GK 400RB mark IV

[pontiacpete]

can anyone tell me if the fan in one of these amps is suppose to come on when powering up or do they turn on after the amp is in use and is heating up?
thanks

[pontiacpete]

Got the schematic and it appears to have a temp sensor, Q27 , LM35. I was in a hurry this morning, trying to decide it I should bring the amp back to the owner at the brunch gig, not knowing if the fan worked or not. He brought it to me to just replace an input jack and make a box for it. I thought I might have broken it.

Out of curiosity,
The data sheet says, (and I'm not sure I'm reading the right part of it)

Specified Operating Temperature Range: TMIN to T MAX
(Note 2)
LM35, LM35A −55˚C to +150˚C

Does this mean the LM35 turns off when temps go above +150º C and therefore turning on the fan?

[Enzo]

I assume the fan is not on now?

Hit the sensor with the blast from a heat gun or lay a solder iron tip on it. Does the fan come on?

You post the schematic, we might analyze the circuit action.

[pontiacpete]

The file is too big, is there a way to send one page? I can't seem to figure it out.
There are 18 pgs
Look at next post

[pontiacpete]

here's the schematic

[pontiacpete]

that's not a very clear copy, sorry. I'll try heating the lm35.
It seems to be located under the fan though.

Is there a way of uploading a large file on this forum? I cant seem to separate the one page schematic of the power amp.

[pontiacpete]

Anyway I give up on the upload .

No the fan isn't working. When powering up it moves a bit, a 1/2" then stays still. The LM35 is buried under a large heat sink then the fan is on top of that.
Only the legs are exposed. I'll try to heat the area some more with a heat gun

[Enzo]

It isn't the number of pages, it is the size of the file.

You can post it over at ampix - Home, a sister site to this one. It is free. Then link us to it there or just tell us it is there. I have posted a number of oversize files there for the group.

That would let the whole group see it, you can send me a copy if you feel like it:

tmenzo at msn dot com

[Enzo]

Thanks for the file.

The LM35 is a temperature sensor with proportional output. it is not a thermostat. You had the operating temperature range, but I think that is what you missed from the data sheet.

The data sheet says the output is 10mv per degree centigrade, so at 100 degrees C there would be 1v at its output as an example. Since it will operate up to 150C, then the output at that temperature would be 1.5v.

The output of the LM35 controls the Q29 base. When the voltage gets high enough there it starts to turn on Q29, which in turn draws current through the base of Q28, and that allows current through the fan.

I couldn;t tell you if the fan will operate on/off, or if there is a linear range where the fan goes from slower to faster before maxing out. But get the sensor hot and the fan should run either way.

[tboy]

How large is the file, and what type is it? I can adjust the limits to allow for it.

[Enzo]

It is close to 5MB. I just posted it at ampix - Home in the Enzo gallery for him.

Pete, please do get yourself all registered at ampix, but I posted the files you sent me over there already.

[pontiacpete]

thanks for doing that for me, enzo

I'm registered with ampix now, , ,will use that next time.

Also, thanks very much for th e explanation of the circuit!

[Mark Black]

Just an FYI - the GK amps with electronic temp-sense fan control I have worked on all let the heat sink get uncomfortably hot before the fan kicks on. Seems to be a design philosophy with them...