[This modification applies to the Uniden 858SSb chassis (PC196BD board), and any other rig with a T/R relay. On sets with electronic switching, it can be made to work with some minor circuit changes.] - 399
Modifying a CB radio to increase its output causes the hi-current devices (supply pass transistor, driver, final, audio amp) to generate more heat. Adding additional heat sinks will help, but the ambient temp in the cabinet still goes up, keeping the devices hotter. Adding a cooling fan is the best way to reduce the ambient temp.
The drawbacks are that the fan can generate a high- pitched whine in receive, and the whirring noise from the fan can be heard in your transmit audio.
On my 139XLR, I wired a small choke to and put a ferrite bead on the fan’s positive lead and connected it directly to the supply B+ output. This eliminated the whine in receive. To remove the noise in transmit, I used the rig’s keying circuit to shut down the fan when transmitting. All this required was a small SPST relay and some lead wire. Refer to the diagrams below:
I connected the small relay’s coil in parallel with the rig’s T/R relay coil, and connected the fan to ground thru the small relay’s Common and NC pins.
When the rig is keyed up, both the T/R and fan relay fire simultaneously. This disconnects the fan from ground, shutting it down while transmitting. Unkey, and the fan starts up again.
Simple and effective.
- 399
Modifying a CB radio to increase its output causes the hi-current devices (supply pass transistor, driver, final, audio amp) to generate more heat. Adding additional heat sinks will help, but the ambient temp in the cabinet still goes up, keeping the devices hotter. Adding a cooling fan is the best way to reduce the ambient temp.
The drawbacks are that the fan can generate a high- pitched whine in receive, and the whirring noise from the fan can be heard in your transmit audio.
On my 139XLR, I wired a small choke to and put a ferrite bead on the fan’s positive lead and connected it directly to the supply B+ output. This eliminated the whine in receive. To remove the noise in transmit, I used the rig’s keying circuit to shut down the fan when transmitting. All this required was a small SPST relay and some lead wire. Refer to the diagrams below:
I connected the small relay’s coil in parallel with the rig’s T/R relay coil, and connected the fan to ground thru the small relay’s Common and NC pins.
When the rig is keyed up, both the T/R and fan relay fire simultaneously. This disconnects the fan from ground, shutting it down while transmitting. Unkey, and the fan starts up again.
Simple and effective.
- 399
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