Magnecord Schematic question

[Aaronrash]

Hi,

I'm in the process of restoring a old Magnecord from 1950 I'm seeing something weird in the schematic for the power rectifier filtering stage. See where it's showing a 30uf cap but then it also says 10uf... Does this mean I can use either a 30 or 10uf or does it mean to connect them in series? Kinda strange, I've never seen this before. I attached the picture.


Thanks

 

[Brian Roth]

They MIGHT mean using a 30 uF in **parallel** with a 10 uF to give a result of 40 uF.  That would make sense if the original used a multi-section can which happened to include both 10 and 30 uF sections.

Bri

 

[Aaronrash]

They MIGHT mean using a 30 uF in **parallel** with a 10 uF to give a result of 40 uF.  That would make sense if the original used a multi-section can which happened to include both 10 and 30 uF sections.

Bri

Hi Brian,

Yes this does make sense but what's even more weird is the next cap in the path is a 10uf after a 10K resistor. There were 2 multi cans on the unit each can had 40,30,10,20 uf. The thing is if they meant for it to be in series I would need to find a 3rd 10uf cap in the unit witch is not in there so I'm not really sure what the schematic means. I've attached a better picture of the schematic. Here is also the link. http://www.thehistoryofrecording.com/Manuals/Magnecord/Magnecord_PT6J-A_Manual.pdf

 

[mjrippe]

In the second pic of the schematic I see (2) 40uf, (2) 30uf, and (2) 10uf all at 450v.  On the cans, check if the 20uf section is at a lower voltage.  Since it is listed last and I don't see any 20@450 it may well be.  So there's two of each and Brian's theory holds.

 

[walter]

I believe Brian nailed it.
You say you're restoring this unit, are the old can caps there? Perhaps you could examine how they are connected and what the values of each section are. 30uF @ 450v and 10uF @ 450v connected parallel is 40uF @ 450v. 30uF @ 450v and 10Uf @ 450v connected in series will work for 900v. The replacement caps will dictate what gets connected in the end if the original values are unobtainable.

 

[PRR]

> connected in series will work for 900v

But the diagram shows 315V off the rectifier. So caps can all be 450V.

It appears they use 450V caps all the way into the 165V parts of the system.

Anyway the units in can-caps usually can't be stacked (they are common-negative).

The first cap is 30&10MFD, or 40uFd made-up from available units.

This feeds 6V6 plates and screens at 290V, also 10K+10u+10K+40u to feed the little bottles.

The bias-oscillator (I assume) is fed separately through 2000 and 40uFd, since its on/off switching should not affect the audio circuits.

None of the filter-cap values is at all critical. Much-less may hum. Much-more costs too much (not so much at today's prices). You can double or triple the values, if you have a few dollars, if they fit nice. Probably little advantage.

Don't worry about peak rectifier current, since there's large resistors between 5Y3 and first caps.

Get a baggie of 40 or 100uFd and a baggie of 10 or 22uFd all 450V and sprinkle them in.

 

[Aaronrash]

Thanks for all the help guys,

Here's a picture of the actual unit I'm restoring. That huge buldging yellow cap is the power filtering cap for 2 tubes on a separate circuit. It's being replaced. I've figured out allot after staring at the schematic for a while. The plates on the 2 6V6 tubes are connected together and then fed to the two 20uf 25V sections to equal the 40uf 25V as shown in the schematic.

Another weird mystery I've discovered though is there is also a extra 25uf 25V cap strapped onto plate of the 6V6 then to ground. In the schematic there is only two 25uf 25V caps and I've found 3 inside my unit. They are in the top left of the picture.

I'm guessing maybe they had extra around at the factory or something and just added a little more filtering then shown in the schematic. Either way I'm gonna take that out and put a 40uf there like shown in the schematic.

The 20uf 25V sections on the can caps were completely dead. The 10uf section on one of them was shot as well. I think the unit will respond well to the new caps and new tubes as well for the entire unit. It wasn't even passing audio when I first got it and the 10K resistor on the filter can burned up within 5 minutes of powering it on.

 

[PRR]

> 25uf 25V cap strapped onto plate of the 6V6 then to ground

That's a 300 Volt jump. Should NOT be a 25V cap across that.

Also the 6V6 plates should not be connected directly together, but via the output transformer.

One other thing to confuse you: I bet they use the "NC" pins of the 6V6 as tie-points for stuff that isn't really about the 6V6.

 

[Aaronrash]

> 25uf 25V cap strapped onto plate of the 6V6 then to ground

That's a 300 Volt jump. Should NOT be a 25V cap across that.

Also the 6V6 plates should not be connected directly together, but via the output transformer.

One other thing to confuse you: I bet they use the "NC" pins of the 6V6 as tie-points for stuff that isn't really about the 6V6.

Opps I worded that wrong. The 25uf 25V is soldered across the Cathode to ground of 1 6V6 but I worded it all wrong. The 2 6V6 cathodes are connected together then into the 2 20uf 25V sections on the filter cans to equal 40uf. It's still strange that they have a extra 25uf 25V there as well that isn't in the schematic...But since the cans are getting removed I'll just put a single 40uf for the cathodes and strap that to GND just like the schematic. I'll report back after finished replacing caps and a few resistors as well as tubes etc... I think this beast will be running again soon

I did also notice they are using unused pins as connection points for other stuff and grounds etc..


Also the 10K resistor soldered between the 30uf and 10uf on one of the cans burned up within 5 minutes of powering the unit on (before replacing anything). But I'm just wondering what could cause a 10K resistor to burn up like that? I mean the thing was completely smoked about to catch fire.

Here's my guesses. Either during shipping since the guy didn't remove the tubes they got jolted around during shipping causing a really bad short inside on of the tubes... This would make sense because all the tubes are bad and if you shake them they sound like a broken light bulb. Or possibly the bad filter cans are shorted. The unit is from 1950 so this would make sense as well. Just my guess

Thanks!

 

[emrr]

Put 100-470 mfd on that cathode these days, it's tiny, cheap, and better, all things they couldn't afford to do reliably back then. 

 

[Aaronrash]

Put 100-470 mfd on that cathode these days, it's tiny, cheap, and better, all things they couldn't afford to do reliably back then.

Thanks for the tip Doug. I'll give that a shot

 

[walter]

That looks really clean. I notice the can caps are mounted on insulator brackets, and then wired to ground. That's a lot easier to work on than if they were soldered directly to the chassis.