[SignalOne] Greetings from 1970 - Two questions.

parinc1 parinc1 at frontier.com
Tue Nov 19 19:37:34 EST 2024 

Hi Scott,
I have not had a CX-7 in years- but you might want to check the passband tuning alignment. If  the hiss cannot be altered by adjusting the PBT, then   that circuit may require alignment.
Dale W4OP
________________________________
From: signalone-bounces at mailman.qth.net <signalone-bounces at mailman.qth.net> on behalf of Scott McDonald via SignalOne <signalone at mailman.qth.net>
Sent: Tuesday, November 19, 2024 4:02 PM
To: Signalone <signalone at mailman.qth.net>; Signal One <signal-one at groups.io>; jerry <jerry at tr2.com>
Subject: [SignalOne] Greetings from 1970 - Two questions.



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.

.
 Jerry, your tales of the last few weeks caused me to liberate my two hamfest beater CX7As from storage.
As always this prompts questions, and I'd really appreciate anyone's feedback on them.
1) Radio number 1 is working nominally it seems.  I had one issue with it in the past never resolved, and I'm not sure if it's really an issue so I thought I'd ask before starting to reacquaint myself with the large pile of documentation here.
The radio has what I think is a very high IF noise level.  RF gain, AGC, sensitivity all good, but the IF hiss is making me a little crazy.  Is that possibly just a characteristic of the radio, or a hint that of someone's tryed to hot rod the radio, perhaps, and that I need to look for a mod to reverse?
2) Radio number 2 is sort of working with 1 vfo only but exhibits serious hum on receive and transmit.  Would it make sense to just go ahead and rebuild the power supply board before doing anything else? Or are there other potential causes I should check first?
I'm not much of a radio mechanic but try anyway, any suggestions really appreciated, it would be nice to have them on this winter.
73 Scott ka9p
    On Tuesday, November 19, 2024 at 11:00:25 AM CST, jerry <jerry at tr2.com> 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.

.
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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