I just rebuilt this amp with all new caps.
I need to set the bias voltage/current.
Anyone that could help that knows this amp?
There are 2 trim pots and I see a DC Test point but I have no clue after that.
thanks
Some images of the recap for interest
I need to set the bias voltage/current.
Anyone that could help that knows this amp?
There are 2 trim pots and I see a DC Test point but I have no clue after that.
thanks
Some images of the recap for interest
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Yes but its an old pyramid with a lousy horizontal scale.
I do have a good RMS DVM.
Is it done by turning to pots all the way off, getting a base current reading then adding xxxMA to the base current in 2 steps?
I could do that. I just need to know how many ma to add.
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This is how I do it when I don't have manufacturer specs.
http://www.bcae1.com/temp/ausettingbias.swf
The DC test point is to allow testing of the DC shutdown circuit, I think (someone should speak up, if this is wrong). It's used on a lot of the ZED amps.
http://www.bcae1.com/temp/ausettingbias.swf
The DC test point is to allow testing of the DC shutdown circuit, I think (someone should speak up, if this is wrong). It's used on a lot of the ZED amps.
Didn't see that ^^^ before I posted this:
I measured the speaker voltage and I get 25mv on left, 14mv on the right.
I measured the speaker voltage and I get 25mv on left, 14mv on the right.
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Somewhere in the green range is where you want it. It's not super critical. All you really need to do is get a very small amount of current flowing through the output transistors.
Ok I will try it.
I noticed when turning up the bias, I get a swooshing noise on my ham radio thats close to it.
Maybe Mr Zed will stop by with a factory method.
If not this will have to do.
There are 6 tip35/36 per side. Maybe a 100ma each for an extra 1200ma over the base current?
Should I ground the line inputs?
Shound I have any load on the speaker outputs?
I will report back after I wake up later this evening. (dont ask)
thanks!
I noticed when turning up the bias, I get a swooshing noise on my ham radio thats close to it.
Maybe Mr Zed will stop by with a factory method.
If not this will have to do.
There are 6 tip35/36 per side. Maybe a 100ma each for an extra 1200ma over the base current?
Should I ground the line inputs?
Shound I have any load on the speaker outputs?
I will report back after I wake up later this evening. (dont ask)
thanks!
Seeing as you are impatient 🙂
If it is bias current you are adjusting then set it on a low amplitude sine signal (with load attached to amp) and adjust until the crossover distortion just disappears.
Use something like 1 to 5kHz and an amplitude of just a couple of volts peak to peak.
Crossover distortion looks like this.
If it is bias current you are adjusting then set it on a low amplitude sine signal (with load attached to amp) and adjust until the crossover distortion just disappears.
Use something like 1 to 5kHz and an amplitude of just a couple of volts peak to peak.
Crossover distortion looks like this.
Attachments
Am I looking for VDC at the output?
Should the scope be ac or dc?
I see a step down on the top and a heel bump on the bottom that seems to flatten out about mid travel on the bias pot.
Ok I think I'm going way past the rounded wave to a square looking one.
Should the scope be ac or dc?
I see a step down on the top and a heel bump on the bottom that seems to flatten out about mid travel on the bias pot.
Ok I think I'm going way past the rounded wave to a square looking one.
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also I may have the wrong tool.
Mine is a function generator at 20khz that does Sine, Square,
Triangle, Skewed Sine, Ramp, Pulse and TTL level square.
I'm pretty noob, I'm sorry.
Mine is a function generator at 20khz that does Sine, Square,
Triangle, Skewed Sine, Ramp, Pulse and TTL level square.
I'm pretty noob, I'm sorry.
OK I used 2vac on the speaker, sine wave at 2khz.
there I could see the sine wave with a bump more like this edited image
I turned the bias up to just before mid way and that lump just went away.
20khz seemed to be hard to get any reading like this.
I hope 2khz is enough.
If not I can try 5khz next
there I could see the sine wave with a bump more like this edited image
I turned the bias up to just before mid way and that lump just went away.
20khz seemed to be hard to get any reading like this.
I hope 2khz is enough.
If not I can try 5khz next
Ok I did 5khz. Bigtime stressing out.
Both trimmers are set to about 11-11:30.
Both lumps "just smoothed out" to make a smooth slope.
I hope its good enough. I'm pretty stressed about this.
The amp is unobtainium.
thanks for all your help I hope this is enough.
I'll check back tomorrow night before I install it for any reasons I screwed up.🙁
Both trimmers are set to about 11-11:30.
Both lumps "just smoothed out" to make a smooth slope.
I hope its good enough. I'm pretty stressed about this.
The amp is unobtainium.
thanks for all your help I hope this is enough.
I'll check back tomorrow night before I install it for any reasons I screwed up.🙁
I guess at dcv on the speaker I was probably flat-topping the amp tring to reach 2v.
Man I feel stupid sometimes.
Man I feel stupid sometimes.
It sounds like you've got the hang of it. Low amplitude and high (ish) frequency tends to show the distortion the most.
Adjusting to the point where the distortion just disappears means that you are playing safe by leaving the bias current low. That's much better than setting it to high which would cause the amp to run hotter and also make the whole amp more prone to 'thermal runaway' where bias increases on its own with temperature... and so on... resulting in a major failure.
So enough to eliminate the distortion but no more.
With no signal there should be no significant DC voltage across the speaker. Less than 100 millivolts is fine.
Adjusting to the point where the distortion just disappears means that you are playing safe by leaving the bias current low. That's much better than setting it to high which would cause the amp to run hotter and also make the whole amp more prone to 'thermal runaway' where bias increases on its own with temperature... and so on... resulting in a major failure.
So enough to eliminate the distortion but no more.
With no signal there should be no significant DC voltage across the speaker. Less than 100 millivolts is fine.
Well back to active duty....
I think I will take it easy and look for another amp that can be destroyed without worry.
This amp is kinda special and its a shame to kill it.
I think I will take it easy and look for another amp that can be destroyed without worry.
This amp is kinda special and its a shame to kill it.
Well the amp works and now with the bias set properly, it produces more rock & roll bass and more bass overall.
The amp was about 45C and I could smell the magic smoke residue heating up and I got nervous. Theres a yellow film of it on the side cover that faces up.
I was pulling over every 5 mins to feel the amp.
It was stone cold till I played holiday and a good level and thats when it was about 45C and the baked on residue was emitting some odor when I smelled up close.
I hate that smell of burning PCB in the morning.
I'll test some more but like I said, this amp has seen enough and needs to be behind a desk.
I did feel the whole amp and the section with the power supply seems to be the warmest.
Is that normal?
If not what would make the power supply freak out?
This is 2 ohms x2 also.
Maybe 2 ohms is too much for this amp?
thanks
The amp was about 45C and I could smell the magic smoke residue heating up and I got nervous. Theres a yellow film of it on the side cover that faces up.
I was pulling over every 5 mins to feel the amp.
It was stone cold till I played holiday and a good level and thats when it was about 45C and the baked on residue was emitting some odor when I smelled up close.
I hate that smell of burning PCB in the morning.
I'll test some more but like I said, this amp has seen enough and needs to be behind a desk.
I did feel the whole amp and the section with the power supply seems to be the warmest.
Is that normal?
If not what would make the power supply freak out?
This is 2 ohms x2 also.
Maybe 2 ohms is too much for this amp?
thanks
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