[HELP] Repair Arcam Alpha 2 - 240v

[CrustyD3mon]

Originally Posted by KatieandDad

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

[KatieandDad]

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.

[CrustyD3mon]

Originally Posted by KatieandDad

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?

[jaycee]

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.

[CrustyD3mon]

Everything mounted and ready for test...only need to give a hand in the circuits os R125 and R129 where the hole was bigger...

[CrustyD3mon]

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

[CrustyD3mon]

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..?

[Ian Finch]

Originally Posted by CrustyD3mon

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.

[CrustyD3mon]

Originally Posted by Ian Finch

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?

[jaycee]

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.