Solved: Problem replacing case fan in Fender amp

[mdombros]

Hi folks:

I have a couple of Fender component amps I bought a bunch of years ago and the fans were getting noisy so I thought I would replace them. They were never that quiet to begin with, so I thought I would buy something equivalent but with a lower db rating.

The orignal fans are Comair-Rotron SU2a1, 115V 50/60 Hz, 11w, 34 cfm, 3250 rpm, 40 db.

I purchased ebm-papst 8500 DP to replace them. 115V 50/60Hz, 11w, 35.9 cfm, 3200 rpm, 34 db

All the spec sheets say this fan is 11w, the sticker on the fan says 12 w.

When I hook it up to the leads, it will not run. It kinda hums a little. Where did I go wrong? Is it the 11w vs 12w?

Thanks a bunch,
Matt

 

[mdombros]

Talked to ebm-papst......

He suggested perhaps the amp had a circuit to limit the fan speed based on temperature. On older amps, he said, they would "flicker" or strobe the current to limit the rpm. He went on to say most modern fans do not behave well with these circuits including theirs.

This made me think of checking the schematic (duh!) and in fact there is some kind of temp circuit on one side of the fan lead. I was able to find an unstrobed terminal I could use for the replacement fan.

This makes the replacement on "full" all the time. It works, but I have to decide if the rpm ramping makes a big enough difference to me. I may end up going back to the Comair-Rotron.

Cheers,
Matt