Hallo all,
I need some help plse. I'm servicing a Pioneer SA 940 stereo amp for a friend. Before I started I switched the amp on through a DBT. I have a 100W lamp in and it glows quite bright and does not go dim. I have tested the OP transistors and the drivers and all are fine. When I remove the heatsink with the OP transistors then the lamp flashes on the inrush current and goes dim which is good. But when I put the OP trannies back the lamp does not dim. I have reflowed a few dry joints on the +/- 15 V regulators. I did not had time to check the bias which I will do tomorrow. Should be between 6 and 20mV. Could it be that it is a NON switching amp that causes excessive current draw for DMT to glow bright ? I do not want to switch it on without the DBT.
Much appreciated
Jan
I need some help plse. I'm servicing a Pioneer SA 940 stereo amp for a friend. Before I started I switched the amp on through a DBT. I have a 100W lamp in and it glows quite bright and does not go dim. I have tested the OP transistors and the drivers and all are fine. When I remove the heatsink with the OP transistors then the lamp flashes on the inrush current and goes dim which is good. But when I put the OP trannies back the lamp does not dim. I have reflowed a few dry joints on the +/- 15 V regulators. I did not had time to check the bias which I will do tomorrow. Should be between 6 and 20mV. Could it be that it is a NON switching amp that causes excessive current draw for DMT to glow bright ? I do not want to switch it on without the DBT.
Much appreciated
Jan
The NSA are known to go into meltdown when open/intermittent bias trimmer or diode string or bad solder joints in this path.
Madis64, the label on the faceplace calls it a NON switching amp, therefore I thought it was something like an class A amp. Trannies are well insulated, thanks mbz, I will check today.
Thanks all, I will let you know of any progress or smoke !😉
Thanks all, I will let you know of any progress or smoke !😉
"analogue" would old school say.
Since when D-class stuff has started to rule the way of writing around here (SS forum) - they have their own dedicated forum?
Not sure what you want to tell us?
The Pioneer is non-switching or 'analogue' and it seems to be in the correct forum.
The Pioneer is non-switching or 'analogue' and it seems to be in the correct forum.
I am expressing a personal view that any reference to "switching" is inappropriate in this forum. Or "non switching".
My personal glitch...
Non-switching is a marketing term for Pioneer's version of clever biasing approaches in the late 70s/early 80s by the major hifi manufacturers. JVC had 'SuperA,' Technics had 'NewClassA' and 'ClassAA.' They were designed to minimize class AB 'crossover distortion.' There are many threads about this in the diyAudio forum. It has nothing to do with a switching power supply or class D (which is still analog).
Hallo Guy's, I had a chance with the amp this morning. I have checked all trannies and diodes, all test good. I have put a single pair of OP transistors on each channel separately to see if it points to a certain channel, no luck, I have tested the bias pots, they test fine. Rail voltages are fine. I will connect the meters on the bias test points and try to see what's happening, I do not want to keep the amp on too long when the lamp on the DBT is burning bright.
Thanks for the input ! 😉 👍
Thanks for the input ! 😉 👍
It usually means that when the current through one side of a class-(A)B output stage gets large, the current through the other side doesn't drop to practically zero, but to some fraction of the quiescent current.
For example, my DIY non-switching amplifier has a quiescent current of about 100 mA. When there is a current of a few amperes flowing out of the output in a signal peak, the current through the upper side of the output stage is a few amperes, the current through the lower side doesn't go below about 50 mA.
Anyway, it is not supposed to make a 100 W lamp glow brightly.
BSST, thanks for posting the schematic, MarcelvdG, thanks, I might get a chance to check again tomorrow, I know both channels works fine with the OP trannies removed, I have checked almost everything. I will put in a 1Khz sinewave through the drivers alone to see if oscillation is a problem, this is a very strange one... I have checked the thermistors, bias diodes, the whole caboodle. Caps checked in circuit all close to specs except ceramics, VCC and VCE is about +-50VDC iso 53VDC, this should not be a problem. I think they would reform as this unit has not been used for a long time. Can the difference in rail voltage cause such a calamity ? I have seen Mark the fixer has posted about this amp and its workings but I cant find it now and I don't understand it fully.
Thanks for the help guy's!😉👍
Thanks for the help guy's!😉👍