hi all,
I upgraded and modified my Marantz 2330 :
- Replaced all electrolytic caps for Power supply P800, PE01, PH01, power amp P700
- Updated to film caps on the signal path and feedback path for tone preamp, and main power amp modules
- For Power supply P800 Updated to fast recovery diode (MUR1620CTG), H801, H802 to MJE15032G
- For main power amp P700, i replaced differential input pairs Q701, Q702, and Q703 to all KSC1845FTA with Q701 and Q702 matched hfe as closed to each other as possible
After all the mods & upgrades.
- I was able to observe a constant voltage of 35 Vdc of b/w J804 and J805 on P800. I noticed almost all of the new caps from P800 pwr supply are pretty warm to a bit hot the receiver is making music. Is it normal for PS caps to get warm to a bit hot with this high of power ? I upgraded the caps to all high temperature (105 C), low ripple, with higher voltage rating than original caps
- When adjusting bias current for both left and right power amps. I noticed 1 side of P700 is able to be adjusted to 13.5 mV and it stayed there after the amp is warmed up. The other side of P700 bias voltage is somehow jumps to 15 mV or 16 mV after it was settled at 13.5 mV as the other side. When it jumps to 15 mV, i readjust the trimpot to bring it back down to 13.5 mV, then the other side starts to slightly increase the bias. By the way, the P700 amp that has the jumping bias voltage is the one with the damaged and burned resistors/diode/transistors as explained below.
- After 2 days of music enjoyment, I smelled a funky smell and smoke coming out of the right channel of P700. This is the PA module that sit closed to power supply P800
Pulling both P700 boards out (left and right) with left was the one that sit closed by P800 board. I noticed
- R730, R732 resistors are complete toast (Please see photo)
- Q724 transistors are damaged with short from B to E
- Diode Q717 was burned. C720, C721 are burned (See photo)
Please see photo and schematic highlighted for the damage signal path.
Instead of replacing the damaged transistors, diodes, caps and resistors, I want to understand why it causes such behavior that leads to the damages for the R730, R732 paths all the way down to Q724, diode Q717
Can someone help me understand the circuit better (P700 schematic attached) ?What is the purpose of transistor Q724 ?
Does anyone know what the actual causes ? it only happens on 1 side of the P700 board. The other P700 board is fine and does not have any issues. I checked the each of the output power transistors and all of them are still alive and functional. Power supply still provide a constant 35V
I have a theory of C718 = 330pf is going bad and short circuit which allows DC current from bias transistor Q708 flow through B of Q724. Thus, it short ckt B to E of Q724 and burns R730 and R732 on its path. C718 does not look burned from visual inspection but I will remove it out of the PCB and check it.
Many thanks,
Tom
I upgraded and modified my Marantz 2330 :
- Replaced all electrolytic caps for Power supply P800, PE01, PH01, power amp P700
- Updated to film caps on the signal path and feedback path for tone preamp, and main power amp modules
- For Power supply P800 Updated to fast recovery diode (MUR1620CTG), H801, H802 to MJE15032G
- For main power amp P700, i replaced differential input pairs Q701, Q702, and Q703 to all KSC1845FTA with Q701 and Q702 matched hfe as closed to each other as possible
After all the mods & upgrades.
- I was able to observe a constant voltage of 35 Vdc of b/w J804 and J805 on P800. I noticed almost all of the new caps from P800 pwr supply are pretty warm to a bit hot the receiver is making music. Is it normal for PS caps to get warm to a bit hot with this high of power ? I upgraded the caps to all high temperature (105 C), low ripple, with higher voltage rating than original caps
- When adjusting bias current for both left and right power amps. I noticed 1 side of P700 is able to be adjusted to 13.5 mV and it stayed there after the amp is warmed up. The other side of P700 bias voltage is somehow jumps to 15 mV or 16 mV after it was settled at 13.5 mV as the other side. When it jumps to 15 mV, i readjust the trimpot to bring it back down to 13.5 mV, then the other side starts to slightly increase the bias. By the way, the P700 amp that has the jumping bias voltage is the one with the damaged and burned resistors/diode/transistors as explained below.
- After 2 days of music enjoyment, I smelled a funky smell and smoke coming out of the right channel of P700. This is the PA module that sit closed to power supply P800
Pulling both P700 boards out (left and right) with left was the one that sit closed by P800 board. I noticed
- R730, R732 resistors are complete toast (Please see photo)
- Q724 transistors are damaged with short from B to E
- Diode Q717 was burned. C720, C721 are burned (See photo)
Please see photo and schematic highlighted for the damage signal path.
Instead of replacing the damaged transistors, diodes, caps and resistors, I want to understand why it causes such behavior that leads to the damages for the R730, R732 paths all the way down to Q724, diode Q717
Can someone help me understand the circuit better (P700 schematic attached) ?What is the purpose of transistor Q724 ?
Does anyone know what the actual causes ? it only happens on 1 side of the P700 board. The other P700 board is fine and does not have any issues. I checked the each of the output power transistors and all of them are still alive and functional. Power supply still provide a constant 35V
I have a theory of C718 = 330pf is going bad and short circuit which allows DC current from bias transistor Q708 flow through B of Q724. Thus, it short ckt B to E of Q724 and burns R730 and R732 on its path. C718 does not look burned from visual inspection but I will remove it out of the PCB and check it.
Many thanks,
Tom
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Q724 and Q725 are part of the VI, volt-amp, limiter circuit. If the output is shorted and enough voltage is developed across R743 and/or R744 that voltage appears on the bases of Q724 and/or Q725 turning them on and removing the drive signal to Q709 and/or Q710 thus limiting the damage to the amplifier due to the short at the speaker terminals. Normally there's not enough voltage in that circuit to fry anything. Make sure all of your parts are installed correctly.
Craig
Craig
Craig, thank you for the pointer.
Based on your explanation, there must be a short in output transistor or the speaker terminal is shorted somehow to create alot of voltage at R743/R744 to turn on Q724/Q725.
I only noticed and smell the burning smoke while listening music for a few hours on so the speaker terminal shorted is eliminated.
I checked each output transistor with DMM across B,E,C and looks like all of them are still working after the damage. In this case, i will go back and check all the caps/transistor that I installed to make sure they are in the right polarity.
Let me know if you have further inputs to what could possibly cause this damage to happen if all the components are installed correctly ?
Thanks,
Tom