The Racal Cougar PRM4515H and L are usually set for tone squelch of 150Hz on receive but can operate OK with other radios set to transmit a 151.4Hz CTCSS tone.
The PRM4515H and L radios can be converted to Carrier squelch by changing the location of one SMD resistor on the RF board. In the case of the PRM4515H it is resistor R163 and for the PRM4515L it is R52. The PRM4515U has a different squelch circuit and cannot be modified.
First remove the battery, antenna and handset then open up the set by removing 9 screws from the case on the serial number side.
The screws are captive in the back casing and don't need to come all the way out of the back casing.
Then remove 4 screws from the RF board
You should be able to see the location of the resistor in the next picture This is for the PRM4515H. It is presently set for Tone squelch.
Move R163 to the opposite pad as in this picture and the radio is now set for carrier squelch
On the PRM4515L it is resistor R52. In this picture it is set for Tone squelch
In this picture it is set for carrier squelch
If when you re-assemble the radio the Squelch stays open at all settings of the Volume control then there is a dry joint and the resistor is not connected properly.
That's it re-assemble the radio and enjoy.
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D. J. MCKAY G1JWG 2017