Dead short across a capacitor, help please

Dazaa

Dazaa

Super Member
This pre amp came to me with one totally dead channel, it's dual mono and one of the transformers would get hot enough to burn your hand in a minute, when hooked up to a dbt the bulb would burn bright, it has not been powered on since my initial checking when I got it. I found loads of shorted transistors in the power supply area of the schematic and replaced all of them, q702,703, 701 ect. However I found a dead short across c702 with my multimeter. I have attached the schematic and highlighted c702. Any idea's what I should check next? I have been able to repair various power amps in the past but this pre amp seems to be more complicated that a simple replacement of a few transistors.
 

Attachments

  • sony schematic.jpg
    sony schematic.jpg
    126 KB · Views: 66
in case the other picture wasn't clear enough
 

Attachments

  • sony schematic1.jpg
    sony schematic1.jpg
    118.4 KB · Views: 56
I'd probably be replacing both C701 and C702. Those appear to be the main power supply filter / reservoir caps. Its possible that the caps just died from heat and old age. It wouldn't hurt to verify that the rectifiers next to the transformer (not on the schematic part, but I assume they must exist) are not shorted. AC on the caps will kill them.
 
gadget73 said:
I'd probably be replacing both C701 and C702. Those appear to be the main power supply filter / reservoir caps. Its possible that the caps just died from heat and old age. It wouldn't hurt to verify that the rectifiers next to the transformer (not on the schematic part, but I assume they must exist) are not shorted. AC on the caps will kill them.
Click to expand...


The main filter caps have been changed, I get a short across the cap even with it removed from circuir and do not get this on the same cap in the working channel. Defo need to check the rectifiers, is there a good tutorial on how to?
 
So, if you read a short between + and - rails on the PCB, without the capacitors in the circuit, you need to isolate the shorted component.

Or, do you get a short ACROSS THE CAPACITOR, with the capacitor outside the board? If that, if could be simply that the capacitor is charged. Your multimeter will beep. Discharge the capacitor with a bleeding resistor, something like 1K or less will work. Measure voltage across the cap terminals, it must be zero or some mV. Also, the capacitors show a short until they charge. If it's discharged, keep your multimeter connected, the resistance will raise until infinite.

I'd disconnect everything from the PS, to see if the problem is the PS itself, or a short in another board.

Follow the + and - rails in the schematic, and analyze what components could be shunting the rail to ground.

Don't discard physical shorts, screws pressing a wire, a piece of solder touching the chassis, some insulation washer missing...

If in doubt about the rectifier bridge, just replace it. You can measure it in diode test, it must allow current flow in one sense only, and not in the other, like any diode.
 
Last edited:
If the power transformer gets hot, someone probably replaced the main fuse with a wire or something. Inspect!
 
If there is an apparent short across ONE of the capacitors , and they have been replaced, is there any chance that whoever replaced them put one in backwards? - with reversed polarity? That is a VERY common n00b mistake and makes it appear that there is a short across one capacitor - which will now be 'toast' by the way.

You say it still shows a short when out of the unit, are you sure it isn't still (reverse) charged and confusing your multimeter?
 
Last edited:
Dazaa.. a 'short' is a term of 'continuity'. A short on a diode will appear as continuity both ways and you will get a reading on a meter.. If you have no readings on either side +/- of a cap you have an OPEN capacitor as in dead. If you have no reading on a resistor you have a 'shorted out' or 'open' resistor. :D
 
Last edited:
right, sorry guys I haven't explained this very well.

When I put the meter probes on the two solder pads for the capacitor (ie, capacitor removed from the circuit) the meter goes beep, indicating a short right?

When the cap is in place it reads something like a steady 200 wheras the same cap in the other channel charges up just as a cap should.

It did this before I swapped any of the mains filter caps so defo not a case of caps being put in the wrong way round.

There are 3 wires running from the board with the rectifiers and transformers on to each channel. I have totally isolated the faulty channel from the rectifier board and still get the beep across the cap. Note I haven't powered it up at all ever since getting it.

I remember finding a few transistors which appears to have small cracks in them, removing these had no effect.
 
Ok, so you disconnected the rectifier and transformer.

Did you disconnect the "-" supply rail from the rest of the device? Is there any way you can break the path from the "-" on that C702 to the rest of the amp, some wire, connector, whatever, or is everything on the same PCB?

I want to know if the problem is between the rectifier and the C702, (what point to some of those transistors between C702 and C708) or if the short is at another part of the circuit.

Do a VERY CLOSE visual inspection. Look for solder bridges, bare wires touching something, components leads touching the chassis... Of course, it could be a burned component. But a detailed visual inspection can reveal many things.

Also, it could be some of the new transistors you installed blew again, so check them again ...
 
elnaldo said:
Ok, so you disconnected the rectifier and transformer.

Did you disconnect the "-" supply rail from the rest of the device? Is there any way you can break the path from the "-" on that C702 to the rest of the amp, some wire, connector, whatever, or is everything on the same PCB?

I want to know if the problem is between the rectifier and the C702, (what point to some of those transistors between C702 and C708) or if the short is at another part of the circuit.

Do a VERY CLOSE visual inspection. Look for solder bridges, bare wires touching something, components leads touching the chassis... Of course, it could be a burned component. But a detailed visual inspection can reveal many things.

Also, it could be some of the new transistors you installed blew again, so check them again ...
Click to expand...


I'm going to have a proper look at it shortly and update.

Defo not the new transistors that are blown, I never powered it up after fitting them :yes:


EDIT
right I was slightly wrong about it being totally isolated, there were 3 more wires from the rectifier board to the phono head amp, I have now disconnected these. I also had a look at the rectifiers and the one for the left channel was behaving totally different to the one for the right channel so I assume it's blown, have removed that too, still getting a buzz across the cap...sigh.

I now remember why I gave up trying to repair this a few months ago, it drove me nuts.

As for removing it from the -ve supply rail, surely by being totally isolated from the rectifier board this has occurred? Or not?
 
Last edited:
FWIW there are 1 uF decoupling caps going to ground on some of the boards. C209 and C508 on the fragment you posted, and probably others. Also, with the DBT can you see any components getting hot (not a recommended repair technique, but ...) My 2cents.
 
As for removing it from the -ve supply rail, surely by being totally isolated from the rectifier board this has occurred? Or not?
Click to expand...

No, you disconnected the transformer and diode bridge from the power supply filtering and regulators (all those transistors and large caps).

But, the -V rail is feeding (connected) to the rest of the preamp. I´d say, the power supply is everything between the transformer and C702, and I'd call "the preamp circuit" all the circuit after C702 (not in the schematic)

We need the full schematic , or a link to it.

Anyway: follow the "-V" rail on the schematic, and narrow on components that can be shorted to ground. Or, figure a way to isolate different parts. Sometimes removing a coupling capacitor or a resistor, you interrupt the rail, isolating 2 parts.

Divide and conquer...
 
Just a mention, capacitors will show very low resistance initially as they charge up. May be that you're just seeing the cap charging up. An analog meter will show this more readily. The resistance will increase as time goes on since the cap will charge up.

If in doubt, use a dim bulb tester to test the unit. This will limit current and prevent things from going nova. The light should dim down if the power supply works normally, it will be near full brightness if there is a short.
 
gadget73 said:
Just a mention, capacitors will show very low resistance initially as they charge up. May be that you're just seeing the cap charging up. An analog meter will show this more readily. The resistance will increase as time goes on since the cap will charge up.

If in doubt, use a dim bulb tester to test the unit. This will limit current and prevent things from going nova. The light should dim down if the power supply works normally, it will be near full brightness if there is a short.
Click to expand...


Nope it's defo not that, all the other capacitors behave correctly, this one just gives me a continuous beep on the dmm, and that is with the cap in place AND when just probing the solder pads, something else is faulty.

very detailed service manual can be found here

http://freeservicemanuals.info/en/servicemanuals/viewmanual/Sony/TAE88/TAE88B/
 
elnaldo said:
No, you disconnected the transformer and diode bridge from the power supply filtering and regulators (all those transistors and large caps).

But, the -V rail is feeding (connected) to the rest of the preamp. I´d say, the power supply is everything between the transformer and C702, and I'd call "the preamp circuit" all the circuit after C702 (not in the schematic)

We need the full schematic , or a link to it.

Anyway: follow the "-V" rail on the schematic, and narrow on components that can be shorted to ground. Or, figure a way to isolate different parts. Sometimes removing a coupling capacitor or a resistor, you interrupt the rail, isolating 2 parts.

Divide and conquer...
Click to expand...


Right, I got you. According to the sm the -31v rail comes off the rectifier and has a few components connected to it, ending with c221. Since I disconnected all the wires from the rectifier doesn't that mean it has isolated the -ve rail? If so would that mean that one of the other components (there aren't many) on the -31v has shorted?
 
I'm getting a continual beep between r707 and the emitter of q711, and 8.2 across r707

r711 is reading 1.3

c209 charges up with my meter leads in one direction, in the other it does nothing

q212 seems to test ok

r223 reads 15k

r220 is 6.8

r219 is 6.8

r 209 is 1.1

So everything seems to be fine on the negative rail?
 
If you can, remove the capacitor in question, so we are sure you are measuring a short somewhere in the circuit.

I can't check your readings right now, but later today.

But you read a short between the V- and ground, so that's not OK.

According to the sm the -31v rail comes off the rectifier and has a few components connected to it, ending with c221.
Click to expand...

Not that easy... Look, from Q709 Gate, there is a dotted line, connecting 2 more circuits, one on top, other at the bottom.

So, many components connected to the -V rail. Try to isolate the different parts.
 
Last edited:
You must log in or register to reply here.

Similar threads

S
Dead Onkyo HT-R560
Replies
5
Views
256
srinath
S
V
How to use a multimeter to test for dead amp parts ??
Replies
4
Views
4K
nashvillebill
N
saabracer23
Crown D-75 failed again, where is this short?
Replies
2
Views
137
gadget73
gadget73
S
Toshiba SA-520 - Right Channel Short
Replies
4
Views
401
sruchris
S
S
SX-1250 Dead Short
Replies
9
Views
356
sruchris
S
Back
Top Bottom