909 Diodes

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18-Dec-2017 01:24 PM

CBK

CBK

Posts: 34

Can anyone tell me is D3 located on the amps boards?

Or is D3 located on the Quadlink board?

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18-Dec-2017 06:10 PM

CBK

CBK

Posts: 34

Sorry i meant D4.

Is D4 on the Quadlink PCB?

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19-Dec-2017 02:07 AM

EJP

EJP

Posts: 1438

Should be on the amplifier board near D5.
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19-Dec-2017 03:26 PM

CBK

CBK

Posts: 34

Thankyou EJP, in addition to de-coupling D1 & D2 (with 100nF MKT), is there any benefit to be had from doing the same with D3 & D4?

Thoughts on de-coupling D3 & D4?


 

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19-Dec-2017 11:24 PM

EJP

EJP

Posts: 1438

No.

D1 and D2 are Zener diodes with 15VDC across them. Decoupling them (or rather shunting them with a capacitor as one end of each is grounded) shunts RF to ground, which improves the stability of the op-amp, although I doubt that it is even necessary in a 909.

D3/4/5 carry audio signal. You *can't* decouple them, as neither end is grounded: if you did, you would be shunting audio to ground. Putting capacitors across them is also pointless.
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20-Dec-2017 01:53 PM

CBK

CBK

Posts: 34

Ok cheers for that EJP, also someone mentioned its also a good idea to replace the diodes if installing new electrolytic capacitors, do you have any opinion on this?

I think he said the new and improved in rush currents from fresh caps often kills aging diodes, or in the least, impedes their function.

Any thoughts?

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20-Dec-2017 11:01 PM

EJP

EJP

Posts: 1438

D3-D5 aren't rectifying diodes so it is all irrelevant. I replace rectifier diodes in 33s but I've rarely if ever had to replace them in power amplifiers. If they ain't broke don't fix them.
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21-Dec-2017 12:29 PM

CBK

CBK

Posts: 34

Ok i'll leave well alone, thanks for the input : )

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