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 Post subject: Seeburg master amp question...
PostPosted: Jun Thu 05, 2014 4:07 pm 
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Location: alameda,CA
I agreed to fix an amp for someone a few weeks back and lo and behold they brought by the amp out of their Seeburg juke box. All it needs is a re-cap as he says it simply hums with some distortion. Anyway, it has an odd 4-blade plug in the back. Here is a link to the amp's schematic:

http://www.jukesales.nl/schemas/seeburg/mra1-l6.jpg

What I'm trying to figure out is how to power it up to test it as the plug for it is attached to the juke box and its too big to bring over.

Here are some pics of the amp in question:
Image
Image
Image

Now as seen in the last pic two of the blades goes to:
A: A fuse
B: one of the leads of the strong of 6V6 tubes

The other two goes to:
A: Input power to transformer
B: A blue wire which goes ( It was late... I forgot to look!!)

Anyway... How would one connect a temporary power source for this thing? Sorry, as I haven't worked on one of these before.


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 Post subject: Re: Seeburg master amp question...
PostPosted: Jun Thu 05, 2014 6:31 pm 
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Location: Warner Robins, GA
On some Seeburg Jukeboxes the Tormat Selector Unit suppies the 6.3Vac to the amplifier tubes while the amplifier's power transformer just supplies power for the HV and rectifier tube.

So you need 6.3Vac or DC for the tube heaters. One of the wires that goes to one of the 6V6 tubes is one of the heater power wires and one of the other wires (maybe that unidentifed wire) is the other tube heater power wire.

More than likely the wire going to the fuse is the other 120Vac power wire.


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 Post subject: Re: Seeburg master amp question...
PostPosted: Jun Thu 05, 2014 6:50 pm 
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Location: North Eastern Ohio
That's a standard Cinch Jones plug that was used in that era. From your picture it looks like you would jumper the bottom two pins the white wire from the fuseholder to the black on the power transformer. The other two pins may be for a muting circuit used between records. you would have to look at the schematic.

A word of advice on this amp if the chassis hangs on the back door and puts the tubes horizontal make sure all of the sockets for the 6v6's fit the tubes tight as well the socket for the 5U4. it looks like the 5u4 socket has a gripper on the chassis to bite into the socket and hold it in there. if not add one. AES sells them for less then a dollar.

Loose sockets on these kind of amps can cause all kinds of problems but more common with miniatures. The combination of heat, gravity and vibration over decades plays hell with the sockets that get hot.

If the 5u4 and the 6v6 do not fit tight in their sockets I would change the sockets to avoid problems.


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 Post subject: Re: Seeburg master amp question...
PostPosted: Jun Thu 05, 2014 6:59 pm 
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Location: North Eastern Ohio
I cant see the model number on your picture but it looks like a MRA3-L6
If so you will need to supply 6.3VAC for the heaters since it was originally supplied from the stepper chassis
here is a link to the schematic http://www.soundup.ru/index.php?option=com_content&view=article&id=2695%3Aseeburg-mra1l6&catid=46%3Alibrary-scheme-ind&Itemid=80


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 Post subject: Re: Seeburg master amp question...
PostPosted: Jun Thu 05, 2014 9:03 pm 
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Its actually a MRA1-L6
So not sure if that's a different animal.
Either way, I need to confirm before I decide to do anything lest I blow all the tubes.


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 Post subject: Re: Seeburg master amp question...
PostPosted: Jun Thu 05, 2014 9:38 pm 
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same difference, the other two pins on the Cinch-Jones connector is the 6.3 VAC heater voltage. if you don't have the stepper chassis that plugs into it you will need to connect the chassis to a separate 6.3 VAC transformer or a bench supply capable of the current. You will have to add up the heater current of the tubes to see what it will need, will be at least 5 amps i imagine.
here is the schematic - http://soundup.ru/index.php?option=com_content&view=article&id=2695%3Aseeburg-mra1l6&catid=46%3Alibrary-scheme-ind&Itemid=78


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 Post subject: Re: Seeburg master amp question...
PostPosted: Jun Thu 05, 2014 10:02 pm 
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That is what I was afraid of. In that case I would rather simply overhaul the unit and bring it to his house and hopefully it will just work.


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 Post subject: Re: Seeburg master amp question...
PostPosted: Jun Thu 05, 2014 10:12 pm 
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Location: Sedona, AZ/ Placentia CA
Bob, your idea is probably the best if you don't wish to supply the filament voltage and A/C on the connector. Not a lot of work if you have a nice filament transformer. If you schematic is the proper one, no testing without the external filament transformer and A/C hook up for 120V and a speaker, which you didn't indicate you had.
It should rock but nice to be able to do a test after your work and during it with a set up for running it at your place. No on/off switch for either without the rest of the system, run it off a variac.
Jerry

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 Post subject: Re: Seeburg master amp question...
PostPosted: Jun Thu 05, 2014 10:16 pm 
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Well, its more that this isn't my amp and as with all things related to juke boxes this thing is probably worth a fortune.I would hate to ruin this. The amp is actually bare-bones simple and knowing that it in fact worked before with only distortion from bad caps means that I'll simply replace all the caps, check the resistors, check the tubes, clean the pots, and then when its done take it over to the guy's house and see what happens. I need to be there too to ensure that it does work and will remain so after a prolonged period.


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 Post subject: Re: Seeburg master amp question...
PostPosted: Jun Sat 07, 2014 6:12 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Beaver Falls, PA. USA
I have a similar amp on the bench right now; it's a later version with two 6L6 tubes in the output. To service those, I use my trusty Heath BE-5 battery eliminator to power the filaments, cranked down to six volts.

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 Post subject: Re: Seeburg master amp question...
PostPosted: Apr Wed 24, 2019 3:52 am 
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Location: Mikwaukee, WI 53207
I know this is a very old thread but I recently was able to locate a copy of the Parts List for the MRA1-L6 and thought I'd post it here.
You can also find the PDF here: https://www.dropbox.com/s/24brtviu4vuc8ur/Seeburg%20MRA1-L6_Parts.pdf?dl=0


Attachments:
USA_Seeburg_MRA1-L6_Parts List_Small.png
USA_Seeburg_MRA1-L6_Parts List_Small.png [ 384.8 KiB | Viewed 604 times ]

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