Optonica SM 4545 Biasing Diode

[Richs_Trains]

Hey all,

Usually, I post in the Marantz forum because I play with them mostly, but have begun to appreciate the Optonica from Sharp. I am trying to find either a replacement or substitute for the biasing diodes on an SM 4545. The parts list shows it as a VHVMV203Y/-1, don't know if that is a Sharp part number or an actual diode part number. Any help would be much appreciated. Thanks Rich

 

[Oilmaster]

It's a MV-203 varistor diode with a forward voltage drop of 1.88~1-98V.

Bit of searching provided:

MV203 - Varactor Diode
Mitsubishi Electric Semiconductor​

V(RRM) Rep. Pk. Rev. Voltage = 30 V
Ct{Cj} Nom. Junction Cap. = 11 pF
@V(T){V(J)} (Test Condition) = 3.0 V
C1/C2 Min. Capacitance Ratio = 4.3
Q Factor Minimum = 300
@freq. (Test Condition) = 50 MHz
P(D) Maximum Power Dissipation = 250 mW
Semiconductor Material = Silicon
Status = Discontinued
Package = DO-35
Military = N
1.88 - 1.98 V (3mA)

Listed as suitable substitutes: BZ102/2V1; BZX75/C2V1; BZV86/2V0; ​

Or check eBay sellers like LittleDiode if they have original MV-203 in stock
Note that the forward voltage does not need to be 0.1V exact; the bias trimmer pot is there to provide an adjustable voltage on top of it.
Though 3x 1N4148 might be too less, and 4x 1N4148 might be too much.

EDIT: some LEDs might be suitable, but in any case it's the current that must be known in that case, as they have a V-I curve

 

[Richs_Trains]

Bit of searching provided:

Thank you for the info! Rich

 

[p4spooky]

I implemented the Sony solution to this using TIP31CG (replacement for 2SD1585) and the Sony circuit to when I rebuilt the SM-4545. I have since moved this to the side of the heat sink and installed shrink tube for insulation (as opposed to the sloppy tape you see in the pic). Amp has been working well - no issues and the bias stable. This may be a cheaper/faster option than try to source the MV203 from questionable sources

Note: TIP31CG MUST be insulated from the heat sink. You can see it in the pic (TO-220 Mica insulator/pad) but want to make sure in case it is not clearly visible.

 

[Oilmaster]

That is a good reminder.
I forgot about that one; it's been a few years

For larger (higher power) amps, it would strongly recommended to mount the bias transistor on edge of the power (output) transisor though, or else the thermal inertia of the heatsink makes the bias swing up and down more.

EDIT: what that Sony service bulletin is missing, is that a insulator must be used to electrically disconnect the case of the TO-220 transistor from the heatsink or power transistor, or else there will be short inside the amp circuit ! This can be avoided by using a molded transistor. The same trick should work with smaller TO-126 transistors which can be found without metallic surface.