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Background: Electronics novice, attempting stereo amplifier repair for educational purposes. Amp (Pioneer VSX-519V) blew when the input was removed while at high volume. Careful visual inspection does not sight any damaged components.

Am in the process of learning how to check a transformer. The first two wires (black/blue) on primary have continuity, 1.7ohms, and 110v AC is present to the terminals that supply power to the transformer. There is no voltage present on any of the output terminals.

There is a second set of thick wires from primary coil: three wires (yellow-red-yellow) that have also proper continuity, but no voltage.

The secondary coils have two groups of three wires, and one group of six wires. Each group has continuity, and expected resistance's, but there is no voltage (AC) present anywhere.

There is no continuity between primary and secondary windings.

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Question: based on the above, does this transformer have a problem? I do not presently understand why all the other readings look good, but I have no AC voltage on any secondary windings.

UPDATE:

When the transformer is taken out and tested on the bench, it is fine. (that was a good lesson). When the transformer is installed, something else is "tripping" and cutting power to the transformers primary coil during startup.

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    \$\begingroup\$ What is the amp? Some readers may have the schematics. \$\endgroup\$
    – user133493
    Apr 3, 2017 at 4:36
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    \$\begingroup\$ Google immediately finds the service manual with schematics. That should help you to proceed. I have repaired a similar model to this. I needed to replace the thermal fuse inside the main power transformer, which required removal of the insulating tape. I mention this so that you don't overlook it, not to say that it is definitely your problem too. \$\endgroup\$
    – user133493
    Apr 3, 2017 at 4:46
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    \$\begingroup\$ When you say the amp does not power on at all, do you mean that even the standby supply is not working? These products have soft power switches, and you should hear the relay clicking on attempted power on. If the startup tests fail, it should turn off again (and click), if memory serves, about 200ms later. \$\endgroup\$
    – user133493
    Apr 3, 2017 at 4:55
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    \$\begingroup\$ By standby I did not mean that you should expect anything on the display, as the filament display supply is derived from the main power transformer not the standby supply transformer. The standby supply powers the main logic IC on the other side of the unit which performs startup tests and drives the mains input relay. Good luck in your repair, it should be a great learning experience. \$\endgroup\$
    – user133493
    Apr 3, 2017 at 5:07
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    \$\begingroup\$ Your update about the transformer being removed from the circuit on startup was exactly what I was referring to with my comments above about the startup sequence. Either the controller is driving the relay to disconnect it so you have a problem downstream, which could take some time to find, or there is a problem upstream which should be quick to find. Do you have a scope? \$\endgroup\$
    – user133493
    Apr 4, 2017 at 1:12

1 Answer 1

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Your update tells me that your initial statement "...110V Ac is present to the terminals that supply power to the transformer..." is misleading. It implies that you are measuring at the transformer terminals - which you are not.
It is clear to me, that there is some safety circuit between the board terminals (were 110V comes in), and the transformer's input terminals.
Your safety circuit is detecting an unsafe condition and disabling power to the transformer primary terminals. There is nothing wrong with the transformer. Most likely, your power output transistors are shorted/blown, causing the safety circuit to remove power to the transformer.

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  • \$\begingroup\$ Thats essentially correct. I was measuring the terminals where the transformer connects to the board (with the transformer unplugged, so I could reach the pins). 110v AC was present on the "transformer input pins". What I didn't realise was that plugging the transformer back in was resulting in power being cut to those same pins - because of the safety circuit as you point out. Good learning experience. \$\endgroup\$
    – Mtl Dev
    Apr 8, 2017 at 2:49

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