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Troubleshooting a power supply circuit - am I on the right track?

chrisv2

Oct 24, 2013
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A friend of mine has a Bose "wave radio" about 10 years old, and it's totally dead. Let me preface the rest of this by admitting that I have basically a high-school electronics shop education regarding electronics theory and practice. From 30 years ago.

I took the radio apart and found that the board is getting 120VAC past the on-board fuse, and the multi-tap transformer seems fine. The bridge rectifier is working.

There are two voltage regulators, a 7805 and a 7810. Neither one of them are showing any voltage at their input pins. I traced the circuit back to two transistors, 2SA1318 PNP whose respective collectors are tied to the input pins of the regulators (with a resistor).

When I measure voltage from ground to either the base or the emitter of the transistors, I get -29V on one and -15V on another. When I measure voltage from ground to the collector of the transistor(s) -- I get a reading of '0'.

So if the collector of the PNP transistor is "dead" or at zero potential compared to the board's ground -- does that mean that the transistor is bad?

I'm willing to risk the few dollars to spend on replacing these two transistors, but was wondering if I'm on the right track at least?

Thanks!
-Chris
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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it would be really useful if you can draw a circuit diagram of this part of the circuit. That transistor you mentioned is a small signal device. I'm not sure what it is doing there.

The 7810 you describe, is it perhaps a 7812 (not that it matters too much right now)?

Can you measure the DC voltage after the bridge rectifier?
 

davenn

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hi chris

there would be no logical reason for there to be transistors on the inputs to the regulators. However the possibility that those transistors are actually "straddling" the input and output pins is more likely so that would be used as current pass transistors.

how about some pics of the PSU poards both sides sharp and well lit

cheers
Dave
 

chrisv2

Oct 24, 2013
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Thanks!

(edit): This is a 2002 model AWRC-1G, PC board is 260128-0001. I searched for a schematic or service manual and no luck :-( I emailed Arfa to see if he might be able to help with that; if anyone happens to have a line on one please let me know...thanks...

Here are two photos of the area of the board I'm looking at. Did the best I could with my phone camera. The bottom view, I annotated the best I could. The top view, the L78M05CV regulator is in the upper right, and the transistor is below it, to the right.

The other regulator/transistor pair is similarly connected. As you can see, like I was saying, the collector of that transistor is tied to the input pin (1) of the L78M05 - and there is zero voltage at that pin, with power supplied to the board.

Since I don't know any better, I suspected the transistor as the culprit. Which would mean both transistors on that side have gone bad.

What do you guys think?
 

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(*steve*)

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However the possibility that those transistors are actually "straddling" the input and output pins is more likely so that would be used as current pass transistors

I googled the transistors in question and I think that's unlikely considering their specs...
 

(*steve*)

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I can't see from those images. Are the centre pins of both of those regulators connected together (probably on the top layer). If you can't tell, read the resistance between the two pin 2's.

Then measure (with the power applied) the voltage between pin 2 and pin 1 for each of these regulators, and also the voltage between pin 2 and pin 3.
 

davenn

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I googled the transistors in question and I think that's unlikely considering their specs...

yup agreed, just low power signal ones :)

got me beat ... never seen a low power transistor feeding a V reg before

D
 
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chrisv2

Oct 24, 2013
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I can't see from those images. Are the centre pins of both of those regulators connected together (probably on the top layer). If you can't tell, read the resistance between the two pin 2's.

Then measure (with the power applied) the voltage between pin 2 and pin 1 for each of these regulators, and also the voltage between pin 2 and pin 3.

Steve, I can't see a physical connection between those two center pins -- but I checked with my multimeter and there is zero resistance between them, so yes they are electrically connected. The center pin is 'ground' - correct? So it appears that they share a common ground.

With power applied to the board, I am getting zero voltage between pins 2 and 1, and 2 and 3 on both the L78M05 and the L78M10 regulators. So, no voltage present at either the input to the regulators, nor at the output.
 

(*steve*)

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OK, that tells us that no power is getting to the regulators. So go back to the transformer and measure the AC voltage at the output, then find the bridge rectifier and measure the DC voltage on the output of that.

If you can't find it take a picture of the entire board (both sides may be useful) and we may be able to point it out to you.
 

chrisv2

Oct 24, 2013
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Thanks...yup I did check all the front end parts of that supply circuit, and all is fine. The trail goes dead at those transistors. I can try replacing them, but if the tiny SMT resistors near them have also been damaged, then it am out of luck...I don't have the tools for that.

Another question - can I temporarily patch in my bench power supply to the input side of those regulators, just to see if I can get the radio to work? Or is that a bad idea?
 
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Arouse1973

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Dec 18, 2013
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hi chris

there would be no logical reason for there to be transistors on the inputs to the regulators. However the possibility that those transistors are actually "straddling" the input and output pins is more likely so that would be used as current pass transistors.

how about some pics of the PSU poards both sides sharp and well lit

cheers
Dave
Dave why would you bypass a 1.5A reg with a 200mA transistor? Maybe I am being a bit thick here.
Thanks Adam
 

Arouse1973

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Dec 18, 2013
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it would be really useful if you can draw a circuit diagram of this part of the circuit. That transistor you mentioned is a small signal device. I'm not sure what it is doing there.

The 7810 you describe, is it perhaps a 7812 (not that it matters too much right now)?

Can you measure the DC voltage after the bridge rectifier?

LM7810 is a standard regulator.
Thanks
Adam
 

Arouse1973

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maybe they are using the transistor as a power switch. Just because the device is rated at 1.5Amps doesn't mean they are using the available current. I would use one for as small as a few mA, they have much better noise performance than a switcher and at low current are quite efficient. And they are cheap and use fewer components that a switcher. Just an old school trick.
Thanks
Adam
 

(*steve*)

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Thanks...yup I did check all the front end parts of that supply circuit, and all is fine.

You might think so, but I'm asking for voltages. And a wider photo.

Your measurement of -15V and -29V at the emitter of these transistos doesn't tell me anything useful for the moment.

Please try to measure the DC voltage at the output of the rectifier(s) (is there one set of rectifiers or two?) DO they have a common ground? Can you identify what is used to rectify the AC?

The whole point is to find out what is switching the DC to the regulators. It may be those transistors, but it might not. If it is, then it seems likely that you have the ground connection incorrect (try measuring the voltages with respect to pin 2 of the regulators)

I hate to be the grinch at Christmas, but I asked those questions for a reason.

The trail goes dead at those transstors. I can try replacing them, but if the tiny SMT resistors near them have also been damaged, then it am out of luck...I don't have the tools for that.

Replacing SMT resistors is not that hard (especially if it's only 1 or 2.

We don't know yet if the transistors are faulty.

Another question - can I temporarily patch in my bench power supply to the input side of those regulators, just to see if I can get the radio to work? Or is that a bad idea?

It is a bad idea for several reasons:

1) The regulators can get quite upset if they have an output voltage and no input voltage.

2) there may be parts of the circuit which operate from the unregulated rail(s).

I wouldn't be resorting to that yet.
 

davenn

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Dave why would you bypass a 1.5A reg with a 200mA transistor? Maybe I am being a bit thick here.
Thanks Adam

you're a bit late Adam!!!!

that was sorted out ages ago!!!!
my Q was asked BEFORE the pics were posted

have a read back through the whole thread ;)

Dave
 

Arouse1973

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you're a bit late Adam!!!!

that was sorted out ages ago!!!!
my Q was asked BEFORE the pics were posted

have a read back through the whole thread ;)

Dave
Sorry dave missed that one.
Cheers adam
 

Arouse1973

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Measure across the base emitter junction of the trannys, that might give us a clue.
Adam
 
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