Working on one of these that presented with intermittent audio between power-ups.
80% of the time it worked. Finally realized opamp rails were seeming to latch up with the + rail sticking near 0.
WEIRD thing is they use 7812 and 7912 LDOs on the audio daughter board, but the incoming voltage to them is +/-13.3v with a 14.4v supply on unit. The incoming supplies are only +/-11v with a 12v supply on unit! Drop outs are 1.3-2v on the LDOs.
Output rails look good at +/-27(ish) v at 14.4. Switching wave forms look great.
Never goes into protect either.
I've thoroughly looked over the pcbs and checked diodes/grounds etc.
Kind of stumped, would they really design something this stupid?
80% of the time it worked. Finally realized opamp rails were seeming to latch up with the + rail sticking near 0.
WEIRD thing is they use 7812 and 7912 LDOs on the audio daughter board, but the incoming voltage to them is +/-13.3v with a 14.4v supply on unit. The incoming supplies are only +/-11v with a 12v supply on unit! Drop outs are 1.3-2v on the LDOs.
Output rails look good at +/-27(ish) v at 14.4. Switching wave forms look great.
Never goes into protect either.
I've thoroughly looked over the pcbs and checked diodes/grounds etc.
Kind of stumped, would they really design something this stupid?
Attachments
Could it be a defective regulator going into over-current protection on startup?
1.3v is a lot of dropout on a LDO. What brand regulator is this?
1.3v is a lot of dropout on a LDO. What brand regulator is this?
I just looked up the standard data sheets for the 7912 and 7812.
https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.CD00000444.pdf
https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.CD00000444.pdf
Is there enough space to insert 1 ohm resistor in series with the output of the regulator to see if it changed anything?
I could try that, sure.
I imagine linear regulators aren't guaranteed to work at all when supplied below their output voltage though?
I imagine linear regulators aren't guaranteed to work at all when supplied below their output voltage though?
I think they would produce output but I've never tested that. Low voltage shouldn't harm the regulator so I don't know why they would make them have a low-voltage shutdown like they have for thermal and over-current protection.
I do have some of the positive regulators already in that package, since that was the one locking up I'll swap it out and see if it makes any difference.
Hard to imagine how the board could be defective in a way that it would have lower than normal voltage feeding the linear regulators. With them disconnected, the voltages are the same though.
Hard to imagine how the board could be defective in a way that it would have lower than normal voltage feeding the linear regulators. With them disconnected, the voltages are the same though.
Yeah, guess it appears the regulators drop 1-2v and mostly function even with less than 12v in.
Not something I would count on though, yeesh.
Not something I would count on though, yeesh.
In older equipment, they used what was called a 'stabilizer'. It was essentially a transistor with a resistor/capacitor filter on the base and the output was taken from the emitter. It let the maximum voltage through but took some of the worst noise out. You don't have anything much better when those regs get too little input. It's a good thing that op-amps have such a high PS rejection ratio.
That said, if it's just for op-amps in the preamp circuit, I wonder why they didn't go with 9v regs. The op-amps will happily run on ±9v.
That said, if it's just for op-amps in the preamp circuit, I wonder why they didn't go with 9v regs. The op-amps will happily run on ±9v.
