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Zener removal?
Zener removal?
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Old 16th August 2018, 04:04 PM   #1
famousmockingbird is offline famousmockingbird  United States
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Default Zener removal?

I am at the end stages of repairing a blown amplifier. All output devices and drive and bias transistors have been replaced along with their associated diodes and any other blown resistors in the path of destruction.

I forgot to order 1N4740A 10V 1Watt Zeners for the output stages and they are shorted. I was wondering what they do anyway and was curious if I can run and bias the amp without them for a short period of time until I order new ones? It looks like to me they are there as protection against too large AC input signal and so they clamp the AC at 40v?


The Zeners in question are D214-217


Thanks!
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Last edited by famousmockingbird; 16th August 2018 at 04:33 PM.
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Old 16th August 2018, 04:15 PM   #2
Andrew Eckhardt is offline Andrew Eckhardt  United States
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Those diodes protect against gate voltage rising beyond what source current can limit in some odd circumstance. I wouldn't run the amp without them unless you want to punch through all the fet gates again. Not worth it.
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Old 16th August 2018, 04:32 PM   #3
famousmockingbird is offline famousmockingbird  United States
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Right but wouldn't that only be during application of a signal? I was trying to bias the amp up without them and not play music though it.


Thanks
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Old 16th August 2018, 04:39 PM   #4
Andrew Eckhardt is offline Andrew Eckhardt  United States
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Hard to say. If it breaks into oscillation for just a microsecond yer done. The only quasi-safe way would be to limit drain voltage to below gate breakdown rating during test for good bias points. That's sketchy to do when the front - end is biased off the output supply rails. You might be fine just throwing the switch. Maybe not.
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Old 16th August 2018, 04:39 PM   #5
famousmockingbird is offline famousmockingbird  United States
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Normally the gate voltage is ~3.5v at quiescent state.

I guess it could oscillate, I did start up briefly not on a scope and nothing blew. I was either lucky or it's fine.


You are the voice of reason right now lol. Thanks I should probably just wait.

Last edited by famousmockingbird; 16th August 2018 at 04:41 PM.
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Old 16th August 2018, 04:43 PM   #6
famousmockingbird is offline famousmockingbird  United States
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I do have 7.5v and 15v zeners but no 10v. I guess I could put the 7.5v in for now and make sure everything else is fine before I order parts.

Basically I just want to only order parts one more time so I want to make sure everything else works and just go ahead and order the zeners.
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Old 16th August 2018, 04:58 PM   #7
JonSnell Electronic is online now JonSnell Electronic  United Kingdom
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A lot of MosFet power amps us 15volt zeners to protect their gates, normally at power on and power off is when the gates need to be clamped, to avoid gate damage.
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Old 16th August 2018, 05:02 PM   #8
Andrew Eckhardt is offline Andrew Eckhardt  United States
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Yeah. Having plenty of driver voltage but no output stage supply is probably the easiest way to fry a gate with no protection in an otherwise good amp.
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Old 16th August 2018, 05:08 PM   #9
famousmockingbird is offline famousmockingbird  United States
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Ahhh I see, that makes complete sense. Think I will be okay to use my 15v zeners instead? I may take a trip to the Electronics store in Manchester, they may have some Zeners.
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Old 16th August 2018, 06:34 PM   #10
famousmockingbird is offline famousmockingbird  United States
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Looks like all I got for the time being 6.8V 1watt


Should work to keep them safe on start up and power off no?
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