[AMRadio] RE: Using Heath SB 610 Monitor Scope to monitor modulation


D. Chester k4kyv at charter.net
Tue Feb 12 12:44:41 EST 2008


> From: "W6OM" <w6om at cox.net>
> ... I have two, one connected to the plate of
> the 2nd IF on an NC 300 for monitoring the incoming signals and for 
> transmit
> it monitors a Ranger driving an SB 220 on AM.
>
> The second on is connected to the grid of the 2nd detector stage on my 
> 75A4
> to monitor incoming signals and also watch my transmit signal on a Bauer 
> 707
> converted to 75 meters.

You are  much better off monitoring your transmitted signal using a sample 
of the signal goind directly to the antenna, than using the output of the 
detector stage of your receiver.  The receiver will inevitably alter the 
waveform of the signal.  There are nonlinearities in the amplifier stages, 
but most significantly, the i.f. transformers and selectivity filters may 
completely change the appearance of the envelope pattern.  The only way to 
avoid this would be to use a selectivity that is many times wider than your 
transmitted signal.  Selectivity in the i.f. will cause phase shifts, as 
well as filter away higher frequency components of the waveform, leaving you 
a false image of your signal.

I don't even like to use the built-in vertical amplifier.  The most accurate 
image is obtained by feeding the deflection plates of the scope tube 
directly with the rf sample.  I use a  highly modified HO-10.  I believe it 
is the same scope, with a different style cabinet.  It even has the same 
problem with those 1600V caps.

Mine had loads of built-in hum and the image was fuzzy when I first got it 
going.  The Heath manual says not to worry about that; it's inherent to the 
design, and won't make the scope any less useful for modulation monitoring 
purposes.  What a  load of crock!  The designers were either incompetent, 
too lazy or in too much of a hurry get the thing into production, and they 
probably didn't want to run up the cost any more by using a real magnetic 
shield for the CRT.  I was able to get a sharper image by adding astigmatism 
control to the scope tube  circuitry, and to get rid of the hum (which was 
modulating not only the horizontal pattern, but the brightness of the 
pattern, too), I re-routed the filament wiring, using a wire lead for both 
sides of the filament line, not the chassis base for one side (figured those 
mods out, one at a time, by trial-and-error).  The piece of tin they wrap 
around the neck of the scope tube is totally worthless as a magnetic 
shield.  I found an old military surplus gadget in my junk pile, that used 
the same size scope tube (I think it was some kind of RTTY monitor), and 
used the black anodised metal magnetic shield that was in that unit.  That 
fixed probably 75% of the  rest of the hum.  I then further reduced the 
remaining hum by beefing up the power supply filtering.


> Hard to imagine a more versatile inexpensive monitor scope available now
> days as they sell for anywhere from $60.00 to $80.00 on eBay and else 
> where.
> If the face plate is good, cabinet paint is available from a couple of
> sources and if needed  to match a vintage IF strip 455Khz IF cans are 
> cheap
> and plentiful at Tubes and More in Phoenix for less than $3.00.
>
> One word of caution, every one I have owned has had the 1600V mylar caps 
> go
> bad, so if you buy an SB610 order some replacements ($12.00 from Mouser) 
> and
> replace them first.

With the HO10, and I assume with the SB-610, the power transformer is 
another troublesome component.  The xfmr in my original one crapped out with 
a dead short somewhere in the winding.  I found another HO-10 at a hamfest 
that had been dropped, for $10.  The cabinet was mangled, but all the 
components were still there and intact, including the scope tube.  I used it 
as a parts rig, and dropped the xfmr into my original one and restored it to 
operation.  Since then, I have found another one for a back-up parts rig. 
So I have a spare power xfmr, compactron tubes, and a couple of spare scope 
tubes.

Since I don't use mine with SSB, and don't use the vertical amplifier, I 
pulled the tubes from both of those stages, just keeping the tubes that 
operate the horizontal sweep circuitry.  That liberated some spare 
compactrons, plus it lightens the  load on the power transformer, and 
hopefully that will make it  last longer.  So for, after nearly 10 years, 
that replacement transformer has held up.

Don k4kyv 



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