Yamaha RX-396 powers off after 2secs

[jurgyman]

Considering repairing a free Yamaha RX-396 Natural Sound receiver.
But I'm no electronics tech, just have basic solder and tester skills.

Symptoms: It turns off after 2 seconds after hitting Standby/On.
I hear the power on relay click then the power off click near the smaller transformer board in rear.

Diags: trying out the Protection Circuit check steps from the manual....
Holding AUX, Preset 8, and Power shows code: PRT-PS
(indicates detection of abnormal DC value from amplifier)

Holding AUX, Preset 7, and power shows code: DC-000
Is that telling me the main transformer isnt putting out any DC?

Not sure what to/do check next.
There doesnt appear to be any caps leaking. No cracked solder joints. All testing above done with nothing (device, speaker) connected.

TIA Jurgyman

[charis]

Re: Yamaha RX-396 powers off after 2secs

I see on schematic that you have 2 relays on main2 (mains and on 12v output circuit )and one at main power who is part of the protection circuit(24v).
The protection circuit signaling direct to Mitsubishi processor with 5v.
The 12v from main2 gives the +5v from regulation circuit and also feed the tuner and reset fuction and then goes to micro with a pull dowh resistance of 100k to 5v.This voltage monitor from DTA114A who control the regulation circuit on main1.Also going to input 2.
The major problem in this units and also on RX 497 is the stand by pcb main1.
The FET Q133 the bridge the 2 zener D109/110 or the protection have problem.
Play around and will see.

[jurgyman]

Re: Yamaha RX-396 powers off after 2secs

Thanks Charis for the reply!

I tested ohms on D109 and D110.
D109 offered no resistance in either direction and appears to be shorted?
D110 read 675K ohms one way and high/open the other way. seems ok?

I am not sure howto test the Q FET. can you advise?

[budm]

Re: Yamaha RX-396 powers off after 2secs

Relay RY102 supplies the main AC to the primary winding of the Main power supply.
Relay RY101 is the speaker protection relay which will not be energized if the detection circuit detect DC offset >1V at the power amplifier outputs.
A quick test, check the DCV between +/- leg the main filter cap C140 when you turn on the unit to so if it is present and then drops out or not.

[budm]

Re: Yamaha RX-396 powers off after 2secs

Thanks Charis for the reply!

I tested ohms on D109 and D110.
D109 offered no resistance in either direction and appears to be shorted?
D110 read 675K ohms one way and high/open the other way. seems ok?

I am not sure howto test the Q FET. can you advise?

What DCV do you have between the two legs of those two Zener Diodes?

[budm]

Re: Yamaha RX-396 powers off after 2secs

BTW, the PROTECTION OPERATION CHECK FUNCTION has to do with AMPLIFIER DC at the outputs.

[tibimakai]

Re: Yamaha RX-396 powers off after 2secs

Shorted final transistors?!

[charis]

Re: Yamaha RX-396 powers off after 2secs

To make correct measurement put the zener out of circuit.
Now on zener you will see a black line in the one size.
Put your meter on resistance(diode tester or 2k scale) the com to the black side of zener the V/Ω το the other side.
You will see a resistance of 750Ω.
In reverse you must have infinity.
This is 15v zener.
Are you the first person on this unit did you see it working?
The 2 second is probably the time that the 2 main caps need to full charge.
Put out the brige rectifier.
Now with a multimeter put the com to + and V/Ω to -.
You see a resistance of 1.5KΩ.
Now put the com to + and VΩ to one of ac input.Do this again with the other ac input.
You see a resistance of 650 Ω.
If you put the V/Ω to + and the com to - or to ac you have infinity.
This is a 200v input bridge.
To test the 2SC4495/Q121 (multimeter on diode test)is npn put your V/Ω to base (first pin at the left) the com to emiter you will see a diode reading and also to collector about 0.6v.Emiter collector you have open.
The Q128 is PNP so put the com to base.
The FET is n chanel the base is the middle pin.

[jurgyman]

Re: Yamaha RX-396 powers off after 2secs

Realizing how much time this troubleshooting will take and so its postponed for now. thanks for all the suggestions.