Broken Sony Stereo Receiver
#1
09-06-01, 07:39 AM
Hi,
I have a Sony STR d-990 stereo receiver with, what I think is a broken power supply. One of the fuses on the PS keeps blowing. It is in line right before one of the lead wires to the large transformer. It is 6.3A 125V fuse. The other fuse on the board is 3 . 15A 250V but it does not blow.
There is a small transformer and a relay on the power supply board that both work. The transformer sends 5V from the board to the front panel of the receiver and gets a switch signal of either 5V (off) or .75V (on) from the on switch. The fuse blows when the switch is turned on.
There is 120V between the fuse contact and the other lead to the big transformer when it is off. When the fuse is put in though, the two leads to the big transformer dont have any voltage between them. Then when it is turned on the fuse blows. I don't know if this is of any importance or just a normal circuit thing.
The large transformer has 0 ohms resistance between the two power leads when there is no voltage.
I don't know much about the elctronics of it, but my feeling is that it is a short of some sort in the power supply circuit or in the large transformer itself. What can I check to narrow down the problem more? Is there something that happens often in these receivers? Any help or suggestions would be really appreciated.
Thanks
Sam Sarcia
I have a Sony STR d-990 stereo receiver with, what I think is a broken power supply. One of the fuses on the PS keeps blowing. It is in line right before one of the lead wires to the large transformer. It is 6.3A 125V fuse. The other fuse on the board is 3 . 15A 250V but it does not blow.
There is a small transformer and a relay on the power supply board that both work. The transformer sends 5V from the board to the front panel of the receiver and gets a switch signal of either 5V (off) or .75V (on) from the on switch. The fuse blows when the switch is turned on.
There is 120V between the fuse contact and the other lead to the big transformer when it is off. When the fuse is put in though, the two leads to the big transformer dont have any voltage between them. Then when it is turned on the fuse blows. I don't know if this is of any importance or just a normal circuit thing.
The large transformer has 0 ohms resistance between the two power leads when there is no voltage.
I don't know much about the elctronics of it, but my feeling is that it is a short of some sort in the power supply circuit or in the large transformer itself. What can I check to narrow down the problem more? Is there something that happens often in these receivers? Any help or suggestions would be really appreciated.
Thanks
Sam Sarcia
#2
09-06-01, 11:43 AM
Good Evening, SSarcia:
If the line fuse is blowing but the B+ fuse is holding, the chances are that either you have a shorted diode in the bridge rectifier on the secondary of the transformer...or, at worst case, a shorted filter capacitor.
The primary windings of the transformer usually don't measure much with a DC ohmmeter.
Incidentally, your post was excellent and the information was very informative.
Smokey
If the line fuse is blowing but the B+ fuse is holding, the chances are that either you have a shorted diode in the bridge rectifier on the secondary of the transformer...or, at worst case, a shorted filter capacitor.
The primary windings of the transformer usually don't measure much with a DC ohmmeter.
Incidentally, your post was excellent and the information was very informative.
Smokey
#4
09-06-01, 08:23 PM
Good Morning, SSarcia:
If I read your first post correctly, you found the transformer in your initial checks. The power line goes to the primary windings of the transformer, the voltage is down converted to one or more lower voltages, rectified by the bridge rectifier diodes, and then filtered to a pure DC
voltage or voltages.
Since you found the primary windings of the transformer, just about everything else on that transformer should be secondary windings. Follow the secondary windings and you will find the diodes in the rectifier stage.
Since a diode only allows voltage and current to pass in one direction, a meter reading on the diode will show a quite high ohm reading in one direction. Reversing the leads on the diode, the ohm reading will be very low. If the diode reads very low in both directions, you have a problem.
Also, read the secondary leads on the transformer to ground. Be suspicious of low readings.
If you need a little refresher on Power Supply design and function, have a look at http://my.integritynet.com.au/purdic/power3.htm
Smokey
If I read your first post correctly, you found the transformer in your initial checks. The power line goes to the primary windings of the transformer, the voltage is down converted to one or more lower voltages, rectified by the bridge rectifier diodes, and then filtered to a pure DC
voltage or voltages.
Since you found the primary windings of the transformer, just about everything else on that transformer should be secondary windings. Follow the secondary windings and you will find the diodes in the rectifier stage.
Since a diode only allows voltage and current to pass in one direction, a meter reading on the diode will show a quite high ohm reading in one direction. Reversing the leads on the diode, the ohm reading will be very low. If the diode reads very low in both directions, you have a problem.
Also, read the secondary leads on the transformer to ground. Be suspicious of low readings.
If you need a little refresher on Power Supply design and function, have a look at http://my.integritynet.com.au/purdic/power3.htm
Smokey
#5
09-07-01, 09:17 AM
broken sony receiver
Hi Smokey,
Your help has put me in quite a situation. Now I am too optimistic about a fix to junk the whole thing but I stil can't figure it out. There are 3 boards that I think it can be on. The first board has the fuse which blows on it and the working 5v transistor. This board sends two lead wires to a big transformer unit. This unit also gets 4 wires coming from the front headphone jack. There are 8 leads coming our of the other side of the transformer. There are 2 more 3.15A 250V fuses on the second side of the transformer thats don't blow.
From there, the wires go to a third board which has what I think are the bridge rectifiers and diodes. There are two large capacitors on this board as well. The bridge rectifiers are flat black thinkgs with 4 leads, the teo outside ones being + and - and the middle being squiggily lines. I have checked all the diodes and they seem to work fine on the board except for two, but when I removed one end from the board they worked fine.
The boards all looked pretty good with no apparent signs of shorting, but I know that doesn't mean much. How do I test the bridge rectifiers?
I don't know very much about electronics; just what I learned in freshman physics so I dont know what you are talking about with primary and secondary windings of the transformer, but I tried to do some reading to little avail.
Anyways, any more help you might be able to give me would be appreciated, otherwise I will just keep messing around with it.
Thanks
Sam
Your help has put me in quite a situation. Now I am too optimistic about a fix to junk the whole thing but I stil can't figure it out. There are 3 boards that I think it can be on. The first board has the fuse which blows on it and the working 5v transistor. This board sends two lead wires to a big transformer unit. This unit also gets 4 wires coming from the front headphone jack. There are 8 leads coming our of the other side of the transformer. There are 2 more 3.15A 250V fuses on the second side of the transformer thats don't blow.
From there, the wires go to a third board which has what I think are the bridge rectifiers and diodes. There are two large capacitors on this board as well. The bridge rectifiers are flat black thinkgs with 4 leads, the teo outside ones being + and - and the middle being squiggily lines. I have checked all the diodes and they seem to work fine on the board except for two, but when I removed one end from the board they worked fine.
The boards all looked pretty good with no apparent signs of shorting, but I know that doesn't mean much. How do I test the bridge rectifiers?
I don't know very much about electronics; just what I learned in freshman physics so I dont know what you are talking about with primary and secondary windings of the transformer, but I tried to do some reading to little avail.
Anyways, any more help you might be able to give me would be appreciated, otherwise I will just keep messing around with it.
Thanks
Sam
#6
09-07-01, 11:55 AM
Good Evening, SSarcia:
Take a deep breath. Relax for a minute!
What you need is a schematic diagram of the unit. Obviously, you have had a small amount of electronic training which you have never used. But, you have training.
(Breath a little). Now, let's get a diagram of the unit so you can find your way through the system.
Believe me! You can fix this thing. We always went to Sams Photofacts for diagrams because it not only gave a schematic of the set but it gave component locations. Try getting a similar set of information from Sams at
http://www.samswebsite.com/
Smokey is here if you need to talk.
Smokey
Take a deep breath. Relax for a minute!
What you need is a schematic diagram of the unit. Obviously, you have had a small amount of electronic training which you have never used. But, you have training.
(Breath a little). Now, let's get a diagram of the unit so you can find your way through the system.
Believe me! You can fix this thing. We always went to Sams Photofacts for diagrams because it not only gave a schematic of the set but it gave component locations. Try getting a similar set of information from Sams at
http://www.samswebsite.com/
Smokey is here if you need to talk.
Smokey
