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Post by hifiman on Nov 30, 2012 12:06:19 GMT -5
I have a Tram D-201 hand wired version. Works great on rec and tx on AM. Works great on rec. in SSB. However when I key the mic in SSB it most the time shows a 4-5 watt DK and swings to 20. When I let go of the PTT it falls back to zero but quickly climbs back up to 4-5 watts DK and somethimes continues to climb up to almost 10 watts DK with no modulation and mic even turned all the way dow. Any thoughts would be great. Oh. I should mention that it has the VFO mod.
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Post by FIXR on Nov 30, 2012 15:41:32 GMT -5
Hifiman,
Sounds like you need to check the neutralization of the final output tube.
Alan
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Post by hifiman on Nov 30, 2012 16:04:15 GMT -5
Good idea. I bet the previous owner replaced the tube without doing so. I will check it out.
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Post by 2600 on Nov 30, 2012 16:18:40 GMT -5
Having a frequency counter in the coax line when this happens can be a big help. Good chance that you would see a frequency that is not anywhere near the channel that you're on. The final stage has a square ceramic-body compression trimmer capacitor mounted to the underside of the chassis between the driver tube and the final tube. The mount bushing and a slotted-end shaft are visible on the top side. This is the neutralization adjustment. It's there to stabilize the final and prevent it from oscillating and producing its own carrier when there is no drive.
On AM transmit, the full-level carrier drive to the final will "swamp" the final tube's internal RF feedback and prevent that second frequency from coming out of it. But on SSB, there is NO drive to the final tube until you modulate it. That makes the final tube more sensitive to small amounts of unwanted RF feedback.
That capacitor will often get tweaked by an AM operator trying to get "just a little" more power. The correct setting is usually fairly "tight", at the end of its travel where there will be friction on the screw head. If it's set at the loose end of its travel, this is your problem.
This control is marked "C706" in the schematic. The factory manual shows a cumbersome procedure for setting it correctly. Good chance that using the "by gum and by gosh" method will work well enough.
Just be aware that turning C706 will change the setting of the driver output coil T700. Any time the setting of C706 gets changed, you'll need to re-peak the slug in T700 to compensate. It will also change the peak setting of the Plate Tune control C711, but not as much.
The VFO in that radio is not stable enough for normal SSB operating. The internal VFO will drift while you transmit, even if it is stable enough for decent receive use. The only way to fix that is to disconnect the heater supply to the VFO tube V302 and feed that heater from a regulated source. More trouble than it's worth, IMHO.
It's a decent SSB radio when using crystals. But the internal VFO is only stable enough for AM mode when you use it to transmit.
73
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Post by hifiman on Nov 30, 2012 16:47:52 GMT -5
Thanks. Here is what I did and it seems to have corrected the issue. There are two 6L6 tubes. I swapped them around. Now I am getting a DK of 4.5 watts and swing to 14 on AM. On SSB I now have no DK carrier and with modulation it swings to 25. Could be someone had put each back wrong. Question. Is there a way to lower the DK without adding a resistor. Can I just detune on the back adjustments. Thanks for all your help.
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Post by hifiman on Nov 30, 2012 17:05:40 GMT -5
Note. Did hook up a Freq counter and it shows correct all the way up the VFO scale on AM and SSB.
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Post by 2600 on Nov 30, 2012 23:27:39 GMT -5
Way cool! Good to hear it was that simple.
And you have discovered the reason for the adjustment at C607.
The "unwanted RF feedback" actually happens inside the tube. You have probably noticed that the shape and arrangement of parts inside the two tubes look a bit different. Those differences between one production line and another won't affect the way an audio amplifier works. As a result, there is no reason to control the small "accidental" capacitors that the tube's internal parts represent to a RF current. Some of your RF output signal feeds back into the tube's input right inside the tube. Those small mechanical differences between tubes will alter the precise level of that RF feedback.
The neutralization circuit feeds a controlled level of negative feedback into the final's input circuit. It's meant to cancel out the positive feedback created inside the tube. Cancels it out so the tube won't oscillate. Differences between production versions of the final tube make C607 necessary, so it can be set to match tubes from more than one factory.
But swapping tubes is way easier than screwing with C607. If you collect spare tubes for this radio, finding one that's a mechanical match to the one you're using could protect you from having to do the neutralization procedure when the final wears out and have to be replaced. Ignore the brand name painted onto the glass. Doesn't tell you who made a tube, only tells you who packaged it. The appearance of the gray or black type number print and the layout/appearance of the tube's internal parts is a better guide.
73
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