Yamaha DSP-AX2 no signs of life.

[r0k1e]

Hi everyone, bought a faulty AX2 receiver hoping for an easy fix. Have soldering skills, searching google skill, but very little experience actually repairing stuff So far no luck with protected/unprotected DIAG using "power", "effect" and "6ch input" - the thing just stays off. Also no relay click and no obvious blown/damaged parts, fuses are ok.

This guy doesn't have a common Yamaha's standby power issue as it has much simpler power supply implementation from what I've learned so far. Downloaded a service manual, tried to check sub voltages (page 63) and found them faulty (mine are green):

Opened the relay cover, it looks clean. Made several blind guesses by iteratively replacing c311, c315 and c318 - no result (circled in red, returned back to original eventually):

This is quite far from open book reading for me as you might already noted What should I do next? Any help would be wonderful. Thanks!

 

[sgrossklass]

Sounds like that part of the circuit is actually OK, but with that 4.8V input missing on the relay driver, it won't turn on. Does the microprocessor receive power at all?

 

[r0k1e]

Fixed images for better quality.
No power on main CPU... I don't think any voltage is outputted from this board at this state since there's even no AC on main large transformer inputs (bottom right corner on the board picture above).

 

[r0k1e]

Ah, see your point now. Need to check +MB signal path and whether MAIN CPU has its 4.8V VCC

 

[r0k1e]

Got an advise from my buddy to check power down detect circuit and voltage regulator NJM78M05FA (page 60):

 

[r0k1e]

+5V VCC is in place, but no PRY from CPU
The whole power down detect and reset circuit voltages from picture above seem ok except for xl501 16 MHz resonator which has 4.9V 0V 4.4V on its legs instead of 2.3V 0V 2.2V. Where should I look further?

 

[r0k1e]

Also verified Main CPU pins for power supply (page 46). One strange thing that i've noticed is sudden voltage decrease on Power detect pin from 4.6 to 0.13V after a minute or two after plugging the receiver in. Is that normal?

 

[sgrossklass]

You might have two different problems here.

#1, µP seems stuck in shutdown (the voltages at XL501 indicate no oscillation). Could you try momentarily shorting out Q507 C-E (pull down /RES) to see whether that brings it to life? If so, suspect C633.

#2, POWER DET going low after a while. What does +MB do then? I almost can't imagine that it would be sagging, but if so maybe bad solder joints at the standby power xfmr. Otherwise power-down detection (Q506 area) seems suspect.

 

[r0k1e]

Thanks sgrossklass for help! Will try that out later today. Just to clarify, should I use a resistor to pull down /RES? If so, will 10 KOhm be fine?

Will also check +MB and re-check transformer / Q506 area.

 

[currentflow]

Thanks sgrossklass for help! Will try that out later today. Just to clarify, should I use a resistor to pull down /RES? If so, will 10 KOhm be fine?

I would use a 10R to 150R resistor for this purpose so as to limit the discharge current of C634 and give a 'good' logic low on /RES.

 

[r0k1e]

Thanks currentflow and sgrossklass! XL501 high voltage state (4.9V 0V 4.4V) corresponds to POWER DET sag! So here's 3 set of measures:
1. Receiver just plugged into AC outlet:
XL501 2.45 0 2.28 PDT 4.6
2. ~2 minutes after:
XL501 4.91 0 4/41 PDT 0.13
3. After /RES pull down, again:
XL501 2.44 0 2.32 PDT 4.6

all three has +MB around 14.5V and no PRY...

Please tell me that reset success means Main CPU and its flash ROM are ok (my main fear is that ROM is damaged). I will also post Q506 area measures before- and after- sag soon.

 

[r0k1e]

I was wrong for +MB to be +14.5 for all states. It is true for state two, but for state one and three I see +17.7V first, then it goes to 16.5, then back to 17, then about 14.5 and so on until it finally settles down to 13.7 with PDT 0.13. All other dependent voltages are also initially slightly higher...

Here's the picture for the state two when everything has settled down:

 

[currentflow]

The PDT (Power Down Detect of the +MB line) seems to be behaving strangely. It shouldn't be dropping to 1.3V after 1-2 minutes as you had reported in post #7 IF the +MB line still has significant voltage on it.

Can you measure the voltage on +MB from switch-on until the PDT line has fallen to 1.3V and report please. +MB must drop to less than 7V to cause PDT to fall as Q506 turns off.

PDT may also be measured at the junction of R673, R674.

 

[r0k1e]

It's worse - PDT drops down to 0.13V after a minute or two and yes, +MB still has 13.7V on it.
I meausred PDT on several positions: top and bottom PCB check points, R673 and R674 pins - all have the same 0.13V. At the same time I'm sure R673 and R663 junction has 7.15V. Does it mean I should try replacing R673 and R674?

I will trace +MB again and post pictures too.

 

[currentflow]

It's worse - PDT drops down to 0.13V after a minute or two and yes, +MB still has 13.7V on it.
I meausred PDT on several positions: top and bottom PCB check points, R673 and R674 pins - all have the same 0.13V. At the same time I'm sure R673 and R663 junction has 7.15V. Does it mean I should try replacing R673 and R674?

I will trace +MB again and post pictures too.

Could I suggest taking some measurements please (with respect to 0V) around Q506 after the PDT line has dropped:
1) +MB
2) Voltage across zener, D503
3) Q506 E,B,C
4) R663 - R673 junction

Regarding R673 and R674, let's see what the readings above produce first.

 

[Mooly]

Quick thought. If you suspect the uP is 'stuck' in a non valid state then it might be worth trying to remove all charge from back-up rails +5M1 and +5M2. +5M1 has a 0.047F super cap fitted.

Do this by connecting a 100 ohm across each supply and leave these in place for an hour or two or even overnight. Unit off and unplugged of course.

Disclaimer... I'm not familiar with this unit but sometimes resetting a uP can reset any regional options that are manufacturer preset such as AM channel spacing etc. Not often that happens but beware.

 

[r0k1e]

Could I suggest taking some measurements please (with respect to 0V) around Q506 after the PDT line has dropped:
1) +MB
2) Voltage across zener, D503
3) Q506 E,B,C
4) R663 - R673 junction

Regarding R673 and R674, let's see what the readings above produce first.

Already did that yersterday:

1) +MB 13.7V
2) Voltage across zener, D503 0V (Anode), 6.52V (Cathode)
3) Q506 E,B,C 13.71V E, 13V B, 13.66V C
4) R663 - R673 junction 7.15V

...but let me double and triple check today

 

[currentflow]

Quick thought. If you suspect the uP is 'stuck' in a non valid state then it might be worth trying to remove all charge from back-up rails +5M1 and +5M2. +5M1 has a 0.047F super cap fitted.

Do this by connecting a 100 ohm across each supply and leave these in place for an hour or two or even overnight. Unit off and unplugged of course.

Disclaimer... I'm not familiar with this unit but sometimes resetting a uP can reset any regional options that are manufacturer preset such as AM channel spacing etc. Not often that happens but beware.

That's a valid point, Mooly. If the firmware's creator(s) took proper care to range-check in the main code the stored variables held up by the super-cap, a stuck condition caused by corrupted option storage should rarely occur. Note I said "if"! Everything is designed and built to a budget in practice.

 

[currentflow]

Already did that yersterday:

1) +MB 13.7V
2) Voltage across zener, D503 0V (Anode), 6.52V (Cathode)
3) Q506 E,B,C 13.71V E, 13V B, 13.66V C
4) R663 - R673 junction 7.15V

...but let me double and triple check today

Thank you. It's important that the readings are taken when PDT = 0.13V.

 

[Mooly]

I've encountered stuck uP's (micro lockout we used to call it) many many times over the years in TV's VCR's, audio gear etc. Even remotes.

The 'big chip' is probably the most reliable part in there but they can lock up. I can give examples where the problem isn't even fixed by removing power for a few hours, the thing has to be left for several days for internal charge to dissipate.