Yamaha DSP-AX2 no signs of life.

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r0k1e

Member
2016-06-19 10:05 pm
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 :D 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):
13079836.png


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):
13079965.jpg


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!
 
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r0k1e

Member
2016-06-19 10:05 pm
+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?
 
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

Member
2016-06-19 10:05 pm
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

Member
2016-06-19 10:05 pm
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:
13108139.png
 
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

Member
2016-06-19 10:05 pm
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

Member
2008-04-15 11:30 am
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.
 
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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
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

Member
2016-06-19 10:05 pm
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

Member
2008-04-15 11:30 am
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.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
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.
 
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