It's a Gyraf Pultec! It's a Radio! You're both right!

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underthebigtree

Well-known member
Joined
Aug 29, 2004
Messages
143
Location
San Mateo, CA USA
With apologies to the SNL sketch of yesteryear...

So I've just finished soldering up most of the cables for my new studio in San Mateo, and sat down yesterday to do my first mixdown. Patched my trusty Gyraf Pultec into the bass track...

And heard the lovely sound of KCSM-FM pumping through my system. And not just a little bit - aside from some sideband noise, it was clear as a bell.

It turns out that KCSM is less than a half mile from my house. And that's normally great, because it is the only true jazz station in the Bay Area. But not great when trying to use my Pultec!

The unit is in a Par Metal chassis, and I've never had a lick of noise before. None of my other DIY units have any radio interference. The whole system is on balanced power.

Any suggestions?

Thanks,

Nick
 
Aside from the usual possibilities (is the chassis ground no longer connected to the other grounds?), the first question is: does the level of the decoded radio signal change when you adjust the controls?

Second question: have you tried using different cables?

Peace,
Paul
 
The fact that it's demodulating an FM station is significant. That points to the involvement of a tuned circuit; add a diode--a triode's grid and cathode will do nicely--and you've got a slope detector. And since we're talking about a box filled with tuned circuits and tubes, it makes perfect sense.

Are you using an input transformer? If not, add a little RF filtering at the input. Also try a couple hundred ohms of "grid stopper" on the amp input. And do check your ground/shield connections.
 
The signal does not change when I change the controls. There are Lundahl trafos on the ins and outs of both channels, and balanced cable to the balanced patchbay.

I'll check the grounding, and will experiment with different cabling to the patchbay. If that doesn't work, I'll try a small resistor as NYD suggested.

Thanks!
 
Is that the station at San Mateo High School, they play all the big band stuff, commercial free?

They must have the antenna on top of the high school instead of Black Mountain.

Just go snip the coax. Wear asbestos gloves. :razz:
 
Hmmm, was expecting to see a freshly finished DIY-Pultec in an old radio-enclosure here... :cool: :roll:

Hope you can get rid of the unwanted stuff....
 
Does your pin 1 of the xlr, or shield terminal go directly to chassis ground with a short wire,, or does it go to the star ground buss?
The Lundahls should be excellent low pass filters below rf. So it's getting onto the ground rail somehow and being demodulated thru rectification somewhere like Dave sez. And it may Not be FM. A lot of poorly tuned FM transmitters emit much incidental AM. Since the removal of a requirement for an FCC license to maintain broadcast transmitters, chimps are often hired to turn the screws.

I'm curious if it goes away with the input lead removed.
 
Problem fixed! You guys pointed me right to it.

The audio ground was in fact floating - not attached to chassis/power ground. It was never a problem at the previous locations this baby lived at. But moving so close to a radio transmission tower clearly made it a different matter.

Wiring the audio and power grounds to the chassis lug at the input XLR solved the problem handily.

And once again, I learned something. One more mistake not to be repeated. :oops:

Many thanks for your help!

- Nick
 

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