I got an inquiry this morning about common causes (if any) of distorted RX audio on a VHF MaxTrac, 16ch, 16-pin.
Existing condition:
1. External speaker audio is also distorted.
2. A known good speaker (internal or external) makes no difference.
3. TX is dead on frequency.
I've already suggested:
1. 2nd oscillator crystal, although they usually just stop working, and only on GM300s).
2. Blown audio PA transistor(s).
3. Leaky capacitors in the RX audio chain upsetting the DC bias.
4. Someone's tweaked the IF or detector coils to make it receive further.
5. Bad IF crystal filter.
Recommended solutions:
1. Replace, repair, or realign RF board.
2. Replace or repair Logic board.
3. Get another radio.
Any other suggestions?
Anyone willing to repair this radio? If so, send me a PM and I'll get you the original questioner's info.
Thanks.
Bob M.
MaxTrac distorted RX audio
Moderator: Queue Moderator
Re: MaxTrac distorted RX audio
The 2nd lo xtal is the biggest failure in the VHF that I have experienced.
I had a VHF 2nd lo xtal stop oscillating on me and then after I fixed it I did a 220 conversion to a Maxtrac and of course after firing the radio up it was also stopped about 2 weeks later.
It was the 2nd lo that stopped working.
I was able to fix it by changing the 820 ohm resistor across the xtal to 1.8K and then pulling the xtal back on frequency slightly by changing the caps coming off of the xtal, so far it's holding frequency fine over temperature.
I had a VHF 2nd lo xtal stop oscillating on me and then after I fixed it I did a 220 conversion to a Maxtrac and of course after firing the radio up it was also stopped about 2 weeks later.
It was the 2nd lo that stopped working.
I was able to fix it by changing the 820 ohm resistor across the xtal to 1.8K and then pulling the xtal back on frequency slightly by changing the caps coming off of the xtal, so far it's holding frequency fine over temperature.
Re: MaxTrac distorted RX audio
If the 2nd LO xtal dies, won't that completely kill the receiver?
The issue here is just distorted RX audio, so the xtal is working somewhat, although it could be way of frequency, as in a few kHz, which does seem odd.
Bob M.
The issue here is just distorted RX audio, so the xtal is working somewhat, although it could be way of frequency, as in a few kHz, which does seem odd.
Bob M.
Re: MaxTrac distorted RX audio
Sorry, I forgot the main part of the story,
The 220 radio drifted off frequency and only became distorted for about a week and a half.
With the VHF I though that it had quit outright but I found that it drifted off enough to where the receive signal was out of the receivers pass band at first making me think the xtal was dead on the 2nd LO.
Playing with the VHF some more I found that if I pulled the 1st local oscillator off frequency of the VHF radio that I could just start to pick the signal up again, but very distorted, which lead me to realize the Xtal had not died but only drifted.
I pulled out the freq counter, I should have done this first but I took the lazy approach, and when I measured the 2nd LO of the VHF radio I discovered that it was off by over 2kHz and stayed off by the same amount over about 1 week of diddling with it.
The diddling mentioned above was playing around with the value of C68, no matter what value I put in C68 I just couldn't get the 2nd LO within 1.2 Khz or it stopped oscillating if the value of C68 was considerably different from the stock value so next I played with R57 across the xtal.
Finally with both radios I finally got the schematic and then started playing with the 2nd LO circuit and found that if I increased R57 across the xtal I could get the 2nd LO back close to within ~800 hz but this time it was always low in frequency, going to low in value of R57 or to high above 1.8 K didn't help out much and it was a combination of R57 and C68 being changed by +-5 pf which put both radios 2nd Lo's within 100 hz.
In both radios the value of R57 was 1.8K.
Then I went thru the alignment to set the reference xtal back on, yes the 7 digit xtal number and the RSS alignment, boy what fun that is.
So far the 220 radio has stayed on frequency. I'm not using the VHF radio anymore but last I checked it was also stable.
As I mentioned R57 got me back within the receivers 2nd IF pass band on both radios and then C68 fine tuned it closer in.
Mike
The 220 radio drifted off frequency and only became distorted for about a week and a half.
With the VHF I though that it had quit outright but I found that it drifted off enough to where the receive signal was out of the receivers pass band at first making me think the xtal was dead on the 2nd LO.
Playing with the VHF some more I found that if I pulled the 1st local oscillator off frequency of the VHF radio that I could just start to pick the signal up again, but very distorted, which lead me to realize the Xtal had not died but only drifted.
I pulled out the freq counter, I should have done this first but I took the lazy approach, and when I measured the 2nd LO of the VHF radio I discovered that it was off by over 2kHz and stayed off by the same amount over about 1 week of diddling with it.
The diddling mentioned above was playing around with the value of C68, no matter what value I put in C68 I just couldn't get the 2nd LO within 1.2 Khz or it stopped oscillating if the value of C68 was considerably different from the stock value so next I played with R57 across the xtal.
Finally with both radios I finally got the schematic and then started playing with the 2nd LO circuit and found that if I increased R57 across the xtal I could get the 2nd LO back close to within ~800 hz but this time it was always low in frequency, going to low in value of R57 or to high above 1.8 K didn't help out much and it was a combination of R57 and C68 being changed by +-5 pf which put both radios 2nd Lo's within 100 hz.
In both radios the value of R57 was 1.8K.
Then I went thru the alignment to set the reference xtal back on, yes the 7 digit xtal number and the RSS alignment, boy what fun that is.
So far the 220 radio has stayed on frequency. I'm not using the VHF radio anymore but last I checked it was also stable.
As I mentioned R57 got me back within the receivers 2nd IF pass band on both radios and then C68 fine tuned it closer in.
Mike
Re: MaxTrac distorted RX audio
Regarding the alignment: you can just go through that first screen, entering the 7 and 8 digit numbers and measuring the 9.6V, then save that with F8, and abort the rest of the alignment, if that's all you need to do. No need to go through all the other steps. The actual ref osc freq can be set with the same procedure that the GM300 uses.
I suppose if the LO xtal goes that far off frequency, it would throw the IF signal off freq too, and that would cause distorted audio. It's just a failure mode that I hadn't thought of. Usually the crystals just stop working, as opposed to moving way off freq. Thanks for the tip.
Isn't working on these radios fun?!?!?! Especially components on the solder-side of the pcb.
Bob M.
I suppose if the LO xtal goes that far off frequency, it would throw the IF signal off freq too, and that would cause distorted audio. It's just a failure mode that I hadn't thought of. Usually the crystals just stop working, as opposed to moving way off freq. Thanks for the tip.
Isn't working on these radios fun?!?!?! Especially components on the solder-side of the pcb.
Bob M.
Re: MaxTrac distorted RX audio
Bob,
Yeah that's as far as I went with the alignment.
I am surprised at how stable it is, the VHF radio was at an unheated site where the temperature varied from 20 at night to over 90 in the day and I didn't measure any drift over 50hz.
Remembering the prices that we used to pay for an off the shelf TCXO 22 years ago, more than 50 bucks, I guess Motorola did it their own way just a lot cheaper.
Have you tried increasing the 820 ohm resistor to see if you can revive your radio and if not then where can you get the 2nd LO crystals from ?
I know the time will come when I run into one xtal that needs replacing instead of re-setting back on frequency.
Mike
Yeah that's as far as I went with the alignment.
I am surprised at how stable it is, the VHF radio was at an unheated site where the temperature varied from 20 at night to over 90 in the day and I didn't measure any drift over 50hz.
Remembering the prices that we used to pay for an off the shelf TCXO 22 years ago, more than 50 bucks, I guess Motorola did it their own way just a lot cheaper.
Have you tried increasing the 820 ohm resistor to see if you can revive your radio and if not then where can you get the 2nd LO crystals from ?
I know the time will come when I run into one xtal that needs replacing instead of re-setting back on frequency.
Mike
Re: MaxTrac distorted RX audio
It wasn't my radio and the guy who asked me about it decided he didn't have the expertise or equipment to mess with it, so I guess it's a moot point at this time. But good info to keep around in case I run into one for real.
Yes, MaxTracs and GM300s are remarkably stable. I think the newer radios (those made this century) all use TCXOs and don't require all the tuning numbers and temperature correction algorithms.
Bob M.
Yes, MaxTracs and GM300s are remarkably stable. I think the newer radios (those made this century) all use TCXOs and don't require all the tuning numbers and temperature correction algorithms.
Bob M.
Re: MaxTrac distorted RX audio
Bob, years ago i used to work in a two way radio shop that had a lot of taxi maxtracs
come in with the very same problem. Most were vhf and some were uhf but they all
developed the same audio distortion.
The fix in the majority of the cases was to remove the top & bottom covers and all
of the shields and go around to all of the torx screws holding down the pcb's and
tighten them all down as some had come loose over time. Some type of grounding
continuity problem. HTH......
come in with the very same problem. Most were vhf and some were uhf but they all
developed the same audio distortion.
The fix in the majority of the cases was to remove the top & bottom covers and all
of the shields and go around to all of the torx screws holding down the pcb's and
tighten them all down as some had come loose over time. Some type of grounding
continuity problem. HTH......
fineshot1
NJ USA
NJ USA
Re: MaxTrac distorted RX audio
You're right. I also forgot about the interconnect pins between the RF and Logic boards since they can cause all sorts of problems when they get corroded, although usually it's the REF OSC that goes off freq, thus affecting both TX and RX, and this happens far more often on GM300s than MaxTracs.
Bob M.
Bob M.