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Old 13th Sep 2018, 1:41 pm   #1
[CRT1972]
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Default Cybernet PCMA001S SSB problem

Hello all you here,
i have an italian CB model, the Alan 88s. Board is Cybernet PCMA001S.
On receiving mode, with no signal, i have a fixed carrier S7 on USB, S2 on LSB.
The radio was off for abt 20 years. My first though is a capacitor leak around SSB detector.
If i touch one of the 2 diodes of SSB detector with my fingers, the carrier vanish out.
Any idea about what and how to check ?

Thank you in advance,
Cristiano - JN61sp, Italy
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Old 13th Sep 2018, 9:15 pm   #2
Biggles
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Default Re: Cybernet PCMA001S SSB problem

Hello, and welcome to the forum.
It may be worth checking the diodes haven't failed due to crack in them. This was proved to be the cause of an s-meter problem in an earlier thread on the forum. It could also be instability in the receiver and could be caused by a faulty decoupling capacitor but I think that would be a rare problem. Does it transmit ok on SSB? That may narrow the fault down a bit.
Alan.
M1CEU.
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Old 13th Sep 2018, 9:24 pm   #3
Sparky67
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Default Re: Cybernet PCMA001S SSB problem

Hi Cristiano,

You could try adjusting RV5, which is the SSB carrier balance potentiometer.

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Old 13th Sep 2018, 11:00 pm   #4
SiriusHardware
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Default Re: Cybernet PCMA001S SSB problem

On the last point I seem to remember that preset sets the TX carrier null. I am away from my diagrams at the moment so I can not really consider the problem in detail. This meter problem usually only happens on one SSB receive mode, either LSB or USB?

This chassis was best known in the UK in another set, the NATO 2000, therefore if you search for typical faults for that radio you may find someone already had this problem along with a solution.

I saw that problem a few times in the past on other Cybernet multimode chassis and I think it was usually down to misalignment of the USB or LSB crystal oscillator frequency rather than a physical fault. To check that you will need a frequency meter with resolution down to 100Hz or ideally 10Hz or 1Hz at 27/28Mhz

Last edited by SiriusHardware; 13th Sep 2018 at 11:13 pm.
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Old 14th Sep 2018, 12:19 am   #5
[CRT1972]
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Default Re: Cybernet PCMA001S SSB problem

Quote:
Originally Posted by SiriusHardware View Post
This meter problem usually only happens on one SSB receive mode, either LSB or USB?
Both L and U. USB is S7, LSB is S3

Quote:
I saw that problem a few times in the past on other Cybernet multimode chassis and I think it was usually down to misalignment of the USB or LSB crystal oscillator frequency rather than a physical fault. To check that you will need a frequency meter with resolution down to 100Hz or ideally 10Hz or 1Hz at 27/28Mhz
Yes, i agree. is not a TX dead key carrier presence - as pointed by Sparky67. Dead key TX is zero emission, and TX balance VR is working correctly.
I try then to do some measure on L/U crystals. my fear are the electrolytic capacitors. Is a new old stock, many years of inactivity...
i keep you guys updated Thanks !
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