Diode confusion NAD C356Bee

[Silas1993]

Hi,

In my Project to fully recap my NAD 356 BEE i noticed, that the amplifier is distorting in the right Channel. Its barely noticalbe in usual hearing but when i run a freq sweep it is clearly noticable.

So i checked the Amp and noticed that one diode in the feedback cuircuit is damaged. It let run the Signal both ways which it shouldnt and the MM diode checker also says, that the Diode is blown.

I identified the Diode "D12" in the service manual and the it just says "4.7V 500mw"

I was looking for the diode on Digikey and found a replacement but im a bit confused what Diode i should pick.

The "4.7v 500mw" diode is available in different types that have different values in the diode impedance and back leakage etc. and im not shure if i have to be careful here to choose the right one with a specific Value.

So i thought a "Vishay 4.7V 0.5W" with the "lowest" 19 Ohm impedance would be the best choice. But it is available up to 100 Ohm and the

"Current - Reverse Leakage" and
"Voltage - Forward (Vf) (Max)"
values are different in the multiple available 4.7 500mw Diodes.

Thats the one i choosed first

https://www.digikey.com/en/products...iconductor-diodes-division/1N5230B-TR/3104101

These are the available Options :

https://www.digikey.com/en/products/filter/diodes/zener/single-zener-diodes/287?s=N4IgjCBcoGwJxVAYygFwE4FcCmAaEA9lANogAsYYAHHAOwgC6+ADqlCAMoYCWAdgOYgAvvjgAmAAyIQKSBhz4ikUmQB0tAAQA1RizaQQAVV7dUAeQBmAWWwBDAM6Z02YfjBwa02fLyESIKQYREDEwCQRoGTQsXyVSAFYJCQ0AWwB1XRBWdmNTSxsHJxdggFoxL2iFP2UQeMYhYPKarW57AAtbAE8NABNuAh7sDTVaLQ1EiRSAd0yYaW4e9hKwkD12VZBUTuYXA2EhIA

So do i have to look close to these values or is any of the 4.7v 500mw fine?

i would be glad if someone could help me out here so i can get the Amp to run again. I also noticed, that the trimpods to adjust the amp to 6mv are not working anymore. Etherway i turn them with my MM on the Test point the value doesnt change anymore. Is that a logic consequense in case of the blown diode in the way?

Here are the shematic infos

​

EDIT:

Entire schematic / Page 17

Thanks and Cheers!
Silas

 

[rayma]

Please post the entire schematic.

 

[Silas1993]

356 BEE cuircuit shematic

page 17 is the schematic of the right Channel

 

[Mark Tillotson]

What is the voltage across the zener? It doesn't look critical, any 4.7V zener of the same power rating should do. I'm guessing its a 1.3W part from the picture?

 

[Silas1993]

Dont know exact voltage across. Had to measure. Its a 0.5watt part

 

[Boydk]

It´s a 4,7 volt Zener-½W (500mW)
Anything with these specifics go.

Dont know exact voltage across.

One pin from DMM on anode and the other on cathode....... voila 😎

 

[Silas1993]

Thanks for the reply, OK! So is it irrelevant if i choose the one with 19ohm Impedance or 100Ohm Impedance? Or does something of that Value matter in any way to the cuircuit?

 

[Mark Tillotson]

Well it will have some effect on performance, but I doubt the original designer specified it...

 

[Silas1993]

I changed all 4 diodes LR Channel today with the Vishay 4.7 0.5w "19Ohm" Version and the Amp works fine again so does the Idle current adjustment. I kind of notice now, that the high freq sound more pushed than ever before. Could that be possible from the diode change, because the changed diode now has different Values in Impedance - reverse leakage and max forward Voltage or are my ears just fooled from listend to my Bluetooth speakers the last weeks?

Im concidering to change them again to the alternative ones ive ordered and listen again.

First one is in now and the 2. is the alternative.

Im just still dont understand what changes these Values would be doing in that position or if they are really that irrelevant .