[SignalOne] Transmit Level

[jerry]

Well,

   The Plot Sickens, as my mother used to say.  It's the ALC.  I
disconnected it and all the gain was back.  Of course, without it
there is no protection for the tube grids, and you have to be very
careful.  But there's full output with the OUTPUT control at 
nine-oclock,
and also full output on 10M.

   I already found one severely drifted carbon comp resistor in the ALC
system;  I wonder if there's more?

   In the table of transmit voltages, the manual says:

5.  This measurement point is ALC voltage.  It must be within +/- 100MV 
P-P of the voltage present at IC1 Pin 1.

What does that even mean?  ALC is a *DC* voltage.  There are multiple 
bypasses - and a 10uF electrolytic to make
sure it's DC.

OK.  With the ALC disconnected and scoped, and the radio in TUNE mode 
and the output control full CCW ( no output ),
the ALC voltage is -400mV.  If I start gingerly turning the output 
control CW, it suddenly jumps to -4V.  Keep turning,
and it jumps to -11.4V.  These jumps seem to occur in discrete steps.  
Turn the knob down, and the ALC lazily drifts back up to -400mV.

I'm guessing that the initial -4V is the "screen current" jump, and the 
second one is the "grid current" jump.  Or
I could be wrong.  What happens when a zener diode conducts into a high 
impedance?  Is the voltage of the zener subtracted?

It's possible that I have simply misadjusted R31, R34 and R19.  I think 
I'll review those settings.

                - Jerry, KF6VB






On 2024-11-19 08:01, jerry wrote:
> This list is for discussions regarding collecting, repair, use and
> sale of Signal/One radios and accessories.
> 
> Documentation on Signal/One radios can be found at
> http://www.hamanuals.com and several other sites on the Web.
> 
> .
> Hello!
> 
>    My CX7A is transmitting well on all bands except 10M.  However, the
> transmit chain seems to have low gain.  For decent output, I have to
> turn the OUTPUT knob all the way CW.  According to the manual, I
> shouldn't have to turn it
> past 9 O'Clock!
> 
>    The list of transmitter voltages in the manual is confusing to me.
> The exact conditions of measurement.
> 
>   SO:  Here I have the radio on 40M.  TUNE mode.  100W output.
> Probing at the input to the RF board ( Pin 267 ).
> The OUTPUT control is slightly past its midpoint.  Seeing 760mV 
> peak-to-peak.
> 
>   Can anybody tell me:  is the low gain BEFORE that input, or AFTER it?
> 
>   OK, I think I answered my own question.  Regardless of conditions -
> the table says that when the voltage at the
> RF board output is 40V p-p ( see some clipping at that level ), the
> input voltage at pin 267 should be 180mVp-p.
> Whereas it is ACTUALLY 940mV p-p.  So something ON THE RF BOARD is not
> performing up to spec.   The board as a whole
> should have a voltage gain of 222 whereas it actually has a voltage 
> gain of 42.
> 
>   Now to do the same exercise inside the board.  I really hope it's
> the CA3028A;  I have two of those here.  The big
> transistors are pretty much unobtainium.  But it's not necessarily a
> semiconductor;  this thing is full of carbon comp
> resistors, and I have already found two that have drifted up in value.
> 
>   For this work, I really love my Hantek 2D72 handheld oscilloscope.
> It has measurement functions that let me just
> read voltages off a tiny numeric display, instead saying "Lemme see:
> We're set at 100mV per division, and we have a 10x probe, and the
> signal starts HERE and ends THERE and...let's nudge it down just a bit
> and look at the top..."
> 
>                         - Jerry, KF6VB
> 
> 
> 
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