Yamaha Stagepas 300 fan question

[Rhod]

Anyone familiar with this mixer amp? In your experience do both the fans run continually? At the moment the power transistors' fan runs but the power supply fan does not - I'm wondering if it should run all the time or if it kicks in when the temp exceeds a certain level.

I've just acquired one of these systems and am checking it out before using it. The unit works but the speakers it came with are kaput so I am proceeding with caution...

Thank you!

[Jazz P Bass]

As far as I can tell from the schematic, both fans run off of the -15Vdc supply.
There is no control.
You may have a faulty psu fan or the wiring .

[Rhod]

Jazz P Bass - thank you - that’s what I suspected. I will check that fan is getting voltage...

Any thoughts why one of the fans has a capacitor across its terminals and the other doesn’t? Is it so that the fan continues to cool for a short time after powering down?

[Enzo]

Are the fans wired in parallel? If so it doesn't matter which one has the cap. No it is not to keep the fan running an extra two seconds. If you wonder, turn the machine OFF and watch how long it takes the fan to spin down. I suspect the cap is there to smooth any noise the fan makes in the power supply. Fans are motors, and motors are noisy electrically.

[Rhod]

That makes sense, Enzo, thanks. Fans are in parallel. The fan was getting voltage ok, so I’ve ordered replacements for both as the other fan wasn’t filling me with confidence either (slightly whiney, slight variation in revs). I’m a little surprised as this unit doesn’t look like it’s been used that much. I’m used to cleaning off dust from fans in units like this but these and the rest of the amp were clean as a whistle.

[Rhod]

New fans now fitted and the
unit seems to be running
fine, although I did have a moment when I wondered if I had replaced them the right way round - and the service manual’s not
much help here! I think they’re round the right way Will put the amp on extended test now...it’s not a bad sounding amp.