Re: OHR 400 "Narrow" Filtering.


Date: Fri Jan 05 1996 - 16:37:40 EST

I have gotten feedback from several folks on the filter mods. There is the
varacter variable bandwidth mod with the associated losses (most people have
noticed some attenuation of signal at all levels) and then there is the
crystal bypass mod. I would be inclined to go with the bypass mod. I really
like the cleaness of the current filter. With the audio filter switched in
and the IF hiss removed, the received signal seems to "jump" 2 s units worth.
There was one fellow that put .2 uf caps on the diodes in the IF and was able
to eliminate most of the high freq hiss. I spoke with Dick at OHR about the
"lack" of AGC and he felt that for the cost and complexity of putting in a
"better" AGC circuit that it just wasn't worth it. He did say that he did
plan on evaluating the variable filter for the 400 in the future. I have no
problem using the radio as is, as the receiver is great and I use a narrow CW
filter anyway, but with the small amount of cold drift that this radio has
and the fact that we all have trouble accurately zero beating, most of the
QRP answers that I get to my CQ are off center just a bit and having the
ability to go a bit wider on the filter would be nice. From what I
understand, the bypass option does not degrade the performanc that much,
other than a small amount of "blow by" on strong signals. I will be
installing the caps on the IF diodes soon and the bypass mod for a bit wider
switchable receive. As far as hooking the switch up to the AGC and hard
wiring the AGC, or adding a new switch for the "wide" filter, not sure what I
am going to do. I will probably add it internal and try it for now and then
decide where to put it.
As far as the radio itself goes, I ahve not even turned on the main station
rig or any of my other QRP rigs since I got the 400 finished. I class act all
the way. The only rub that I have with the rig is the size, it's a bit large,
and all of the point to point wiring that is in it. With the great
instructions that Dick has in the manual and the way that the radio is wired,
this is really a minor point. It's just the fact that with the lid off the
radio is a little "busy" under the hood. I plan on adding the filter caps to
the IF, the "wide" filter bypass mod and a KC-1 keyer. If you got Dicks
Curtis keyer, you will find that the 56k resistor in the weighting loop needs
to be decreased. I found that a 5 k resistor in there will give the same
weighting as most of the common keyers like the MFJ and the AEA.
On alignment. I did not use a scope or freq counter for the alignment. In the
places where the alignment called for a freq counter, I used my TS140 and had
it hooked up to either a dummy load or a external antenna, depending on the
strength of the signal. As far as the power level adjustments, I just used
the TS140 on the dummy load and tuned for max S units. I went back and
checked all of the alignment and adjustments using a scope and freq counter
and they were so close that did not have to touch up any of the adjustments.
The reason that I mention this is that I notice that there are a lot of folks
that have sent their rigs back to Dick for alignment since they did not have
a freq counter. If they have a main station receiver, then they should be
able to do an acceptable alignment.

Jess N0TFI

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