These pics are ok, thanks.
1. the +/-60V supply all the 5 channel at the same time via the white flat cable, it will draw the max current every time you power up your amp, even you only connected the +/-55V power to one channel.
2. the voltage on C609/C610 will be more than 100V dc, we assume each amp channel draw 25~30mA current from the +/-60V supply rail, 5 channel total minimum draw 125mA current, so minus the 60V output voltage, the voltage drop on transistors Q601/Q602 C/E pins will be more than 40V and it will dissipate around 5W on each transistor, the small heatsink apparent not sufficient for that. The solution is adding an extra large resistor before Q601/Q602 to reduce their power dissipation, the resistor need share around 16V~20V voltage drop, so that you should measure and calculate the suitable resistor value.
3. the amp production very long time, the capacitors using for the +/-60V supply rail should be all replaced by reliable new capacitors. also the Q601/Q602 overheated last time and therefore also need new replacement.
4, after the new cap. and trs. replacement and confirm the 5 channel amp working ok, the repair is success.
1. the +/-60V supply all the 5 channel at the same time via the white flat cable, it will draw the max current every time you power up your amp, even you only connected the +/-55V power to one channel.
2. the voltage on C609/C610 will be more than 100V dc, we assume each amp channel draw 25~30mA current from the +/-60V supply rail, 5 channel total minimum draw 125mA current, so minus the 60V output voltage, the voltage drop on transistors Q601/Q602 C/E pins will be more than 40V and it will dissipate around 5W on each transistor, the small heatsink apparent not sufficient for that. The solution is adding an extra large resistor before Q601/Q602 to reduce their power dissipation, the resistor need share around 16V~20V voltage drop, so that you should measure and calculate the suitable resistor value.
3. the amp production very long time, the capacitors using for the +/-60V supply rail should be all replaced by reliable new capacitors. also the Q601/Q602 overheated last time and therefore also need new replacement.
4, after the new cap. and trs. replacement and confirm the 5 channel amp working ok, the repair is success.
Thank you for your suggestion.
I have already changed Q605, Q603, R615, R620 because they burned out after I replaced the burnt R609 with a 1W non-fuse resistor (maybe Q605 and Q603 were already burned out, but I changed only R609 and this caused R615 and R620 to burn out).
I changed also IC601, D604, D605 and Q601 to stay on the safe side, but till now I have'n changed any capacitor. I will look for capacitors with the same charateristics and I will replace C615, C616, C617, C618 and C619. I'm wandering if change also C6087, C608, C609, C610, C611, C612. It depends also on the time needed to find the replacement capacitors.
I have already changed Q605, Q603, R615, R620 because they burned out after I replaced the burnt R609 with a 1W non-fuse resistor (maybe Q605 and Q603 were already burned out, but I changed only R609 and this caused R615 and R620 to burn out).
I changed also IC601, D604, D605 and Q601 to stay on the safe side, but till now I have'n changed any capacitor. I will look for capacitors with the same charateristics and I will replace C615, C616, C617, C618 and C619. I'm wandering if change also C6087, C608, C609, C610, C611, C612. It depends also on the time needed to find the replacement capacitors.
I have changed all the capacitors, Q601 and Q603. Unfortunately, nothing has changed. I have always a voltage (current) peak on the R+ (100 ohm) that I put in series to R609 (see picture 85). This gives an abnormal current absorbtion on the +60V supply resistors and transistors.
But I've seen a strange thing; if I touch with a finger the dissipator of Q601 or the cathode od D603, the current stabilizes and on R+ I find 2,2 V, which is more or less the voltage drop that I expected. Picture 101 is the voltage on the cathode of D603, picture 102 is the same voltage when I touch the dissipator and picture 103 is the voltage when I touch the D603 cathode (more or less the same measure, conside the different scale).
It seems as there is an oscillation which stops when I touch D603. Or something is folating and with the finger I fix...
Any idea?
But I've seen a strange thing; if I touch with a finger the dissipator of Q601 or the cathode od D603, the current stabilizes and on R+ I find 2,2 V, which is more or less the voltage drop that I expected. Picture 101 is the voltage on the cathode of D603, picture 102 is the same voltage when I touch the dissipator and picture 103 is the voltage when I touch the D603 cathode (more or less the same measure, conside the different scale).
It seems as there is an oscillation which stops when I touch D603. Or something is folating and with the finger I fix...
Any idea?
yes, it seems like an oscillation, it maybe caused by Q605 or Q603, since Q605 pins B,C had a capacitor C613 which against oscillation, but you can add an extra 100pf 200V ceramic capacitor on the same location of C613, keep C613 and maybe solder the cap under the PCB. after that If oscillation still there, add another 100pf 200V ceramic capacitor on Q603 pins B,C directly and test again. Normal disc type ceramic capacitor voltage over 150V is ok.
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I've changed the C613 capacitor with a 100pf one and changed also diode D603. The oscillation decreased greatly! With only two channels installed from 39 Vpp has become 0,5 Vpp! Adding the other channels increases the oscillation, but with all 5 channells it is still less than 2,5 Vpp, which I assume is acceptable. See the following oscillogram (Ch1 is the voltage across the 100 ohm resitor in series with R609, Ch2 +60Vdc regulated output):
The strange thing remains that touching with a finger the heat sink of Q601 ot the cathode or D603 or the C613 capacitor, the oscillation goes nearly to 0 (lower trace):
The circuit behaves like an antenna... I'm wondering if there is something floating.
I had already added a 100 ohm 2W resistor in series with R609 and R612. The voltage drop across this resistor is about 9,8 Vdc (i.e. about 100 mA), so the dissipation is about 1 W. With this setting, the 100 ohm resitors reach a temperature of 90° C, R609 is at 70° C and the heat sink of Q601 is at 62° C.
I don't have the temperature reached by R609 and Q601 heat sink when the amplifier was new, but I remember that this part of the chassis has always been the hottest.
The strange thing is that the R609 fuse resistor is a 0,5 W one and with 100 mA it is at the limit. Perhaps when it was new the absoption was less because it seems strange to me that the working point of R609 have been set so near to its dissipation limit.. This was the first component that I found burnt at the first failure of the amplifier.
Now I'm going to reassemble everything and enjoy the music!
Do you think that I should check or do anything else before closing the chassis?
Thank you very much for your valuable advice.
The strange thing remains that touching with a finger the heat sink of Q601 ot the cathode or D603 or the C613 capacitor, the oscillation goes nearly to 0 (lower trace):
The circuit behaves like an antenna... I'm wondering if there is something floating.
I had already added a 100 ohm 2W resistor in series with R609 and R612. The voltage drop across this resistor is about 9,8 Vdc (i.e. about 100 mA), so the dissipation is about 1 W. With this setting, the 100 ohm resitors reach a temperature of 90° C, R609 is at 70° C and the heat sink of Q601 is at 62° C.
I don't have the temperature reached by R609 and Q601 heat sink when the amplifier was new, but I remember that this part of the chassis has always been the hottest.
The strange thing is that the R609 fuse resistor is a 0,5 W one and with 100 mA it is at the limit. Perhaps when it was new the absoption was less because it seems strange to me that the working point of R609 have been set so near to its dissipation limit.. This was the first component that I found burnt at the first failure of the amplifier.
Now I'm going to reassemble everything and enjoy the music!
Do you think that I should check or do anything else before closing the chassis?
Thank you very much for your valuable advice.
Basically you can reassemble it and enjoy the music.
But if you still have time or like to try, you can increase capacitor C613 to 220p at the same time also add another capacitor (100pf 200V ceramic capacitor) at Q603 between pin B and pin C, soldering the capacitor under the PCB pads.
B.rgds
But if you still have time or like to try, you can increase capacitor C613 to 220p at the same time also add another capacitor (100pf 200V ceramic capacitor) at Q603 between pin B and pin C, soldering the capacitor under the PCB pads.
B.rgds
Dear Patrik,
I've added a 47p capacitor between B and C of Q603 and the final result is that now the voltage across the R100 resistor is stable as when I put a finger on the dissipator of Q601:
I've reassembled the amp and started listening to music. The only concern is the high temperature of the resistors: I hope they will last over time, especially during summer time. But there isn't a way to limit the current drained by the audio channels on the +/- 60V, so this is the power that has to be dissipated either on the transistors or on the resistors.
By the way, I readjusted als the bias current of all the channels via the RV301 trimmer: I used the setting from the MA240 amp service manual that is 15 mV +/- 2 mV on the dedicated test point (TP301A and TP301B on the audio channel schematics). The power section of the my amp and the MA240 are very similar.
Thanks again for your support.
Best regards.
I've added a 47p capacitor between B and C of Q603 and the final result is that now the voltage across the R100 resistor is stable as when I put a finger on the dissipator of Q601:
I've reassembled the amp and started listening to music. The only concern is the high temperature of the resistors: I hope they will last over time, especially during summer time. But there isn't a way to limit the current drained by the audio channels on the +/- 60V, so this is the power that has to be dissipated either on the transistors or on the resistors.
By the way, I readjusted als the bias current of all the channels via the RV301 trimmer: I used the setting from the MA240 amp service manual that is 15 mV +/- 2 mV on the dedicated test point (TP301A and TP301B on the audio channel schematics). The power section of the my amp and the MA240 are very similar.
Thanks again for your support.
Best regards.
Hi guys,
I have a Myryad MA500. I want to recap it (23+ years old and it runs a bit warm). During dismantling I discovered that no bleed resistors are built in: the psu pcb shifted a bit and the filter caps discharged unplanned and uncontrolled on the psu's mounting bracket. Optically, there's no damage other than two black spots on the pcb. But I can't test the unit at the moment. During the recap, I might as well replace any and all components around the fried spot on the pcb. Including some diodes I know nothing about, except a voltage rating of 22V (D609, D610). So a service manual might come in handy to identify risk-areas and components. Does anyone know if there is such a thing as an official service manual for this amp? Or if there's anything special I need to know about said diodes?
Thanks in advance!
James.
I have a Myryad MA500. I want to recap it (23+ years old and it runs a bit warm). During dismantling I discovered that no bleed resistors are built in: the psu pcb shifted a bit and the filter caps discharged unplanned and uncontrolled on the psu's mounting bracket. Optically, there's no damage other than two black spots on the pcb. But I can't test the unit at the moment. During the recap, I might as well replace any and all components around the fried spot on the pcb. Including some diodes I know nothing about, except a voltage rating of 22V (D609, D610). So a service manual might come in handy to identify risk-areas and components. Does anyone know if there is such a thing as an official service manual for this amp? Or if there's anything special I need to know about said diodes?
Thanks in advance!
James.