"Jim Shorney" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- On Fri, 11 Jul 2008 23:13:07 -0500, Jim Shorney wrote:
>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. >I'm wondering if anyone has been down this road before or has any suggestions? The silence is deafening. So, diving in to the problem with wild abandon and a hot soldering iron.... Googleing the problem turned up two or three references to notes from other hams in various forums that were experiencing the 40 meter oscillation problem. The only fix that was offered was to turn down the power level, along with speculation regarding the PIN diodes as the source of all evil. This was not satisfactory. I also found references to the power output of a properly working TR7 being 140-150 watts on 80/40 meters, gradually decreasing to a little over 100 watts on 10 meters. From my experiments, it seems that the PA deck is easily capable of this, especially with the upgraded transistors. Looking at the schematic of the (version 2) predriver board, it looks like the R2203-RFC2202-C2204 network from collector to base of buffer Q2201 is designed to flatten the gain of the predriver board across the HF range by providing increasing negative feedback as frequency is decreased. I added a 1K resistor across R2203, which should reduce the net resistance to 338 Ohms. This did decrease the gain somewhat, and I am now able to adjust the ALC for 150 watts CW output on 40 meters at the point where the ALC LED *just* comes on. There is no longer any apparent instability or oscillation on 40 meters at this point. Power on 10 meters is now 105 wattts at 28.3, decreasing to 95 watts at 29.8, with ALC action across the entire band. The predriver gain pot was adjusted at 29.8 MHz, as I previously described. This provides plenty of ALC at 28.3, and just enough at the high end of 10 meters. So far, the PA has been rock stable on all bands after this modification. As the manual states, for proper operation under normal operating conditions the carrier level should not be set beyond the point where the ALC LED just comes on. Observing the output on a scope, power does not increase but the carrier does start to garbage up beyond this point. Better yet, back it off so the ALC light just goes out. On SSB, I like to stay out of ALC as much as possible for the reasons cited here: http://www.nitehawk.com/sm5bsz/dynrange/alc.htm So in a nutshell, to cure a TR7 that oscillates at full power on 40 Meters, try this: 1. Decrease the value of R2203, either by adding a 1K in parallel or replacing it with a 330 Ohm resistor, and readjust ALC and predriver pots per service manual spec. 2. If output on 10/15 meters is still low, try the PA board grounding improvements described by DL7MAJ at http://www.dl7maj.de/TR- 7.html. This MAY NOT work for all cases - this is what worked for me, it may not work for you. YMMV. Standard disclaimers apply. It's your responsibility if you let the smoke out. In stereo, where available. Contains no trans-fat. Note: I do not believe this problem to be a reflection on the upgraded transistors suggested by K8AC, VE3EFJ, and others. Rather, the notes I found seemed to indicate that it can happen in a completely stock TR7, and it seems likely that it is related to component and build variations in individual radios. The work these gentlemen have done is excellent, and I humbly thank them once again. Now, to see what other havoc I can wreak in my poor TR7 while the covers are off... 73 -Jim NU0C ---------------------------------------------------------------------- Submissions: [email protected] 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 ----------------------------------------------------------------------
