AVT150 dead at gig but works when I got home

[SteveF]

Hi All

My AVT 150 was working at a gig then just stopped. All lights still on and fans spinning (but loud as usual). None of the channels worked. I tried to take the line-out to the PA, but it was very quiet and very distorted. Impossible to test properly in a gig situation so I ended plugging my guitar straight into the PA which was awful. Went back to the AVT after a while and it worked, but just for a few minutes. Got home and tried it and it works. It sounds like an overheating problem.

Is this a common fault?

I'll leave it on overnight and see if it cuts out by the morning. If so I'll change the valve to see if that helps. Anyone have any ideas? I'm gigging again soon so I need to sort it out quickly.

[Jazz P Bass]

The fact that the signal out of the send jack is distorted. tells me the problem is in the preamp or the power supply.

[km6xz]

When it is not working, try sending a signal into the efx return jack. If there is undistorted signal at the output, you can concentrate on the preamp section. When it is not working, make sure there is heater glow in the tube. It could be as simple as a loose tube socket connection or intermittent tube. Anything else is going to require delving deeper into the circuit and taking some measurements to find what is normal.
If the efx loop test did not work, the problem is in the output section or power supply. There are a couple connectors running between the preamp and power amp that can be wiggled to see if they are intermittent. The output ICs ( TDA7293 ) are pretty rugged but still are the most common cause of total output failure. This case is not total failure so unless highly unusual avoid an attempt to replace them. The problem is going be be simpler than that.
Do you have access to test equipment, an audio frequency signal generator and multimeter? Using a signal generator allows applying a stead state pure tone that will allow easy detection of any change in amplifier function while doing other tests. The efx loop jacks are a frequent cause of intermittent signal, try plugging in and out a 1/4" phone plug to exercise the contacts, if that helps, apply a de-oxidizing chemical on the jack contacts. A good one is Caig Laboratories D-5 DeXit.
A useful tool for what might be a mechanical intermittent is a wooden dowel to prod and flex the PC board slightly in suspected areas to reveal broken solder joints, cracked traces etc while listening to a steady tone from the signal generator. Try the efx loop test first and report back. Don't be temped to replace anything until you precisely pinpoint the cause. A lot of beginning techs are anxious to do something, replacing perfectly good power output ICs or tubes needlessly as the first victims of that poor technique.

[SteveF]

Thanks Jazz P Bass and km6xz

I have a correction to my first post - sorry Jazz P Bass. I guess the ambient sound was to load for me to hear this correctly on the night of the gig, but the headphone out, FX send and line-out ARE working when the main output is not. The tube is always glowing. I managed to replicate the fault by leaving the amp on for about 6 hours so I am convinced it is heat related. When I played the guitar the output was distorted a bit and then just went dead. I left it on and after about 2 hours I heard a little pop and it worked again for a few seconds.
When it is not working, the guitar feeds through to the headphone out, line-out and FX out. A keyboard into the FX return feeds through to headphone out together with the guitar.

Given that the pre-amp seems to be working, where do I concentrate on next? I do have a signal generator. km6xz - I will take a look to see if I can identify any dry joints or problems with the fx sockets (I doubt it as I never use them, but I will check).