"Jim Shorney" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ----------------------------------------------------------------------
You may recall from our previous episode, I was experiencing instability in the 7-10 MHz band of my TR7 when operating at or near full power levels (140-150 watts) with the upgraded transistors. The instability seemed to be related to the setting of the predriver gain pot, so I was guessing that the predriver board was breaking into oscillation. I have been unable to confirm this. Thinking that improving the high end gain of the PA might result in needing less predriver gain, I performed the grounding improvements described by DL7MAJ in the document he published at http://www.dl7maj.de/TR-7.html. The net result appears to be a wash - although I was able to achieve higher power output on 10/15 meters with somewhat less gain (104 watts @ 29.7), the 40 Meter instability was still there. I ended up right back where I was, with the rig set for about 130 watts out on 40 Meters and dropping to about 88 watts at 29.8 MHz. Above about 140 watts or so, she starts to want to take off. Previously, I had measured the frequency of oscillation at about 9.6 MHz with a frequency counter. Tonight's testing showed that the PO on 30 Meters was slightly higher than on 20 or 40 meters, suggesting a gain peak in the 10 MHz area. Tuning across the bands, it seems that there is a broad but definite peak from about 9.3-9.7 MHz. This matches up nicely with what my frequency counter says. Whether the peak is in the predriver or PA, I can't tell. The problem with testing at high power levels is that RF tends to get into everything, so my scope hasn't been much help. I'm thinking that if the gain were rolled off a little bit more below 20 meters (where there seems to be plenty), this problem would go away. I'm pretty rusty on my power amp theory, so I'm not sure where to start. I'm wondering if anyone has been down this road before or has any suggestions? Or should I just forget it and be happy? This 40 meter instability just *bugs* me.... 73 -Jim NU0C ---------------------------------------------------------------------- Submissions: drakelist@zerobeat.net Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ----------------------------------------------------------------------
