intermittent noise

For some reason I am getting noise, but not always. It's a solid state preamp. Some times when I turn it on it will have a lot of RFI and noise coming through. Then other times it won't. Even if nothing has been changed, same cables, input, signal flow... It's definitely the preamp.

Volume doesn't affect the level of the noise. So it's either after the attenuation stage or in the power supply. What do you think?

Logical Trouble Shooting: Start at the front stage and work your way back.

Use a cap to short out the signal. Be careful with polarity and voltage rating. Easier than pulling wires. 1 uf or even a .22 poly might be enough. Dicharge the cap inbetween stages so you don't zap a transistor.

You can also work from the back to the front. If you start at the front, and you immediately notice the noise disappear, thats where the troble is most likely to be. Does not always work, but a good place to start.
Scope out your pwr supply for ripple/nasties.

When you find the trouble spot, then it's parts swapping time. Hope you can find replacement parts. That's why I hate working on transitor stuff. So many different part numbers. With tubes it either a 12AX7A, a 12AU7, a EF804S, a 12BH7, a 5Y3 or a 6L6. Plus a few more.
:grin:

I think first I'll start with the power supply and put it on my scope... I have a hunch it might be there.

Thanks for the advice, just want to see if this sounds familiar to anyone. But when I think about it... it's probebly the most common problem in the world but with so many different causes.

Is it old? I have seen this sort of instability in old equipment. My guess would be either bad solder joints or other bad connections, or perhaps electrolytics or carbon resistors drifting a lot out of spec.

Best regards,

Mikkel C. Simonsen

yes it's old from '98, and i figured it was just time to recap. but i did just notice that one of the RCA ground wires had just one strand fraying on it. I think that it might have been arching once in a while. ... all soldered up and heat shrinked now. I plugged the pre back in and no noise... but it was intermittent so i guess time will tell.

old means at least 1960.

heh... yeah

What type of power supply single or + and -.
If + and - sometimes if not designed correctly they can power up wrong. There have been a few threads with this kind of problem.

Next time it does this measure the + and - rails if this preamp uses them when working and not working.

Check for bad solder often a bad connection will cause strange things that come and go.