"moreradio" wrote in message
. ..
Hello,
as a winter project I'm restoring an SP-600, just finished
the mechanical part I'm working on the electronics.
I wonder if anybody in BAland has tested the BFO waveform
and say if I'm right or not.
Checking the point between C132 and L46 the waveform
appear ugly and distorted, only .5 Vpp.
Everything is correctly wired and components are good, as
witness, the grid of V13 shows a magnificient 8 Vpp
signal.
Can you help me?
73's de Claudio
I never thought to test this. I just looked at my
SP-600-JX-17. I looked at two places: the first is the point
you specify above except that I looked at pin-1 of V-12, the
BFO buffer tube. At that point the waveform shows a lot of
ripple, probably harmonics. I also looked at pin-5, the
output of the buffer. At that point the waveform is a fairly
clean sine wave. It shows some distortion but not much and I
doubt if its of significance. A warning about this
measurement: the plates of the BFO buffer and IF driver are
connected directly together so that the IF ouput to the
detector exists at this point. In order to see the BFO
waveform clearly you must put the AVC in Manual and turn the
RF gain all the way down. Otherwise you will see the IF
output and its noise at this point.
I checked this with a Tektronix 453 scope, which is good
to 50mhz so it will show a 455khz waveform and all
harmonics.
I used a standard probe with the X-10 switch on. This
increases the input impedance of the probe to a very high
value. If used in X-1 the waveform shows flat-topping due to
loading, this may be the source of your bad waveform because
this is a quite high impedance circuit. It may be that the
slight distortion I am seeing is due to the load impedance
being too low even with the probe in X-10. With the probe
in X-10 the voltage at this point is not quite 40V p-p.
Other models of the SP-600 have an adjustment for the
cathode bias of this stage in order to provide a level
control for BFO injection. Normally, its set from the
factory for zero bias but normal bias for maximum gain of a
6BA6 tube is about 1.0 volt and I set my JX-21 for this
value. Probably running the 6BA6 at zero bias shortens its
life a bit and the extra injection level is not necessary.
In the JX-17 the bias and level are fixed. I am not sure why
the level was made adjustable. In some older receivers it
was done to prevent the BFO from activating the AVC and
causing desensitization.
I also re-checked the waveform at the cathode of V-12
and get the same harmonic rich signal I do at the grid.
Evidently, the plate choke coupling of this stage is
selective because the plate waveform is very much better.
Since there is already a buffer for the BFO I don't
think adding an additional one is of much utility. These are
pretty well engineered receivers and, I suspect, most
modifications do not improve them.



--
Richard Knoppow
Los Angeles
WB6KBL