[HELP] Repair Arcam Alpha 2 - 240v

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1% tolerance is better than 5% tolerance so they will be fine.

I had to laugh but it's a bit like saying I cant find a 1L bucket, will a 5L bucket hold the same liquid.


AHAH no problem...;) like i say i have no experience on electronic...i already made some DIY kits and that stuff...but all the references i have it's only curiosity that make me search on internet...

Sorry for the noob question:p
 
If you look at sites like Farnell, Mouser, RS and many many others you will now see that the original range of resistor values E12 in the early days has been improved to (I think) E49.

Basically the E was followed by how many different values were avaiable in each decade of resistors.

E12 gave you 1K0 1K2 1K5 etc because 5% covered the differences.

As manufacturing has got better the differences have got smaller and the tolerances tighter.

Designers back in the 5% days had to allow for these poor resistors. Nowadays we can buy far better components.
 
If you look at sites like Farnell, Mouser, RS and many many others you will now see that the original range of resistor values E12 in the early days has been improved to (I think) E49.

Basically the E was followed by how many different values were avaiable in each decade of resistors.

E12 gave you 1K0 1K2 1K5 etc because 5% covered the differences.

As manufacturing has got better the differences have got smaller and the tolerances tighter.

Designers back in the 5% days had to allow for these poor resistors. Nowadays we can buy far better components.

Thanks for the explanation...now i got it!! :) so...only need to wait for ZTX comming...

Regarding the adjustment of the bias how you recomend to do it?
 
The manual says to adjust for 30mA, which is 7mV across the emitter resistors. So you would put a voltmeter, on a low range, across the pins of R41, switch on, and adjust RV1 until you see 7mV. Start with RV1 set so that it is set to maximum resistance.

You do this with no speakers connected and with the input shorted to ground. Keep watching this voltage for 15 minutes or so as the amp warms up, it will drift so correct the setting of RV1 until it stabilises at 7mV. Do the same for the other channel. If the voltage keeps drifting higher as it warms up and will not stabilise, check Q6 is as close to the heatsink as possible.
 
Ok.....IT WORKS...eheh already turned it on...and tested with headphones.....it works perfectly...

With speakers i don't tested yet...because i try with a small speakers from a junkyard TV and the sound cames out with lots of noise...but only when i level up the volume knob...in very low levels there's no noise....it's weird...need to get proper speakers to test....

The bias it's not adjusted yet...because my multimeter can measure low values like 7mv.... :S
 
already checked with proper speakers and that's something wrong....

_the sound that cames out from speakers is very bad...its almost only noise...and it gets better with the rise off volume....

has i said before....with the phones everything its ok....

Can you guys give some sugestions for troubleshooting..?
 
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Ok.....IT WORKS...eheh already turned it on...and tested with headphones.....The bias it's not adjusted yet...because my multimeter can measure low values like 7mv.... :S
You need to recheck the small DC voltage at the speaker terminals, called DC offset (when the signal and speakers are disconnected). It should be < 40mV

Also, you cannot avoid the proper measurement and setting of bias current. You need to borrow or buy even a cheap DMM with 2V scale or less, such that it indicates mV. Even if everything else is now working correctly, the sound quality will be poor until you follow the manufacturers specification. (when the signal and speakers are disconnected!)
Follow Jaycee's directions.
 
You need to recheck the small DC voltage at the speaker terminals, called DC offset (when the signal and speakers are disconnected). It should be < 40mV

Also, you cannot avoid the proper measurement and setting of bias current. You need to borrow or buy even a cheap DMM with 2V scale or less, such that it indicates mV. Even if everything else is now working correctly, the sound quality will be poor until you follow the manufacturers specification. (when the signal and speakers are disconnected!)
Follow Jaycee's directions.

Its a good tip, but i already try to adjust and that is a problem, i don't know in what level of volume it's supposed to be 7mv in R41 for example...

When i increase the volume, the value increases too....i cannot fix only one value...its that normal?
 
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

An externally hosted image should be here but it was not working when we last tested it.
 
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!!!!
 
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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!!!!


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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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.
 
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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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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