Grounding in CB radios - UK Vintage Radio Repair and Restoration Discussion Forum

Jameson · 15th Dec 2022, 3:49 pm

Hi all I have another novice question if anyone can answer. Basically I have a Cybernet 134 CB radio (Sapphire x4000) that im trying to make some sense of. The schematic shows various points around the set are grounded to a certain trace, which im assuming is called the ground plane, is that correct? This ground plane, should it connected to the chassis ?, because im getting no continuity between the chassis and anywhere on the PCB (sweeping the probe across), other than the screws holding it to the chassis case. I am getting continuity between the negative terminal and various points thou; this suggests the ground plane is connected to the negative but isolated from the (Ground I suppose its called

). This is a 12v set. If the chassis is indeed meant to be isolated from the negative (ground

) then why is this ? Is it to stop interference ? Thanks all

GMB · 15th Dec 2022, 3:59 pm

It is possible that the chassis is earthed using a capacitor as the aerial coax may be connected to it, and you do not want the power earth joined to the actual aerial in case the set is being powered by an earthed mains PSU. (dangerous on PME mains).

FYI an earth plane is normally one of the PCB layers that is unbroken and goes all over it. It's purpose is to provide the optimal route for return currents for every signal wire in the PCB thus limiting external fields and hence EMC issues. The chassis box may be a further EMC enclosure or not depending on the design.

Jameson · 15th Dec 2022, 4:09 pm

Ok, when you mention the "power earth" you mean the 12v negative ? Its not a mains set

GMB · 15th Dec 2022, 5:54 pm

Yes, the power earth sounds like it is the 12V negative.

Sets like this can be used in vehicles or in a house with a mains PSU.

G6Tanuki · 15th Dec 2022, 6:22 pm

Some CBs did have a 'floating' ground-bus, linked to the actual chassis via capacitors so the ground-bus and chassis were at the same RF potential but could be not at the same DC potential.

This same approach was used by quite a few 60s/70s VHF/UHF two-way mobile radios, so they could be used with old cars with positive-earth. The Pye Westminster had a multi-way power-plug that had to be wired in sympathy with the earth polarity of the installation.

The 'floating' approach also had the advantage that it stopped destructive current-flows via the coax and the radio's negative-lead if the battery-to-chassis earth-lead went high resistance or open-circuit.

[Nothing good has ever been reported from having your radio's coax/negative-power-lead and PCB tracks trying to pass the hundreds of amps taken by a starter motor]

SiriusHardware · 15th Dec 2022, 6:23 pm

The chassis in those radios is always DC isolated from and 'RF connected' to the circuit ground track via a number of parallel ceramic capacitors.

I haven't thought it through before but this arrangement was probably adopted to allow them to be used in both negative earth and positive earth vehicles.

Edit: Crossed with Tanuki

15th Dec 2022, 3:49 pm	#1
Jameson Triode Join Date: Nov 2019 Location: Kent, UK. Posts: 29	Grounding in CB radios Hi all I have another novice question if anyone can answer. Basically I have a Cybernet 134 CB radio (Sapphire x4000) that im trying to make some sense of. The schematic shows various points around the set are grounded to a certain trace, which im assuming is called the ground plane, is that correct? This ground plane, should it connected to the chassis ?, because im getting no continuity between the chassis and anywhere on the PCB (sweeping the probe across), other than the screws holding it to the chassis case. I am getting continuity between the negative terminal and various points thou; this suggests the ground plane is connected to the negative but isolated from the (Ground I suppose its called ). This is a 12v set. If the chassis is indeed meant to be isolated from the negative (ground) then why is this ? Is it to stop interference ? Thanks all

15th Dec 2022, 3:59 pm	#2
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,099	Re: Grounding in CB radios It is possible that the chassis is earthed using a capacitor as the aerial coax may be connected to it, and you do not want the power earth joined to the actual aerial in case the set is being powered by an earthed mains PSU. (dangerous on PME mains). FYI an earth plane is normally one of the PCB layers that is unbroken and goes all over it. It's purpose is to provide the optimal route for return currents for every signal wire in the PCB thus limiting external fields and hence EMC issues. The chassis box may be a further EMC enclosure or not depending on the design.

15th Dec 2022, 4:09 pm	#3
Jameson Triode Join Date: Nov 2019 Location: Kent, UK. Posts: 29	Re: Grounding in CB radios Ok, when you mention the "power earth" you mean the 12v negative ? Its not a mains set

15th Dec 2022, 5:54 pm	#4
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,099	Re: Grounding in CB radios Yes, the power earth sounds like it is the 12V negative. Sets like this can be used in vehicles or in a house with a mains PSU.

15th Dec 2022, 6:22 pm	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Grounding in CB radios Some CBs did have a 'floating' ground-bus, linked to the actual chassis via capacitors so the ground-bus and chassis were at the same RF potential but could be not at the same DC potential. This same approach was used by quite a few 60s/70s VHF/UHF two-way mobile radios, so they could be used with old cars with positive-earth. The Pye Westminster had a multi-way power-plug that had to be wired in sympathy with the earth polarity of the installation. The 'floating' approach also had the advantage that it stopped destructive current-flows via the coax and the radio's negative-lead if the battery-to-chassis earth-lead went high resistance or open-circuit. [Nothing good has ever been reported from having your radio's coax/negative-power-lead and PCB tracks trying to pass the hundreds of amps taken by a starter motor] __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

15th Dec 2022, 6:23 pm	#6
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,577	Re: Grounding in CB radios The chassis in those radios is always DC isolated from and 'RF connected' to the circuit ground track via a number of parallel ceramic capacitors. I haven't thought it through before but this arrangement was probably adopted to allow them to be used in both negative earth and positive earth vehicles. Edit: Crossed with Tanuki