1290 crackling

Hi there,

I am the happy owner of a 4 channel 1290 mic preamp that I built 6 years ago.
I have always been happy with it but yesterday I used a configuration (which I never used before) and my preamps two of them started crackling and even saturated., they also produced a lot of noise.

I said this was a new configuration, because I used two Coles 4038, 6 feet away from their source (which was very quiet, = acoustic guitars) . These mics (which by the way have been re-ribboned) need a lot of gain, habitually they feed into some ampex that I have (which were not available at that time).

The point is that I had to push the gain of the 1290 practically to full rotation (minus 2 stops to be precise).

So my questions :
- is it normal that I got this kind of behaviors having pushed the preamps too far.
- if not, what (in the circuit) could cause such problem, what should I be looking at to fix my preamps ?

Thanks a lot.
Alexandre

This is not normal. Do all 4 channels behave the same? Try cleaning the PCB with some cleaning solutions or isopropanol and see if this helps.

Two stops from max is 70dB gain which is probably about right bearing in mind the mic and its distance from the sound source. The first thing to check is that the 1290 is at fault. To do this you need to make up an XLR with pins two and three shorted together. Plug this into the 1290 mic input and set the gain to the same value and listen to the output. If the crackling is there it is the 1290. If it is not then the source of the crackling is external.

Cheers

Ian