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Old 11th February 2005, 03:20 PM   #1
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Join Date: Feb 2005
Location: Kingston, ON
Default Troubleshooting Help

Hey, there!

I'm trying to troubleshoot my Yamaha PF-85 keyboard, hopefully you guys can help. Yes, I have schematics, no I don't have a scope.

Symptoms:
1. Right speaker does not work
2. Output II (right channel line level) does not work
3. Right headphone does not work

Additional Troubleshooting info:
1. Speaker itself is okay (swapped left-to-right)
2. When power is on and volume is zero both speakers hiss (VERY quietly)
3. When volume is increased, left speaker hiss increases; right speaker hiss stays constant
4. During power-on, BOTH speakers receive an inaudible "pop" (I saw it on my analogue voltmeter)
5. Turning the "internal speakers" switch off kills the hiss on both speakers.

Design info:
1. The audio generated is normally mono, except when the cho
rus button is selected. Hitting chorus doesn't activate the right speaker.
2. The generated sound goes into a "stereo" volume sliding pot, into a pair of 4558 op-amps, through a 4016 quad bilateral switch, and out a pair of STK4131-II 20W power amps.
3. The 4016 appears to be there to kill power-up pop (it's connected to "internal clear".
4. It's possible that the input to BOTH 4558s was shorted to ground at some point, by plugging the damper pedal into Output II (I didn't own it when it died).
5. No obvious signs of physical damage

Thoughts as to how to troubleshoot, without a scope? Plan is currently:
1. Bypass 1/4 4016 with a 300 ohm resistor or diode (will it matter?)
2. Replace 4558
3. Build a small LM386-based amp and "follow" the audio signal 'till I find where it dies
4. Beg/borrow steal a 'scope, perform diagnostics as above

..in no particular order. Replacing the 4558 is not high on my want-to-do list, as it is no socketed (and the board is quite dense).

Any feedback is appreciated (is this the right forum?)

Wes
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Old 12th February 2005, 07:22 AM   #2
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
Replacing parts like that is just guessing. A scope is best, but you already have what I call the poor man's scope - your meter. Set up a signal and you can measure it on the AC scale. If you hold down a key in a voice that will pay continuous like organ, the steady signal will be in the signal path for you to monitor with the meter. Can't measure at the input pins of the op amps, you get no readings there, but certainly at the outputs and everywhere else. If all that thing plays is piano, then keep hitting the key or enlist some help. If it had a demo mode, trigger that. A matchbook wedged between keys will hold one down for you.

Ther is also the poor mans signal injector - your finger. Hold a piece of wire or a nail or something in your fingers. Touch it to the signal path you inject loud hum. Great for testing an amplifer path for breaks. A dirty 60Hz is as good as any other signal for continuity testing.

Number one suspect is the slider. Got signal at the top AND at the wiper on boith sections? Or is it a control voltage silder to an electronic pot chip?

From your description, the amp is working - the thump and hiss say so. The slider increases noise in the good channel but not the bad channel. I suspect it is not passing the system noise from before itself - bad slider.

With your injector, touch the inputs to the 4558 - sound? Touch the slider wipers - sound? Was there signal voltage at the top of the sl;ider on the bad channel side? If not, touch there.

If you find signal on the 4558 out and suspect the 4066, it won;'t hurt it to short between in and out of it.. For that matter, with the system rrunnig, measure the resistance through a cell of the 4066 with your ohm meter. That would tell you if it was stuck open. But if you can inject a signal before the 4066 and it comes out the speaker, then it is OK - it passes signal.

DId I mention that sliders are the weakest link in the system. Of course my ranting about it will make it be something else.
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