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QED A 240 CD Can Anyone Pls Help With Repair
QED A 240 CD Can Anyone Pls Help With Repair
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Old 7th March 2018, 05:37 PM   #31
Mooly is offline Mooly  United Kingdom
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QED A 240 CD Can Anyone Pls Help With Repair
Its essential you do understand it because you could be dangerously over biased. Under biased is safe, its just the distortion figures that suffer. Over biased runs hot and in more severe cases could enter thermal runaway where the extra heat allows more current to flow... more current more heat... more heat more current and so on until the amplifier fries and fails.

You need to locate these two resistors, and they will be near the output transistors and typically a much larger part than all the others. The marked value will be in the 0.22 to 0.47 ohm region. So very low value.
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Old 7th March 2018, 07:17 PM   #32
Rol G is offline Rol G  United Kingdom
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i measured across the points shown in the oval which says 10-20mV.
these two small resistors are in the path of the large ones which you circled.
or do the test probes actually have to touch the large ones to get a correct reading?
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Old 7th March 2018, 07:36 PM   #33
Mooly is offline Mooly  United Kingdom
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QED A 240 CD Can Anyone Pls Help With Repair
The probes have to measure across the two resistors to get the reading. If you look at the circuit you will see that those points are actually the same as measuring between the two emitters of the output transistors. The emitter is the lead with the arrow. Knowing that should help you locate the correct points.

Now its a critical adjustment and I'll admit its not easy if you are not confident and you are trying to hold leads in one hand while reading a meter and tweaking the pot.

A good method can be to solder wires to the points concerned and bring those wires out to the meter so that you have a good and permanent connection.

Although you need to confirm this I think you are going to be looking at these two points (or same on the other channel).
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Old 7th March 2018, 07:38 PM   #34
Rol G is offline Rol G  United Kingdom
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directly across each of the large resistors you circled it reads just over 3 mV.
If I alter the presets to raise it to 10-20mV will that alter the 1.4V reading between R45/74 and R47/76 ?
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Old 7th March 2018, 07:50 PM   #35
Mooly is offline Mooly  United Kingdom
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QED A 240 CD Can Anyone Pls Help With Repair
That sounds a realistic value although a little on the low side. Slight movement of the preset will change the reading considerably. Check your original channel in the same way to gauge the factory set value.

Changing the preset will alter the 1.4v slightly but that is not the reading we are interested in and mustn't be used as such.

I'll have to leave it for tonight but remember you are only moving the pot a little and the voltage reading you are interested in will change. If it doesn't seem to work as expected then leave it and we'll go over it again tomorrow.
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Old 7th March 2018, 07:56 PM   #36
Rol G is offline Rol G  United Kingdom
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my amp has on each channel 4 large resistors, in parallel pairs, i assume i need to touch the two resistor ends which go to the two outside pins of the pair of output transistors, making sure not touch any the 4 transistor legs in between ??
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Old 7th March 2018, 08:12 PM   #37
Rol G is offline Rol G  United Kingdom
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Thanks, i think now i figured out how to do it, even a very tiny adjustment on the preset pots changes the mV reading drastically. the heat sink is very cool, i'm leaving it on full volume on mm with inputs disconnected to see if it gets any warmer.
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Old 8th March 2018, 12:53 AM   #38
Ian Finch is offline Ian Finch  Australia
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Quote:
Originally Posted by Rol G View Post
my amp has on each channel 4 large resistors, in parallel pairs, i assume i need to touch the two resistor ends which go to the two outside pins of the pair of output transistors, making sure not touch any the 4 transistor legs in between ??
Yes, that seems like the right points to measure the test voltage. The resistors are arranged just as the circuit diagram shows when you include those shown in broken outline.
Below is a pic to make certain we have the right ones and distinguish those in either channel.

PS. Unless the amplifier is working harder (producing output current to a load), turning up the volume control won't do anything. Just let it idle with the cover on.
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Old 8th March 2018, 07:30 AM   #39
Mooly is offline Mooly  United Kingdom
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QED A 240 CD Can Anyone Pls Help With Repair
sounds like you are getting there.

Just make sure the amp is at normal temperature for the adjustment and don't be tempted to overbias it. The recommended range is 10 to 20 millivolts and so I would settle on somewhere in the lower half of that range.
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