Fender M80 (bass) problem

[slideman82]

Well, the fact is when I played for some time, when it heats up, the preamp starts to sound like unbiassed, it has no problem with the PA, I plugged the bass directly to the PA and sounded ok. I also installed a couple of cooler fans pointing directly to the OT transistors, but I'm going to disconnect them and see what happens. Any ideas, any similar trouble like this?

thanks!

[J M Fahey]

Unfortunately, these "it works but ... " type of problems are the ones that require a scope, you do have an output power signal driving your speaker, but certainly it must look funny.
Have you tested the speaker with another head, just to discard it?

[slideman82]

The output stage works well, bit certainly there's something wrong with the preamp related with increasing temperature... The speakers are alright too, I have some different cabinets... I thought could be a bad zener, but voltage still works ok.

Should we talk in Spanish? Ja!

[J M Fahey]

De dónde sos?
Anyway, to benefit most members, let's continue using English.
Or writing bilingual text.
I assume you take signal from the preamp, send it to another amp, and it still sounds fuzzy or whatever, is that so?
You can check for thermal problems using a "Freeze spray" and a hot air gun.
Really the best tool here is a scope.

[slideman82]

Yup, the best tool could be a scope! But I don't have any! I'm from La Plata. I will defeat this problem, it can't beat me! Thanks for all!

[J M Fahey]

1) Lacking a scope, the best next thing is build an audio probe/audio tracer : a .01x400 or 600V polyester cap to isolate it from DC, an LM386 with a 1M Log pot at the input,  driving headphones.
Inject a clean sinewave at the preamp input, follow it stage by stage until it dies or distorts and measure DC voltages in that stage to find the culprit.
2) Si andás por Capital pegate una vuelta por mi taller con el enfermo y lo revisás ahí.
No hace falta traer Baffle ni Bajo.

[slideman82]

Thanks for all! That looks good! I have to try this and some other stuff just to see if it works!