# What can I use to replace an SV-04F bias diode?

I had a nice Wega V3840-2 audio amplifier, but sadly, it broke. During my attempts to test what was wrong, a diode, attached to the heat sink, failed with a nice whiff of magic smoke. According to the Kundendienstanleituing (user service manual), this particular diode is a SV04F. I cannot check the markings on diode itself, since it's embedded in epoxy in a little clamp that attaches it to the heat sink. I cannot find this particular diode at my local stores, so my question is, what can I use to replace it?

For your convencience, I also added part of the schematic. The top rail is +32V (also the leftmost connection coming from above), the bottom rail -32V. The connection center left is the actual audio signal, the output on the right goes to the speakers.

The failed diode is D201, that's attached to the heat sink of T210 (which is in turn attached to the main heat sink of all four final stage transistors).

Note about the circumstances it broke under for those interested: I replaced T210 and T211 with a BD2430 and BD244C in an earlier attempt to fix the amplifier (see first link of this post). I now realise that the trafo is set for 220V even though we have 230V these days, perhaps I'll set it for 240V. Other than that, I still don't know how or why my amp is no longer working...

• Find its specs, find another one with similar specs. – PlasmaHH Jun 30 '16 at 10:20
• My guess would be that it's a 4 V zener diode OR a diode consisting of 4 diodes in series. This because it will need to generate a voltage of at least 4x Vbe + 2 x 25 mV. I would use 4x 1n4148 in series, then turn Rt201 to lowest resistance, then power the amp from a lab supply, carefully increasing the supply voltage while monitoring the voltage across RT281. – Bimpelrekkie Jun 30 '16 at 10:21
• @FakeMoustache Yes, some sources on the Internet seem to agree that this is not just a simple diode. Someone recommended 3x 1N3595 for an SV04 (without the F, nobody knows what it means...), what are you thoughts on that? Thanks for your reply, either way. – Sanchises Jun 30 '16 at 10:29
• @PlasmaHH If I could find the specs (closest I get is this which is the SV04YS rather than the SV04F), and more importantly, if I knew which specs really mattered (what is 'similar' enough?), I wouldn't be asking here... ;) – Sanchises Jun 30 '16 at 10:31
• The SV04 is indeed 4 diodes in series. I expect that any SV04 (or SV04S) will work fine, it is not that critical. Parameters of importance are Vf, the forward voltage and the maximum current. I still bet that 4x 1n4148 (or 4x 1N3595) in series will do the job. I expect 3x 1N3595 to give too little voltage drop, your amp will work in class B then. Do mount them isolated on the heatsink because the diode is a thermal feedback for the output transistors ! – Bimpelrekkie Jun 30 '16 at 10:44

This is from an old Sony service bulletin. It mentions SV04S (not F) but part numbers show SV04, SV04S and SV04F as equivalent, so it should work.

It's a Vbe multiplier, also known as an adjustable diode or rubber diode. It's made from an NPN silicon transistor (in this case 2SD1585) and two resistors.

You could replace both the 1k and 3k3 resistors for one 5k linear trim pot, so you can precisely adjust the DC bias for the output transistors, like so (fig 2.5b):

I built a similar device to fix an old Sony receiver before I bumped into this solution, so I know it works fine. I'm not sure if the resistor values are ok for your particular application, so maybe the 5k trimpot should be the way to go, at least until you find the exact resistance values to use.

As it was said before, it's important to thermally couple this assembly to the output transistors (mounting it to the heatsink close or at the same place as the output transistors should do) to get thermal feedback.

I just replaced my previous fix (BC550 NPN transistor + similar resistors) with this very device on my Sony TA1630, and can confirm it works fine so it should work in your Wega amp as well. I built the device right on top of the D1585 transistor (you may prefer using a PCB, but I think it's overkill. YMMV). With some patience and 1W metal-film resistors, it can be built lifting the base pin, and putting both resistors in front of the transistor. Then, I tapped two holes on the heatsink, on top of the PNP power transistors and fixed the device there.

• This is a much more authoritative answer than the currently accepted answer. Welcome to EE.SE and I hope you have more good content to provide! – user2943160 Aug 31 '16 at 23:45
• Thank you, this seems like a nice solution especially since it provides a means of adjusting the current. The amplifier board is indeed Sony, so I guess this should work quite well. – Sanchises Sep 2 '16 at 11:26
• Your Wega V3840-2 seems to be functionally identical to my Sony TA1630, down to the chassis, schematics, component numbering scheme, rear connectors panel, front panel item location, etc. so this fix should work a treat. Actually, I'll probably try this very fix for my amp next week... – Luncheon Ticket Sep 2 '16 at 22:02

The purpose of D201 is to bias the push-pull darlington output stage. Since that output stage has 4 B-E drops between the bottom and top of D201, this is probably 4 diodes in series.

It's not clear how much current the diode needs to handle since it's hard to tell how much of the total 64 V drop T206 and T207 eat up. The remainder of the 64 V that isn't across the diode and transistors is across the sum of R275 and R276, which is 200 Ω. The absolute worse case is 300 mA, but the actual maximum could be substantially less.

You can replace D201 with 4 diodes in series. These should be rated for 1/2 A, which rules out the 1N4148. Just about any ordinary silicon diode that can handle the current should work here. These diodes are always forward biased and are only used as voltage sources.

If D201 fried, then probably at least one of T206 and T207 has failed shorted.

• These should be rated for 1/2 A I disagree since the current through D201 also flows through T207 which has an Ic,max of 200 mA. Also the Pmax of T207 is 320 mW meaning that it must run at a much lower current than 100 mA since it must also drop part of the 30 V supply voltage (assuming D201 operates around ground level +/- some volts, actually, Vout). So you don't need beefier diodes, 1n4148 will do the job. – Bimpelrekkie Jun 30 '16 at 13:15

Not sure if this is the same device but I came across this while reverse-engineering an old calculator:

I took some measurements and it looks to be consistent with 4 diodes in series:

  Current (mA)   Forward Voltage
0.01266        1.871
0.0522         2.057
0.1022         2.143
0.360          2.31
0.402          2.325
0.719          2.407
1.002          2.454
1.226          2.485

• Just out of curiosity, what would you say is the cross-section? – Sanchises Nov 18 '16 at 22:20