Teac audio receiver shutting off intermittently

[xav_dude]

I have a TEAC AG-V8520 that works intermittently. When everything works fine and the receiver is turned off, the standby light is on, ready to be turned on. If I turn it on, main display goes on, standby light goes off and I can listen to music.

What happens is that at any moment, receiver will completely shut off whether on standby or while turned on listening to music. The standby light will disappear, so will the main display. Pressing the power button won't change anything nor unplugging the AC cord. It'll stay in this state for half a second or for hours literally, come back for a few seconds or minutes or hours then goes off again, this is totally erratic.

I was suspecting some kind of protect mode going on but measuring voltage at the Protect pin on the main board never showed any voltage when the receiver shuts off. I tried comparing voltage at different spots when problem happens and when it works fine but this is too complex for me.

I'm uploading the "service manual" for a TEAC AG-V8060 that seems to share the exact same configuration as my 8520. Hope someone can help me pinpoint the issue!

[retiredcaps]

Re: Teac audio receiver shutting off intermittently

I have never worked on any audio equipment, but I do watch MaxxArcade's youtube channel. He is also a member here on badcaps.

On his channel, he does a lot of audio equipment repair and invariably a lot of the equipment has poor solder joints. You may have the same?

http://www.youtube.com/user/Maxxarcade/videos

[rievax_60]

Re: Teac audio receiver shutting off intermittently

Have you checked the ST-BY 5.6v on page 16?

[xav_dude]

Re: Teac audio receiver shutting off intermittently

I checked the "+5.6V" pin located between the GND and F MUTE pins, is that the one you're referring to? If yes, there's constant 5.6V with or without the problem occuring.

[rievax_60]

Re: Teac audio receiver shutting off intermittently

Its difficult to make sense of the circuitry on the front panel because of the number of option circuits in the dotted lines.
See if you can check for any change of state of pin 33 Reset, pin 26 Back-up, pin 1 Vdd
on IC 81. Be careful to not short any pins together, some have -30v on them for the display.
Look for corrosion caused by possible leaking capacitors. Backup capacitors like to leak although Im not certain where it is.

[xav_dude]

Re: Teac audio receiver shutting off intermittently

pin 1 and pin 26 are always 4.74V and 4.47V respectively no matter what. When the receiver is not working, pin 33 is 0.08V and when working (standby light on) 4.30V.

I also noticed that pin F MUTE on main board (the one beside +5.6V you asked me to check) has 4,63V when working and 0V when problem occurs. Not sure if that means anything.

I'm not seeing any signs of corrosion or capacitors leak otherwise. It's pretty clean inside, no dust as well.

[xav_dude]

Re: Teac audio receiver shutting off intermittently

So it seems that the system is going into reset mode. If I understand correctly, the reset path should have constant 4ish Volts. When issue occurs or if I press the reset button on the Tuner board, voltage drops along the path. I guess it's now all about finding the component that stops sending the 4V, is my thinking right?

[rievax_60]

Re: Teac audio receiver shutting off intermittently

Yes. Also 4.74v/4.47v seems too low.
Usually when there is a 5.6v rail, it gets dropped to 5v after it passes through a diode on its way to the microcontroller.

[xav_dude]

Re: Teac audio receiver shutting off intermittently

I've noticed that resistor R837 (100K) on the Front board gets the 4.7V but only 0.08V gets out of it. When the system works, it outputs about 4.5V.

I was thinking this was the problem but if I press the reset button manually on the tuner board while the standby light is on, that resistor voltage output drops to zero as well which makes me think this is the common behavior when the reset path is broken... I'm a little confused now

Edit: The Back Up optional circuit in dotted lines is not my system.

[rievax_60]

Re: Teac audio receiver shutting off intermittently

If Q823 is being turned on during the problem, try to find the reason.

[rievax_60]

Re: Teac audio receiver shutting off intermittently

This is the type of problem that is likely to disappear when the faulty part is disturbed.

[xav_dude]

Re: Teac audio receiver shutting off intermittently

Q823 is part of an optional circuit that is not on the board, should I be checking Q811 instead?

[rievax_60]

Re: Teac audio receiver shutting off intermittently

Q811 provides power fail detection. Its collector should always be high.
What provides reset to the micro?

[xav_dude]

Re: Teac audio receiver shutting off intermittently

You lost me with "What provides reset to the micro?"

[rievax_60]

Re: Teac audio receiver shutting off intermittently

I cant properly follow the schematic because of the options that may or may not be present. Can you tell me which components the reset signal comes from?

[rievax_60]

Re: Teac audio receiver shutting off intermittently

You said before that the Back Up Option is missing so that only leaves R837 and C806 in the reset circuit. You need to find what is causing the reset voltage to sometimes be shorted to ground. The capacitor could be faulty.
Try disconnecting the path to the tuner PCB.

[xav_dude]

Re: Teac audio receiver shutting off intermittently

I disconnected one of the front board connector going to the tuner board, the one carrying the reset wire (CN18) and instantly the standby light went on. Reconnecting it makes the issue happen again. Does that mean the reset switch is faulty? I'm not sure how this component works, it seems to let current through but it's not part of any circuit loop, that reset switch seems to be on a dead end. Unless its function is to go to ground only which would prevent the current from flowing all along the system when reset button is pressed?

[rievax_60]

Re: Teac audio receiver shutting off intermittently

It looks like a simple normally open press switch. You will need to have a close look. The only possibility I can think of is that the switch is growing tin whiskers. Try cleaning the switch with a toothbrush.

[xav_dude]

Re: Teac audio receiver shutting off intermittently

Is there a way I could test bypassing the switch by going straight to the ground of the circuit board?

[rievax_60]

Re: Teac audio receiver shutting off intermittently

No, the switch is a normally open type.