Lowband Maratrac noisy speaker audio

[MP 6.7]

I just picked up a lowband Maratrac (T81XTA7TA7BK) that I'm in the process of moving up to 6 meters. When I powered up the receiver through the green wire, there's constant low-level noise, sort of a buzz, in the speaker when the audio PA is on. There's also a little click every second or so, also when the circuit is on. It also makes clicks when I push buttons on the head, and a lot of clicks and noise when I hold down the volume buttons. The audio circuit also seems to be susceptible to RF interference; I get buzzing and noise when I key up my VHF MT2000 next to the unit. I took out the board the audio PA is on and couldn't find anything obvious. I tried two different power supplies thinking the switch mode supply I was using was inducing noise, but using my linear supply didn't change it. From my pro audio background, it sounds like a grounding/shielding issue. Any ideas?

[maxkelley_kc2spy]

Perhaps the speaker output leads were shorted or something in the past? Those audio amplifiers are pretty sensitive to shorts and weird loads and stuff like that... After shorted, they'll either sound crappy (like you describe) or just not work at all!

[MP 6.7]

I might just replace them anyway, Digikey has TDA2003's for $2.35 which is cheap enough to give it a shot. Come to think of it, one of my Spectras did something similar when the audio PA went bad.

I was using one of my 3.2 ohm Spectra speakers on it which made the noise sound a little louder, but it's still above what I would consider acceptable.

[maxkelley_kc2spy]

Yup, according to the service manual, it'll do 10 watts into 3.2ohms @ 5% distortion max, so indeed, that's the proper impedance!

[jackhackett]

Those TDA2003s are pretty rugged, supposedly designed to handle a short on the output, and the outputs are capacitor coupled to the speaker so they don't have the same type of problem from shorting speaker leads to ground that a Spectra/Radius/CDM would have.

Your problem might be caused by bad caps, such as in a Maxtrac the output coupling caps would dry out and you'd get low, distorted audio.

[MP 6.7]

I did a little bit of troubleshooting and it looks like the noise is coming from the head's audio circuitry. The audio coming out of the drawer sounds great at the drawer connector and at the head connector, but is noisy coming out of the head. I opened up the head and listened at several points and it sounds like the noise starts happening downstream of Q1009. Any ideas before I go find a different head?

[jackhackett]

Took a quick look at the schematic (I'm assuming you have the advanced control head), the most obvious thing I can see to look at is the VAG voltage (U1002 pin 7), that's the virtual ground for the op amps, should be about 4.8V, look for noise on it... C1005 could be the problem if you have noise there.