Batboard

I have a MaxTrac which has been converted to use as a 902 receiver. I found out the hard way you have to be very, very careful with the wires coming from the accessory plug! After replacing the fuse inside, I have found that the audio is very low and distorted. I have tried pin 11 and 16 on the accessory plug and both of the audios are very low and distorted. Did I blowd something up inside the radio? Thanks in advance for any help, 73 KK5FM

Sounds to me like you possibly grounded one of the external speaker wires from the accessories jack and blew up the audio PA.

The Maxtrac has a floating audio PA, neither side should be grounded!

[quote]Sounds to me like you possibly grounded one of the external speaker wires from the accessories jack and blew up the audio PA.
[/quote] This sounds, very, not-good. Thank you for the reply. It sounds like I need to order the service manual. :(

Just get another radio, the 800 ones are a dime a dozen!

I'd like to dispute some of what was said above.

The MaxTrac logic board uses discrete transistors for the audio power amplifier, and a 1000uF capacitor is in series with the output. The internal speaker connects to pin 15 and the other side goes to ground through a 1 ohm resistor in the radio. An external speaker would go from pin 16 to pin 1 (ground).

The RX Audio output (pin 11) is lower level but is similarly capcitively protected from most destructive acts.

A GTX mobile, on the other hand, DOES drive both sides of an external speaker. Neither side goes to ground. They get more power out of it this way.

MaxTracs are pretty indestructible. You should get about 0.25 to 1VAC (depending on the audio being fed into it) at either the headphone audio pin on the MIC jack, or pin 11 of the accessory jack. Also note that pin 11 can be fed either flat audio (before de-emphasis) or muted, filtered audio (after squelch and de-emphasis). The latter is exactly the same as what you'd get at the MIC jack. A two-position jumper on the logic board selects this option. Neither one, however, should sound distorted unless there's really something else wrong with the radio. Both are way before the audio power amplifier.

If the radio is way off frequency, or programmed for the wrong freq, that would result in some serious distortion. Not much can go wrong when converting the radio to receive 902 MHz.

A good signal generator and an oscilloscope will tell you pretty quickly whether the RF board is having problems and producing distorted audio, or the audio circuit on the logic board is killing it. Pin 3 of the connector between the RF board and logic board has detected audio. It will probably be sitting at around 4VDC but the audio should definitely be clean there. Also, some of the audio coupling capacitors eventually go bad, which will give you distorted and low audio when the radio is first turned on. Usually it becomes normal within 15 seconds however.

Could a previous owner of the radio have twiddled with anything on the RF board? There's one coil in particular that's part of the audio detector circuit and messing with that WILL give you grief.

Bob M.