Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Jim -
"I" would connect a good ground between the SP-75 and TR-7. My years as an RFI/EMC Engineer with NASA convinced me that the first step when RF is afoot is to have everything connected together with low impedance ground straps. Multiple ground paths can cause problems too, but in this case I would say that having a low Z connection from the SP-75 to the TR-7 is the best way to keep RF out of the audio. Let the SP-75 get it's ground return through the TR-7 ground return rather than directly to the supply. Make sure the wire between the supply and the TR-7 is big enough. The TR-7 can draw a LOT of current, more than modern transceivers, and voltage drops in the ground lead can cause all kinds of problems.
73, Garey - K4OAH Glen Allen, VA Drake 2-B, 4-B & C-Line Service Supplement CDs <www.k4oah.com> Jim Shorney wrote:
"Jim Shorney" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Hi gang, Since the 8BN8 mystery was so quickly solved, here's another question. Both this year and last year at Field Day, I found that if the SP75 was not very carefully adjusted, the PS75/TR7 system was plagued by RF feedback on the transmit audio. It seemed more dependent on output level than input gain of the SP75. IIRC I had to turn the output level up and decrease the mic gain on the TR7 to tame things. I had not noted this effect using the SP75 at home. The processor is connected per the manual, directly to the power supply with no extra grounds in the system. Antenna was a 4-element 20 meter monoband beam on a tower, probably 35 feet in the air and maybe about the same distance from the comm shack. No earth ground at the station or the antenna, and the antenna was resonant around 14.2 according to my MFJ box. Before I re-invent the wheel, does anyone have any suggestions or ideas about what's behind this? Looking at the SP75 schematic, it looks fairly well filtered and bypassed, but there may be room for improvement. Or perhaps the mic input on the TR7 could use some help...? TIA, -Jim -- TR7/RV7, TR6/RV6, T4XC/R4C, L4B, NCL2000, SB104A, R390A, GT550A/RV550A, HyGain 3750 - all vintage, all the time!
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