Maxtrac 800 goes deaf after warm up.

[va3cp]

I have a Maxtrac 800 that goes deaf after it has been on for 30-45 mins. When first turned on, it receives perfectly. As it warms up, reception fades away and the radio becomes deaf. Only very strong signals are detected.

This radio was converted by another party to receive 896mhz packet transmissions. I am using the discriminator output from the 16 pin accessory connector into a sound card on the computer. Another radio works fine in the same service.

Does anyone have an idea what to check. Apparently, the person who I got the radio from has several radios which exhibit the same symptom.

Regards....

[DJP126]

The first thing that I would suspect is VCO drift. Did this other person also convert the radio that works fine? As he said that he has several radios that have this problem, maybe it is something in his conversion.

[va3cp]

Dave,

From what I understand, the conversion is done by removing the band pass filters in the radio and re-programming the radio for the 896mhz band. An external band pass filter is used for selectivity as these radios are in a "recieve only" service.

[va3cp]

Sorry Dave,

Forgot to say that the same person has converted about 50 rigs. He has a handful that exhibit this problem. It is possible that the defective radios all have a similar component problem.

[jackhackett]

I'd check to see if the frequency is drifting first, as that's not too unusual for a maxtrac, if someone has played with the reference oscillator coil it will throw off the temperature compensation. If that's okay I would try to narrow down where the signal is being lost by injecting IF signal, there's a high IF around 45MHz I believe, and a second one at 455KHz. You'll need a schematic to see where to inject the signals.

The second oscillator crystals go bad, but that usually causes a complete loss of receive, I got the impression yours is fading out but a strong signal will still come through, is that correct?

I've also seen the discriminator IC go bad and cut out when it gets warm, you could try hitting it with some freeze spray.

[va3cp]

Thanks....

I will see what I can do with your suggestions.

I'd check to see if the frequency is drifting first, as that's not too unusual for a maxtrac, if someone has played with the reference oscillator coil it will throw off the temperature compensation. If that's okay I would try to narrow down where the signal is being lost by injecting IF signal, there's a high IF around 45MHz I believe, and a second one at 455KHz. You'll need a schematic to see where to inject the signals.

The second oscillator crystals go bad, but that usually causes a complete loss of receive, I got the impression yours is fading out but a strong signal will still come through, is that correct?

I've also seen the discriminator IC go bad and cut out when it gets warm, you could try hitting it with some freeze spray.