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Post by frogman on Feb 10, 2009 13:09:28 GMT -5
My VFO is open.I used to get up to 27.555. Now I can olny get to 27.510 then the numbers on my freq counter strart flipping all over the place. Any ideas
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Post by Night Ranger on Feb 11, 2009 9:55:17 GMT -5
My VFO is open.I used to get up to 27.555. Now I can olny get to 27.510 then the numbers on my freq counter strart flipping all over the place. Any ideas I've seen some PLL circuits that would lose lock as the channel frequency increased. Usually the VCO voltage was too low. Increasing the VCO voltage to spec corrected the problem. As far as I know the Tram D201 uses crystal synthesis instead of phase lock loop, but checking that the oscillator voltage(s) on the Tram are within specification is a good place to start. If you don't how to do that then it is time for a trip to a qualified technician. |
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Post by frogman on Feb 23, 2009 13:42:13 GMT -5
Thanks... I am tired of sending the old girl to the Tech.I could of bought a new nice radio by now.Oh well thats how it goes.New problem now is when I unkey on ssb the freq drifts.I am about ready to pack it up.Although,these radios are not the most stable for ssb.
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Post by 2600 on Mar 13, 2009 2:14:24 GMT -5
The SSB drift problem has a cause that's not terribly obvious. I remember back in the day folks would put big, fat 10-Watt zener diodes onto the VFO tube to regulate the B+, and it would still drift. Eventually they figured out that this doesn't help enough to tell, if at all.
The problem is caused by the radio's HEATER voltage dropping while you transmit. The added load on the radio's power supply during transmit causes the voltage on ALL the transformer's secondary windings to fall slightly when the mike is keyed, including the 6.3-Volt heater winding. If you look inside an old lab-grade RF signal generator from the 50s or early 60s, they would use a tube as the RF oscillator. ALL of the lab-quality generators had a separate regulated power supply section for the heater in that tube. It's the only way to make it stable.
On AM, this amount of drift is impossible to hear on the other end. But on SSB, it's enough to make you sound terrible after you have been transmitting more than a few seconds. And the drift you hear on SSB receive is from the heater in the VFO tube responding the slightly-higher voltage it gets when you unkey. The tiniest rise or fall in the temperature of that tube's cathode will cause the VFO to drift.
I have worked for some devoted Tram fans over the years, but nobody ever volunteered to pay the development cost to put a regulated power source in the radio for just the heater on just that tube. But this is the only way that the 'drifts while keyed on SSB' problem will ever go away.
Oh, and I think your VFO problem with the reduced dial coverage is being caused by bad feedback capacitors on the VFO tube, C322 and C324. They are the big 510 pf silver-mica caps just towards the front from V401. One of them will start to lose capacitance and cause this. Best to replace both at the same time.
73
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Post by dj406 on Mar 25, 2009 9:16:05 GMT -5
I wonder if adding a separate filament transformer would stabilize the heater voltage.
I havent played with my 201a for a while, maybe Ill drag it out and see if it still works
Doug
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