Friday, 27 May 2016

Amateurizing the PRC-351?

I mentioned in my last post that suitability for amateur use of the PRC-351 is somewhat limited by the way the Loud/Whisper audio system works, and how I planned to look into setting the Tx deviation up in such a way as to make it more usable on both settings.

Now, theres a few problems with this. The first is to understand how the two level systems operate. On receive, in Loud mode, a two stage amplifier is at work. In Loud mode, pin 5 of module 7 is grounded, to enable both stages. In Whisper mode this pin is not grounded, which disables the stage and reduces the gain by 20dB. On transmit, the mic amplifier module 13 pin 9 is floating when in Loud mode, and so the mic gain is standard, but is grounded in Whisper mode, enabling an additional 20dB amplifier.

The result is that in Whisper mode, the earpiece audio is quiet, and the mic gain is high. In Loud mode, the earpiece audio is increased, but the mic gain is lower.

Now, what we need to do is have the same mic gain level in both Whisper and Loud mode, whilst maintaining the two different levels on the receive side. As it happens, the two control lines for module 13 pin 9, and module 7 pin 5 (and in fact the 'noise on' control line, which is concurrent with the Loud command) are sourced directly from the mode switch itself. And, as luck would have it, it seems that they are independent!  If we examine the mode switch, 2S1, we see that this is a 3-pole 4-way rotary switch. In the OFF position, all three poles go to unused contacts. In any of the three ON positions, one of the poles connects the battery supply through, whilst the other two poles control the audio command lines.  One of these poles has only one contact connected, which provides the Noise On signal. The remaining pole controls the audio mode. In Whisper mode, the control line to module 13 pin 9 is grounded, but the lines to module 7 pin 5 are untouched. In Loud and Noise On modes, the control line to module 13 pin 9 is left floating, and the line to module 7 pin 5 is grounded.

Now, unless somewhere else also uses the same control line, it seems that in order to make the mic amp operate as in Loud mode all the time, whilst retaining the two options for the receive audio, all that is needed is to disconnect the contact for that line from the switch! And, this line is much more easily accessible at the switch than at the modules! If it happens that the extra gain is needed to allow the deviation to be set as required, then simply adding a permanent ground line to this contact will fix that. In fact, this would be the easiest trick to try first, requiring just a small jumper wire!

The second problem, is that this radio uses two VCO's, one for the lower band and one for the higher band, and as the set uses reaction modulation of the VCO frequency, there are two deviation controls to adjust! Not an insurmountable problem.

I need now to study the schematic and make sure that the '0v W' line as its notated, only goes to just pin 9 on module13.

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