This page is dedicated to the modification of the Racal/Cougar radio PRM 4515L (70 Mhz).
All information and pictures were copied from;
http://amateurtele.com
For me there re 4 major modifications:
1. Removing the Time Off Timer
2. Disabeling the CTCSS Tone Squelch
3. Disabeling the, annoying, No Crypto tone
4. Reversing modification 01 in order to put the Amplifier back on 20 Watt.
01. Removing the Time Off Timer
Because I have the PRM4515L instead of the PRM4515U, I have investigated whether this modification can also be applied. It has been found that the design of the transmitter / receiver print is different. Physically, the print is different, but the effect is the same. Resistors R201 and R200 fulfill the same function in the "L" model as in the "U" model. As shown in the pictures below, the movable SMD resistors of 1 M Ohm (print 105) are placed to set 60 seconds.
My plan was initially to adjust the time to 120 seconds. In the end I decided to switch off the time-out timer completely, especially because the warning tones of exceeding the time are no longer audible. But this insight can vary per person, of course. All possibilities are shown below.
02. Disabeling the CTCSS Tone Squelch
It is possible to turn off the tone squelch so that the squelch switches on the carrier. This is only hardware-changeable and not firmware / software. There is a 12 K (imprint 123) SMD resistor R52 at position "T" (of "tone") placed on the transmitter / receiver board. This resistor connects two of the three solder islands. By moving the resistor to the other two islands "C" (from "carrier") we switch from the 150 Hz NATO PL tone to carrier squelch. For PRM4515H the relevant resistanc is R163
03. Disabeling the No Crypto Tone
By removing one resistance (R81) from the main board, all warning tones are off. In other words, the tones (TNA signal) from the CPU (pin 7) are now no longer injected into the HEF4053 chip of the audio amplifier. The disturbing clear tones can no longer be heard through the loudspeaker. A downside is that the time-out timer indication has now also expired and the indication that there is no communication with the remote control. But in my view these side effects are not objectionable. The time-out timer has already been switched off (set to limit) and if the remote control does not work anymore, this is also shown on the display
