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Hello All,
Since a couple of days I have an annoying problem when operating 6m,
I have the power output limited to 30W, but sometimes I see the A-meter of my PSU going all the way up while transmitting. Power output and ALC go to max, no matter what, even without modulation. This happens on all modes.
I checked with a spectrum analyzer, and found out that when this happens, it outputs a high signal on around 47MHz, and nothing on the expected frequency. This happens irregularly, sometimes it is ok for hours, and suddenly it's not, for a couple of transmissions. Often it resets after a few transmissions.
I believe I have seen it on other HF bands as well, but recently I have only been able to replicate it on 6m. In any case, it seems to happen more frequently on 6m.
It looks like some oscillation is going on in the unit, causing maximum output on a wrong frequency?
I removed all the input and output connections (except the antenna and mike), but this does not change the behaviour. My antenna is reasonably well adjusted, VSWR <1,5. I haven't checked it with a dummy load yet, will do that later.
Does anybody recognizes this?
73
Peter/PA3FQH
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It is known that the 857 tents to have spurious oscillations once in a while. I think it is a feedback from the filters into the first stages of the RF generation. What I suggest is: ?- remove top and bottom enclosure ?- clean all mating surfaces, I use sandpaper
?- I add thin 1 mm gaskets (foam over fabric) to improve contacts ?- check all screws that hold the board down.
?- close the unit, add the four screws from mounting into a car. Be careful not to use too long screws ?- I also add ferrite loaded elastomer on the inside of the enclosure on the RF power amp side. I do not know if this helps and is needed.
Overall, I think very good contact of the PCB to the frame and the lids to the frame are needed. Last time when I had oscillations I found that I had forgotten to add a screw to the lid
David?? OE6NZM / AD0PY / DL7ZM
Hello All,
Since a couple of days I have an annoying problem when operating 6m,
I have the power output limited to 30W, but sometimes I see the A-meter of my PSU going all the way up while transmitting. Power output and ALC go to max, no matter what, even without modulation. This happens on all modes.
I checked with a spectrum analyzer, and found out that when this happens, it outputs a high signal on around 47MHz, and nothing on the expected frequency. This happens irregularly, sometimes it is ok for hours, and suddenly it's not, for a couple of transmissions. Often it resets after a few transmissions.
I believe I have seen it on other HF bands as well, but recently I have only been able to replicate it on 6m. In any case, it seems to happen more frequently on 6m.
It looks like some oscillation is going on in the unit, causing maximum output on a wrong frequency?
I removed all the input and output connections (except the antenna and mike), but this does not change the behaviour. My antenna is reasonably well adjusted, VSWR <1,5. I haven't checked it with a dummy load yet, will do that later.
Does anybody recognizes this?
73
Peter/PA3FQH
|
I seem to recall that one way to do something about this is to open up the rig, then clean all the grounding points for the circuit boards, tighten all the screws securing the cct boards or improve them, this was an issue many years ago. Interesting that these old issues with power cords, interfacing and oscillations are resurfacing. I haven't searched Google yet, but I wonder if some of the websites detailing these issues are still up and running.
John VE7KKQ
Hello All,
Since a couple of days I have an annoying problem when operating 6m,
I have the power output limited to 30W, but sometimes I see the A-meter of my PSU going all the way up while transmitting. Power output and ALC go to max, no matter what, even without modulation. This happens on all modes.
I checked with a spectrum analyzer, and found out that when this happens, it outputs a high signal on around 47MHz, and nothing on the expected frequency. This happens irregularly, sometimes it is ok for hours, and suddenly it's not, for a couple of transmissions. Often it resets after a few transmissions.
I believe I have seen it on other HF bands as well, but recently I have only been able to replicate it on 6m. In any case, it seems to happen more frequently on 6m.
It looks like some oscillation is going on in the unit, causing maximum output on a wrong frequency?
I removed all the input and output connections (except the antenna and mike), but this does not change the behaviour. My antenna is reasonably well adjusted, VSWR <1,5. I haven't checked it with a dummy load yet, will do that later.
Does anybody recognizes this?
73
Peter/PA3FQH
|
I remember this issue coming up on
the group from time to time, over the years.? So far as I
remember, it's usually caused by a loose screw holding the PCB
to the chassis which isn't making good electrical contact.
???? Good luck,
????????????? cheers,
??????????????????????? Darrel,
aa7fv.
On 6/28/2021 07:07, David wrote:
toggle quoted message
Show quoted text
It is known that the 857 tents to have spurious
oscillations once in a while. I think it is a feedback from
the filters into the first stages of the RF generation.
What I suggest is:
?- remove top and bottom enclosure
?- clean all mating surfaces, I use sandpaper
?- I add thin 1 mm gaskets (foam over fabric) to improve
contacts
?- check all screws that hold the board down.
?- close the unit, add the four screws from mounting into a
car. Be careful not to use too long screws
?- I also add ferrite loaded elastomer on the inside of the
enclosure on the RF power amp side. I do not know if this
helps and is needed.
Overall, I think very good contact of the PCB to the frame
and the lids to the frame are needed.
Last time when I had oscillations I found that I had
forgotten to add a screw to the lid
David?? OE6NZM / AD0PY / DL7ZM
Hello All,
Since a couple of days I have an annoying problem when
operating 6m,
I have the power output limited to 30W, but sometimes I see
the A-meter of my PSU going all the way up while
transmitting. Power output and ALC go to max, no matter
what, even without modulation. This happens on all modes.
I checked with a spectrum analyzer, and found out that when
this happens, it outputs a high signal on around 47MHz, and
nothing on the expected frequency. This happens irregularly,
sometimes it is ok for hours, and suddenly it's not, for a
couple of transmissions. Often it resets after a few
transmissions.
I believe I have seen it on other HF bands as well, but
recently I have only been able to replicate it on 6m. In any
case, it seems to happen more frequently on 6m.
It looks like some oscillation is going on in the unit,
causing maximum output on a wrong frequency?
I removed all the input and output connections (except the
antenna and mike), but this does not change the behaviour.
My antenna is reasonably well adjusted, VSWR <1,5. I
haven't checked it with a dummy load yet, will do that
later.
Does anybody recognizes this?
73
Peter/PA3FQH
|
Thanks all,
Will give it a try next week. I will post results when done
73
Peter/PA3FQH
|
I experienced a similar situation where the ALC display would max out on 6m when operating digital modes and the supply voltage displayed on the rig would drop to approx 9 volts, but the voltage displayed on the power supply was normal (13.8).? I didn't think to switch the power supply meter to show current.
It looks like I might have experienced a similar, if not the same issue.
I searched the group for "ALC maxing out" and similar terms but didn't find anything relevant so gave up and?swapped the rig out with an FT100D that was in my mobile so I didn't ever get to the bottom of the issue.
I'll pull the covers off the 857 again and check the contacts on the mounting screws etc.
Thanks for this info.
--
73
Rob
VE3RXH
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I have opened the unit, and re-tightened all the bolts that hold the PCB's
Most seemed OK, but some could be turned a little.
Tested it, and until now (2 days) I haven't seen the fault re-appearing, while before it was almost permanent on 6m.
So it seems to do the trick.
I tried to understand what happened. I can't believe that the screws are actually too loose. I noticed they are (probably) zinc plated. I guess that over time some oxyde layer develops,causing high impedance at certain positions. Moving the screws a bit (losing and re-tightening) will probably damage the oxydelayer enough to lower the impedance to within tolerances to avoid oscillation.
Another explanation can be oxydation in the tapped holes in the alumin(i)um frame they are connected to?
The solution may be temporary, will keep an eye on it.
73
Peter/PA3FQH
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