[SignalOne] Greetings from 1970 - Two questions.
wes Bolin
k5apl41 at gmail.com
Thu Nov 21 17:54:33 EST 2024
Sorry I added my Group notification to Jerry's original Post and its
replies.
I am the one with the relay problem and Jerry has given me some ideas to
troubleshoot it.
Scott- I am an advocate for the CX7B PS that I built from the schematic on
the internet. After two years of failures and fixes and my
causing problems, my PS has held up in great shape. I have only lost one
regulator (+5V) in all that time. YMMV. At any rate, good success.
Wes
On Thu, Nov 21, 2024 at 4:36 PM Scott McDonald via SignalOne <
signalone at mailman.qth.net> 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.
>
> .
> Dale/Wes, thanks for the clues. My noise is definitely being generated
> between the antenna and passband tuning, so that's narrowed it quite a bit,
> and I will go after the 34 V supply first, after I reconcile all sorts of
> conflicting info about power supply board mods.
> Unfortunately the number one radio that was working, was not working
> consistently in all modes after having time to thoroughly check it out, and
> contact cleaning did not resolve the issues. So I have a bit of a mess on
> the bench at the moment myself trying to identify the issues without
> wreaking havoc on anything unnecessary.
> The gent I bought that radio from many years ago referred to it as Our
> Lady of the Perpetually Undocumented Modifications, and I'm beginning to
> understand that.
> Jerry - congrats on your QSO and resolving that most recent relay issue.
> I feel like I've started down this process about half a century too late,
> but I learn something every time you post.
> 73 Scott
> On Tuesday, November 19, 2024 at 06:37:53 PM CST, parinc1 <
> parinc1 at frontier.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.
>
> .
> 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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