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Old 19th March 2013, 03:50 PM   #71
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Quote:
Originally Posted by jaycee View Post
The power amp's input should be 0 when you measure DC offset and set the bias, so turn the volume all the way down. If you try to measure it with some signal present at the input, you will get a varying reading because the amplifier is producing a signal.
Ok...i already do that...

- Input 0 + volume all the way down;
- My multimeter sets do 200m
- Measuring the R41 i have 0,2 i can regulate the RV1 but nothing happens on this value;
- Same thing for R141 but with the value of 0,3

I don't know if i was doin'g this properly....because this multimeter its a cheap one like this...and i was setting it to 200m

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Old 19th March 2013, 05:19 PM   #72
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You have to read dc offset from speaker terminals and have to short-out input (RCA) terminals when you are doing this. To read the bias is the one from those big resistors that are next to the output transistors. you have to read voltages across those resistors, be careful no to short out output transistors when you measure bias. Bias and dc offset = to different things!

PS: For bias, Read voltage across from one 0.22ohm emitter resistor and adjust voltage to 8mv. ALL this is measured in DC mode on your meter!!!!

Last edited by lanchile; 19th March 2013 at 05:48 PM.
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Old 21st March 2013, 08:42 PM   #73
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Originally Posted by lanchile View Post
You have to read dc offset from speaker terminals and have to short-out input (RCA) terminals when you are doing this. To read the bias is the one from those big resistors that are next to the output transistors. you have to read voltages across those resistors, be careful no to short out output transistors when you measure bias. Bias and dc offset = to different things!

PS: For bias, Read voltage across from one 0.22ohm emitter resistor and adjust voltage to 8mv. ALL this is measured in DC mode on your meter!!!!

So i was doing some tests:

DC offset, i set the front knob of amplifier on Tunner, and put some cables in tuner imput...volume sets to minimum levels and short-out the RCA input terminals, the value is pratically the same that without shortout:
- I measure between 175mV and 180mV fixed values always but every time i test again i measure -1 (180-179-178...and on..some times plus 1)

Regarding the BIAS:
- in R42 i measure .573 mV and .in R142 .544mV
- This measures without speakers...with speakers i measure about .692mV

- In R41 i measure 0.02 mV and in R141 0.03 mV
- Both measures done without Speakers

I notice that withoutt any input, and with speakers connected, if i turn up the volume generates noise....

Another think that i notice...is that with the phones, independently of the input that i have...doesn't generate noise on sound...

Can you guys help a little more :P
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Old 21st March 2013, 09:22 PM   #74
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Well, first 180mv in the dc offset is on the high side
Second, you have some voltage missing for the bias
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Old 21st March 2013, 09:52 PM   #75
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Originally Posted by lanchile View Post
Well, first 180mv in the dc offset is on the high side
Second, you have some voltage missing for the bias

:S thoughts!?!? :S
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Old 22nd March 2013, 12:49 AM   #76
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First thought I have is that you are using speakers on the amplifier before testing and setting it up correctly. This is likely to cause more accidents and failures. Don't connect speakers until the measurements are correct!

You don't need any inputs connected. Remove inputs, turn volume completely down and select a line input such as aux. Make sure that tuner or phono is not selected as these are noisy. Preferably short the remaining active input, centre to outer, as well. You can do this with a cable fitted if necessary

Second.
Read the lowest meter scale as just mV, millivolts or multiples of 0.001 V, so there is no .xxxmV - just mV. We are getting confused about what you post.

Third
Have you got the service manual and are you following the schematic diagram for the voltage measurements? Retest offset voltage with absolutely no connections to the amplifier apart from the power. You need to test the amplifier, not the input signal.
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Old 23rd March 2013, 12:49 AM   #77
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Quote:
Originally Posted by Ian Finch View Post
First thought I have is that you are using speakers on the amplifier before testing and setting it up correctly. This is likely to cause more accidents and failures. Don't connect speakers until the measurements are correct!

You don't need any inputs connected. Remove inputs, turn volume completely down and select a line input such as aux. Make sure that tuner or phono is not selected as these are noisy. Preferably short the remaining active input, centre to outer, as well. You can do this with a cable fitted if necessary

Second.
Read the lowest meter scale as just mV, millivolts or multiples of 0.001 V, so there is no .xxxmV - just mV. We are getting confused about what you post.

Third
Have you got the service manual and are you following the schematic diagram for the voltage measurements? Retest offset voltage with absolutely no connections to the amplifier apart from the power. You need to test the amplifier, not the input signal.
Ok..i just do it again...and the values in the conditions that you refer are completly the same...:S

my multimeter do'nt have another scale....
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Old 1st April 2013, 05:45 PM   #78
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Sorry for neglecting to reply - I had to replace my PC.
If you read your #73 again, you will see you have written variously "between 175 and 180 mV" - that's fine, but later you say .573 ....and .543 mV . I don't think this what you meant because your meter cannot resolve this extremely small voltage. I think you may mean 573..... and 543 mV, no? Or you could have said 0.573 V....and 0.543 V which is the same thing.

The problem for us is knowing what you are really seeing on your meter. A 3.5 digit model will still give readings down to 1mV on the lowest, 1.999V scale, even if inaccurate. When you say that you read .02 mV, it tells me you can't be reading the meter correctly or it has a bottom range that is far too sensitive to be useful. I think you must be seeing perhaps 2 or 20 or 200mV. If it were 200mV, the emitter resistor being tested would be quite warm and dissipating 1 Watt. Otherwise, quite cool.

I will ask again if you have a service manual and have followed the schematic to find the various test voltages. You can check your readings against a working channel or in the preamp or the power supply sections to verify that what you read is of the correct order of magnitude. The circuit description will be helpful too, if you are OK reading English. You will have to register at the following site but this manual has good information.
Arcam Alpha 2 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine

I also have a question regarding the driver transistors, FST239 & 240 types. Did you replace these with same pinout parts? Whenever output transitors fail so catastrophically, you can be sure the drivers have also or are going to fail soon. However, you should check the pinouts are the same on any replacements so that they will work properly. Just Google the part number followed by "pinout" for most replacement types. The next concern is the limiter transitors, Q9 & 10 which often fail in this meltdown situation. If not already replaced along with D1 & 2, do so.

In summary, there may be some more work yet in sorting out component faults and connections. Don't despair as repairing damage like this is not viable for commercial technicians because of the tedious testing, replacements, time and parts cost. As you found, it is not a simple job even if you have plenty of time and enthusiasm. Used amp buyers should beware that the amp they buy will likely be sold for a reason. That is often because the owner doesn't want to pay for repairs that are nearly the cost of buying a new one.
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