The AM Forum

Hi

As seen in my first post, I have obtained an old RCA OCXO from an AM broadcast band transmitter. I am carefully cleaning and restoring this unit. First, what product if any should I use to remove 80 years of grime from the chasis?

Second, I am going to build a power supply for this, and I am concerned with RF going down the filament wires and into the mains. I am thinking about using a large ferrite antenna rod from an AM radio, and winding two 12 GA wires in a bifilar manner on the ferrite rod. Does this sound like a viable means of eliminating this problem?

Thank you again for sharing your wisdom.

Joe

[quote author=KB0TXC link=topic=49099.msg346056#msg346056 date=1742354447

Second, I am going to build a power supply for this, and I am concerned with RF going down the filament wires and into the mains. I am thinking about using a large ferrite antenna rod from an AM radio, and winding two 12 GA wires in a bifilar manner on the ferrite rod. Does this sound like a viable means of eliminating this problem?
Joe
[/quote]

How do you know you're going to have a problem?
Build it, test it, and then worry/fix any issues. Over thinking the design is sometimes not the best answer.

You can make it easy on yourself by using a line filter module. Perhaps a builder in your area has one in his junk box. One style would have the IEC/C14 plug in one end and would eliminate the hassle of the line cord. Whatever you do, don't have the input leads and outputs double back on themselves as this negates a lot of the HF filtering.

Here's an example of what one look likes, there's many models;
 from Mouser:

https://www.mouser.com/ProductDetail/Wurth-Elektronik/810912001?qs=sGAEpiMZZMv0DJfhVcWlK5mYDxK9NoKRXUb5Sd5jUz%252BkQ3nzLVlDwQ%3D%3D

Not easy for RF to pass thru a power transformer, also RCA built RF bypass capacitors on the filament legs of all the RF stages to conduct any RF at that point to ground. I ran an RCA BTR-1 on 160 and worked on many broadcast transmitters for years and never had a issue of RF on power lines with the exception of RF being picked up on the AC mains because of the proximity of the stations antenna and that has never been a concern. But then again, I only worked around little 1 and 5 kW AM stations, don?t know what it would be like at a 50 kW property.

   I agree with Pete; getting it up and running is your first priority. Worrying about things that are extremely unlikely to happen will only be a distraction. It's an oscillator, not a high-powered transmitter, and the 807 uses an indirectly heated cathode with the heater most likely bypassed for RF. Whatever feeble RF energy makes it beyond that would have the proverbial snowball's chance of getting through the filament transformer and into your AC service. If it doesn't happen with broadcast transmitters, like KA3EKH said, it's not likely to happen with your oscillator.
   

^^^This right here. Filament transformer is going to block any RF that might be riding on the filament lines.

Thank You!

I remember back in the early 70s when CB was a huge phenomenon, and this op near where I lived bragged about his "lin-ear", and there was splatter and rfi everywhere... I never wanted to be that individual, so I always used filters and spent an inordinate amount of effort to only have a very clean and legal signal.

I guess I was being a tad bit retentate with that idea:-)

Thank you again,

Joe

Weird thing about RF on AC power lines is that in college back in the seventies and eighties it was common for most schools to have what was called carrier current transmitters. They were small AM transmitters that would broadcast a AM signal on the broadcast band for student radio on campus. Most transmitters were ten to fifty watts and built by LPB and they would be installed in the dorms where the AC power came into that building. On a campus with several dorms you would have a transmitter in each dorm because as soon as the AC power left the building it would hit a power transformer and disappear.  We used dry pair phone lines to feed all the transmitters on campus from one central studio location, usually in the student union. The transmitters were designed for fifty Ohm outputs but they feed a coupler unit that connected them directly to the incoming three phase power for the building. You would be able to pick up the station in the building and maybe part of the parking lot but not much beyond that.
Hard to believe today but by taking care of the student radio station, all its transmitters and that sort of stuff that paid for my time in college.
By the nineties everyone started to listen to FM and forgot about AM, many schools put in FM transmitters on there cable TV plants and some went so far as to do LPFM over the air stations and carrier current radio disappeared from the scene. I have been told also that AFRTS used huge transmitters on military bases back in the day for carrier current operations but never saw one myself.

I just checked a receiver transformer for inter-winding capacitance between the primary to the secondary HV and filament windings in parallel; I came up with 370 pF.

As a simple line filtering, just put a 0,047 uF or around that value connected to ground from hot line after the switch and fuse, should be a Y class rated capacitor.

You mentioned filament wires plural so it sounds like both filament wires are floating at the OCXO end, good. Check with a capacitance meter to see if there is a reasonable r.f, bypassing on the filament wires to chassis. If you don't see a total of 0.1 uF or more, go in there and add two r.f. bypasses.

Even with the filter, if the rest of the circuitry is not shielded or the power line is not shielded, the free-space RF will just couple onto the power line on the other side of the filter.