Magnavox A9304 amplifier project

[guitargrump]

I'm posting these for reference...


Apparently the online consensus is that the 272fx is the correct a9304 replacement power transformer...

But the mounting orientation is wrong...

The 273cz look to be really close??? but the voltage rating is 650vct @ 172ma versus the 272fx 600vct @ 172ma

To be honest I'm confused by the 272fx documentation how does 600vct equal 654v one of these numbers is wrong or I'm misunderstanding them...

I assume that it's the 600vct number listed on the Hammond200 series data sheet that's wonky but assuming stuff is trouble.

Opinions?

 

[trainbuftony]

654 when nothing is drawn.
It sags to 600vac when loaded to its rated current.
That makes it easy to check for overload and to get an idea of the power supply's ability to self regulate.
If you're not getting to 600vac out of the transformer then its pulling too much current.
If its shooting over 600vac that's because you're not using all of the power its making.
Most amplifiers run in this condition.
Pulling the correct current in the output stage should have your B+ spot on or slightly over.
Nothing wrong with that. Tubes are extremely forgiving of minor variations like that and the fact is most of these amps start to sag the B+ once you drive them out of class A.
That's typical, that's how they work when they're working right.
You're shooting at a moving target.
So if your B+ Is high because you're not pulling any power youve got little to worry about, it evens itself out once it's under load.
Can't burn anything up with 0 current...

 

[guitargrump]

@trainbuftony @6DZ7 So then I was misunderstanding the abbreviation NLV thank You...

The 9304 circuit diagram specifys 300v on red so I'm thinking the 272fx is the right transformer just the wrong orientation to match the original PT...

I can live with that. We don't have endless choices in new power transformers manufacturing Hammond Canada is in stock and available Edcor is a six week lead time

I just want to be dead certain before I totally blow my mad money out the door...

 

[guitargrump]

I've never seen a riders diagram on my projects just the sam's. Sams is what is always floating around the forums...

The 272fx has a 125vac tap so no bucking the power is required...

I've always thought that power sag was the enemy and the primary reason for top end distortion...

it's getting hard to get tubes and longevity of the filiment is probably way more important these days...

Unless everything works right to start a console is just a amplifier with speakers to me and without a proper circuit diagram I'm useless to do anything but find and replace out of spec parts, I do that pretty well.

I'd never really considered the radios draw on the circuit as a major design factor I should probably start doing so...

Lots to consider thanks for helping me out...

I'm ordering the 272fx it's closest to the sam's it's all I have to go by... I'm so glad that the info in the forums was right this time...

 

[trainbuftony]

Power sag is the enemy and likewise, Class A operation is your friend.
Power supply gets the same draw all the time in class A.
Full throttle, or silent rest. Your B+ won't change.
Class A is operating the tubes such that they never shut off. The current averages to be the same across the same load, means the power supply voltage stays virtually the same.
People tend to agree Class A sounds fantastic. Class A doesn't get you any "free" or "cheap" headroom, but the power supply sag will never happen, and it will sound good.

 

[FlaCharlie]

There's something wrong with one half of the main power transformer one side of the red puts out 330vac the other red side is putting out 2.97vac I believe that I missed that period in 2.97 when I was checking it my eyes aren't what they used to be...

When you measured the HV secondary voltages did you also measure the resistances? According to the Sams, one side should measure ~50 ohms from the red/yellow center tap and the other one should be ~54 ohms.

Also, were the red wires still connected to the rectifier tube socket? I'd suggest that you do the resistance measurement with both the red wires disconnected from the socket. I've had bad sockets that resulted in odd readings, once even with a NOS socket. Then maybe re-measure the voltages.

Probably a long shot, but it won't hurt to double check.

 

[guitargrump]

When you measured the HV secondary voltages did you also measure the resistances? According to the Sams, one side should measure ~50 ohms from the red/yellow center tap and the other one should be ~54 ohms.

Also, were the red wires still connected to the rectifier tube socket? I'd suggest that you do the resistance measurement with both the red wires disconnected from the socket. I've had bad sockets that resulted in odd readings, once even with a NOS socket. Then maybe re-measure the voltages.

Probably a long shot, but it won't hurt to double check.

Excellent suggestion I'll do that the ohms were definitely wrong though the one side read 85ohm the other side was 0ohm

 

[trainbuftony]

I'm wondering if that 2.97v measurement was actually half of the 5v rectifier filament and the power transformer is probably fine.
I've made that mistake and that's what you'd expect to measure when you measured the wrong thing. Only know it because I've done it.
A spare power transformer isn't the worst thing you could possibly have on the shelf...

 

[guitargrump]

I'm wondering if that 2.97v measurement was actually half of the 5v rectifier filament and the power transformer is probably fine.
I've made that mistake and that's what you'd expect to measure when you measured the wrong thing. Only know it because I've done it.
A spare power transformer isn't the worst thing you could possibly have on the shelf...

I'll triple check the measurements before I order the new transformer but I am pretty sure that the transformer is toast I was just checking the two red leads to the center ground point the red& yellow wire...

 

[guitargrump]

Ya it says red/yel on the diagram but mine is 95% Y and 5% little red dots. Hope its not misswired with one of the Y/Y heater winding.

I just checked again got the wife involved with the camera so everyone can see...

The colors are pretty clear in my shop/porch sunlight it's the red & tan side to the red & yellow center tap ground of the coil giving me really bad readings... The voltage was higher this time ...



Maybe there's a problem with the wires in the transformer where I can't see it...

I'd have to replace all the transformers wires if I open the transformer the wire casing is brittle and the wax potting is dry and crumbly... It's gotten very hot at some point before I got the amplifier



As far as I can tell nobody but me has been in the underside all the parts look original

 

[FlaCharlie]

You're still measuring at the socket pins. If you remove the wires from the socket that will eliminate the possibility that the socket is causing some problem.

After de-soldering the wires from pins 4 and 6, check resistances first, then check voltage again.

That PT looks like it did get pretty toasty, though. So this is probably just a shot in the dark.

 

[gadget73]

I'm a chicken with this stuff. I feed transformers 12 volts on the primary and measure my voltages that way. If its basically 10% of what it should be, its good. Not too fond of poking at 300+ volts if I don't have to.

 

[guitargrump]

@6DZ7

The guide grove is between the 2&8

@gadget73

Excellent suggestion how many amps does the 12vac supply need to work the transformer coils properly? I'll have to buy One...

@FlaCharlie

I did the ohm measurements after desoldering just now same results on the ohms I'm definitely convinced that the main coil or circuit path is bad on the red&tan side just 5 ohms


I'm j hooking the wires back in place as placeholders until the new transformer arrives thank you so much for helping...

I'll tear it apart after I have the new one in place... It might just be a partial inside short on the wires... The transformer wire cloth casing is brittle and crumbly...

Maybe I'll get lucky and be able to fix a simple short but I am replacing the transformer first regardless at this point...

I learned that I should never take or leave stuff laying apart until the replacement is on hand a long time ago. What if I forget where the wire's went or lost something..

 

[gadget73]

Don't need much. Unloaded a transformer consumes almost no current. The trick it it must be AC volts. Only thing special about 12 volts is that it makes the math easy. 35 volts on the HV secondary means its probably going to be awful close to 350 volts when plugged in to the wall.


I use my variac because its convenient to just plug things in and dial it to 12v though. If its not something already in a chassis with a cord, then I just use some wire nuts and a lamp cord to give it one.

 

[guitargrump]

Don't need much. Unloaded a transformer consumes almost no current. The trick it it must be AC volts. Only thing special about 12 volts is that it makes the math easy. 35 volts on the HV secondary means its probably going to be awful close to 350 volts when plugged in to the wall.


I use my variac because its convenient to just plug things in and dial it to 12v though. If its not something already in a chassis with a cord, then I just use some wire nuts and a lamp cord to give it one.

I don't have a Variac yet it's on my list of tools along with a ocilliscope

 

[gadget73]

happen to have a model train transformer? One of those should be able to adjust to 12vac

 

[guitargrump]

happen to have a model train transformer? One of those should be able to adjust to 12vac

Nope no model train controller here... I need to just buy a Variac great idea though Thank You

I kinda blew my mad money out the door for this month but a brand new Hammond 272fx PT is on the way from digi-key should be here around next Saturday... $127 with shipping and taxes...

Digi-key isn't the cheapest route just the most security and privacy respecting...

Amazon even had three sellers with the Hammond 272fx but I avoid doing business via Amazon because of their twisted AI algorithm that won't show the results your looking for without stuffing a million other semi related things in front of a exact query... A very effective Bait and Switch tactic...

So small piddly stuff while I wait for the power transformer...

Oh well I guess that I need a short break from the PTSD flashbacks and anxiety attacks anyway...

I'm doing better than before. it took me almost 30 years to get myself up to facing these memories...

So you and everyone else who is helping me here is doing way more for me than just helping me fix a old amplifier...

I Thank You All... You guys are definitely Truly Awesome...

 

[guitargrump]

The postman brought more supplies today... My RCA jacks and the multi cap can from amplified parts... I was really super surprised that the multi cap arrived today...



I've got the old multi cap cut out with my sorta Dremel ish tool and the new can soldered in place...

I found the first evidence that someone else had done repair work to the amp module before while desoldering the multi cap wire's... That means I need to check everything way closer than if it was untouched...


I haven't resoldered the wires yet because I am seriously thinking about the recommendation @FlaCharlie made to swap the 40uf and 60uf sections...

I don't know because I am trying to do two projects here first get it working then do @dcgillespie 9300 mods to it... I'd like it to be still repairable after Its out of my hands by a future tech... I intend to document everything but documents tend to get misplaced over time...

So could someone please explain to me why the 60uf section should go after the choke in as plain English as possible... So I understand why I am making this change...

 

[gadget73]

A big cap right after the rectifier puts a lot of stress on the rectifier tube. The technical term is inrush current, but basically the bigger the cap, the more current is required to charge it up. The rectifiers have a max current rating and beyond that it significantly shortens their life.

 

[guitargrump]

A big cap right after the rectifier puts a lot of stress on the rectifier tube. The technical term is inrush current, but basically the bigger the cap, the more current is required to charge it up. The rectifiers have a max current rating and beyond that it significantly shortens their life.

Okay Thank You I understand why now... I should do this mod to my Magnavox 175-67 too then... Awesome thanks I got this now...