Gallien Krueger fan not working

[edge59]

Hi all, I have a Gallien Krueger 700RB 1-15/tweeter combo with a fan in the top that does not seem to be working. I checked resistance on the fan (about 1.9 meg) I am not sure where its supposed to read, I also hooked it up to my bench DC supply and set it for 20 vdc, its a 24vdc brushless fan and does not work with my DC supply so I figure its dead anyway and I checked the connector on the pcb where the fan coonects, there is not volatage at the pins on the PCB either. Does it only come on under exreme demand? Please let me know what you think, we need it for a gig this weekend. Does anyone have a schematic on this that I can look at? The bass player says the sound was cutting out the other day in the heat and the fan did not come on at all obviously... It looks like a computer power supply fan.
Thank you
kindly,
Ed

[edge59]

I tried another fan with the same rating that works in another amp and it still does not come on in this amp, is there a heat sensing circuit that activates the voltage to turn the fan on??? PLEASE HELP

Edge59

[Mark Black]

Well it does sound like the original fan is bad if it won't work on your bench supply at 20V.

I don't have schematics for the 700RB, but in other relatively newer models they do have thermal sense & fan speed control. On at least some models the fan is driven with the positive connected to ground and the negative fed through a temp-sense network from the negative rail.

Try hooking your voltmeter (DC) across the fan hookup pins and using a hairdryer or heat gun on the heatsink to see if you can get a voltage change with increased temp.

As I remember the heatsink has to get uncomfortably hot before the fan will actually kick in.

Good luck!

[edge59]

Hi Mark, thanks! I tried a heat gun over the heatsink area, but did not know if I could get it too hot before the amp blewup... I hooked it up with a similar fan from another bass amp (Hartke 2000) but did not come on. I was informed by a repair tech the fan should come as soon as the amp warms up, but they may be trying to score a repair job as well... I will try the meter across the terminal from the pcb and heat it up again tomarrow.
Thank you so much for responding!
Take care,
Ed

[Bull Handy]

Edge, did you ever get an answer of this problem? My buddy recently has gone through the same steps/issues as you.

[nevetslab]

Hi all, I have a Gallien Krueger 700RB 1-15/tweeter combo with a fan in the top that does not seem to be working. I checked resistance on the fan (about 1.9 meg) I am not sure where its supposed to read, I also hooked it up to my bench DC supply and set it for 20 vdc, its a 24vdc brushless fan and does not work with my DC supply so I figure its dead anyway and I checked the connector on the pcb where the fan coonects, there is not volatage at the pins on the PCB either. Does it only come on under exreme demand? Please let me know what you think, we need it for a gig this weekend. Does anyone have a schematic on this that I can look at? The bass player says the sound was cutting out the other day in the heat and the fan did not come on at all obviously... It looks like a computer power supply fan.
Thank you
kindly,
Ed

These schematic files should suffice.

700RB-II Power Amp Schematic.pdf

700RB-II_1001RB-II_PREAMP_PN206-0251.pdf

1001RB-II_POWER_AMP_PN_206-0260.pdf

[The Dude]

I've had several of these with the same problem. Even when the circuit works, it comes on far too late to keep the amp cool. I've just wired the fan to a permanent supply so it runs constantly and that takes care of it. IMO, it's not worth repairing the fan drive circuit because it doesn't keep the amp cool anyway.