RCA 6-EY-3A Question

[Hi-Fi-Mogul]

Hello all,

I may take on my first RCA 45 rpm record player servicing.

Looking at the schematic, I see a tap on the OP transformer
that is not Center tapped.

Is this another type of negative feedback ?

And why in that section of the coil ?

Inquiring minds....

Thanks.

[electra225]

They used the center tap for hum bucking. They go open. If found open, you can go around them.

[Hi-Fi-Mogul]

Yes, I know about the center tap use, but the
diagram shows a different location of the tap.

Usually the tap is in the center of the coil on the
diagram from my experience.

[electra225]

Typically, a center tapped output transformer is used with push pull audio output tube. Since this amplifier is single ended, there will not be a center tap.

[Hi-Fi-Mogul]

Good information.

So, is this tap a negative feedback design for SE amps ?

Thanks !

[electra225]

RCA liked this circuit. When I made my comments earlier, I was at work on my phone. After I got home on my computer with its large monitor, I noticed that this design is a tad different than that used in radios. In radios, they fed back a portion of the audio into the screen of the output tube. Here, they feed a portion of the audio into the grid of the 25L6. Typically one finds a portion of the audio output being fed to the grid of the first audio tube, here a 6AT6. This is found more commonly in higher-output amps with push-pull audio and a phase inverter stage. No matter, the purpose is the same, to control the gain of the amplifier and to stabilize the output. Stabilize against distortion, oscillation, and, in this case, hum. The way this feedback loop controls the gain is to drive the grid of the tube more positive. For maximum gain, the plate must be highly positive in relation to the grid. When the grid is driven positive, this cuts down the amount of electrons reaching the plate. This lowers plate current and the gain of the stage. Changing the value of R5 will change the amount of feedback. This resistor should be pretty close to value for the stage to work correctly. Oscillation may be the result of too much feedback. Too little feedback may cause distortion, since the stage will be over-driven. We can refer to this setup as negative feedback, since it controls the gain of the output stage.

[Hi-Fi-Mogul]

I took to the dis-assembly of the RCA.

For this model, after the screws are removed from
the changer/amp mounting base, lift up the
motor board from the rear.

This enables the amplifier to clear the cabinet.

The amp is attached to the motor board by 2
brads with their ends folded over to hold the
amp on the board.
The speaker connections are push-on plugs, so
the amp OPT wires are easily dis-connected.

The motor rotor would barely turn by hand.
I wanted to separate the changer from the motor,
for ease of service.

I unsoldered the 2 audio leads from the amp to
the terminal strip on the changer.
I did not remove the amp from its mount, but
I freed the motor AC switch from the changer
base.

That made removing the motor from the changer
base fairly easy, and allowed the changer to be
separated for servicing.

This unit was serviced in Aug 2017, and it has
"new" idler and drive wheels.
I found that by manually turning the spindle, the
changer functions seemed correct.

So, outside of the complete motor clean/lubrication,
I did some cleaning of critical points in the cycling
mechanism, without any changer dis-assembly.

One thing I was glad to learn about, was the idler
wheel being held on its shaft by a spring.
I learned that from Chris Cuff's (RIP) videos on the RP-190.

I cleaned the surfaces of the inside rim of the platter,
idler, and drive wheels with alcohol.

I re-mounted the motor, re-attached the AC switch, and
wired up a spare speaker for a mechanical test with 45s
on the record spindle.

The little RCA works like a champ.

I ordered a new stylus for the P-188 cartridge
that someone had installed.

The most challenging parts, was removing the old
Pfanstiehl 793 stylus.

The person who installed the new cart, had glued the
end of the stylus body onto the end of the tonearm.
Not sure if that was intentional, or a bit of sloppiness.


.

[electra225]

I was going to recommend you watch that Chris Cuff video, but I see you already have. Those little changers are not really that tough, but everything has got to be right. Changing the needle, even in the original cartridge, is/was a pain. On mine, I'll need to remove the tone arm to change the needle. I used a kit from VM Enthusiasts, so maybe Gary has a better idea. I think I remember reading an article where you can set the changer cabinet on edge, then extract the needle that way. Those are amazing little changers. Mine is the 168, so yours is a more modern version. I have a sapphire needle in mine, I think a diamond would be a better investment. Nice job, Mr. Mogul, as usual....

[TC Chris]

Kind of odd to have vol. & tone controls inside the cabinet!

Chris Campbell

[William]

I have one of those, same cabinet, same changer, but I think mine has three tubes and I don't remember the amp being attached to the motor board. I will check to make sure, old age and memories.

Bill

[Hi-Fi-Mogul]

The Pfanstiehl 793-D7M stylus came in,
and I installed it on the cartridge.

I put eight 45 records on the spindle, and they
played through fine.

I did tweak the tonearm set down position with the eccentric
adjustment stud underneath.

The customer will pick up Saturday.

I'm glad this was not a grueling service item. [ : )

[William]

Congrats, Mr. Mogul, it is nice when one turns out ok and you will have a happy customer.

Bill