GK 400rb fan issue?

[methodofcontrol]

hey guys, i've got my friends' bass amp here and i think the problem it's having is heat related. turn it on, it plays fine but every once in while after playing for a good amount of time it will start to lose power and sound a little fizzy. it could be something else i'm sure. but i found something pretty curious inside....the fan doesn't spin. the weird thing is, the connector that supplies the fan w/ voltage (P4 on a GK schematic, which i don't have) is reading plenty of power to get that fan moving. when i pulled the fan out and hooked it up to a 9v battery it worked fine. it's a 24v fan, and the GK starts off around 5v or so at that connector, and after a few minutes it will read 25v or so, and just now it was reading 56v! but the fan still won't spin when it's it in the amp. i tried loosening the screws and giving the blade a push-start, but nothing's working. the amp has been on and working fine for the last 30 minutes, but that fan won't spin. the fact that it works w/ the extra ventilation is what makes me think this fan issue is part of the problem. anyway....anyone know the proper voltage for the fan power source connector on a GK400rb III?

[Enzo]

Did you measure the voltage across the fan on the amp or did you measure it to ground? The schematics don't seem to cover the fan, so I can't tell if there is a thermal detection circuit to drive it or if it should just spin.

Trace the fan circuit back. it either goes right to a power supply rail, or it goes to something like a transistor that controls it.

[methodofcontrol]

the fan is on the power amp circuit board which is a bit of a pain to get at. but i will check that out. i measured the voltage to the fan from the two pin connector w/ the fan disconnected. one lead on each pin.

[Steve Conner]

Fan hooked up backwards? That seems the most likely explanation to me. Check the polarity of the voltage on the amp's fan connector matches the red-and-blackness of the fan wires. Also try measuring it with the fan connected, since the other likely explanation is the driver going bad and not being able to push enough current to spin the fan.

[Mark Black]

Don't know if they used the same idea on the 400RB, but a 1001RB I am working on has a very similar problem. I also measured voltage when the fan was disconnected but it mostly went away with the fan in circuit.

On that one the fan positive terminal goes to ground and they use voltage from the negative rail through a transistor to drive the fan. The fan drive transistor is driven by another smaller transistor which has a PTC resistor from its collector to the negative rail and also across 2 series diodes to the base of the larger transistor. On this particular specimen I finally traced the problem to a partially bad PTC resistor, which I ordered from G-K and will probably install tomorrow. The PTC resistor should be about 100 ohms at room temp and go to near infinity when hot.

Hope this is some help...

[Rcubed]

As I recall the GK fan control uses current control as opposed to voltage control or a series resistance.

When the unit (actually the ptc) is cold the resistance is low and the current is low. When the unit is hot the resistance is high and the current is high.

If you take the load off the current source and try to measure the voltage you will see a high voltage because the current source is driving into a high impedance. If the unit is cold you might see a low voltage because the current source is essentially cut off. It can be a little tricky interpreting measurements with current sources and current source circuits if you are not used to them.

It will be more useful if you measure the fan current. You can do that by measuring the current directly with a multimeter, or, you might be able to measure the voltage across a low valued resistor on the fan control circuit (it will be a 1ohm to 10ohm value) then use fan_current = resistor_voltage / resistor_value, or, you can measure with both methods. If the fan is open the sense resistor voltage will give you the wrong current. If there is a voltage across the sense resistor but no actual fan current that would indicate an open circuit (wiring, conectors or fan). There could be a fan problem if the motor is not spinning but the current is high.

[Mark Black]

Update - for what it's worth the PTC resistor cured the problem on the 1001RB I was working on...

[methodofcontrol]

hey mark is that PTC under the heatsink?

[methodofcontrol]

nevermind, it's right next to the connector for the fan. it reads 100 ohms cold.
the +pin for the fan goes to the collector of Q26(big transistor), then out the base and into the collector for Q27 (small transistor). the emitter of Q27 goes through the PTC and to the "G" leg of a componant i can see because of the heat sink. (the componant has 3 legs labled "O", "G", and "V".) then the "O" leg of the mystery componant goes to the base of Q27.
so the real question is, how did you determine that the PTC was the problem?
and also, what were the symptoms of the 1001 you were working on?

[Enzo]

If you want to test the fan control circuit, hit the sensor with a heat gun. After a few seconds the fan ought to come on.

[Mark Black]

The method I used to isolate to the PTC resistor was embarrassingly dumb brute force: I eventually replaced every single other component in the fan drive circuit because I had them all in stock and it didn't get better, leaving the PTC as the likely culprit.

When I had it out and started checking I found it read a bit high at room temp (about 120 ohms, and it ain't that warm in my shop), and acted erratically when heated with a heat gun - never really going to infinity. It also reacted to physically squeezing with wild changes in resistance. I must say the fact I checked the PTC last was partly due to to the fact I had the other components in stock, but even more due to the fact I have never before encountered a bad one excepting when it had a broken lead.

The initial symptom was heating with no fan turn-on, further mystified by observing increasing voltage at fan connector (fan disconnected) with temp rise, but almost total voltage collapse when fan was plugged in. Fan was verified to work properly out-of-circuit on another DC source.

Sounds like the 400RB fan drive is a bit of a different circuit than the 1001RB I was working on. Still - manufacturers tend to recycle the essential bits of utility circuits if they have been proven to work. The 1001RB uses negative rail voltage as the source and it sounds like the 400RB uses positive voltage, but both circuits apparently use a similar detector/driver/regulator set of components.

G-K has been most accomodating in sending schematics for me - might be worth a friendly call. Just have the model, serial number, and more importantly the rev number of the actual PCB you are dealing with. Better yet - have the thing opened up and in front of you when you call - they can tell you what numbers they need.

Best - O -Luck!

[methodofcontrol]

can i test the thermistor w/out current running through it? can i just use a blow dryer and my ohm meter to see if the resistance goes up under heat w/out the circuit having any power?

[Mark Black]

That should work. You may not read the actual resistance of the thermistor due to other things being across the same points but it should give you some idea if the thing is changing.

Another thing to try: go ahead and remove or at least lift one leg of the thermistor and see if the fan then works. That was something the G-K tech guy suggested I try to make sure the rest of the circuit was functioning properly, and it makes perfect sense since the thermistor is supposed to rise to near infinite resistance when hot.

[methodofcontrol]

smart thinking, mark. i'll lift one leg right now and see what's up. another question for you though, can i order the replacement thermistor from mouser if it turns out i need one? they seem to offer a variety of 100ohm thermistors, and i'm not sure what other specifications i need. any suggestions?

[methodofcontrol]

ok well, nevermind that mouser question because i lifted the thermistor and the fan doesn't go on. i guess that proves that there is a problem w/ the fan circuit, but just that it's not the thermistor. i haven't figured out how to lift the heat sink off the board yet, so i really can't trace the fan circuit out. anyone have some suggestions for me? specifically, how many componants are involved w/ the fan circuit? (it looks like 2 transistors and that thermistor, but i can't see under the heat sink, so i don't know if there's more.)