. But it's really nice to know that the amplifier would probably work just fine as is...

Not much more I can get done until the other parts arrive... I would pick a holiday weekend to order parts...
 
Those were optional depending on the specific model in question. If you look at the Magnavox schematic it has a decoder chart that shows the values of certain parts and which models didn't have other parts. It really doesn't affect things all that much.
 
guitargrump said:
Everything is in really awesome condition it has me debating on just replacing the coupling/filter capacitors and the few out of spec resistors and calling it done...

I probably won't be satisfied with anything but a 100% complete rebuild. But it's really nice to know that the amplifier would probably work just fine as is...
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Most console amps have relatively few parts so I often find myself replacing most all of them, even though I might start out thinking otherwise. On the last one I worked on I think the only original parts I left were a wirewound resistor and some carbon comps that were used as grid stoppers, since their exact value is not critical.
 
FlaCharlie said:
Most console amps have relatively few parts so I often find myself replacing most all of them, even though I might start out thinking otherwise. On the last one I worked on I think the only original parts I left were a wirewound resistor and some carbon comps that were used as grid stoppers, since their exact value is not critical.
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My last build a Magnavox 175-67 I started out thinking the same way I ended up replacing all the resistors and capacitors at the end anyway because they just didn't match up with the new components...

I'm not sure why I want to preserve the amplifier with original components... I saved all the used parts from the last build even...
 
I'm sort of 50/50 on things. Most of the stuff I actually use, I want right and much of the time that involves replacing a lot of components that are out of tolerance. Things that I just want because of what they are I'd prefer as original as possible. My Magnavox doesn't even run the original circuit but my Zenith T/O will retain all of it's original parts from 1950 since it still works as-is, and I know I'm not really going to make use of it. I just can't justify the effort of making it not original to have it sit on a shelf when I can leave it totally original and do the same thing with it. Besides, they're only original once.

if someone had already been in the T/O I'd have probably just overhauled it.
 
I'm pretty sure that it will work as is... I'll be dead certain though before I bring it up to power...

Your right once you start modifying it's no longer original... It's for full time use so I'll just do everything...

I guess it's sentimental foolishness to want this stuff to work properly and reliably but still be all vintage original...

I go through this mental wrestling match about changing anything from the original designs on any of my projects no matter how simple or practical the change is...

It's a desire to preserve the treasures of the past and the sure knowledge that time is not on my side...
 
Don't stress too much about originality. If you replace all the drifted, and out of spec parts with new ones of the same values you preserve the original sound or improve it, and it's still essentially original. Knowing you can use it daily and knowing it will work every time is a much better feeling than knowing it's all original and the next failure is probably well underway...
Do what you want, but I think the "all original" thing mainly applies to techs who dont want to clean up after 4 hair brained patch jobs.
It's much easier to get working again when it's all original, vs totally screwed with...
 
Your right as usual @trainbuftony

I guess that I don't want to psychologically ruin the magical properties of the amplifier.:)

I spent some time today going over my module and the A9304 Sam's schematics and rereading @dcgillespie more fun with the Magnavox 9300 series thread again...

There's a few minor differences between the a9302 Dave used and the a9304 I have... Not sure if that's a problem yet...

The main things I see different are a 4 or 5 lug turret that my model doesn't have and a balance control and no 100ohm 5watt grid resistor going to ground and a bunch of capacitors that this model doesn't have yet...

So I started on the basic stuff, that I know that I need to do no matter what I decide to do...

I started out by cutting out the molex connectors wires and capping the molex opening with pop rivets and a scrap of aluminum...

IMG_20220601_174837.jpg IMG_20220601_174907.jpg

I guess that corner looks alright. I have a few more holes to cover...

I'm still waiting on the fuse holder and indicator lamp so I can do the power connections and I want to replace the power chord so it can only be plugged in one way...

The seller cancelled my 6eu7 purchase he couldn't find one of the tubes. Ugh.

The tube prices are still up there like the tubes were made of unobtainium.

I was trying to buy a new CE Manufacturing multi cap like the one that I used on the Magnavox 175-67 amp build but it's out of stock... So I guess I will try re stuffing the old can left over from my last build...

It's a good thing that there isn't any customer waiting on this besides me...
 
guitargrump said:
I was trying to buy a new CE Manufacturing multi cap like the one that I used on the Magnavox 175-67 amp build but it's out of stock... So I guess I will try re stuffing the old can left over from my last build...
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There's no need to go that route unless you're just a glutton for punishment.

With all the room under the chassis it's much easier and much, much cheaper to mount individual caps instead. I usually go with Nichicons or Panasonics that are rated for 10,000 hours at 105 degrees as their other specs are also excellent. Much better specs than a CE replacement can.

Just make sure you source them from a major supplier like Mouser or Digi-Key. Digi-Key has a free shipping option. No minimum order and no handling charge:

http://www.audiokarma.org/forums/in...hipping-on-parts-orders.739513/#post-10018562

You can just add a terminal strip and mount the caps on it.

Or, if you use radial caps with wire leads, you can even mount them where the original can is. Just cut off the positive terminals from the original can. Then you can solder the negatives of the individual caps to the twist tabs of the original can. Usually, two of the tabs have holes in them, which makes it easier. As long as the leads are cut very short the caps won't move around. Then add the necessary resistors and wires between the positive leads, which should also be cut very short.
 
what you are starting with isn't really all that relevant. It all gets gutted if you use Dave's schematic so it doesn't matter which specific model or variation it was.
 
FlaCharlie said:
There's no need to go that route unless you're just a glutton for punishment.

With all the room under the chassis it's much easier and much, much cheaper to mount individual caps instead. I usually go with Nichicons or Panasonics that are rated for 10,000 hours at 105 degrees as their other specs are also excellent. Much better specs than a CE replacement can.

Just make sure you source them from a major supplier like Mouser or Digi-Key. Digi-Key has a free shipping option. No minimum order and no handling charge:

http://www.audiokarma.org/forums/in...hipping-on-parts-orders.739513/#post-10018562

You can just add a terminal strip and mount the caps on it.

Or, if you use radial caps with wire leads, you can even mount them where the original can is. Just cut off the positive terminals from the original can. Then you can solder the negatives of the individual caps to the twist tabs of the original can. Usually, two of the tabs have holes in them, which makes it easier. As long as the leads are cut very short the caps won't move around. Then add the necessary resistors and wires between the positive leads, which should also be cut very short.
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I just read your parts thread excellent info... I like the idea of using the original can cap tabs as a cap mounting point... Very Innovative...

@gadget73 excellent point thank you...
 
Using the original can caps as a mount point makes your new cap just as good as the old one.
The old one shorts then the new one provides zero filtering.
You need to remove the old cap from the circuit if you're going to use those terminals.
 
You could pull the guts out, lop off the old "cap roll" and solder discrete replacement caps to the terminals.
Stuff the wafer back in the can and nobody will see that youve been there. Everyone will wonder how you seem to be the only guy around who gets amps to be dead silent on seemingly 70 year old electrolytic caps.
Tell them or don't tell them, your choice.
Somewhere theres a thread about it on audiokarma which I'm the author of.
It takes some work but comes out beautifully every time.
 
normally the only tab I recycle on the old can is the ground soldered to chassis, and mostly because its a lot easier to solder to that vs to the chassis in a new spot. The other terminals get unhooked.
 
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