Idling current in Rega Mira 1, 2 or 3

[2N6658]

Hello,

I'm looking for the idling current of the final stage of the Rega's amplifiers Mira (it's the same final stage for Mira1, 2 and 3).
If anybody has a Mira and can juste give me the tension on the two big white emitter resistors it would be very nice.

Thank's.

EG

[mashley_nad]

It should probably be about 20mA, and if your R5,6,7 and 8 are 0.22 ohm, then the voltage would be 4.4 mV; 5mV would give 23mA.

Out of interest, what value did you settle on?

[2N6658]

Hello,

Thanks a lot for your answer!

R are 0,22ohm, I ajust the voltage at 4,4mV after 1 hour, and it's still at this value after 2 hours.
In fact there was a problem on one chanel, and I had to change the two Power Transistors. But I was not sure that the biais of the other channel was good, in fact it was (4,4mV).

Do you have a Mira? Which model?

[mashley_nad]

Sorry for the late posting.

I do have a Mira - a clamshell, the main PCB says "ver 2" so I assume it's a Mk II. tested in 1998. I bought it after my last post.

It has an electromechanical rotary switch for remote control input switching - works OK, but they forgot to add muting circuitry to silence the inputs you're passing over.

When I got it one channel ran very hot - the 'OK' channel was biased at 7ma, so I set the hot channel to that, and it has worked OK.

[mashley_nad]

Just heard from a guy who works at Rega that the bias should be 2.5 mv (across the 0.22 ohm emitter resistors) so about 11 ma it seems!

[mashley_nad]

Info for Clamshell Maia (Mira minus the preamp) from Rega:

Setting up the Maia’s Quiescent Current.


Before any changes can be made, please make sure the amplifier is “cold”, ambient temperature of 20º C - 25º C and switched off!

Equipment:


1x Mutli-meter (DVM)
1x Trimmer
2x Crocodile clips.

Open the lid of the Maia, and locate the potentiometers in the power stage, turn them fully anti-clockwise and then adjust according to the following procedure.


Turn on the amp and wait for the relay to click over.


Attach the crocodile clips to the right hand output resistors on test points TP1 and TP3.


Switch DVM to read mV’s, it should be reading nothing! Then turn potentiometer until the DVM reads 2.5mV tolerance of +/- 0.1mV. - Don’t take too long doing this as the left side amp is starting to warm up too!


Now attach the crocodile clips to the left hand output resistors on test points TP4 and TP6.


Then turn left hand potentiometer until the DVM reads 2.5mV tolerance of +/- 0.1mV.


Now leave amp on for 15 minutes to stabilise.


Check the voltages on both channels, they should read between 6.5mV – 7.5mV, if they don’t then re-adjust to correct voltage. – This reading is dependant on ambient temperature so may vary!


Leave amp for another 5 minutes to stabilise if adjusted then recheck voltages again.


If all is ok then replace the lid and panels.