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Yamaha 120v voltage crash when transformer connected

thebeekeeper

thebeekeeper

New Member
So I'm trouble shooting a Yamaha amp that appears flat dead. It was working, was moved a few hundred miles and is now dead.

Quick check with my multi-meter shows 120vac coming in, fuse fine, but from the first power filter board to the transformer I have 0 volts. I pulled the connector to the transformer, and read 120 v on the pins. I put on the connector and read 0 volts. I thought it was the connector. So I went with some jumper wires and proved as soon as the transformer is connected, it goes from 120 to 0 v. Odd.
I disconnected the 2ndary from the transformer. Repeated. 0 v. This does not compute with my electrical engineering brain. The fuse is not blowing, the transformer is not shorted. Resistance is 2-3 ohms on the primary, and resistance is not impedance when it comes to Xformers.

My first guess is some self protection circuit cuts off voltage. But I don't ever hear the relay clicking. So i'm stumped.
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thebeekeeper said:
My first guess is some self protection circuit cuts off voltage. But I don't ever hear the relay clicking
Click to expand...
Probably right. The amp may have a standby mode where 12V/5V is provided via a small transformer to microprocessor/IR etc, during power on the mcu makes a health check, detects a protection issue and inhibts power on of the main transformer.
 
The RX-V/HTR amps are a real PITA to troubleshoot, you might get lucky but I doubt it...

Just for clarification, when you attempt to power on the unit, there is not sign of any activity, eg, powers
up for 1 second then falls over?

You could try reading the diagnostic/protection info, see page 17 of the service manual, probably will show
nothing...

Working through the schematic, page 55 MAIN-2 board...
Standby power is provided by (small) transformer T371, power is then sent to regulators/mcu via cut wires CB371, S12 wire

#### Measure dc voltage at D371 cathode(=S12), it's located beside the relay opposite side of fuse. Expect 8-13Vdc.
Unit needs to be in standby state, ie, plugged into wall socket and any wall socket switch turned on.

If S12 is 0Vdc then measure Vdc at D373 cathode, about 18Vdc?
Then you have a problem with Q373, Q372, Q374, C373, C371 or....

If S12 is ok,
When you attempt to power on the unit the order is sent to the mcu who makes some checks, if all good then an order is
sent to connect the main power transformer/Operate Relay on CB371 PRY wire

#### Unplug the unit, carefully clip you multimeter red leed to Q371 base then plug in the power cord, Q371b should read 0Vdc.
Then hit the units power on button(STBY?) while watching the meter, wanting to see a brief flash of digits
 
Just a thought.
I`ve had yrs. ago a 8 trk. R to R`s original main`s fuse look perfect, but had developed a very high internal resistance when the unit powered up drawing very low idle audio electronics current dropping the 120 v down to almost nothing.
New fuse, and all worked perfect.
 
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