Power supply query

[Yazoo]

There are schematics available online for the Roland JC-20 which have not been completely confirmed. I am looking at the power supply section, specifically the supply (Vdd) going to the MN3207 and MN3102 which has to be around 9V but definitely not above 10 volts. It uses a capacitance multiplier to filter noise out and is preceded by a Zener diode. The value of the Zener is unknown but I guessed at 9.1V. I can't understand how Vdd, the supply to the MN3207/MN3012, is regulated to around 9V as the schematic stands in the first attached picture.

I have amended the schematic, shown in the second picture, so that the transistor collector is connected at the mid-point of the resistor R44 and the Zener diode. Would this guarantee that Vdd would then be correct, i.e. 9.1 V for the MN3207/MN3012?




Amended version:

[antonis]

Basically, Vdd is Zener voltage minus Q3 V_BE (minous a little voltage drop across R45..)

In amended version, V_CE is restricted to 9.1 minus V_BE..
(less power dissipation for Q3 but too much power dissipation inconvenience for R44..)

P.S.
Both configurations DON'T guarantee 9.1V..!!
(amended version Vdd is strongly dependent on Q3 h_FE..)

[PRR]

It uses a capacitance multiplier to filter noise out and is preceded by a Zener diode.

The transistor also "multiplies" the Zener. The voltage on the Collector hardly matters; the Emitter will "follow" the Base which is fed no-more-than 9.1V.

The original looks right/workable.

Your "correction" mostly ignores the usefulness of the transistor and will sag badly on load much more than a BBD chip.

See also the second paragraph here. (The 3rd pp wanders away from factors we need in simple power supplies.)

[Yazoo]

Thanks for the information. I have built this and tested it with a 12V bipolar supply. The real thing will be around 15V. I wanted to check voltages before I put in the BBD so I left the MN3102 socket empty and put a led with resistor in the GND and V+ socket pins of the MN3207. I measured about 10.3 volts, too high for the BBD.

I want to regulate Vdd to a definite 9 volts without adding additional noise. Any advice would be very welcome.

[antonis]

7809 (1A) or 78L09 (100mA) regulator..

[Rob Strand]

I have amended the schematic, shown in the second picture, so that the transistor collector is connected at the mid-point of the resistor R44 and the Zener diode. Would this guarantee that Vdd would then be correct, i.e. 9.1 V for the MN3207/MN3012?

Interesting, in another thread I just noticed the 15V and 27V rails look whacky as well.

Either version of those circuits would work.  None are conventional connections really.    The original will produce a slightly higher output voltage.

As suggested in the other thread,  look at some other versions of JC schematics.

[Yazoo]

I went ahead and added a 78l09 to get the correct voltage for the BBD. I was reluctant to do this because I did not want to add any additional noise. I'm in the process of boxing it up now. I managed to fry the two TDA2030s using a 15-0-15v transformer. I am now using a 10-0-10 transformer and it is working, maybe a loss in volume but for what I want, not a problem.

The puzzle is how did the BBD, MN3207, work in the original circuit without getting fried.

[ElectricDruid]

Absolute max Vsupply from the MN3207 datasheet is 11V, so 10V won't kill it. 10V is the recommended maximum working supply voltage.