Electronics > Beginners
Identification of diode in a form I've never seen before
RoGeorge:
Other transistors on the board looks like Si, so I'll assume the diode is also Si based.
Never seen a diode in such a package.
Does it measures like a diode?
Vovk_Z:
What voltage drop it is? If it is a diode then voltage drop will be as in a diode.
SeanB:
Those are normally a double or triple diode in a common package, so that the voltage drop will compensate 2 or 3 base emitter junctions in the output stage, depending on topology you will have quasi complimentary output or complimentary, meaning you have to compensate for 2 junctions if it is NPN PNP output, or for 3 if it is a Darlington Slizaki stage. Used to offset the voltage drop of the junctions between the output stage. You can measure the forward voltage of the good one at 1mA, 5mA and 10mA, and find it will be around 1V2 to 1V8, and then select an appropriate number of 1N4007 diodes to match it within 50mV at the same current. Don't use a 1N4148, they will not track the power devices over operating temperature range, and the new diode array will need to be thermally coupled to the heatsink, but insulated from it, easiest done with a strip of Kapton tape between them and the heatsink, and another bit of tape over the top.
andy3055:
I have seen these in old audio gear. Mainly, Sony/Sanyo etc. What other members have stated about multiple diodes in series is correct. What is the make/model of the unit? I have some old schematics and might be able to find but just not in a hurry as I am away from home till the end of the month.
magic:
This is not the bias spreader.
The track which goes to the output transistor goes to its collector and also to the collector of the driver, the two clearly arranged as a Darlington pair, so this track has to be a supply rail.
The other end of the diode goes through R405 (left channel) to the collector of one of the input transistors and presumably the base of the gain stage. I will have a look what's going there, it's probably some kind of clamping, protection, anti-saturation or whatnot.
edit
Looking at the markings, these green transistors appear to be PNP (NPN is in the center of the board), so it has to be the negative rail. The diode is mounted in forward direction from the VAS (TR405) input to the rail, in parallel with the VAS and its 2.7Ω emitter resistor R413. What's going on?
edit
Maybe they wanted to make the VAS a "precision" current mirror, with current gain exactly defined by the ratio of R413 to R405, so D401 ought to be a single diode. Too bad that the ratio of those resistors appear to be 1800 :wtf:
D403, the similar diode used in the bias generator, doesn't generate the whole bias alone. It's in series with TR407, presumably a VBE mulitplier. Quite possibly a single diode too.
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