[ARC5] Visual alignment of BC-454

[Richard Knoppow]

    A couple of thoughts:
    The retrace will not be a straight line unless the 
output of the sweeper is blanked. If the signal that is 
producing the sweep on the scope is also sweeping the 
generator and it drops to zero between sweeps, as it should 
if its a ramp, it may cause some sort of transient in the 
generator. The generator can't move back to the starting 
frequency instantaneously.
    Rather than a small cap on the probe a resistor may be 
of help. Its the same idea as the resistors on the input 
probes of DC VTVMs, reduce the capacitance at the test 
point.  Might make no difference but worth trying.
    The voltage at the probe may make a difference. Diodes 
become square law for small voltages and become peak reading 
as voltage becomes higher.  Again, my not apply here.
    The single peak you show looks like a typical under- or 
critically- coupled IF transformer.   Not sure what you are 
actually measuring because I can't find a schematic for the 
BC 454 on line.
    I ran into a very curious problem recently when trying 
to align an RCA AR-88 IF stage by stage.  The second and 
third IFs tune fine but when I tried measuring the first IF 
at the input to the second stage the reading bounced all 
over the place. Yet I don't see that bounce further on. I 
have to look at this again because something is going on 
that I don't understand.  The reason for making these 
measurements is that the coupling transformers between the 
first and second IF and between the second and third IF are 
expandable and are overcoupled in both bandwidth settings. 
There is a method for tuning them without a sweeper which I 
wanted to try. The method works fine in the first 
transformer but I couldn't get a clean reading from the 
other.
    I tried a couple of different methods, the one that 
worked best was to inject the test signal using a X10 scope 
probe and pick it up with a similar probe going to a 
Hewlett-Packard 400-F AC voltmeter.  This IF runs at 455 
khz.
    One other thing, as has been noted the response of the 
IF filters is not instantaneous; they store and release 
energy (all tuned circuits do this).  If the rate of 
sweeping is too fast both the leading and trailing edges of 
the sweep will be displaced and the trace distorted.  BTW, 
this is a problem with spectrum analyzers too and is the 
reason the sweep rate, deviation and bandwidth are usually 
interlocked to prevent combinations that give false 
readings.  The shape of the bandpass filter is critical if 
distortion is to be minimized.




--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com
----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "Dennis Monticelli" <dennis.monticelli at gmail.com>; 
"ARC5" <Arc5 at mailman.qth.net>
Sent: Thursday, May 23, 2013 1:07 PM
Subject: Re: [ARC5] Visual alignment of BC-454


> On 23 May 2013 at 9:21, Dennis Monticelli wrote:
>
>> Ken,
>>
>> That is not jitter because it would then be random.  The 
>> double trace
>> is an artifact of the triggering for each successive 
>> sweep.
>
> That may be so, all right: another explanation is residual 
> hum or audio on the
> signal.
>
>>  In any
>> case, the shape of the passband is not in question, that 
>> much is
>> clear.
>
> Thanks! At least I got something right. ;-)
>
>>
>> The asymmetry within the passband is what I would expect 
>> when only one
>> IF stage undergoes regeneration while the coupled other 
>> half does not.
>>  Hard to tell how much overcoupling is going on because 
>> of the
>> asymmetrical IF gain of the respective coupled stages.
>
> Yes. In fact, when I reduced the regeneration, the display 
> reverted very
> quickly to the single-hump classic shape, but gain was 
> severely reduced. I
> am thinking of temporarily removing the gimmick-capacitor 
> and then
> sweeping it.
>
>>  I wonder how
>> much better it would look if both IF stages were 
>> regenerated?
>
> Hmmmm.....might not be a bad idea to try.
>
>>  Perhaps
>> it would be a prettier passband if less regeneration was 
>> applied but
>> it was applied equally within each stage.
>
> I'll bet you're right. I may be able to give that a try 
> soon.
>
> Ken W7EKB