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I am restoring a Japanese Teisco mixing amplifier from the 1970s. It's a solid state 5-channel mixer with 16ohm speaker outs and a line out. Amongst other problems, I have discovered a couple of the bridge rectifier diodes have shorted out.

Here is a photo of the diode bridge:

unkowndiodes

Now i am trying to find a suitable replacement for these diodes as they have no markings. I cannot find any circuit diagrams or info on the unit at all. The diodes and the PCB both have no markings. If we can't identify this component ~ Is there a common equivalent used for typical solid state amps from this time? Surely. The filter capacitor is 2200uf 63V. Would a 1N4007 do the job?

Thank you in advance, Looking forward to a reply.

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    \$\begingroup\$ Its been a long time, but I seem to recall that that shape was a 3 amp diode. Digikey has 3A and 5A in that shape. If you compare the size, you should be able to estimate the current. Knowing the DC voltage that the bridge creates would help confirm. If you don't know the DC voltage, a 100V diode should be enough, hard to imagine that it would require more than that. \$\endgroup\$
    – Mattman944
    Mar 20, 2020 at 3:01

3 Answers 3

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1A is a bit wimpy for an audio amplifier; 1A into 8 ohms is only 8W. I would concur with Mattman that these were probably 3A (1N5403 rings a bell, or 5405 for higher voltage) in that case shape.

A Google image search for "3A diode" shows several in this case shape (merely illustrating my confirmation bias; "1A diode" shows some too, but fewer I think)

After eliminating avalanche diodes and other fluff, these show several possible 3A rectifier diodes :

BYW 95A Rectifier diode, SOD64, 200 V, 3 A

BY228 Fast Diode 3A 1000V

SF5408 SOD64 3W1000V 75NS Ultra-high Speed Fast Recovery Diode

enter image description here

(image from BYW95A link above)

Fast recovery is not needed here but now we have the package style SOD64 allowing us to search a supplier for SOD64, in this case giving 27 results; mostly 3A rectifiers.

So you can even keep the original look if you are so inclined.

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Those were a very common diode at one time.

My very strong recollection is that they were rated at one Amp. 1N4005 should replace them just fine. Be sure to replace all four diodes.

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Put in the highest peak-repetitive forward current diodes you can.

And install a small fan.

The only way for heat to exit is by the PCB foil, and there is very little of that.

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  • \$\begingroup\$ Or, if you're not to concerned about keeping the appearance, maybe put in a bridge rectifier module, even a 4A one e.g. KBL405G or similar. They have a much larger surface area to dissipate heat, should only cost a dollar or two, and will work forever \$\endgroup\$
    – elchambro
    Mar 20, 2020 at 3:54

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