A727 laser gain control.

[steerpike]

Hi All,
Please allow me to write in English, my technical German will not be good enough to get my meaning accross:

On the Servo board of the A727 is variable resistor R11, which seems to have the abilty to alter the laser power in two steps, IF the microcontroller sets the control line <LASER GAIN>, (TP1).
However, there is no discussion of this in the service documents.

This is not described in conjunction with the more usual laser curent adjustment on the underside of the player (R13).

Is there any description of this section of the circuitry anywhere, and under what conditions the <LASER GAIN> signal is set high or low?

Thank you.

[borchee]

AW: A727 laser gain control.

Hi,

i have no clue what you are talking about...but...does your's have a Philips CDM-1 drive? - there is a service manual for the drive in existance, just not from Revox/Studer...and a few service bulletins for the A727, although i don't know if they cover what you are looking for...might help, nevertheless.

Regards
Borut

[eckibear]

AW: A727 laser gain control.

,,,This is not described in conjunction with the more usual laser curent adjustment on the underside of the player (R13).

Is there any description of this section of the circuitry anywhere, and under what conditions the <LASER GAIN> signal is set high or low?

This might be a secret Willi Studer took into his grave. R11 obviously shunts the laser monitor diode when activated by Q2, the same way as R13 does for the normal laser power adjustement. The big question is, why and when the uC would issue this particular command, i.e. alter the nominal laser power.
It might just be an additional built-in service or diagnosis feature for the laser diode.

The Philips TDA5708 data sheet and application circuit does not show such a feature, nor does any of the CDMx manuals of Philips.
Revox B227 has same IC, but no extra control either.

[steerpike]

AW: A727 laser gain control.

This might be a secret Willi Studer took into his grave. R11 obviously shunts the laser monitor diode when activated by Q2, the same way as R13 does for the normal laser power adjustement. The big question is, why and when the uC would issue this particular command, i.e. alter the nominal laser power.
It might just be an additional built-in service or diagnosis feature for the laser diode.

The Philips TDA5708 data sheet and application circuit does not show such a feature, nor does any of the CDMx manuals of Philips.
Revox B227 has same IC, but no extra control either.

Thank you.
Your conclusion is exactly like my own. That data flag from the uC on my player never seems to change state - even with discs that I know have low reflectivity.