Adcom GFA-4402 fan circuit problem ? - diyAudio
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Old 7th March 2012, 10:56 AM   #1
azvrt is offline azvrt  Netherlands
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Default Adcom GFA-4402 fan circuit problem ?

I purchased another Adcom, but this one appears to have a problem in the fan circuit.

Normally, at start-up, the voltage to the fan is approx. 4 volts.
On this one, it's only 2.86 which isn't enough to make the fan start turning.
With the wires to the fan cut (fan disconnected), we have about 12 volts by the way.

On other Adcoms of mine, we replaced the 220 ohm 1/4 watt resistor R336 by a 500 ohm multi-turn pot, which enables us to control fan speed.

Since voltage was low on this one, we replaced it by a same pot to see whether we could increase voltage by decreasing resistance.
The strange thing is that when turning the pot one way, the voltage increased only a little, but instead of keeping increasing the whole time, it would decrease sometimes while still turning the same way, then increase again !

Then, suddenly, with the voltage still too low to jumpstart the fan, suddenly while turning the pot the voltage would jump right from 4 volts to over 12 volts (I think the same voltage as the power supply voltage feeding the amp, could be wrong though) instead of increasing gradually, and the fan would start turning. Then, when turning back the other way, it would not decrease a lot until it would suddenly drop back to below 3 volts and the fan would stop turning.

Any idea ? I can provide the schematics if anyone wants to have a look.

Last edited by azvrt; 7th March 2012 at 10:58 AM.
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Old 7th March 2012, 12:49 PM   #2
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Is the voltage on the positive fan wire a constant 12+v?

When measuring the voltage on the fan, did you have the meter probes directly on the two fan wires?
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Old 7th March 2012, 01:03 PM   #3
azvrt is offline azvrt  Netherlands
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When the fan is disconnected (wires to the fan cut) we measured over 12 volts on the wires that normally go to the fan.
When the fan is connected, we measured 2.86 volt on the same wires, which is too low.

In both cases the voltage is constant.
We also tried with an identical fan from another GFA-4402.

Yes the probes were directly on the wires to the fan.
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Old 7th March 2012, 01:08 PM   #4
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With the black probe on the amp ground, do you have 12+v on the positive fan wire at all times?
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Old 7th March 2012, 01:11 PM   #5
azvrt is offline azvrt  Netherlands
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I will have to check and will let you know.
Thanks.

By the way my guess is the answer is no, cause if the answer was yes, wouldn´t the fan be turning ?

But I will check.
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Old 7th March 2012, 01:20 PM   #6
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If the voltage isn't constantly at 12v (approximately), that could be part of the problem.
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Old 7th March 2012, 10:24 PM   #7
azvrt is offline azvrt  Netherlands
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We measured with the negative probe to ground.
Without the fan connected, there is the same voltage (+12V) as the power supply, which is normal as the positive wire to the fan seems to be coming straight from the +12V, it's the negative wire that has all the circuitry on it.

We measured a voltage drop of 9 volts, so there was only 3 volts left which is not enough to start the fan. On the other amps there is 4 volts which does start the fans.

We think TIP61 might be faulty, or thermistor R337. Both are not very common parts.
We'll try and repair the circuit. If we can't we'll bypass it and make our own circuit with a relay hooked up to the +12V, gnd and remote terminals and a 1 watt resistor or 1 / 1.25 watt potentiometer in series with the fan.

We experimented a bit with some resistors, we need approx 120 ohms. Two 1/4 watt 200 ohm resistors in parallel got a little warm, so 1 watt should be better. A 200 ohm 1.25 watt potentiometer might be best.

But I will try and get the TIP61 or an alternative and maybe the thermistor if I can and see whether that's the problem. Uncommon parts are a bit harder to get over here.
If we can't find them, we'll bypass the circuit.

By the way, when I purchased the amp the circuit was already bypassed by a similar idea , but it was very clumsily done so I removed it.
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Old 7th March 2012, 10:26 PM   #8
azvrt is offline azvrt  Netherlands
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By the way, when looking at the schematics, the positive wire to the fan seems te be connected to something called SW RADIO.

My father thinks it's connected straight to the remote. Is this true ??!?! If it is, then it's a very strange design as that would imply the fan would run straight off the remote of the head unit which would be bad if you hook up multiple of these amps to your HU.

In my car that wouldn't be a problem since I use a relay for the remote anyway since I use 6 amplifiers, but not everybody uses a relay.
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Old 8th March 2012, 12:27 AM   #9
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Your schematic is incomplete. Adcom added a small PCB right on top of the turn on lead connection that used two transistors to alter the turn on lead current draw issues the original amp design had. Try looking at any other schematics you might have I am sure it will be in one of them if the schematic is updated.

Originally Adcom had a turn on lead current draw of almost 100ma. This presented issues to HU's so they did a add-on board that was screwed right to the top of the turn on lead edge connector and the trace on the board was cut to allow the add on to work properly. The add-on board required +12 and ground and one out of 12 volt to the amp so its a three wire hook up plus the connection to the turn on lead lug.

If not I will try and find my old schematics and relay the turn on circuit board mod to you....
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Old 8th March 2012, 09:26 AM   #10
azvrt is offline azvrt  Netherlands
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Thanks for the information. We will have a look.

The parts list mentions TIP29TC can be used instead of TIP61CP.
Now my electronics store apparently has TIP29C. Is that okay ?
Just want to try swapping it, see whether that one is causing the problems.
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