"K8AC - k8ac" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Jim - Just received this message via email and it appears to have come from
the drakelist at zerobeat.net site, but when I go there, your message is not
in the forum there. Maybe you can tell me the exact URL of the site where
you posted it?
My TR7 with the rebuilt PA ran just fine for quite a while, until I erected
a new 40 meter vertical. The first time I used that antenna with the TR7 on
40, the final oscillated. Switching back to a dipole antenna - no
oscillation. Really odd, since there is virtually no reactance seen on
either feedline and the antennas are a ways from the rig. Anyway, dialing
back the gain pot in the pre-driver eliminated the oscillation but of course
reduced my output on 10 meters again. I'll have to try your change and see
if that changes things.
73, Floyd - K8AC
- Original Message -
From: "Jim Shorney" <[EMAIL PROTECTED]>
To:
Sent: Sunday, July 20, 2008 12:40 PM
Subject: Re: [drakelist] TR7 oscillation
"Jim Shorney" <[EMAIL PROTECTED]> made an utterance to the drakelist
gang
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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 gentle
