Morning Sakis 
Yes, we've been here before with these old amps.
(a small DC offset of less than 1 volt or so may not trip the protection but would seriously imbalance the bias current as measured in each resistor if a load were connected. The clue (apart from the actual offset) is that the current in one resistor would be low or zero and high in the other. Polarity of the offset determines which way round of course)
Yes, we've been here before with these old amps.(a small DC offset of less than 1 volt or so may not trip the protection but would seriously imbalance the bias current as measured in each resistor if a load were connected. The clue (apart from the actual offset) is that the current in one resistor would be low or zero and high in the other. Polarity of the offset determines which way round of course)
Yep, we need to know......
Measurement of what is actually happening is key to diagnosing this.
Hi Guys, Thanks for the help
I agree I am just your robotic surgeon on this one. This all started because i love to learn very difficult things. And no tech( in my area) would touch this amp due to the previous tech makeing a mess. And the level of damage
The diode d701 is VD1212 in service manuel. ZD701 is the zeener
I measured the voltage drops.
D701 1.260 LEFT ( Repaired channel in question)
D702 1.250 LEFT ( Repaired channel in question)
D701 1.26 Right Good side
D702 1.29 Right good side This diode goes down to 1.26 when I adjust bias down to match left side
Still only about 1 mv adjustment possiable on left channel .1-1.0 mv
Also ZD701 LEFT -7.6
Right -7.99
I agree I am just your robotic surgeon on this one. This all started because i love to learn very difficult things. And no tech( in my area) would touch this amp due to the previous tech makeing a mess. And the level of damage
The diode d701 is VD1212 in service manuel. ZD701 is the zeener
I measured the voltage drops.
D701 1.260 LEFT ( Repaired channel in question)
D702 1.250 LEFT ( Repaired channel in question)
D701 1.26 Right Good side
D702 1.29 Right good side This diode goes down to 1.26 when I adjust bias down to match left side
Still only about 1 mv adjustment possiable on left channel .1-1.0 mv
Also ZD701 LEFT -7.6
Right -7.99
The voltage across D701 looks to be in the right ballpark. The crackling sound could be a problem with the power amp stage... I'm wondering if its distorting badly as soon as the stage conducts.
Lets carry on with definites... some of your readings are a bit confusing to interpret
The DC offset you say 0mv - 10mv. Well if its no higher than 10mv then that's excellent however that also means that adding a load like a speaker should not alter things, and yet you say it does.
Look at the output stage and Q711.
The voltage across Q711 determines the bias current. When that voltage exceeds the four base-emitter volt drops (two drivers and a pair of outputs) then the stage should begin to conduct heavily.
So you need to measure that voltage across Q711. Once you reach 4*0.7 or so volts (so around 2.8 volts) then the stage should draw current and you should be able to measure a rise in bias current.
Does the voltage across Q711 adjust that high ?
Lets carry on with definites... some of your readings are a bit confusing to interpret
The DC offset you say 0mv - 10mv. Well if its no higher than 10mv then that's excellent however that also means that adding a load like a speaker should not alter things, and yet you say it does.
Look at the output stage and Q711.
The voltage across Q711 determines the bias current. When that voltage exceeds the four base-emitter volt drops (two drivers and a pair of outputs) then the stage should begin to conduct heavily.
So you need to measure that voltage across Q711. Once you reach 4*0.7 or so volts (so around 2.8 volts) then the stage should draw current and you should be able to measure a rise in bias current.
Does the voltage across Q711 adjust that high ?
nikko 1415 again
Hello
After a small break with this amplifier im back.
It actually sounds ok when turned up high enough to not hear the crackling static noise
If I touch my ocilliscope probe to the base of Q702 or anywhere in that area of the negitive feedback loop, The crackilng goes away. Thats just the tip of the probe, ground clip not attached to anything
When the speakers are not connected and no signal I measure 23mv across the bias test point. Same both sides
With speakers connected The side in question bias drops to 1.5v. And only adjustment with pot turned all the way up and down is .00mv - 1.6mv
Also the crackling/buzzing comes with the signal voltage. I hear it as the music or voices get louder. when there is a quiet period the noise is quiet
Its not a constant noise, but it is modulated with signal. Or something like that
Ill make the q711 measurements next
Hello
After a small break with this amplifier im back.
It actually sounds ok when turned up high enough to not hear the crackling static noise
If I touch my ocilliscope probe to the base of Q702 or anywhere in that area of the negitive feedback loop, The crackilng goes away. Thats just the tip of the probe, ground clip not attached to anything
When the speakers are not connected and no signal I measure 23mv across the bias test point. Same both sides
With speakers connected The side in question bias drops to 1.5v. And only adjustment with pot turned all the way up and down is .00mv - 1.6mv
Also the crackling/buzzing comes with the signal voltage. I hear it as the music or voices get louder. when there is a quiet period the noise is quiet
Its not a constant noise, but it is modulated with signal. Or something like that
Ill make the q711 measurements next
Its been a while the train of thought goes.
Your last few posts suggest that you might have a problem with the amp oscillating... and that could be because of all the different devices you have fitted. All the evidence is pointing that way, mainly that connecting speakers alters your bias current. If it is oscillating then all the DVM readings become meaningless... its 'scope territory from here on in.
Connect the scope across the speaker output. Is there ANY sign of low level oscillation. Is your scope a "proper" one or a USB PC thingy ? The oscillation could be in the 5 to 50MHz region.
the train of thought goes.Your last few posts suggest that you might have a problem with the amp oscillating... and that could be because of all the different devices you have fitted. All the evidence is pointing that way, mainly that connecting speakers alters your bias current. If it is oscillating then all the DVM readings become meaningless... its 'scope territory from here on in.
Connect the scope across the speaker output. Is there ANY sign of low level oscillation. Is your scope a "proper" one or a USB PC thingy ?
The oscillation could be in the 5 to 50MHz region.I have a rigol ds1104z.
And i noticed when i probe around and i touch the base of Q2 the static goes away. I use the secound channel to probe around while the first channel is connected to the base of Q2. And the static is gone
But as soon as i disconnect the probe from base of Q2 , the static comes back
Im a newbie when it comes to using oscilliscopes to look for oscilations in audio circuts ( or anywhere) But i have been learning for over a year now .
Also i see a lot of static on the sine wave when probing certain areas , is that what oscillation will look like.
The good channel of the amplifier has no static and it looks much cleaner on the scope, like a nice thin line representing the sine wave
And i noticed when i probe around and i touch the base of Q2 the static goes away. I use the secound channel to probe around while the first channel is connected to the base of Q2. And the static is gone
But as soon as i disconnect the probe from base of Q2 , the static comes back
Im a newbie when it comes to using oscilliscopes to look for oscilations in audio circuts ( or anywhere) But i have been learning for over a year now .
Also i see a lot of static on the sine wave when probing certain areas , is that what oscillation will look like.
The good channel of the amplifier has no static and it looks much cleaner on the scope, like a nice thin line representing the sine wave
The fuzzy signal i get when probing around in the amp are oscillation
I can compare the good side with the bad side and its obvious the static I hear and blurry line i see on the scope is oscillation.
Now im just not sure what to do with the info
But whenever i touch the oscilliscope probe to the base of Q2 or any of the components that are attached to Q2 the oscillation goes away.
I can compare the good side with the bad side and its obvious the static I hear and blurry line i see on the scope is oscillation.
Now im just not sure what to do with the info
But whenever i touch the oscilliscope probe to the base of Q2 or any of the components that are attached to Q2 the oscillation goes away.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.