Sony STR-D511 power transformer

[Deadly Sones]

Hello,

I have this stereo (STR-D511) sitting here and it appears that the transformer is shot... when I power it up there is no display but the video led, muting and "DBFB" lights are on and can be turned on and off by pressing the buttons. The relay clicks and the transformer powers 120v on one side. The other side only outputs about 4.5v on each of the other 8 pins...

Here is a schematic of the input section... Is there anything else that could be causing this or is the transformer shot? T902 appears to be working properly... T901 not so much...

Thanks in advance,
Paul

[Mooly]

If the diagram is correct then disconnect power and just measure on ohms range across one of those switched outlet sockets as they are directly across the primary when the relay is off. Should be a low reading, maybe a couple of ohms only.

[Deadly Sones]

Hi, thanks for your response...

I tested both the switched output plug and the 1-423-574-11 transformer with the main power plug disconnected and I get OL on my DMM. With power they both read 122v.

[Mooly]

If you get OL (what your meter displays when the probes are not connected) then that indicates the primary is open circuit.

[Deadly Sones]

Time for a new transformer? Nothing else would make this happen right? I suspected the transformer from the start but did go over the complete reset and stop switch sections. I also checked all the outputs as I thought it might have been in protect at first...

Thank you very much Mooly. :-)

[Mooly]

If you are certain the primary is open then the transformer has failed. Usually there is a cause... often a short on the secondaries somewhere, maybe the output stages of the amp or a rectifier etc. Read across the plus 50 volt rail and the -50 volt rail to chassis ground. A dead short would show up... anything other than and it would require faultfinding but worth checking.

In practice there may well be a non resetable thermal fuse in the primary winding which you can often see near the pins. From a safety point of view these should always be in place.

[Deadly Sones]

I pulled the transformer right out of the circuit... the primary coil is definitely open. I might not be doing the other test properly, but with the black lead connected to a board ground screwed to chassis (or even the rail labelled GND) and the red on just about any rail labelled +50/-50, +25/-25, or +5/-5 I only get 0.000 for a couple seconds then it shows really high fluctuating resistance numbers.

Any idea what I should check next?

The owner said he thinks it was a power surge that killed it... I think he cranked it too long personally, lol. All fuses (F1, F2 & main) are good. The output transistors appeared to be good. I even removed them to be certain.

I have the full schematics if you need anything further.

Thanks

[fastvideo]

Originally Posted by Deadly Sones

I pulled the transformer right out of the circuit... the primary coil is definitely open. I might not be doing the other test properly, but with the black lead connected to a board ground screwed to chassis (or even the rail labelled GND) and the red on just about any rail labelled +50/-50, +25/-25, or +5/-5 I only get 0.000 for a couple seconds then it shows really high fluctuating resistance numbers.

Any idea what I should check next?

The owner said he thinks it was a power surge that killed it... I think he cranked it too long personally, lol. All fuses (F1, F2 & main) are good. The output transistors appeared to be good. I even removed them to be certain.

I have the full schematics if you need anything further.

Thanks

Make sure you have no short circuit on the secondary side.
Check if you can find the thermal fuse on the primary winding, and measure if this fuse is open. Replacing the fuse should power the unit again.

[Deadly Sones]

The transformer is completely out of the radio... I can not see any thermal fuse on the transformer. There is a small 4A fuse on the input power board that is good and two 4A slow burning fuses (fluorescent indicator tubes) that are on the secondary side between the main board and the secondaries... they are both good also.

[mrfeedback]

From memory open circuit primary was a commonish fault, caused by primary winding open circuit thermal fuse.
Take a close look and you should see two tinned wires disappearing in insulation sleeves into the transformer primary winding and connected to termination pins on the transformer.
You can bridge the thermal fuse with a thin wire and you will be operational again although you will have no thermal protection.
If you are really skillful you may be able to replace the thermal fuse but you may also destroy the primary winding.

Eric.