Raytheon RR-10 hum

I have one of these on the bench with a pretty major hum. All caps have been changed. Hum appears to be coming from the first stage, goes away with grid grounded. This one came in missing the input transformer, so I installed a UTC CG-135 connected for 250ohms/120kohms. UTC claims 30dB of hum rejection for this transformer. There must be about 90dB of pure hum if it's shielding 30dB!

I noticed that the (factory installed) PSU was miswired. The 6X5-GT was connected as half-wave rather than full wave. One of the plates was wired to ground instead of the other half of the secondary. I redid it as a full-wave and I think I have less buzz, but same amount of hum.

If the noise goes away when you ground the first grid, the problem is in your input termination. Try a 10K only on the first grid, is it quiet? Is your transformer secondary open circuit? All metalwork and sheilds on transformer grounded?

radardoug said:

If the noise goes away when you ground the first grid, the problem is in your input termination. Try a 10K only on the first grid, is it quiet? Is your transformer secondary open circuit? All metalwork and sheilds on transformer grounded?

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Right now, I have the secondary unterminated, as recommended by RCA for use with ribbons. We use ribbon mics almost exclusively.

Not the point.

Remove the transformer from the equation and listen with 10k grid to ground. 

EmRR said:

Not the point.

Remove the transformer from the equation and listen with 10k grid to ground.

Click to expand...

10k R to ground gives much improved amount of noise. Still a little hum present.

I have the CG-135 transformer connected as follows: Pri - 2 & 5 / Tie 3 & 4 , Sec - G & G / Tie F & F.

I have used this transformer with same terminal arrangement on another preamp with no hum problems. I determined the connections with a meter, as I was unable to find documentation on this transformer.

Catalog description attached.  Humbucking design, so perhaps it is wired wrong after all?

Terminals are labeled 1 - 5 on primary and G-F, G-F on secondary. It has worked as I have it wired in another preamp.

Dmichel123 said:

Terminals are labeled 1 - 5 on primary and G-F, G-F on secondary. It has worked as I have it wired in another preamp.

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Sorry, I don't doubt it works as wired, but perhaps the humbucking effect requires a different arrangement.  Just guessing here.  The other preamp may not be so prone to hum.  I've looked through my files but can't find the CG series pinouts and schematic.

On that transformer, which pin is shield? Have you tied it to ground?

I found a different Primary connection which is giving less hum. 1 & 3 , None tied. Still not quiet enough, but much better.

radardoug said:

On that transformer, which pin is shield? Have you tied it to ground?

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No pins tied to shield, as far as I can tell. I assume the case makes contact through the chassis screws.

Look for a pin that doesn't connect to either primary or secondary. That is likely to be a shield.

There are no shield pins, as far as I can tell. I measure 28 ohms between pins 1 & 3, with a center tap at pin 2. Pins 4 & 5 are a separate primary winding, 14 ohms.

It is frequently through the case. 

I'm also getting a high frequency oscillation at higher gain settings. Grounding one side of the OT secondary gets me a lot more pot rotation before the oscillation shows up, but it still happens.

I may have a original input xformer for this if you still need it.

Peace,
DrRick

Does anyone have a schematic for the RR-10?

Thanks,
David

mrtnasty said:

Does anyone have a schematic for the RR-10?

Thanks,
David

Click to expand...

I may, let me know if you need it,
PEACE,
DrRick

DrRick said:

I may, let me know if you need it,
PEACE,
DrRick

Click to expand...

Yea that would be awesome!

Best,
David