NAD 7400 bias limit

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Jakobsson

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Joined 2009
#1
Repairing this amplifier with blown outputs and Q016 bad. Replaced them and also measured everything else.
Is there a way of limiting the bias to almost zero but have most of the components in place?
I am a little bit worried because i already replaced the transistors once but they blew up instantly, found the bad q016 after that(might have missed that the first time)
Tried the amplifier without the outputs and that was ok but alot of negative voltage around R825 but i guess the amp is not supposed to be use without outputs.
Any other ideas?,tried with a 100w bulb in series but maybe that is not big enough, manual says 200-250w when any work has been done.
Maybe i should just go for it but this is my last pair of outputs.
 

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#2
Always always use a bulb. 100w is plenty.

Do not connect speakers.

Q016 is in the switched rail (Glass G) circuitry and any faults here should not affect the main amp besides running it at the higher rail voltage when not needed.

Apply a link across C809 to force zero bias.
 
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#4
You're in good hands with Mooly!

With the outputs absent, the amp is open-loop--- no bias feedback to center the output. But there should be roughly 1.2V across R825 and it should respond to the bias pot.

With outputs absent and link across C809 removed, you might try this: install two 12k resistors in series paralleling R825. Connect the center tap of these resistors to feedback bases Q801, Q802--- i.e. temporary feedback in lieu of R812. I believe this will bias amp output to nominal 0V and allow confirmation of front-end and inspection of bias circuit.

Good luck!
 
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#6
In my entire working life, I was a bench EE, did all kinds of R&D, RF, synthesizers, audio digital filters, etc. Last ~ 30 years was VP engineering at Valentine Research where we made police radar detectors, but still a bench guy. Loved it all, still miss it.

The avatar and the BSST moniker are my brag on my son: Boom Supper Sonic Transport. Visit BoomSupersonic.com for good entertainment. He's founder and CEO.

He's leaving his old man in the dust.
😀
 
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#7
Jakobsson said:
This means i still have som problem beacuse it made no difference.
Click to expand...
If the bias is high the bulb will light. Is that what you mean?

You may need to reduce the bias on the good channel (or just turn the bias down) to bring that channels current right down as well and so stop the bulb lighting.

If the bulb lights because of a fault in the amp (and you have shorted that cap) then you need to first just confirm the current really is flowing in the output transistors by measuring the voltage across each pair of the 0.22 ohm emitter resistors. If that confirms high current flow then you must have a faulty driver or pre driver stage (or output transistors). With the cap shorted there should be virtually no voltage across each 0.22 ohm.

Shorting that cap means there is no possibility of the output transistors turning on together and so drawing current.
 
#8
Problem with the bulb is that the amp has some sensitive standby/remote/protection board so i have to give it 3 startups with bulb before it turns on.(with one channel working)
I tried to link the working channel capacitor(bias) to limit startup current but that did not help.
If i desolder Base from Q817/Q818 then amplifier turns on but stays in protect, about +27v on output
 
#11
OK. I would say you need to determine where the high current actually is under those first start up conditions. That means leaving it with the lamp bright and measuring the voltage across the four 0.22 resistors on each output pair. Do the measurement on both channels, good and bad and see if any are passing significant current.

As you mentioned, is this an artefact of usingthe bulb or is it something else. There mut be no load attached for this.
 
#12
As it is it i not possible to light up the bulb contantly, it only flashes and then turn off. If i connect defective channels output transistors or drivers then it just flashes and turns off and it is not possible to get it running even if i try multiple times. My ideas is either "BSST":s idea of a simulated feedback or disable the protection. I have not checked the small capacitors on the amplifier board, yet.
 
#13
The amp itself should not be affected by the protection circuit. All the protection does is engage (or not) the speaker relay.

I thought you meant the bulb was lighting up permanently on the first and second attempts and only on the third attempt would it go out 🙂 If that is not the case and the bulb goes out on the first attempt then you must fault find in that state. If there is something like an intermittent transistor junction then that could be why it seems to work after a few attempts at powering on and off.
 
#15
Connected eveything back but removed power going to outputs, power on the schematics to D023/D024. Everything seems to be measuring ok with proper voltage over R825. Also responded well to bias adjustment.
Reconnected power to the outputs and jumped the protection relay and now i finally have a lit bulb.
Q820 gets pretty hot and voltage across R825 very high 11v.
I wish i had jumped the relay from the start for easier fault finding.
 
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