Hoffman Amplifiers Tube Amplifier Forum

Other Stuff => Solid State => Topic started by: dpm309 on October 01, 2022, 10:53:22 am

Title: GK MB200 Cooling Fan Issues
Post by: dpm309 on October 01, 2022, 10:53:22 am

Working on a GK MB200 with the temperature sensor fan that is running constantly. Looks like I need to replace the LM35CZ temperature sensor (U1). I can get one of these off of eBay and was wondering if I should be looking at the other components in the cooling fan circuit. I have attached the schematic of the preamp showing the cooling fan circuit.

Title: Re: GK MB200 Cooling Fan Issues
Post by: sluckey on October 01, 2022, 11:13:18 am

Look at Q1. Temporarily short the base to emitter. If the fan stops then Q1 is working so just replace the LM35CZ. If fan continues to run then Q1 is bad.

Title: Re: GK MB200 Cooling Fan Issues
Post by: shooter on October 01, 2022, 12:58:01 pm

What Steve said^^


also, look real close at J5, make sure there is continuity across each pin

Title: Re: GK MB200 Cooling Fan Issues
Post by: dpm309 on October 01, 2022, 05:00:23 pm

Pulled the cover off and found the thermal sensor and J5 (See picture). Reinserted the sensor into J5 and it the fan seems to be functioning properly. When I turned it on, the fan did not start up until I started playing through it at a fairly high level. Turned the gain down and the fan slowed down and then stopped after a few minutes. Thanks for the suggestions about J5. Can't believe that an amp this small can produce 250 watts! Notice the small toroidal transformers.