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Identification of diode in a form I've never seen before

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floobydust:
These were a unique Japanese part, as a bias diode string mostly for audio power amplifiers.
They were called "Silicon Varistor" or "Silicon Varistor Temperature Compensating Diode" or "Varistor Diode", with the SV part number prefix i.e. SV-3A, SV03YS, SV-04F, Ohimuzi MV11Y etc.
They ended up in weird locations in the circuit, wherever the engineer could stuff in temp comp.

Long obsolete, people are just making diode arrays from 1N4148's as a sub. I made SMT version with BAV99S and bolt to the heatsink. I recall seeing some that appear to be several series diodes plus thermistor all in one package, so language translation of varistor vs thermistor is my guess what happened. Those measure like a mushy pair of diodes.

I don't think they ever worked super great for bias temp. compensation because they weren't always mounted on the heatsink (extra manufacturing labour) and accurate tracking of bias with temperature is extremely difficult to do. A VBE multiplier is what the industry moved to using. Here Sony subs out an SV-04S with a VBE multiplier but no trimpot.

bob91343:
Sometimes they used thermistors, so don't rule out that possibility.

moothefish:
Well this has certainly been a more interesting thread than I thought it would be! The receiver in question is branded as a Rank-Arena ra-402, which is either a rebrand or bespoke design using NEC boards. I wasn't able to find an NEC receiver that looked the same, I did find one which looked similar but used a different circuit in the amplifier board.

So am i correct in thinking that I will need to somehow create a new diode stack to replace each of these? How many diodes need to be in the stack? I'm definitely in over my head at this point...

floobydust:
OP, what is the amplifier not doing, that you suspect D401, D402 in each channel?
Zener D405 is common to both channels, looks odd with heatshrink so it might be two diodes in there?

I did a rough schematic but don't have much for part numbers and values. If you can check, it would help us figure out the mystery diodes.

edit: corrected errors in schematic rev0 -> rev1

Jwillis:
Doing a little searching and putting together bits pieces here's a short version of the history. Rank merged with Arena to create Rank Arena in 1971.  Arena audio components were manufactured by Hede Nielsen in Denmark until it's merger into Rank-Arena in the UK in 1971.Rank also had holdings in NEC Japan. So that would explain the PCB branded with NEC. So maybe you could cross reference the stereo with a similar one Branded NEC from the same time period. Might be able to find a service manual .Some basic performance specs might help to narrow the search.

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