Batboard

I have a GM300 that when it warms up the rx audio is distorted.
It's like it is off freq (tx is OK, I have done a complete alignment).


I have adjusted the quad coil and it is better, but I forget the proper way to adjust it...( I replaced one a long time ago and vaguely remember a procedure).
I know it does have a very slow lag when adjusting.

I have never seen the quad coil in one of these radios need adjustment unless someone messed with it. Probably the simplest way to adjust it is to inject a 455 KHz signal modulated at about 60% with a 1000Hz tone, monitor the received audio with a scope and adjust for the best looking sine waveform, it should be pretty obvious when you've got it centered, you should also be able to tell pretty well just by listening.

Using the 455KHz second IF signal will eliminate any problems in the RF and first IF stages from affecting the adjustment.

I would also suggest that you check to see that the radio has the newer type feedthrough connector between the RF and Logic boards. Do a search of the forum here, those connectors have been discussed numerous times.

If you still have a distortion problem the next thing I'd check is the 2nd oscillator crystal, if it's off freq it will throw off the 455KHz signal, in which case adjusting the quad coil might appear to correct it, but would only really be masking the problem.

It has the newer feedthru pins, which I cleaned. No help.
I'll check the 2nd oscillator xtal and report back...

I posted this way back, it's called Slow warm-up on Maxtrac Radius.

http://batboard.batlabs.com/viewtopic.php?
t=4959&highlight=slow+warmup

There is a coupling cap, 33uF from the discriminator output to the input of the audio filter opamp on the Logic board. The distortion you hear ican be due to the DC voltage offset on the input of the opamp due to the leaking cap.

Change it to a 10uF 25 volt tantalum radial leaded cap.

Will wrote:I posted this way back, it's called Slow warm-up on Maxtrac Radius.
There is a coupling cap, 33uF from the discriminator output to the input of the audio filter opamp on the Logic board. This one seems to leak and the distortion you hear is due to the DC voltage offset on the input of the opamp due to the leaking cap.

Change it to a 10uF 25 volt tantalum radial leaded cap.

That fix is for slowly rising/distorted audio when the radio is first turned on, is it not? His wording indicates the opposite...the audio is fine when the radio is first turned on, but becomes distorted when warmed up. Not to say it can't be the same capacitor giving the problem though.

Todd

Will wrote:Todd,

He specifically states "have a GM300 that when it warms up the rx audio is distorted. It's like it is off freq (tx is OK, I have done a complete alignment). "

And yes I have seen the 33uF cap continually leak too, but that causes distorted RX audio all the time.

]

Ummm, yes...that's what he says. Your fix is for slowly rising or distorted audio when the radio is NOT warmed up, but works fine after a minute or so, is it not?

OK, we fixed it.
And yes, it had good audio when cold, distorted when warm.
The oposite of the slow warm up audio symptom in Maxtracs.

BUT it WAS the infamous C551 on the logic board.
However, it was a 10uf tantalum! (vs the 33uf in the Maxtrac).