Maestro 612p – no sound issue

[mhutcherson]

Hello, I have a Maestro 612p and am looking for troubleshooting advice. There is no sound off the reed rail (except a faint hum) via any output. I have confirmed both fuses are good and the power amp is functioning (ran external audio through the PU-1 input). using the wiring diagram as a guide I confirmed voltage is correct at R40.

What other test points should I measure voltage and what voltage numbers should I look for?

Many thanks!

[theseacowexists]

What is the voltage at R1? You should have 200V at the point marked "B".

[mhutcherson]

Thanks! I am getting 50V at point B. Time to replace all those caps in the power supply?

[theseacowexists]

Yessir. Check the resistors and the diodes in the power supply too, to make sure that none of them are shorted or too far out of spec. If you don't have 200V at point B, there is not enough voltage for the pickup to produce any sound - just like a wurly.

[mhutcherson]

Will do, thank you!!!

[mhutcherson]

You cannot view this attachment. I re-capped the power supply section and am still getting 50 V at point B. Any other ideas? Wondering if the transformer is shot...

Yessir. Check the resistors and the diodes in the power supply too, to make sure that none of them are shorted or too far out of spec. If you don't have 200V at point B, there is not enough voltage for the pickup to produce any sound - just like a wurly.

[Jenzz]

Hi .-)

Looking at the pics, it seems that the leads of the diodes have melt into the housing of silver caps, probably touching the inside foil?

Jenzz

[theseacowexists]

Wouldn't hurt to pull those diode leads so they aren't touching the body of the silver caps, like Jenzz pointed out. Are those silver caps new?

Remove the diodes and test to see if they are still working.

What is the voltage at the red lead in the top right corner of your picture? That should be the 100V from the transformer.

[mhutcherson]

Thanks for the feedback. Finally had a chance to replace the resistors and I'm now getting 140 V at point B. Still not 200 V! I tested AC voltage coming off the transformer and am getting 112 V. The diodes aren't making contact through the foil of those silver caps (yes, they are new) and they are functioning correctly (removed and tested). What could be the cause of insufficient voltage (~60 V) off the power supply at this point? Huge thanks for the advice everyone! I have had this piano for a very long time and would love to get it working again.

[theseacowexists]

140V should be enough voltage to power the pickup. My Pianet gets about that much at the pickup and it works fine.

Are you measuring 140V on the pickup itself, or just at the power supply? It could be that the voltage is not making it from the power supply to the pickup due to a broken wire or bad connection.

Is the pickup shorted to the reeds at all? In that case there would be no voltage at all on the harp, and therefore no sound. Check for continuity between the pickups and the reed. If there is continuity, then they are shorted somewhere.

If that is all well and good, then we should check the preamp, since you've confirmed that the power amp "PU" preamp work. Can you insert a signal at the junction of C1 and R2?

[mhutcherson]

Thank you! Voltage at the pickup is 76 volts. There is not a short between the reeds and pickup rail. I just ran some audio through the C2 and R2 junction. I do get sound, but it is faint. There is also a fairly pronounced hum whenever the piano is on. UPDATE: I just discovered an odd phenomenon, that if I hover a probe or my finger anywhere near C8 the hum gets much louder. If I make contact with a probe to end of the cap it makes a terrible noise. No other cap is doing this.

140V should be enough voltage to power the pickup. My Pianet gets about that much at the pickup and it works fine.

Are you measuring 140V on the pickup itself, or just at the power supply? It could be that the voltage is not making it from the power supply to the pickup due to a broken wire or bad connection.

Is the pickup shorted to the reeds at all? In that case there would be no voltage at all on the harp, and therefore no sound. Check for continuity between the pickups and the reed. If there is continuity, then they are shorted somewhere.

If that is all well and good, then we should check the preamp, since you've confirmed that the power amp "PU" preamp work. Can you insert a signal at the junction of C1 and R2?

[theseacowexists]

Although that cap isn't in the part of the circuit we're troubleshooting (it's in the external input preamp), you should replace it. Have you replaced any of the electrolytics on the board? You should do all of those.

What voltages do you get a point A, and at all the points labeled along the top edge of the schematic?

I realize I was wrong about the 140V at point B being directly connected to the pickup. R1 is between point B and the pickup, and should knock the voltage down. So 76V makes sense.