Thank you a lot Dieter for providing your thoughts on my problem, this is much appreciated. You may well be right, most likely there is more needed than an adjustment of the trimpots to fix my problem, however it would still be nice if someone could make these old Technical Bulletins from Mark Levinson available to us so that we can maintain and enjoy our ML 431/432 amps for the years to come.
Anyway I had a friend around yesterday who knows a lot more about electronics than I do and he too could not locate the root of the problem based on a visual inspection of the boards, nothing seems to be burnt or broken. Also I measured Q119 to Q122 for resistance on the left channel and the right channel and both sides give the same reading. Only that Pin 1 and Pin 2 of Q221 show no resistance when I measure the resistance between them, but looking at the schematics this seems to be okay and it is the same on the left and the right channel.
What I am trying to work out now is which audio channel has the fault, left or right?
And I am trying to work out which logic circuit is responsible for detecting a signal-related fault, "LED Behavior - Rapid blinking (on half of the time)". Because this is the type of blinking I saw.
Currently I am not sure if the signal-related fault is detected by the "ELEC_FAULT" signal or by not receiving the "AMP_ON" signal on the Micocontroller U100.
Optocoupler U3 I believe is responsible for the "AMP_ON" signal (HIGH signal equals amp is on) and Optocoupler U102/U103 areresponsible for the "ELEC_FAULT" signal (HIGH signal equals an electrical fault).
Moving forward I have the following plan:
1. My unit is still set to 240V which was the voltage at the time when I lived in Scotland when I had bought the unit, I now live in Germany. Having looked through the schematics I see that I can convert the amp to 230V by bridging posts D-E instead of D-F so I will do this.
2. I will load the large caps to 20V with a low current to check if they keep their charge.
3. I will put the amp together and insert wires into the P100 connector of each audio channel to see what voltage there is on Pin 1, 2 and 4 shortly after powering the unit on.
4. If the fault points towards Pin 2 of P100 then I will see if I can solder another 4N33 on top of U3 to find out if it is simply the optocoupler that has failed as per your suggestion Dieter.
By the way, I wonder what the POS_16V circuit is used for. Does the amp use 16V to detect if all is well before switching on the power transistors? (Apologies for my very limited knowledge of electronics.)
Cheers,
Mischa
Anyway I had a friend around yesterday who knows a lot more about electronics than I do and he too could not locate the root of the problem based on a visual inspection of the boards, nothing seems to be burnt or broken. Also I measured Q119 to Q122 for resistance on the left channel and the right channel and both sides give the same reading. Only that Pin 1 and Pin 2 of Q221 show no resistance when I measure the resistance between them, but looking at the schematics this seems to be okay and it is the same on the left and the right channel.
What I am trying to work out now is which audio channel has the fault, left or right?
And I am trying to work out which logic circuit is responsible for detecting a signal-related fault, "LED Behavior - Rapid blinking (on half of the time)". Because this is the type of blinking I saw.
Currently I am not sure if the signal-related fault is detected by the "ELEC_FAULT" signal or by not receiving the "AMP_ON" signal on the Micocontroller U100.
Optocoupler U3 I believe is responsible for the "AMP_ON" signal (HIGH signal equals amp is on) and Optocoupler U102/U103 areresponsible for the "ELEC_FAULT" signal (HIGH signal equals an electrical fault).
Moving forward I have the following plan:
1. My unit is still set to 240V which was the voltage at the time when I lived in Scotland when I had bought the unit, I now live in Germany. Having looked through the schematics I see that I can convert the amp to 230V by bridging posts D-E instead of D-F so I will do this.
2. I will load the large caps to 20V with a low current to check if they keep their charge.
3. I will put the amp together and insert wires into the P100 connector of each audio channel to see what voltage there is on Pin 1, 2 and 4 shortly after powering the unit on.
4. If the fault points towards Pin 2 of P100 then I will see if I can solder another 4N33 on top of U3 to find out if it is simply the optocoupler that has failed as per your suggestion Dieter.
By the way, I wonder what the POS_16V circuit is used for. Does the amp use 16V to detect if all is well before switching on the power transistors? (Apologies for my very limited knowledge of electronics.)
Cheers,
Mischa
Hi Mischa,
Q119 and Q120 keeps the power dissipation away from the darlington Q117 and Q118 otherwise they will melt away.
Maybe you take away one power amp and try if the you can switch on the amp, if not you may put the other in and try again to find out which channel is faulty (if the reason for fault is inside the power amps).
To change the voltage of the unit from 240 to 230 I would do as the very last activity.
The POS_16V (as well as the NEG_16V) is needed for supplying the servo amp U101 and theoretically the U1 and U2 optocouplers, but according the circuit diagram they are not used (NU). As I don't have the full schematics there may be another need in the logic circuitry.
best regards
Dieter
Q119 and Q120 keeps the power dissipation away from the darlington Q117 and Q118 otherwise they will melt away.
Maybe you take away one power amp and try if the you can switch on the amp, if not you may put the other in and try again to find out which channel is faulty (if the reason for fault is inside the power amps).
To change the voltage of the unit from 240 to 230 I would do as the very last activity.
The POS_16V (as well as the NEG_16V) is needed for supplying the servo amp U101 and theoretically the U1 and U2 optocouplers, but according the circuit diagram they are not used (NU). As I don't have the full schematics there may be another need in the logic circuitry.
best regards
Dieter
1. Connect a DMM positive lead to TP8 and connect the negative lead to the BLACK
(GROUND) speaker binding post. TP8 is on the daughter board under diode CR120
(diode is mounted on spacers).
Test:
Initial Left Servo Adjust: Adjust the Servo (R133 on backside of the LEFT Channel daughter
board) for +500mV at TP8. Confirm that the voltage at the RED binding post is -1volt. NOTE: If
this voltage is more than 500 mv above or below -1 volt there is a problem with the channel and
it must be repaired before proceeding further.
Initial Right Servo Adjust: Adjust the Servo (R133 on backside of the RIGHT Channel
daughter board) for +500mV at TP8. Confirm that the voltage at the RED binding post is -1volt.
NOTE: If this voltage is more than 500 mv above or below -1 volt there is a problem with the
channel and it must be repaired before proceeding further.
Operate Mode: Press the STANDBY button. You should hear the 431/432 amplifier
make a series of clicking noises. The amplifier is now On (in OPERATE mode).
Observe the front panel’s RED LED. It should be glowing steady and bright.
4.0 Setting the Amplifier Bias Voltage
Setup:
Set the DMM to “mVDC” and connect it between the hook on the RIGHT channel’s RED binding
post and one of the emitter resistors on the RIGHT channel (R322, R318, R314, R310).
Test:
Left Bias Adjust: Set R213 for the LEFT channel (R213 on channel motherboard to 9mVDC.
Test:
Right Bias Adjust: Set R213 for the RIGHT channel (R213 on channel motherboard – to 9mVDC.
(GROUND) speaker binding post. TP8 is on the daughter board under diode CR120
(diode is mounted on spacers).
Test:
Initial Left Servo Adjust: Adjust the Servo (R133 on backside of the LEFT Channel daughter
board) for +500mV at TP8. Confirm that the voltage at the RED binding post is -1volt. NOTE: If
this voltage is more than 500 mv above or below -1 volt there is a problem with the channel and
it must be repaired before proceeding further.
Initial Right Servo Adjust: Adjust the Servo (R133 on backside of the RIGHT Channel
daughter board) for +500mV at TP8. Confirm that the voltage at the RED binding post is -1volt.
NOTE: If this voltage is more than 500 mv above or below -1 volt there is a problem with the
channel and it must be repaired before proceeding further.
Operate Mode: Press the STANDBY button. You should hear the 431/432 amplifier
make a series of clicking noises. The amplifier is now On (in OPERATE mode).
Observe the front panel’s RED LED. It should be glowing steady and bright.
4.0 Setting the Amplifier Bias Voltage
Setup:
Set the DMM to “mVDC” and connect it between the hook on the RIGHT channel’s RED binding
post and one of the emitter resistors on the RIGHT channel (R322, R318, R314, R310).
Test:
Left Bias Adjust: Set R213 for the LEFT channel (R213 on channel motherboard to 9mVDC.
Test:
Right Bias Adjust: Set R213 for the RIGHT channel (R213 on channel motherboard – to 9mVDC.
Thanks a lot LJYS, the fog disappeared now 
So the bias adjustment (R213) results in 40mA per power transistor pair - sounds considerable; the servo (U101) adjustment with R133 now seem to be clear as well, as V_out *(-0.5)= V on TP8.
I will check soon the output DC Voltage in the "ON" stage as -1V at the output binding posts seem to be a little high under normal working conditions.
One - final - question is the adjustment of R2 and R5, which defines the POS_REG and NEG_REG voltages, which from my understanding should be as high as possible to allow a maximum of voltage swing in the pre driver.
best regards
Dieter
So the bias adjustment (R213) results in 40mA per power transistor pair - sounds considerable; the servo (U101) adjustment with R133 now seem to be clear as well, as V_out *(-0.5)= V on TP8.
I will check soon the output DC Voltage in the "ON" stage as -1V at the output binding posts seem to be a little high under normal working conditions.
One - final - question is the adjustment of R2 and R5, which defines the POS_REG and NEG_REG voltages, which from my understanding should be as high as possible to allow a maximum of voltage swing in the pre driver.
best regards
Dieter
L
You could help me?
I'm looking for the schematic of the Proceed HPA amplifier. I have an HPA3 that goes into protection ...... I hope you can help me with this! ..... Thank you!
LJYS........Thank you very much for the schematics !. It is very similar to 431 ....... right?
Hi Symphony,
your description of the fault is a bit general; have you opened the amp and checked if something is burnt, have you checked the voltages on the main caps?
Maybe you should check the thermofuses TS1 and TS2 in the transformers?
Also here there are some missing circuit diagrams like power supply and logic (if they have a logic pcb which I don't know)
best regards
Dieter
your description of the fault is a bit general; have you opened the amp and checked if something is burnt, have you checked the voltages on the main caps?
Maybe you should check the thermofuses TS1 and TS2 in the transformers?
Also here there are some missing circuit diagrams like power supply and logic (if they have a logic pcb which I don't know)
best regards
Dieter
Dieter.
I have the remaining information. I've been working a bit on the HPA3. I have detected the channel that does not work. This one has 2.5v DC on the output. The voltages are correct, the voltages after regulation are correct, the voltages after regulation with zener is correct.
I have compared all transistors with working channels and found no differences.
At the output of the IC that works as a Servo there is 42v in the channel that has DC in the output and in the channels that work correctly there is 21v.
in R303 and R305 there is 2v and in R315 and R316 there is 0.2v (in the channels that work well and in this defective channel)
Something that catches my attention is... I turn on the amplifier with the Power button, the transformers and amplifiers turn on but the protection does not activate and there is 2.5v at the output. When I press the Standby button, at that moment the protection activates and turns off the amplifiers and transformers...............
What is this protection for?...if when I turn on the amplifier I have DC at the output and it does not activate until I press the Standby button?.
I really don't know what could be going on.
I hope you can help!
Thanks!!!
Rolando
Hi Rolando,
2,5V DC on the output is definitely too much and I don't understand that the protection does not react immediately; one thing I do not understand: you found at the output of the servo chip (U300) 42V at the faulty channel and 21V at the working ones. Both is impossible as the supply voltage of the servo chip is +/- 16V on pin4 and 7, so I suggest that your measurement is related to a wrong ground level. The max swing of the output of U300 is about +/- 13V as CR317/318 are Zehners with 12V. One more thing is a bit confusing: C11 (470uF) is an electrolytic cap but as Vout swings to positive as well as negative compared to ground it should be a normal cap not to become reverse polarised.
As already mentioned there are missing some circuit diagrams, as for instance I don't know what is behind the P300 connector which seems to be part of the protection circuitry?
best regards
Dieter
2,5V DC on the output is definitely too much and I don't understand that the protection does not react immediately; one thing I do not understand: you found at the output of the servo chip (U300) 42V at the faulty channel and 21V at the working ones. Both is impossible as the supply voltage of the servo chip is +/- 16V on pin4 and 7, so I suggest that your measurement is related to a wrong ground level. The max swing of the output of U300 is about +/- 13V as CR317/318 are Zehners with 12V. One more thing is a bit confusing: C11 (470uF) is an electrolytic cap but as Vout swings to positive as well as negative compared to ground it should be a normal cap not to become reverse polarised.
As already mentioned there are missing some circuit diagrams, as for instance I don't know what is behind the P300 connector which seems to be part of the protection circuitry?
best regards
Dieter
dieta,
Así es, estaba tomando mal el terreno. En la salida del servo OpAmp hay -12.6v.
Adjunto el circuito de protección.
Hola Rolando,
thanks for the additional schematics - it is from the Proceed AMP5 five channel amplifier; my Spanish is a bit limited but when I understand correctly you get -12,6V at the output of U300 which is the clamp voltage due to the Zehners CR317/318. Is this voltage in standby as well as in operating mode the same? If POS_16V and NEG_16V is correct it may be a faulty U300.
To avoid triggering the protection mode in the faulty channel you may try to disconnect R24 which senses the output DC voltage to get possibility to measure the voltages also in standby mode without active protection circuitry. You also may shortcircuit C317 - this should give zero Volt at the output of U300 - if not this would strengthen my idea that U300 is faulty
Hope these hints help a bit
atentamente
Dieter
thanks for the additional schematics - it is from the Proceed AMP5 five channel amplifier; my Spanish is a bit limited but when I understand correctly you get -12,6V at the output of U300 which is the clamp voltage due to the Zehners CR317/318. Is this voltage in standby as well as in operating mode the same? If POS_16V and NEG_16V is correct it may be a faulty U300.
To avoid triggering the protection mode in the faulty channel you may try to disconnect R24 which senses the output DC voltage to get possibility to measure the voltages also in standby mode without active protection circuitry. You also may shortcircuit C317 - this should give zero Volt at the output of U300 - if not this would strengthen my idea that U300 is faulty
Hope these hints help a bit
atentamente
Dieter
Dieter,
Thank you for your response!.....I followed your recommendation. I connected the OpAmp input to ground and everything was the same. I have removed the OpAmp from another channel and placed it on this one and now it works fine!
I'm thinking of C11.....should I change this to a non-polarized capacitor?
This is not an AMP5...it is an HPA3, but the control board is the same for both models.
R504 and R505 raise their temperature quickly and they look somewhat overheated. I am going to change these and I will also change the electrolytic capacitors. The manual recommends putting an 8k66 resistor in parallel with R504 and R506.....I think this is to adjust the voltage between +/- 16v. In +16v are the relays and they generate an extra consumption with respect to -16v.
Rolando