Hi,
I am looking for a circuit drawing for a C-Mark MR2450 amp.
I have a problem with the cooling fans. One runs all the time and one does not run at all. Should they be running all the time ?
There are two thermal switches on either bridge heatsink. One on the top and one on the underside. Should these be N/O or N/C ?
I have swapped the suspect fan over to the *always on side and this runs ok, so I have two good fans and a problen with the control side.
Also, the fans are 24v, so is there a relay or do the temp sensors switch 24v ?
thanks
Dan.
I am looking for a circuit drawing for a C-Mark MR2450 amp.
I have a problem with the cooling fans. One runs all the time and one does not run at all. Should they be running all the time ?
There are two thermal switches on either bridge heatsink. One on the top and one on the underside. Should these be N/O or N/C ?
I have swapped the suspect fan over to the *always on side and this runs ok, so I have two good fans and a problen with the control side.
Also, the fans are 24v, so is there a relay or do the temp sensors switch 24v ?
thanks
Dan.
Thermal switches are often either for 1 disconnecting speaker when plug is shorted or 2 controlling fans.
In the 20 year timeframe they are not very reliable.
You should be able to trace the wires from thermal switch to either 1 the fan or 2 the speaker terminal and the emitter resistor chain. The emitter resistors are large white boxes, an inch long or more.
Fan control from thermal switch can go to the base of one transistor, or directly to the power capacitor of the 24 VDC circuit. That capacitor will have a voltage rating of between 25 V and 50 V.
Dates of production on power transistors will tell you something. First 2 digits of 4 digit code like 8843 for 43rd week of 1988. Thermal switches for control went out about 2000. After that, they mostly used a NTC resistor (a little colored drop with two wires pointing out from the contact surface) series a regular resistor, going to the gate of a NFET. (TO220 package).
If the thermal switches are for fans, the side that works right is the one that will tell you if it is NO or NC. You need a DVM with ohms scale for this test.
As with any device, don't use two hands poking around while the power is on. 24v across your heart from one hand to the other can stop it. Use a grounding alligator clip lead for the minus probe and poke around with one hand only. No jewelry on hand wrist or neck, 1 v can burn your flesh to charcoal through metal.
In the 20 year timeframe they are not very reliable.
You should be able to trace the wires from thermal switch to either 1 the fan or 2 the speaker terminal and the emitter resistor chain. The emitter resistors are large white boxes, an inch long or more.
Fan control from thermal switch can go to the base of one transistor, or directly to the power capacitor of the 24 VDC circuit. That capacitor will have a voltage rating of between 25 V and 50 V.
Dates of production on power transistors will tell you something. First 2 digits of 4 digit code like 8843 for 43rd week of 1988. Thermal switches for control went out about 2000. After that, they mostly used a NTC resistor (a little colored drop with two wires pointing out from the contact surface) series a regular resistor, going to the gate of a NFET. (TO220 package).
If the thermal switches are for fans, the side that works right is the one that will tell you if it is NO or NC. You need a DVM with ohms scale for this test.
As with any device, don't use two hands poking around while the power is on. 24v across your heart from one hand to the other can stop it. Use a grounding alligator clip lead for the minus probe and poke around with one hand only. No jewelry on hand wrist or neck, 1 v can burn your flesh to charcoal through metal.
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