Hello everyone,
I have a problem with a preamp. The thing is, when I try to turn on my preamp it does not, but if I let it connected to energy sometimes start. Seems like magic, so I checked the circuit and a capacitor located at the power stage (2200 uF 42 V) seems a little fluffy, so I think I would maybe the cause of the problem, but I rather ask here first instead of go right away and replace it. Any thoughts?
Thank you!
I have a problem with a preamp. The thing is, when I try to turn on my preamp it does not, but if I let it connected to energy sometimes start. Seems like magic, so I checked the circuit and a capacitor located at the power stage (2200 uF 42 V) seems a little fluffy, so I think I would maybe the cause of the problem, but I rather ask here first instead of go right away and replace it. Any thoughts?
Thank you!
What do you mean by fluffy ? If its white papery stuff coming out of the top or bottom then it sounds like its failed.
I Mooly thanks for your time and help.
By fluffy I mean swollen, it has 2 capacitors in the power stage just after the diod bridge imgur: the simple image sharer I've check continuity in all parts of the circuit at it seems it has.
By fluffy I mean swollen, it has 2 capacitors in the power stage just after the diod bridge imgur: the simple image sharer I've check continuity in all parts of the circuit at it seems it has.
Swollen caps or caps where the vent has split open (signifying excess pressure) are a sure sign of failure. They need replacing.
(You could attach a photo of them if you want to this thread... that would help confirm the diagnosis)
If you do a Google or Bing image search for "failed electrolytics" you will find lots of examples.
(You could attach a photo of them if you want to this thread... that would help confirm the diagnosis)
If you do a Google or Bing image search for "failed electrolytics" you will find lots of examples.
It doesnt apreciate so well in the image, but one is a little bit swollen than the other. So I think it could be a sympton of not turning on, and maybe too that when it stay long periods of time connected to energy it turn on, right?
I attach two images:
I attach two images:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
I'm not convinced seeing those. Its common for caps (and other items) to be glued down and that looks a bit like that. If it were in front of me I could tell in an instant... in a picture... well it looks like glue.
Do you have a circuit for this preamp ? A few basic voltage checks would help locate the problem.
Do you have a circuit for this preamp ? A few basic voltage checks would help locate the problem.
This is just a part of the technical manual that I've downloaded, but it explains well the power stage.
If those capacitors aren't the cause of the problem what else do you think could be the problem here?
An externally hosted image should be here but it was not working when we last tested it.
If those capacitors aren't the cause of the problem what else do you think could be the problem here?
Thanks... that's a board layout... I can follow that as far as it goes but what is really needed is the actual circuit diagram 🙂
From this image you should be able to measure an AC voltage between BL and both RE terminals (top left) I would expect at least 15 volts AC between each, and it could be higher depending what voltage rails the preamp runs on. That voltage should be there though.
You should see a DC voltage of at least 20 volts across each of those large caps. Again it could be higher... that's why a circuit diagram helps it all make sense 🙂
See if you can measure those voltages first.
I suspect your problem will be a little more involved but we have to start somewhere...
If you can come up with a circuit diagram then there is a real chance of fixing it. Common problems could be something as simple as a poor joint on a regulator transistor or IC. It happens all the time on commercial stuff.
Look at the last pictures in post #1 here. That is typical of a poor connection.
http://www.diyaudio.com/forums/digital-source/226288-sony-cdp790-kss240-restoration-project.html
From this image you should be able to measure an AC voltage between BL and both RE terminals (top left) I would expect at least 15 volts AC between each, and it could be higher depending what voltage rails the preamp runs on. That voltage should be there though.
You should see a DC voltage of at least 20 volts across each of those large caps. Again it could be higher... that's why a circuit diagram helps it all make sense 🙂
See if you can measure those voltages first.
I suspect your problem will be a little more involved but we have to start somewhere...
If you can come up with a circuit diagram then there is a real chance of fixing it. Common problems could be something as simple as a poor joint on a regulator transistor or IC. It happens all the time on commercial stuff.
Look at the last pictures in post #1 here. That is typical of a poor connection.
http://www.diyaudio.com/forums/digital-source/226288-sony-cdp790-kss240-restoration-project.html
I've checked the values you gave me, and the values directly from my multimeter are around those. So, I decided to check the PCB from the other side and I found that two transistors are kind of burned. The transistors models are:
2SB949
2SD1913
I'm afraid that I can't get any circuit diagram, it is an old preamp 🙁
2SB949
2SD1913
I'm afraid that I can't get any circuit diagram, it is an old preamp 🙁
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
The schematic should be in the technical manual you downloaded unless it has been deliberately removed. I have owned number of Yamaha components in the past and got the manuals.
Discolouration isn't necessarily a problem, its a normal reaction of the board material to heat.
Is the preamp totally dead at first. No lights or anything ?
You need to check the basics first which is all the supply voltages. Can you report back with the voltages on point A and point B. Be extremely careful measuring, particularly around that transistor. One slip of the meter lead between the pins could result in major damage. It is best to find a convenient point it goes to and measure there.
When you switch on, do you hear a relay click ?
Attachments
From the symptoms I would guess that at least one of the small-fry electrolytics in the regulator or the relay driver circuit is leaky and shorting out at power-up until the dielectric has recovered. Looking at the various voltages should clear that up. I see only a handful of such caps anyway. I would replace with good quality parts of same capacity and about the same physical dimensions (which should mean a voltage rating that's a step higher or two).
If the big ones were bad, there would be issues like hum, or the fuse would burn out.
If the big ones were bad, there would be issues like hum, or the fuse would burn out.
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