[ARC5] Visual alignment of BC-454

[Kenneth G. Gordon]

On 23 May 2013 at 14:34, Richard Knoppow wrote:

>     A couple of thoughts:
>     The retrace will not be a straight line unless the 
> output of the sweeper is blanked.

OK. It isn't. Rats!

> If the signal that is 
> producing the sweep on the scope is also sweeping the 
> generator

...which it is....

> and it drops to zero between sweeps, as it should 
> if its a ramp,

Well, it is a ramp, all right, but not a very good one, especially at slow (1 to 2 
Hz) sweep rates.

> it may cause some sort of transient in the 
> generator. The generator can't move back to the starting 
> frequency instantaneously.

Ah! I had not thought of that! That makes sense. Thanks.

>     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.

I tried that, and didn't get a very good trace. I may revisit that though. Right 
now, I am using a Heathkit PK-3 RF probe. This uses a .001 mfd disk 
ceramic blocking cap, then three diodes (1N61s) in series, connected in 
parallel with the cap to ground, then a 4.7 meg resistor to the cable.

>  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, may not apply here.

Another thing to think about. So far, the voltage at the IF is under 90 V, but is 
certainly not "low".

>     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.

The BC-454 circuit is a simple superhet with an RF amp, mixer/hfo, 2 
transformer-coupled IF stages, to a transformer-coupled diode detector and 
an audio stage. There are three IF transformers between the mixer and the 
detector. Each IF transformer has two coils in it. The IF frequency is 1415 
KHz, nominally.

According to the maintenance manuals, the transformers in receivers with 
1415 and 2830 KHz IFs are a bit overcoupled to get the necessary 
bandwidth.

Also, according to the maintenance manuals, the bandwidth for the 1415 Khz 
IF at 60 db down is something like 26 KHz.

The Navy manual gives the following for the 1415 Khz IF receivers:

2X - bandwidth=7.3 KHz, 10X - BW=13 KHz, 100X - BW=19 KHz, 1000X, 
BW=26 KHz. I haven't figured out the relationship between values and DB, 
but as I remember it (NOT a good idea) 1000X is 60 db. while 2X is 6 db.

>     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).

I call it "ringing". ;-)

>  If the rate of 
> sweeping is too fast both the leading and trailing edges of 
> the sweep will be displaced and the trace distorted.

Yes. According to everything I have read on the subject up to now, a sweep 
rate of 30 Hz or less is acceptable. At the moment, I think I have my sweep 
rate set somewhat lower than that.

Ken W7EKB