Yes, you are absolutely correct. Actually I was referring to the missing components, the resistor is just burnt in an evident way. I have also found a picture on the web depicting another board from ML9 with a layout different from mine and @perelman 's too.
Honestly this makes me feel a bit more relaxed because this means that the board I have is not the result of someone playing the little electronic engineer 😀
Honestly this makes me feel a bit more relaxed because this means that the board I have is not the result of someone playing the little electronic engineer 😀
An interesting paragraph about the functions of this ML-9's board (full paper here):
"The power supply of the ML-9 shows attention to some unusual details. The a.c. power input immediately goes through a Sprague LC line filter, which filters both sides of the line in respect to the chassis or third-wire power ground.
Additional line filtering is provided by a balanced network consisting of a capacitor between hot and neutral, a choke in each side of the line, and another capacitor after the chokes between hot and neutral.
A network consisting of a relay-contact shortable resistance is in series with the transformer primary to reduce inrush current. This relay is operated by a transistor time-delay circuit, which shorts out the series resistance after about 2 S.
A detector circuit on the rear-panel output board monitors the d.c. offset at each channel's output, and it will trip the a.c. line-switch/circuit-breaker if offset becomes excessive.
This detector and the time-delay circuit share a small, separate power supply, with its own full-wave bridge rectifier and capacitor filters, which is fed from the switched a.c. line."
With an explanation of the TIP125 Darlington transistor as a time-delay operator. I really need to draw the schema.
"The power supply of the ML-9 shows attention to some unusual details. The a.c. power input immediately goes through a Sprague LC line filter, which filters both sides of the line in respect to the chassis or third-wire power ground.
Additional line filtering is provided by a balanced network consisting of a capacitor between hot and neutral, a choke in each side of the line, and another capacitor after the chokes between hot and neutral.
A network consisting of a relay-contact shortable resistance is in series with the transformer primary to reduce inrush current. This relay is operated by a transistor time-delay circuit, which shorts out the series resistance after about 2 S.
A detector circuit on the rear-panel output board monitors the d.c. offset at each channel's output, and it will trip the a.c. line-switch/circuit-breaker if offset becomes excessive.
This detector and the time-delay circuit share a small, separate power supply, with its own full-wave bridge rectifier and capacitor filters, which is fed from the switched a.c. line."
With an explanation of the TIP125 Darlington transistor as a time-delay operator. I really need to draw the schema.
This helps a lot to understand the role of the TIP and of the relay, great.
I am glad to share with you that I decided to keep the ML9 and you have a good part of this conclusion, thank you again. As soon as I have some spare time I will replace the mandatory caps and I am sure I will enjoy it a lot.
By the way, this is the picture I found on marklev that shows another version of the power management board. In this one the engineer redristributes the components to fill the void space and reduce the heat on the relay, maybe a more recent revision?
Gaetano
I am glad to share with you that I decided to keep the ML9 and you have a good part of this conclusion, thank you again. As soon as I have some spare time I will replace the mandatory caps and I am sure I will enjoy it a lot.
By the way, this is the picture I found on marklev that shows another version of the power management board. In this one the engineer redristributes the components to fill the void space and reduce the heat on the relay, maybe a more recent revision?
Gaetano
Indeed, much fewer components. The 10Ohms RCL T10 get very hot and also the TIP, I touched them after afew minutes with power on.
The shell of the relay is brown probably for the spark that came out when the pin of the 33Ohms resistor underneath melted down.
What about the bias? The link you provided here tells something useful, is that the right value?
The shell of the relay is brown probably for the spark that came out when the pin of the 33Ohms resistor underneath melted down.
What about the bias? The link you provided here tells something useful, is that the right value?
I'll check what I measured on mine, but from memory, it was not above 25-30mV for each channel, and the temperature was around 50-55°C on each side. Depending on the room temperature, since I did that during summer if I remember.
If you set the current high with a voltage > 30-40mV, I guess it will be more linear than if you set the voltage lower than 20mV, but then you're at risk of having a (very) hot amp, which stresses the components more (temp above 55°C).
There is another way to choose the right bias position: you measure the temperature of one side (right or left) after setting this side's trimmer and waiting for a full stabilization after 10-15'. Saying for example that 50°C is a sustainable temperature for this amp, then you measure the voltage drop on the 0.56Ω resistor which is set, and apply this voltage to the other channel. You might also decide that 55°C is the right temp (even if it will - on a long-time base - stress a little bit more the amp and components) and do the same. What's important is to set the voltage drop at the same value for each channel (balancing).
Correct me if I'm wrong!
If you set the current high with a voltage > 30-40mV, I guess it will be more linear than if you set the voltage lower than 20mV, but then you're at risk of having a (very) hot amp, which stresses the components more (temp above 55°C).
There is another way to choose the right bias position: you measure the temperature of one side (right or left) after setting this side's trimmer and waiting for a full stabilization after 10-15'. Saying for example that 50°C is a sustainable temperature for this amp, then you measure the voltage drop on the 0.56Ω resistor which is set, and apply this voltage to the other channel. You might also decide that 55°C is the right temp (even if it will - on a long-time base - stress a little bit more the amp and components) and do the same. What's important is to set the voltage drop at the same value for each channel (balancing).
Correct me if I'm wrong!
Maybe this could help in some way.
It's the power supply from the ML-2.
You see the same 3x10R resistors for surge control when switching on.
And also the two diodes and the Tip125 to switch relay K1 on after some delay, thereby shorting the 3x10R resistors.
So the assumption that the circuitry was meant to block DC was wrong.
Hans
P.S. I think that Perelman is correct in tuning the temp on both sides to between 50°C and 55°C
It's the power supply from the ML-2.
You see the same 3x10R resistors for surge control when switching on.
And also the two diodes and the Tip125 to switch relay K1 on after some delay, thereby shorting the 3x10R resistors.
So the assumption that the circuitry was meant to block DC was wrong.
Hans
P.S. I think that Perelman is correct in tuning the temp on both sides to between 50°C and 55°C
Attachments
Hi and thank you for the step-by-step guide. I need just one more information (sorry if it is a basic one). I assume that the trigger is the yellow little one which is on top of each channel PCB but I need to understand which is the resistor where I should measure the voltage drop.
Dear @Hans Polak , this is much appreciated. I think now it is almost consolidated that this circuit is to limit the inrush current and the schematics from ML2 confirms this.
Hi, I decided to open the case again to adjust the bias according to the instructions above. At the beginning I measured 12mV across the resistor on the left channel and about 33mV on the right. At the end of the adjustment I had about 24mV for both with a temperature on the case so the finals of about 45C.
After about 90 min of low volume sound the voltage drop was around 27mV for both. Is this raise normal?
After about 90 min of low volume sound the voltage drop was around 27mV for both. Is this raise normal?
I understand, I'm always very cautious when doing things inside when power is on.
You can do that but just put the cover above without putting the screws back right now. The cover won't be perfectly closed but you don't mind. The trimmer sensitivity is not easy to feel exactly so sometimes it's too much or not enough, you'll see that tomorrow. Even if a 1mV delta between the 2 channels is almost nothing and cannot be really heard I'd say (at least with my old ears. If it is satisfying for you, then it's OK, and il seems to be OK like it is right now.
And your amp is glorious.
You can do that but just put the cover above without putting the screws back right now. The cover won't be perfectly closed but you don't mind. The trimmer sensitivity is not easy to feel exactly so sometimes it's too much or not enough, you'll see that tomorrow. Even if a 1mV delta between the 2 channels is almost nothing and cannot be really heard I'd say (at least with my old ears
. If it is satisfying for you, then it's OK, and il seems to be OK like it is right now.And your amp is glorious.
Good morning. It is now fun time with the amp, I moved the bias up a little bit, now it is about 28mV across the emitter resistor . It is holding the value quite well, at least for the time being. The temperature on the dissipator is about 48C so, given @Hans Polak 's suggestion I should rise it more.
The sound is somewhat dry if compared to the Quad previous power amplifier I used, do you have the same experience?
Moving to the broken resistor that @perelman highlighted, I still wonder what can be the consequence of using the amp without it, as I am doing it now
. It is holding the value quite well, at least for the time being. The temperature on the dissipator is about 48C so, given @Hans Polak 's suggestion I should rise it more.The sound is somewhat dry if compared to the Quad previous power amplifier I used, do you have the same experience?
Moving to the broken resistor that @perelman highlighted, I still wonder what can be the consequence of using the amp without it, as I am doing it now
What I understand by examining the available pictures is that, the missing 33Ohm resistor makes the relay always open so the three resistors RCL TIO 10Ohm are never shorted (by the way, they are quite hot during the amp operation). This means that the current is always flowing through the resulting 3.3Ohm resistor down to the board connector array. This is definitely not nice
Hi,
hope everything is fine with all of you.
One more note about the bias topic. In the thread Mark Levinson ML-9 bias (thank you @perelman ) it is said that the idle current of the power stage should be 260 mV. Then "This is divided over 4 transistors giving 65mA", why 4 transistors when I see 5 of them? Probably I am misunderstanding...
Have a nice day
G
hope everything is fine with all of you.
One more note about the bias topic. In the thread Mark Levinson ML-9 bias (thank you @perelman ) it is said that the idle current of the power stage should be 260 mV. Then "This is divided over 4 transistors giving 65mA", why 4 transistors when I see 5 of them? Probably I am misunderstanding...
Have a nice day
G
Hi Gaetano, you're right, there are 5 transistors, but as far as I understand the schematics, one is a pre driver. This would let 4 transistors as the output stage. That's what I understand, more experienced and experts will correct me if necessary. But anyway, this calculation would give around 36 mV which seems little bit high to me. As much current will make the amp heat quite a lot. I would stay below 30 mV, at least that's the maximum I went to on mine.
Good day!
Good day!