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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 11th Apr 2019, 6:46 pm   #1
Damo666
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Join Date: Jan 2010
Location: Blackburn, Lancashire, UK.
Posts: 399
Default Radiomobile CB201 intermittent RX/TX fault

I've just picked up a Radiomobile CB201, new old stock, still with tamper seals intact.

I believe these share the same internals as the Cybernet Beta 1000, Roadmaster 500, and maybe the odd other rig.

At initial switch on, there's no Transmit and no Receive - but after a few minutes, it will sometimes receive & transmit. This doesn't happen all the time, though, and it eventually reverts back to doing nothing after a few minutes.

The CB always makes the usual white noise loud rushing sound with the squelch turned down, though.

If anyone can point me in the direction of where to look for the fault(s) I'd be very grateful.

I have a DMM, Soldering iron, and an old Black Star Meteor 600 Frequency counter to assist.


I've attached a schematic of this CB here.
The schematic is quite difficult to make out, but I can take another image zooming in on a specific section if required.
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Old 16th Apr 2019, 6:50 pm   #2
SiriusHardware
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Default Re: Radiomobile CB201 intermittent RX/TX fault

The number one hardware cause of strange intermittent working / not working problems on the PTBM135 was the VCO trimmer capacitor CT1 going faulty. The circuit is really just a miniaturised version of the Cybernet 134, but the actual type of trimmer used in the 135 chassis was different, and it proved rather unreliable.

First things first though - in receive mode for the moment - on the PLL (LC7137) IC pin 14, what voltage do you see (with respect to 0V)

-When the fault is present?

-When the fault is not present?

Still in receive mode and when the set is working, what voltage do you see on test point TP1

-On channel 1?
-On channel 20?
-On channel 40?

Are your fault symptoms the same regardless of the channel selected, or do you find that it actually works reliably / straight away at one end of the channel coverage, but not in the middle or at the other end?

Last edited by SiriusHardware; 16th Apr 2019 at 6:55 pm.
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Old 21st Apr 2019, 3:03 pm   #3
Damo666
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Thumbs up Re: Radiomobile CB201 intermittent RX/TX fault

Hi Sirius,

Nice to see you still around here.

I've only just had chance to visit this forum, so sorry about the late reply.

I decided to lift the lids off the rig a couple of days ago, & do some poking around with the end of a plastic trimming tool on the solder side.

The fault was poor soldering around the VCO area near the trimmer Capacitor you referred to.

It's now working flawlessly & has been since I looked at it.

As always, though, I'm very grateful for your valued input.
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Old 23rd Apr 2019, 12:20 am   #4
SiriusHardware
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Default Re: Radiomobile CB201 intermittent RX/TX fault

I'll add that to the list of potential problems on the rear of my 135 diagram.

Glad you got it sorted.
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