Maxtrac With Intermittent Receive

Victor Xray · Post by **Victor Xray** » Mon Mar 12, 2007 2:10 pm

OK here's one for you Maxtrac gurus. I have a Maxtrac 300 with a receive problem I believe to be related to the PL/DPL decoder.

The radio receives transmissions from another radio perfectly if the transmitting radio has an unmodulated carrier (i.e. keyed up but no talking). However, as soon as audio is applied to the transmitted signal, this screwy maxtrac mutes itself as if either the carrier is gone or the subaudible tone has gone. Well since the carrier of the transmitting radio obviously hasn't gone, the PL/DPL detect circuit must be going whacky when audio is applied to the carrier. Oh and this problem only appears when the receive channel is set for any PL or DPL code; if the channel is set for CSQ then the receiver never mutes for as long as the transmitting carrier (with audio) exists.

Has anyone seen this before? Any ideas on where to start troubleshooting? Also, I'd be really appreciative if anyone has a PDF or scanned copy of the maxtrac service manual!

Thanks

Max-trac · Post by **Max-trac** » Mon Mar 12, 2007 7:18 pm

I've seen them do really screwy things if the 44.645 xtal goes off freq (although they usually fail completely).
You should be able to measure it or pick it up with a receiver.

Also if the VCO is mistuned or you are near the edge of the band (you could try another freq).

jackhackett · Post by **jackhackett** » Tue Mar 13, 2007 6:11 am

I don't think I'd suspect either of those if the radio is receiving okay in CSQ or with PL but no audio. More likely is either too low a PL signal, distortion on the RX audio, or a problem in the audio circuits.

Is the transmitting radio set up for 25 or 12.5 KHz deviation? If it's 12.5 there might not be enough PL deviation for the Maxtrac to recover it reliably.

If the quadrature coil is misaligned the audio could be getting distorted, look at the signal on pin 3 of the RF to Logic board connector (pin 1 is towards the back of the radio), with a sine wave generated into the radio make sure the signal there doesn't look distorted.

There's an issue in the maxtracs with a capacitor, C551, which couples the audio coming from pin 3 to the receive filters where the PL is separated out, if that cap is bad it might be the cause, do a forum search for info about that cap.

Victor Xray · Post by **Victor Xray** » Tue Mar 13, 2007 8:16 am

Thanks for the tips. I'll start probing later this week. FWIW, the receive issue appears on any transmitting radio (tried various models, some not even Motorola) and any PL/DPL code, all at 25khz.

kc7gr · Post by **kc7gr** » Tue Mar 13, 2007 8:34 am

Victor Xray wrote:Thanks for the tips. I'll start probing later this week. FWIW, the receive issue appears on any transmitting radio (tried various models, some not even Motorola) and any PL/DPL code, all at 25khz.

One other thing to check: Make sure the reference oscillator is on frequency (as evidenced by transmit frequency error). If it's off, the whole radio will be off.

Happy Tweaking.