Update on problem

[Mark]

Thanks for your suggestions Leonard and Mark. The idea that the processor
may being held in a RESET state makes sense, and is consistent with the
observed symptoms. So, I found the components associated with the reset
circuit and by implication, the reset pin of the processor (pin 63 if I'm
counting them correctly). Pin 63 is normally held high by 100k-ohm pullup
resistor R921, and is pulled low by npn reset transistor Q909. When the unit
is unplugged, then reconnected to the AC line, pin 63 goes low for a few
hundred milliseconds, then high again. So it would appear that the reset
circuit is working correctly (darn!).

I can also reset the processor by grounding pin 63 directly. When I do this,
the 25 Hz square wave (40 ms period) on pin 34 stops for the duration of the
reset, and resumes when the reset is finished. Again, this appears to be
normal behavior. However, once in a while the waveform at pin 63 will change
to a narrower pulse after the reset, and when it does, the power-on relay
kicks in and momentarily turns the unit on (for only tenths of a second,
however).

So, it appears that sometimes after reset the processor attempts to execute
the proper set of code, and other times it gets lost or hung up or
something.

And yes, Mark, you are correct. This model does not scan the front-panel
keys. It uses a voltage-encoding scheme like you describe that requires only
2 wires to connect 10 pushbuttons. Very clever.

Anyway, I'll keep plugging away. If anyone has any suggestions, please pass
them on.

Mark

 

[Mark]

Sorry; I didn't post this in the right place.

 

[Leonard Caillouet]

Mark,

Happy to help, but a couple of suggestions. First, post the model number in
the subject line. Second, include the symptom and relevant troubleshooting
info from previous posts. When reading news I don't keep all of the
previous posts because there are so many. I'm happy to help and will look
up the manual and my notes when I get to the shop on Monday, but I am less
likely to bother if I have to keep looking back to recall what we were
talking about.

I think Mark Z was right to suggest looking to the status of the oscillator
if the reset circuit is working. I don't recall, but did you check the
state of the protect lines? Seems like you were dealing with micro lockup,
not shutdown, but if it is shutting down it is likely in protection.

Leonard Caillouet

 

[Mark]

Sorry, Leonard. I intended to post this to its previous thread (see subject
above), but hit the wrong button.

The oscillator seems fine; I see a nice, steady 10 MHz signal at the clock
pins of the chip. And yes, this appears to be a lockup problem, not a
shutdown. The STOP line from the processor always stays low. At this point,
the unit is still off, so there is no power to the output stages and
therefore no protection signal.

Mark

Mark,

Happy to help, but a couple of suggestions. First, post the model number in
the subject line. Second, include the symptom and relevant troubleshooting
info from previous posts. When reading news I don't keep all of the
previous posts because there are so many. I'm happy to help and will look
up the manual and my notes when I get to the shop on Monday, but I am less
likely to bother if I have to keep looking back to recall what we were
talking about.

I think Mark Z was right to suggest looking to the status of the oscillator
if the reset circuit is working. I don't recall, but did you check the
state of the protect lines? Seems like you were dealing with micro lockup,
not shutdown, but if it is shutting down it is likely in protection.

Leonard Caillouet

First post:

As a favor to my son's friend, we are trying to repair his Yamaha HTR-5440
Receiver. The symptom for the past few weeks was that the unit would play
for a while and then stop responding to all front panel buttons and the
remote; then the only way to turn the receiver off was to unplug it.
Re-plugging it in would restore operation until the next time that the unit
froze up.

When we brought the unit into our shop, it wouldn't work at all. The low
voltage power is present (+5V, +15V and -15V), but the power-on relay won't
activate, so the display and everything else is dead.

Probing around, I find a clock signal on the microprocessor, running at 10
MHz. I can see the signal from the power-on switch; it goes into a pin on
the microprocessor (+5V when the button is pressed, 0V otherwise). The
output from the microprocessor that is supposed to drive the power-on relay
is always low regardless of the state of the power-on button. Other than the
clock, there is only one other signal on one other pin of the processor that
isn't +5V or ground, and it is a square wave with a 40 msec period. I would
have expected to see other lines transitioning from data transfers, but
nothing.

I don't see any obvious signs of damage, and all of the solder joints look
good. Does anyone have any experience servicing this receiver or ones like
it? Can anyone shed any light on the problem?

 

[Dave Moore]

them on.

Mark

Might help if you'd mention make and model no#

 

[Dave Moore]

don't have a clue what you're working on , but
I've seen micro's locked up from bad controls.

if the micro keeps gettting a false que from a
control it's polling, then it will stay stuck
in that control polling loop.

 

[Tim Kettring]

Also , check if you have a "soft" power supply .

If there is not adequate voltage regulation to all supplies to a U-processor
, it can be very erratic ( intermittant operation ).

The senior tech "fixed" a U-processor in an audio system by installing a big
Zener diode at a voltage supply pin that I didn't even consider to be "soft"
( 1% or so regulation I think )

tim

 

[Mark]

Dave:

Sorry; I accidentally put this post in the wrong place. It was supposed to
be with its previous thread from 10/25/2003 6:13 PM "Frozen Microprocessor
in Yamaha HTR-5440 A/V Receiver". All of the background information is
contained in that thread.

I've verified that it isn't a front-panel button that is causing the
problem, but it may be from another input to the processor. I'm still trying
to get a service manual so that I can figure this out; it's a little hard
guessing! Was just hoping someone recognized this model and its symptoms and
could offer a clue that would help out.

And Tim, thanks also. I think that I've ruled out the power supply. The +5V
going into the processor seems rock-solid but maybe it's worth trying a
large electrolytic across the 5-V regulator output.

Mark

 

[Dave Moore]

Also , check if you have a "soft" power supply .

If there is not adequate voltage regulation to all supplies to a U-processor
, it can be very erratic ( intermittant operation ).

The senior tech "fixed" a U-processor in an audio system by installing a big
Zener diode at a voltage supply pin that I didn't even consider to be "soft"
( 1% or so regulation I think )

tim

IMHO, all supplies to static sensitive devices like CMOS
U-processors should be like you described, of the shunt
regulator type. Why? To shunt out voltage spikes of course.
better imMunity from ESD
=^^= DM