Hello all, I have that amp malfunctioning.
I try to understand how it works, see the schematic with some voltage (green if I think it's what expected, and red otherwise). When I plot the two channels on the oscilloscope, everything seems fine
The input stage is simple, next we have a classic VAS stage with a Vbe multiplier and a constant current source. A simple emiter follower driver and output stage. But I have no clue about what the transistor between the driver and output stage are doing. It looks like some kind of current limiter thing. I understand it tends to lower (stabilize ?) the gain of the output stage
To my understanding, 3 things are needed to release the protection : not overheating, not having too high DC offset on either channels, and… the third one is plugged to that circuit I don't understand.
What is the name of this circuit ? Would you have some resources I could use to understand ? How would you advise me to investigate the default of the amp ?
Thank you all !
With kind regards
I try to understand how it works, see the schematic with some voltage (green if I think it's what expected, and red otherwise). When I plot the two channels on the oscilloscope, everything seems fine
The input stage is simple, next we have a classic VAS stage with a Vbe multiplier and a constant current source. A simple emiter follower driver and output stage. But I have no clue about what the transistor between the driver and output stage are doing. It looks like some kind of current limiter thing. I understand it tends to lower (stabilize ?) the gain of the output stage
To my understanding, 3 things are needed to release the protection : not overheating, not having too high DC offset on either channels, and… the third one is plugged to that circuit I don't understand.
What is the name of this circuit ? Would you have some resources I could use to understand ? How would you advise me to investigate the default of the amp ?
Thank you all !
With kind regards
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OMG, I was impressed by the fact I didn't understood everything. Yes, it's a current limiter, the two transistor start conducting when the current crossing the power emitor resistors gets too high, and then they lower the driver's base current.
unfortunatly, this doesn't help me with my problem 🙂
unfortunatly, this doesn't help me with my problem 🙂
I think I got it ! usually C221 get charged through Q304 and D305, and is at vcc - 0.7.
If the current limiter trigger, it pulls current from C221 and D204, so it lower the potential and trigger the protection.
So, my problem is that somehow, this current limiter seems always active, or C221 is leaky, or something like that. I'll manage to find that tomorow.
Sorry I brothered you, it seems like I just needed to write down my problem. 🙂
If the current limiter trigger, it pulls current from C221 and D204, so it lower the potential and trigger the protection.
So, my problem is that somehow, this current limiter seems always active, or C221 is leaky, or something like that. I'll manage to find that tomorow.
Sorry I brothered you, it seems like I just needed to write down my problem. 🙂
Where even is C221?
Anyway, this is how I think the OCP triggers protection:
Q304 is being activated by pulling its base low through either R133 or R233 (it thn pulls protection input high via R304). This happens when the voltage across any of the 5 W emitter resistors gets high enough to turn on Q113/114 or Q213/214.
I would suggest checking R222, which may be bad or suffer from bad solder joints. Failing that, Q213 may no longer sustain rated voltage. If you can't obtain a 2SC1815 or similar general purpose Japanese transistor, try BC639, it's not very critical.
Anyway, this is how I think the OCP triggers protection:
Q304 is being activated by pulling its base low through either R133 or R233 (it thn pulls protection input high via R304). This happens when the voltage across any of the 5 W emitter resistors gets high enough to turn on Q113/114 or Q213/214.
I would suggest checking R222, which may be bad or suffer from bad solder joints. Failing that, Q213 may no longer sustain rated voltage. If you can't obtain a 2SC1815 or similar general purpose Japanese transistor, try BC639, it's not very critical.
He was hiding because he was GUILTY ! (between the base and the collector of Q213)
I guess it becomes leaky at high-ish voltage. I removed it, it works now 🙂
(my process was to remove some components until it starts to work again 😛)
To my understanding, its purpose is to delay the over-current protection for some few instants ? To give some charge to Q213 before it starts takings the drivers one's ?
Thank you very much !
On my amp it's a 103Z ceramic disc. I don't have the same exactly, I see it is rated for 50V. I have some few disks ceramic caps, but there is no mention of their ratting. Does it worth a try ?
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I'm stil only seeing C211, but assuming you mean that, it's probably a combination of thwarting oscillation and turn-on delay.
The voltage rating on ceramic disc caps is generally "high enough to not matter", but if in doubt go by size, that should be a pretty decent indicator given the same capacitance value and dielectric (usually Z5U or similar). IMHO a 50 V rating is a bit on the tight side for that spot, which may have contributed to the part failing in the first place. Ideally I'd like to be seeing about a nominal 100 V, as well as a transistor rated 80 V for Vceo (it's 60 V for the 2SC1815, a bit off but a real part may still have enough headroom to make it unscathed).
The voltage rating on ceramic disc caps is generally "high enough to not matter", but if in doubt go by size, that should be a pretty decent indicator given the same capacitance value and dielectric (usually Z5U or similar). IMHO a 50 V rating is a bit on the tight side for that spot, which may have contributed to the part failing in the first place. Ideally I'd like to be seeing about a nominal 100 V, as well as a transistor rated 80 V for Vceo (it's 60 V for the 2SC1815, a bit off but a real part may still have enough headroom to make it unscathed).