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 Post subject: Help for my Magnecord
PostPosted: Jan Mon 14, 2013 7:54 pm 
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I have an early 50's Magnecord PT6-j recording amplifier. It was originally used in conjunction with the Magnecord PT6-a reel-to-reel tape deck. When I first powered it up, it gave off that unsettling multi-stage hum that so many worn out tube electronics produce, and the VU meter went off the charts. I re-stuffed the filter cans with new capacitors, reconnected everything as usual, and replaced the rotten wall cord as well. Now it powers on fine without noise, but it doesn't make any noise at all. I've tried various different mics with it, and I don't even get a reading on the meter.

The old MF ratings for each of the two cans were 40, 30, 10, 20 with voltages of 450, 450, 450, 25 respectively.
The new caps I installed have MF ratings of 47, 33, 10, 22 with voltages of 450, 450, 450, 25 respectively, each with %20 tolerance.

There is a large wax electrolytic cap that should probably be replaced (1000MF @ 15v), as well as a burned resistor, but (correct me if I'm wrong...I'm new at this stuff) I don't see how these would prevent even the meter from getting a signal.

Any advice?


Attachments:
File comment: Under the chassis
Screen Shot 2013-01-14 at 1.41.34 PM.png
Screen Shot 2013-01-14 at 1.41.34 PM.png [ 253.18 KiB | Viewed 3219 times ]
File comment: My rig of re-stuffed caps. The negatives are all grounded to a twist tab on the can.
restuff caps.png
restuff caps.png [ 209.15 KiB | Viewed 3219 times ]
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 Post subject: Re: Help for my Magnecord
PostPosted: Jan Mon 14, 2013 7:58 pm 
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Location: Raleigh NC USA
It depends on where in the circuit those defective parts happen to be.

The unit should be purged of ALL defective parts before testing.

Meanwhile, verify the work already done. No offense, but even the most expert wiremen make mistakes. No one is immune.

Good luck with your project.

:wink: Larry

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 Post subject: Re: Help for my Magnecord
PostPosted: Jan Tue 15, 2013 11:09 pm 
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Could you post the schematic. It is in Sams CM6 (changer manual). This way suggestions
might be possible.

I have dim memories of those machines, probably concerning radio station use.

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 Post subject: Re: Help for my Magnecord
PostPosted: Jan Tue 15, 2013 11:46 pm 
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Allied Radio carried the complete Magnecord line thru the fifties. The 1954 Allied catalog gives some info about this unit. Not much, but enough to make me suspect that there's a DC heater supply serving at least the tube(s) of the preamp stage(s). I suspect that's what that 1000 uf, 15v electrolytic cap is associated with. The burned resistor too, if it's associated with that cap.

10 watts output (pair of 6AQ5's or 6V6's?) to either 4 or 16 ohms. Also a balanced 600-ohm zero-level output. Supplied with a plug-in 7.5-IPS tape speed equalizer stage with a plug-in 15-IPS tape speed EQ available.

It all looks fairly sophisticated for its day. Certainly worth restoring, especially if one could have the restored PT6-AH tape transport to go with it.

Recapping this beast can't stop with just the lytics. ALL the tubular (paper/wax) caps must also be replaced, for it's a cinch that they're all leaky as sieves. Every other cap (ceramic, mica) should at least be given a quick leakage check, and then you get to the resistors, many of which are sure to have drifted off value.

:wink: Larry

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 Post subject: Re: Help for my Magnecord
PostPosted: Jan Wed 16, 2013 5:20 am 
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I remember working on these in the 1960's. Also remember the related PT6 tape decks did a fine job of testing the integrity of 7" plastic tape reels. The high speed rewind sometimes threw pieces of plastic reels into the acoustic ceiling tiles of the studio.

I sold my PT-6J amp in a moment of weakness, been kicking myself ever since! I did keep or acquire several pieces of Magnecord equipment, a PT-6A deck with amp and monitor speakers in a portable carrying case (heavy!), and a Magnecordette Stereo deck with record electronics in a portable case. Also re-built 2 CM-6K 6V6 amplifiers that are now providing music in the work room.

PT-6J schematic in Sams 387-11:


Attachments:
Magnecord PT6-J.jpg
Magnecord PT6-J.jpg [ 186.53 KiB | Viewed 3172 times ]

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 Post subject: Re: Help for my Magnecord
PostPosted: Jan Wed 16, 2013 6:21 am 
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Just a preliminary note: Be sure to adjust R35 to 12.6 volts at the 12SJ7 socket, and not the 5879.

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 Post subject: Re: Help for my Magnecord
PostPosted: Jan Wed 16, 2013 1:57 pm 
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Aha! It DOES have a DC heater supply for the front ends, and that 1000uf cap is associated with it! And it's supposed to be 25 volts instead of 15!

With that DC heater supply down, the front ends are down, and of course no signal is getting through to the meter. The entire DC heater supply must be rebuilt, beginning with that bridge (selenium?).

Then take things from there.

:wink: Larry

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 Post subject: Re: Help for my Magnecord
PostPosted: Jan Wed 16, 2013 4:04 pm 
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MAG wrote:
I remember working on these in the 1960's. Also remember the related PT6 tape decks did a fine job of testing the integrity of 7" plastic tape reels. The high speed rewind sometimes threw pieces of plastic reels into the acoustic ceiling tiles of the studio....


There must have been many snapped tapes too, since those were the acetate days.

The original small-hub 7" Scotch reels (remember those?) would crack in a heartbeat. Drop one, full or empty, and you'd almost be sure to have a crack around the rim somewhere unless it landed on thick carpet. The spokes also liked to crack free of the hub.

The small-hub 7" reels made by American Molded Products were of a different style (6-eye, three large, three small, alternating one and one). They didn't break nearly so easily. (Irish tape was marketed on those reels for some time).

I still have a goodly number of both reel styles for use with my oldest tape machines (Ampro 731's). They hold a good bit more tape than the 7" reels with larger hubs that became standard by the late fifties, and stayed standard from then on.

:wink: Larry

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 Post subject: Re: Help for my Magnecord
PostPosted: Jan Sun 20, 2013 7:20 am 
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Thanks for all the great advice!
Well I met up with a local electrician who has been working with these tube machines since his days in the Air Force in Vietnam. He tested all of my tubes and found that the 5879 was bad, but all the rest were great. Strangely enough, the 5879 isn't used when the machine is switched to "amplifier" mode, so we started testing things in that circuit. someone put a 1/2 watt resistor where there should have been a 1 watt, so we switched that out (the other one wasn't handling the load when warmed up). Realized that pins 6 and 8 on the 12SJ7 tube are getting wayyyy too much voltage, whereas pin 5 on the 6SN7 isn't getting enough. He took some readings from my filter cap re-stuff job and said it was fine...suggested I reheat the solder points to make sure nothing had come loose over the years (I did this to just about every point in the circuit...still no signal).

@BigBandsMan: It is indeed a 10 watt with two 6V6's, along with the 4, 16, and 600 ohm jacks! My model only has the power plug to the tape deck..no extra adjustment for speed (I know some models have this). And the 1000MF is still 15V in the schematic

@MAG: I think I saw that schematic online, but it is for a later model (but yep...still very heavy!) I have attached my schematic below. Glad you found that one, though!

@radiotechnician: I'll definitely have to take a look at that now that you mention...


Attachments:
File comment: here is a web link to it: http://www.thehistoryofrecording.com/Manuals/Magnecord/Magnecord_PT6-A_Manual.pdf
good schematic.png
good schematic.png [ 236.23 KiB | Viewed 3109 times ]
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 Post subject: Re: Help for my Magnecord
PostPosted: Jan Sun 20, 2013 4:53 pm 
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Congrats on finding the manual, and many thanks for the link! Didn't realize we were looking at the wrong schematic awhile ago, although I should have. A pair of 5881 tubes is overkill for only 10 watts output. Good to see the right schematic.

Good luck fixing up this unit and keep us posted.

:wink: Larry

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 Post subject: Re: Help for my Magnecord
PostPosted: Feb Sun 17, 2019 7:02 am 
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Old thread, but just bought a pt6 same as OP, with 12sj7 and 5879 preamp tubes.

On that last schematic posted by OP, one of the capacitor symbols, coming after the 5y3 rectifier, is annotated as "30mf & 10mf". Is this just two capacitors wired in parallel?

BTW, I'm pretty sure this particular model originates to 1949 or thereabouts.


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 Post subject: Re: Help for my Magnecord
PostPosted: Feb Fri 22, 2019 11:39 am 
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Location: Powell River BC Canada
The instruction book with schematic is here:

There seems to be a difference in versions between the OP posted
one and the instruction book. I cant find the deck schematic. I think
the bias and erase oscillator is on the deck.

http://www.thehistoryofrecording.com/Ma ... Manual.pdf

http://www.preservationsound.com/wp-con ... Notes1.pdf



Attachment:
magnecord pt6 J snip.JPG
magnecord pt6 J snip.JPG [ 23.31 KiB | Viewed 1040 times ]

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Amateur Radio Literacy Club. May we help you read better.
Steve Dow
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 Post subject: Re: Help for my Magnecord
PostPosted: Feb Sat 23, 2019 6:18 pm 
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Tape drive PT6-A schematic...
https://www.radiomuseum.org/r/magnecord ... pt6_a.html

Photofact ref: Set-190 Folder-6
https://www.samswebsite.com/en/photofac ... msid/190_6


Greg.


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 Post subject: Re: Help for my Magnecord
PostPosted: Feb Sat 23, 2019 7:33 pm 
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@radiotechnician . . . As you may know, there are a bunch of different versions of the pt6. I do believe however, that Amoore's post of the schematic is the same as that in your first link of instruction manual. I believe this is a very early and probably the simplest version of the pt6. Also, compared to other schematics I've seen of other versions, this early version applies comparatively less voltage to the power tube plates.

Regardless, this early schematic doesn't have actual part numbers. So, I'm still wondering, if anyone cares to educate me, what does the schematic indicate when it says "30mf & 10mf" next the first filter cap after the tube rectifier? Also, on the schematic, I can't find the henry choke that the instruction book refers to. Anyone? . . . . Just to be clear, I am not talking about the schematic that MAG posted, but the one that Amoore posted and that radiotechnician linked to that appears at the bottom of the instruction book.

Also, I'm an electronics beginner, so please forgive if my questions or comments seem errant.


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