[R-390] No Peak on T701

[Jim M.]

www.mouser.com
takes small orders.


----- Original Message ----- 
From: "Barry" <n4buq at knology.net>
To: <r-390 at mailman.qth.net>
Sent: Sunday, April 30, 2006 3:07 PM
Subject: Re: [R-390] No Peak on T701


> I'm seeing about 1.8V peak-to-peak, unterminated into my 'scope.
>
> The more I think about this, the more I'm wondering if C709 might be bad.
> Wouldn't this cause low output?  It's spec'd as a 2% so I'm thinking if it
> has changed, even slightly, it might be causing a problem.  If I recall,
> C327 (the cap across L230/T207) is of the same type and it has been known 
> to
> cause low output from the 1st crystal oscillator.
>
> I haven't found a 510pF, 2% cap yet, though.  :-(   Surplus Sales of
> Nebraska has them, but they also have a $10 minimum order and I really 
> don't
> need anything else from them right now.  Any sources, anyone?
>
> Barry - N4BUQ
>
> ----- Original Message -----
> From: "Tim Shoppa" <shoppa_r390a at trailing-edge.com>
> To: <shoppa_r390a at trailing-edge.com>; <r-390 at mailman.qth.net>;
> <n4buq at knology.net>
> Sent: Sunday, April 30, 2006 6:47 AM
> Subject: Re: [R-390] No Peak on T701
>
>
>> "Barry" <n4buq at knology.net> wrote:
>> > I've been doing some more tweaking on one of my R390As.  T701 (output
>> > transformer from the PTO) will not peak.  I'm getting slightly less 
>> > then
> 2V
>> > peak to peak no matter how I adjust the slug.  Anyone ever seen this
>> > behavior?  If so, any ideas what's wrong?
>>
>> 2V RMS is not awfully low. When I scope the PTO output I see 6-8V Pk-Pk,
>> the max in the middle of the range and falling off towards the edges
>> (especially the high freq/low counter edge). This is the
>> raw unterminated PTO output; putting a 50-ohm terminator on it
>> drops the value by a little more than a factor of two.
>>
>> Let's see, 6V Pk-Pk is a little above 2V RMS. Maybe your PTO output
>> is a little bit low but I can't say that it's "bad".
>>
>> It's been a while since I screwed around with it but IIRC there
>> was not a lot of variation to be had by swapping PTO tubes around.
>> (I was mostly looking for frequency stability not PTO output
>> level as I swapped tubes around.)
>>
>> I seem to recall past postings on this list that suggested that
>> the PTO output network was there to "squash" any resonances and
>> make the output roughly flat across the PTO range. So actually
>> peaking it up for one particular frequency may be not good (may
>> in fact be impossible!)
>>
>> As another data point, most of my crystal oscillator band outputs
>> are in the 8 to 10V Pk-Pk range, with some going up to 12 or 13V
>> and a few bands as low as 5 or 6V (I guess the variations are
>> due to crystal activity or just the way picking off the harmonics
>> works; the units have been peaked up on each band.)
>>
>> Getting back to my mixer design obsession, I guess this means
>> that most of the mixers were designed with LO inputs in the
>> range of a few (1-3?) volts RMS in mind and that a factor of two
>> variation in level throughout the frequency range is not out of
>> design specs.
>>
>> I'm not claiming that my values are golden reference standards
>> either, just what I see on my two 390A's. Is there an official
>> output level that the PTO is supposed to meet? And is that
>> value into a terminator or not?
>>
>> Tim.
>>
>
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