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rroscoe
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Joined: Tue Feb 23, 2010 8:29 pm
Location: Northborough, MA

Rek-O-Kut R-8A amplifier schematic wanted.

Post: # 8002Unread post rroscoe
Wed Feb 24, 2010 9:36 am

Folks, just discovered your website! It's great!

Can anyone help me find a schematic for the Rek-O-Kut R8A recording amplifier? I have searched high and low on the internet!

I just purchased an M-12 cutter on a TR-12H turntable along with the R-8A, which doesn't work. I suspect the pretty blue Aerovox plastic-enclosed capacitors are the culprits, but I'd rather not remove them all before I have a schematic so I can check voltages, etc. All the tubes test good on a Hickok 539C.

This is my first foray into the joys of disc cutting, something I have wanted to do all my life!

Thanks for your help!

Ron Roscoe

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Matt
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Joined: Tue Jan 13, 2015 11:21 pm
Location: Orlando, FL

Re: Rek-O-Kut R-8A amplifier schematic wanted.

Post: # 34785Unread post Matt
Wed Apr 29, 2015 8:26 pm

I am looking for the same thing. Did you ever find one?

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markrob
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Joined: Wed Nov 28, 2007 1:14 am
Location: Philadelphia Area

Re: Rek-O-Kut R-8A amplifier schematic wanted.

Post: # 34786Unread post markrob
Thu Apr 30, 2015 7:56 am

Hi,

Congrats on the find. As far as restoration goes, here are some tips. Take them with a grain of salt.

The electronics for these machines are pretty simple. At the end of the day, its just an audio power amplifier. If you have electronics experience, you should have no trouble getting this up and running without a schematic. If you get a copy of the RCA tube manual and lookup each tube, you will be able to work stage by stage from input to output and look for proper operation. Even with no electronics experience, you can generally make you way through to restoration with some systematic steps.

If the electronics are parallel string filaments (all 6 and 12 volt tube numbers) and the AC mains are transformer isolated, it is much easier and safer to work on. I suspect this is the case for your unit. This will allow you to pull all of the tubes, isolate each stage one section at a time.

Typically, you should replace the electrolytic filter caps and any paper coupling caps. They may be good, but do not age well. Rather than trying to determine which units are bad, its cheaper and safer in the long run to replace them all. There should not be that many. In the case of the multi-section cans, you can find typically drop in replacements, but they can be pricey. The alternative is to disconnect the can and leave it in place for cosmetics. Add new axial lead single units under the chassis. Note that the actual capacitance value is not super critical (better to go a bit higher in value rather than lower). Just make sure the working voltage is the same or a bit higher. Take your time and replace one at time so you don't make a error.

Before you dive into that, pull all of the tubes and check the windings of the power transformer with an AC voltmeter. Make sure all of the filament windings have the proper voltage (you can do this at the tube sockets using the tube manual to ID the filament pins. Next check the B+ windings they should go directly to the plates of the rectifier (the tube manual should give you some idea of the voltages to expect). Use caution as these voltages will be in the 200-600 Vac range! If the amp is fixed bias, try to see if that is functional (usually a low negative voltage). The RCA tube manual has typical audio amp circuits in the back that cover most of the types of amps you'll find. Also, try to measure the primary and secondary taps on the audio output transformer with an ohmmeter. Check out the Hammond Transformer web site for some typical values for the windings. They spec them based on typical output tubes (e.g.push-pull 6L6's). Again, the RCA tube manual will give you info on the approximate output power for a particular tube running in class AB push pull fixed or self bias.

Don't rely on tube testers alone. Other than a completely dead tube, they do not always predict how well the tube will function in-circuit.

If you take you time, you should be able to get cutter up and running. Hope you find that helpful.

Mark

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