With the info on the PCB pictures I could complete the schematic.
The two 10.000uF are not in series but parallel, so positioning is correct in the picture.
I have re-arranged D3 and D4, to make it more obvious that D4 is to catch the flyback pulse from the relay when switched off and that D3 is to discharge the 6.8uF cap through the relay resistance at switch off.
With the given component values and knowing that the 24V Relay has 600R DC resistance, I could simulate the on/off time, see second attachment.
Blue is the rectified voltage from TR1 at switch on and switch off, I assumed it to be 28V.
Red is the voltage on the Relay, limited at 24V by Z1.
Green is the current through the relay while switching on and off.
Switch-on time is ca 0.8sec and switch-off is almost immediate.
Hans
The two 10.000uF are not in series but parallel, so positioning is correct in the picture.
I have re-arranged D3 and D4, to make it more obvious that D4 is to catch the flyback pulse from the relay when switched off and that D3 is to discharge the 6.8uF cap through the relay resistance at switch off.
With the given component values and knowing that the 24V Relay has 600R DC resistance, I could simulate the on/off time, see second attachment.
Blue is the rectified voltage from TR1 at switch on and switch off, I assumed it to be 28V.
Red is the voltage on the Relay, limited at 24V by Z1.
Green is the current through the relay while switching on and off.
Switch-on time is ca 0.8sec and switch-off is almost immediate.
Hans
Attachments
You're incredible Hans!
Your schematic is absolutely right, and its presentation is much more "didactic" than mine. And much easier to understand. And your calculation of the off/on time is probably exact too. I'll try to check with a chronometer if I can
And it's also exact that switch-off is nearly immediate, as far as I can hear.
I don't know where you found the coil resistance value (data sheet?), but it is correct since I measured it at nearly that (597ohm if I remember). I should have this figured on the schema.
Please allow me a newbie question? what's the use of those 2 x 10000µF capacitors? Since it appears that they are not present in all ML-9 (cf Gaetano's own amp), but we can see them in the 23 and the 27...
Thanks once again, I'll redraw the schema I've got with consideration to your (more than) valuable contribution.
Your schematic is absolutely right, and its presentation is much more "didactic" than mine. And much easier to understand. And your calculation of the off/on time is probably exact too. I'll try to check with a chronometer if I can
And it's also exact that switch-off is nearly immediate, as far as I can hear.
I don't know where you found the coil resistance value (data sheet?), but it is correct since I measured it at nearly that (597ohm if I remember). I should have this figured on the schema.
Please allow me a newbie question? what's the use of those 2 x 10000µF capacitors? Since it appears that they are not present in all ML-9 (cf Gaetano's own amp), but we can see them in the 23 and the 27...
Thanks once again, I'll redraw the schema I've got with consideration to your (more than) valuable contribution.
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First question should be: what are those diodes doing ?
They are to block a possible DC voltage offset on the mains supply, thereby preventing the transformer going into saturation.
But since the on-off switching of these diodes may produce HF spikes, the two 10mF caps are there to suppress these spikes.
When using soft recovery diodes, this effect may be diminished but it will not completely remove the spikes.
So maybe ML has been experimenting with different diodes.
Hans
It's very clear Hans. I'm therefore surprised as such a value just to suppress some HF spikes. Would it be that such a capacitor's role is to prevent short-term variations in the load current from becoming voltage spikes? Is its function analog to a capacitor/diode snubber circuit? Would it be interesting then to bypass them with smaller (like film-type) capacitors to improve their HF filtering capacities, or am I completely out of the question? Or even design a RLC filter?
I'm not specialist at all, so pardon my (probably stupid) questions, it's just curiosity. I guess that ML engineers knew what they did when designing such circuits. Have a nice weekend.
I'm not specialist at all, so pardon my (probably stupid) questions, it's just curiosity. I guess that ML engineers knew what they did when designing such circuits. Have a nice weekend.
Probably from a given time on they decided to suppress the little network at all. In the case of my amp the Frako caps as well as the two diodes are just and simply missing, they have never been soldered. I also saw another board from a ML9 in which the area of the caps and the diodes D1 and D2 was occupied by the 33Ohm resistor which is normally right below the relay.
Jerome, given that your amp dates back to 1985, mine probably is from the same year or from 1986, which is already in the Madrigal era. One more little confirmation is coming from a little sticker on the back cover of my amp saying "Mark Levinson" and "Madrigal" right below
Yes Gaetano. Mine has no sign at all of any Madrigal signature. The faceplate is purely Levinson, old address. Madrigal took control of MLAS end of 1984 or at the very beginning of 1985, so I guess it's a madrinson. But I suppose they had numbers of faceplates already made in the factory.
As Hans says, "maybe ML has been experimenting with different diodes"... and different circuits.
As Hans says, "maybe ML has been experimenting with different diodes"... and different circuits.
In my case, trying to estimate relay activation time by a chronometer, I obtain about 1.8 s to switch-on and 0.5s to switch-off.
Gaetano