Fan noise

[lowell]

Got a carvin R600 which when turned on you hear the fan rev up like a motorcycle and then it just sits there humming away while the fan is on. With fan uplugged it's quiet. There is 640mv of DC on speaker jack when fan is on and plugged in, however this DC goes away w/ fan unplugged out of circuit. I can't seem to figure out how the fan is connected and how its ac signal is injecting into the output.

[Enzo]

Wait. Is there some mv of DC on the output or is there SIGNAL on the output, which is AC? And does this offset go away with a load present?

ANyway,measure the voltage across the fan. The schematic shows the fan running on the 12v regulator VR3, and then when the amp gets hot, it turns on Q301, which shunts the 24v across the regulator. If you have 24v on the fan, it is stuck on high speed. That means either the VR3 is shorted or Q301 is shorted, or the control circuit has an issue. I am assuming the amp does not get instantly overhot at power up. Since the thermal sensor circuit also controls the speaker relay, if the relay turns the speakers on, then I must assume the thermal sensor part is OK. At least as far as A302.

[MarkusBass]

Lowell,

As Enzo has written, measure the voltage accross the fan. It should be -12V when you turn on the amp, and -24V when the amp gets hotter. If there is -24V, the problem is quite simple; either VR3 or Q301 is shorted, or the comparator A301.A has problems. The comparator can be easily verified by measuring voltages accross TP2 and TP3 (check this first).
Appart from this, you can test the fan with any 12V power supply (separately from the amp). If it makes the same noise, it is to be replaced (BTW, is there only one fan in the amp? I thought there are two - one for each power amp).
The issue with DC on the output is strange (please clarify whether this is DC or AC). The only thing that comes to my mind is that the fan has a separate ground (separate from the amp) and it is connected to the power amp ground only is one point - on the power supply board. Would it be possible that someone has modified (incorrectly) this connection?

Marek

[lowell]

I had my meter on DC setting when taking measurement. If the signal is AC, which it probably is, would the DC meter setting still read something?

The voltage across the fan is -12V. The DC voltage between TP2 and TP3 is only 1.4V.

[Enzo]

OK, so it is just a noisy fan. Put a drop of oil on the shaft bearing and see if it helps, otherwise, new fan.

If there is some small AC voltage on the output, it should not read on the DC meter. SO I am a little confused just what you measured when you wondered to us how the AC signal got on the output by taking a DC reading.

If the fan loads down some low voltage supply a hair, I suppose it is possible for that to offset the amp a hair. I would expect that to servo itself away, but maybe not. In any case, that would not be an AC signal from the fan. Seems unlikey though since the fan has its own -24v rail. Well, not its own, the relays run on it too, but it is not part of the signal circuits.

Aer you using a speaker load while taking these readings?

[lowell]

Enzo no speaker connected when measuring the DC on output. Lubed fan w/ oil, didn't help.

[lowell]

where can i get 24VDC 80x80x25mm fan? can't find one mouser or digikey.

[MarkusBass]

Have you checked farnell.com ? I can buy them in Poland, for example this one: http://www.tme.pl/dok/a22/kd2408ptb3...6200g-00)0.pdf
It's only $8 but I assume this it to far from you . The delivery would cost more.

Marek

[Enzo]

perhaps Carvin still has one?


Look at stock number 599-0046 at Allied, 9 bucks.

www.alliedelec.com

[lowell]

thanks!

[lowell]

well I put the new fan in, connected a speaker and the fan came on immediately and the problem was gone i.e. no loud motorcycle sound from fan. I cannot remember if I had mounted the fan in the amp yet at this point. I then plugged in and made sure the amp worked... it did. I put it back together and set it aside for a couple days. Now today I tried it and the motorcycle sound is back! Could it be just happening because of physical location reasons? Could something have happened to kill the new fan? I will try it again w/ the fan out of the amp and see if that makes a difference, I don't know maybe the fan motor is injecting noise into the outputs...??

[lowell]

i checked the output again just to clear things up. there is 24DCmv on the output without the fan and 184DCmv with the fan in and on. is this a problem? the fan still measures -12v across the connector so according to previous posts there is not a problem there.

[lowell]

if I were to bias this amp correctly what do I measure as I turn the servo pot/s? Also, could it be that the -12v measures correctly but something is still wrong w/ the ICs you mentioned? I just measured the mvDC on the speaker jack and turned the servo pot, I was able to bring the 184mvDC down to 0dc. The fan noise diminished greatly but is still there... is this how to bias it?

[MarkusBass]

Lowell,

If you look at the schematic, you will see that the voltage across R435 is amplified by A302 IC and later used as power supply for the fan. So there is direct relation between bias current of output transistors and the fan speed.
But I don't see any reason why the DC voltage on the output jack is dependand on the bias voltage. Anyway, I would just set up the bias correctly and later tacle with the fan. I don't know what is the correct voltage to be measured across R435 but looking at similar output stage in G-K you had there 5mV. So I would start with 5 mV and measure voltage on the fan. It should be 12 V and it should increase when you provide some signal to the amp. And I mean signal like 20V on the ouput. It's because the fan should get 24V only with almost maximum power. Of course it would be the best if you find a service manual for this amp - bias procedure should be described there in details.

Marek

[lowell]

Marek,
Ok I asked Carvin to send me the u-manual. In meantime what do you think I can check or test for the fan. As in previous posts I have swapped out for a brand new fan and voltage is -12vdc across fan test points. I played the amp for a while fairly loud and -12vdc did not change to -24vdc, I might not have been pushing it hard enough though. But being that there is a new fan should I test VR3, Q301, or similar?