Msg#:22462 *SW ECHO* 03-07-89 20:27:00 From: STAN GAMMONS To: ALL Subj: RFI I seems that we are at the RFI question again, so here goes. I don't pretend to be an expert in this field but here is some of my experiences. If you are using a PC compatible, first determine what the source(s) of interference is. Since most "clones" and true blue IBM are pretty much similiar, most everyone has attached devices. I.E. printer, monitor, modems and etc. Start by disconnecting the power and adapter cables to the monitor. Most likely the adapter cable is the radiator. If the interference is still present, continue disconnecting devices until you find the one that is the major source. Once you have found it the cable to the device will need to be "choked" with a Toroid(s). I recommend one around 2.5" in diameter for use on the modem and power cables. This size allows you to wrap several turns, usually 5 or more , without too much difficulty. The Amidon FT-240-77 seems like a good choice. I have all the toroids on my cables about 4" from the PC end of the cable Now that one source is attenuated or eliminated, start on the other cables. The serial ports on my AT clone were a major source of interference. Com1 is an IBM serial/parallel board connected to a USRobotics Courier HST. The HST seems to be pretty quite RF wise. The RF comes from the serial port. A toroid with a couple of turns eliminated the RF. Com2 is an IBM async card used for the PK232 and still needs to be tended to. The pk232 radiates a little RF but a shielded cable usually will cure most of it. The keyboard was the next big source. A toroid and 6 turns took care of it. The more turns you have around the toroid the higher the impedance to RF, thus reducing the RF. If the RF is bad you made need more than one toroid. The toroids used in the correct places have cured most of my RFI. I can still hear it at various frequencies but it's usually S1 to S3, not too bad. If you have every cable disconnected and there is still interference, then it can get tough. I HIGHLY recommend that if you are building your own "clone" that you get a GOOD case. I choice a Bandy Inc. "tower" case. It appears to be the same one that NorthGate uses for there 80386 machine. It's a good solid case that doesn't radiate alot of RF. If you aren't fortunate enough to have a "low RF" case then other measures need to be taken. I don't have much experience here but I have read that there are sprays that can be sprayed onto the inside of the case to help attenuate the RF. If you've ever opened an IBM monitor and looked on the inside you know what I'm talking about. The spray contains tiny particles of what appears to be metal. The metal when sprayed on creates a shield around the device and RF can't (well not as easily) get through. Since it contains metal care should be taken to avoid getting it on the electronics or you'll likely blow up something. Inside the case there are several sources of RF. The Disk/diskette controller, power supply, display adapter and etc. The disk cables can be fixed with some "split core" type ferrites. The power supply can be fixed with the toroids. The display adapter will probably need to be fixed externally with a toroid. If you are using an outside antenna, the wire from the antenna to the receiver can pick up the RFI. I used some RG-6 that I had lying around for the lead-in. RG-6 has a double shield (braid and aluminum foil) and it dropped the RFI 3 or 4 S units As a good rule of thumb, most every device can be a suspect for RFI. Do not overlook anything. Consider it guilty until proven different. You may also find that the interference is at different frequencies. This can be a long drawn out process, especially with 30+ mhz receivers. The interference from my AT clone was primarily above 10 mhz but it can be anywhere between .1 and 30 mhz. To do the "RF eliminating" I just got on a frequency that had a high level of RFI and started tinkering with the cables and toroids. The Toroids can be found at various sources. Amidon, Palomar, MFJ and even your local Radio Shack has some of the snap on variety. As for the sprays, well I don't know. Perhaps someone with more experience in this area can provide a source. The Amidon catalog offers some suggestion on eliminating RFI. The catalog alone is useful for this information. This is all that I can think of of the top of my head, so good luck. --- ConfMail V4.00 * Origin: The Unknown Opus - Lebanon, Tn. 615-443-4306 (1:116/4)