if you remove the darlington's you will still have 20 volts across the zeners.(they are feeding base's of the darlingtons) Q851and it's associated components are the error amp the darlington's are the pass elements.
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probably not a correct term to describe it.
how would you describe Q851's operation/function in this circuit?
how would you describe Q851's operation/function in this circuit?
Its a constant source, albeit one with slightly modified characteristics because of R873. Any value of zener, or even a short circuit in place of the zener would see the same current flow.
Diode D859 and R873 hold the B-E voltage of Q851 reasonably constant. The value of R855 now determines the current flow because this resistor sees this voltage less Q851's B-E volt drop. The operation is slightly modified by including R873, possibly to allow for better thermal tracking.
Diode D859 and R873 hold the B-E voltage of Q851 reasonably constant. The value of R855 now determines the current flow because this resistor sees this voltage less Q851's B-E volt drop. The operation is slightly modified by including R873, possibly to allow for better thermal tracking.
No problem 🙂
The whole PSU arrangement on that board all looks a bit "fussy" to me... they must have had their reasons though.
The whole PSU arrangement on that board all looks a bit "fussy" to me... they must have had their reasons though.
A little update for you: Some but not all of the caps arrived today, I changed C851/C852 first & tentatively powered up... 10k across the positive rail now measures +19.8v & 10k across the negative rail measures -19.7v. Still too high for those 18v opamps though... Time to swap in some 22v NE5532's ?
So it looks as though the caps were the problem then. Well done 🙂
(the opamps in your first picture are actually fed via 100 ohms in each rail and so the voltage is automatically pulled down at the opamp depending on the opamp current consumption)
(the opamps in your first picture are actually fed via 100 ohms in each rail and so the voltage is automatically pulled down at the opamp depending on the opamp current consumption)
It looks that way 🙂
Thank you for your help, when one doesn't know the in's & out's of how something works it can be very tempting to just replace everything on the board in hope of success...
Have a good new year !
Thank you for your help, when one doesn't know the in's & out's of how something works it can be very tempting to just replace everything on the board in hope of success...
Have a good new year !
Mooly, remember you guessed there may be more than one problem due to those resistors blowing ? Well... the +/-18v is behaving itself since the cap change but the +/-57v while OK without load blows R859 & R860 with that same 10k resistor across the rails.
All capacitors are new [with much higher voltage ratings] so we can discount them this time... Another dose of wisdom would be much appreciated 🙂
All capacitors are new [with much higher voltage ratings] so we can discount them this time... Another dose of wisdom would be much appreciated 🙂
You mean with the 10k across the 57 volt rails ? Again that doesn't really make sense. The 10k can only pull around 6 milliamps from those rails which won't blow anything.
Is the board still free standing and not coupled up to the rest of the amp ? In your picture I can see what looks like the white glue securing the large caps. That stuff can become conductive/carbonised with age. Make sure its not in contact with other parts, best just to pull it all off.
Is the board still free standing and not coupled up to the rest of the amp ? In your picture I can see what looks like the white glue securing the large caps. That stuff can become conductive/carbonised with age. Make sure its not in contact with other parts, best just to pull it all off.
Yes, with 10k across the 57 volt rails. The board is still free standing [no "outputs" connected] but you have to connect J851 as that supplies AC for the 57v part of the circuit... Here's another picture to show you where that feeds back to:
I did remove the white glue when fitting the new capacitors but I'll double check & make sure absolutely none remains.
I did remove the white glue when fitting the new capacitors but I'll double check & make sure absolutely none remains.
Just wondering how Q856/Q857 come into the equation ? -57v connects to the base of Q856 via R868 [10k] which looks discoloured... Its definitely been asked to dissipate more heat than it can handle.
R868 and surrounding circuitry is again high impedance... the discolouration of the resistor is probably normal, with 60 volts or so it will dissipate around 0.3 watts. That part of the circuit looks as though it generates a fast logic level change (the power off line) probably to mute the output quickly and perhaps to hold the system in a reset state on power on for a fraction of a second.
With the PCB free, only a problem around the bridge rectifier or the 330uf caps could see off those 1 ohms. The supply doesn't go anywhere else apart from the high impedance of the reset generator.
With the PCB free, only a problem around the bridge rectifier or the 330uf caps could see off those 1 ohms. The supply doesn't go anywhere else apart from the high impedance of the reset generator.
Two secondaries on same core, if one suddenly short it blocks magnetic flux through iron core and cause voltage peak, maybe it burns resistors?
Okay thanks, not related to the problem then ! I know the 330uf caps are fine so will have a look at the rectifier diodes.
overall feedback said:
The transformer does make a slight buzzing sound when the short occurs... I know, I should have sorted out the dim bulb tester 😱
Do the resistors go up in smoke violently ?
It might be worth permanently monitoring the voltage across the 330uf caps and seeing what actually happens. Perhaps monitor the AC too from the tranny. What overallfeedback mentions... well its rare (very very rare) for a tranny to develop a fault but the problems you are getting are so unrelated that all possibilities need looking at.
Maybe I'm going cross-eyed looking at that circuit but those 1 ohms seem to go to different tappings on the tranny. One goes to fuse F902, the LV winding ? (its a bit blurry) and the other goes to the HV winding. They would both have to go to the HV windings to get -/+ 56 to 60 volts DC.
It might be worth permanently monitoring the voltage across the 330uf caps and seeing what actually happens. Perhaps monitor the AC too from the tranny. What overallfeedback mentions... well its rare (very very rare) for a tranny to develop a fault but the problems you are getting are so unrelated that all possibilities need looking at.
Maybe I'm going cross-eyed looking at that circuit but those 1 ohms seem to go to different tappings on the tranny. One goes to fuse F902, the LV winding ? (its a bit blurry) and the other goes to the HV winding. They would both have to go to the HV windings to get -/+ 56 to 60 volts DC.