[TenTec] Omni VI problems, continued

[VE1BN]

Ken -

Please refer to my earlier post.

I erred when I said it was the PBT board.  It is actually the 9 MHz Filter 
board.
Sorry about my faux pas.  Please change board nomenclatures.

73 -  Don

---- Original Message ----- 
From: "VE1BN" <ve1bn at eastlink.ca>
To: <ken.d.brown at hawaiiantel.net>; "Discussion of Ten-Tec Equipment" 
<tentec at contesting.com>
Sent: Wednesday, May 28, 2008 10:34 AM
Subject: Re: [TenTec] Omni VI problems, continued


> Ken -
>
> I have had power vs band output problems on my Paragon II.  I suspect the
> same
> correction to make all bands full output could apply to your OMNI 6. The 
> P2
> uses can
> shielded slug tuned coils versus capacitors in the OMNI 6.
>
> The input crystal filter on the PBT board required some adjustment to 
> bring
> the power
> level to 100%.  There are two adjustment capacitors on either side if the
> crystal.  By
> tuning these miniature caps you can vary the drive level.
>
> All this came about when I found the RF out on my P2 was half power on 
> 160,
> 15, 12
> and 10M.  I tried to adjust the input slug in the first can (in my case)
> which refused to
> turn back up when I turned it farther into the coil,  Had to send the rig 
> to
> Ten-Tec who
> graciously did the coil replacement for me.  The rig came back 100% RF
> output all bands.
>
> Fortunately my OMNI 6+ has no problems.  Am referring to an OMNI 6+
> schematic to
> advise you of the PBT input adjustment that could be the main problem.
>
> Hope this is of some use, Ken,
>
> 73 -  Don,  ve1bn at eastlink.ca
>
>
>
> ---- Original Message ----- 
> From: "Ken Brown" <ken.d.brown at hawaiiantel.net>
> To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
> Sent: Wednesday, May 28, 2008 4:46 AM
> Subject: [TenTec] Omni VI problems, continued
>
>
>> Hi all,
>>
>> My Omni VI is not producing full power on all bands. I have not fixed it
>> yet, though I am probably closer to understanding the problem than I was
>> when I last posted to the reflector about it. I have barked up several
>> wrong trees, and by describing the process I might be able to save
>> someone else experiencing similar problems a bit of time. So here goes:
>>
>> When I first noticed that my TX power output was low on 160 meters, I
>> also noticed that the ALC LED was coming on in CW mode on all bands at
>> all PWR knob settings, whenever the key is closed. This is something
>> that I had never paid attention to before, so when I noticed it, I
>> thought that the ALC circuitry was cutting back the drive. I spent many
>> hours focussed on the inputs to the ALC circuit, trying to figure out
>> which input might be causing the problem. Eventually I got in touch with
>> Ten-Tec service on the phone and found out that the ALC LED always comes
>> on in CW mode during key down. I have not found any mention of the ALC
>> LED in the CW operation portions of the manual. I have learned a lot
>> about the ALC circuitry in the Omni VI,  though this has not got me any
>> closer to fixing the low TX power problem.
>>
>> Ten-Tec service suggested looking at the LOW PASS FILTER BOARD. One
>> suggestion was that there are diodes on that board which can fail and
>> cause a reduction of transmit power output. Another possibility is a
>> failed relay on that board. By disconnecting the PA output coax from the
>> LPF board and connecting it directly to my watt meter and 50 ohm
>> termination I proved to my satisfaction that no significant amount of TX
>> power was being lost in the LPF board. This method may be flawed, since
>> those low pass filters are not exactly symmetric. They don't have the
>> same capacitances on their inputs as on their outputs (except the 10/12
>> meter one), so they may provide some impedance transformation as well as
>> filtering.
>>
>> I probed around using a 50 MHz scope, to see how the RF voltage levels
>> compared at various places, from the ALC circuits on the 9 MHz IF board,
>> through the low level driver and on to the input of the PA. Finally I am
>> starting to get some interesting data. The 9 MHz IF TX output is the
>> same regardless of band selected, so there is not something going on in
>> the ALC circuit that cuts the power back on some bands and not on
>> others. The TX RF voltage at the input and the output of the low level
>> driver board is NOT the same on all bands. This could be in part due to
>> a not quite properly compensated 10X probe and oscilloscope input,
>> except the voltages correspond so well with the power outputs on each
>> band. I have decided the lack of TX power on some bands is due to lack
>> of drive to the low level driver.
>>
>> The low level driver board gets it's TX RF drive from the BPF/FRONT END
>> BOARD, and that gets TX RF from two sources, the 9 MHz IF and the XTAL
>> OSC LO MIXER BOARD. Now I am suspecting that the LO power is lacking on
>> some bands, though BPF switching problems might also be the cause of low
>> power on some bands. I've had some frequency instability on some bands
>> too, so I'm leaning towards a fault in the XTAL OSC. That board is not
>> easy to get to. I finally saw it for the first time tonight. It is a
>> really well built, good looking board, and the schematic shows only a
>> single electrolytic capacitor on the whole board. That electrolytic
>> capacitor is in the part of the circuit that "sets the bias point of the
>> oscillator stage to stabilize the oscillator output level", so it seems
>> plausible that a failure there could cause the problem I am having.
>>
>> I'll let you know what happens after I change C19 on the XTAL OSC LO
>> MIXER BOARD.
>>
>> DE N6KB
>>
>>
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