Technics PX Digital Keyboard Dead

[NickFil]

Hi Everyone:

This is the first time I am posting on any Forum, so please bear with me.

I have a Technics PX103M Digital Piano, which is, for lack of better words, dead. It has nothing lighting up, no noise, nothing! I opened up the unit and tested the fuse, which is O.K. I also put my multimeter on the input of the Power Transformer (Part Number QTPG1M022A) and there is power going to it. However, there is no voltage on the output side of the transformer. It is, as well as I can tell, supposed to be +15V -15V on the output side, but I get no voltage at all. I have the Service Manual for the unit, with schematic diagrams, but I never really understood electronics very well. I have attached on schematic diagram which I think is relevant to this problem. If anyone can help me identify the transformer specifications, I would appreciate it since Panasonic no longer makes this part available and I have searched the internet for many days with no success in finding a replacement part. I am thinking of buying a compatible transformer from an electronics supplier like Electro Sonic (located in the Toronto area), which sells Hammond transformers, but I cannot identify which one I need, since I don't know the output specs. Thanks in advance for any help provided.

Nick

[Enzo]

Transformers make AC, not DC, so there will not be +15 and -15v coming from the transformer. The transformer takes the 120vAC from the wall and makes a lower AC voltage which is then rectified and filtered by circuits that then make the two 15v supplies.

I see three wires leaving the transformer for this. Looks like the center one should be grounded, and the other two would have about 20VAC on each. Got 20VAC? Or 40VAC between the two hot ones?

You say you find 120VAC getting TO the transformer, but it could have an open primary. Power off and unplug from the wall. Looks like you have a small board for incoming 120VAC mains. On the end of the board I see a two wire connection to the transformer. I assume that is where you founf 120VAC getting to. Now with power removed, set your meter to resistance and see if the transformer shows some lowish value or if it is opan. Open means bad.

[NickFil]

Transformers make AC, not DC, so there will not be +15 and -15v coming from the transformer. The transformer takes the 120vAC from the wall and makes a lower AC voltage which is then rectified and filtered by circuits that then make the two 15v supplies.

I see three wires leaving the transformer for this. Looks like the center one should be grounded, and the other two would have about 20VAC on each. Got 20VAC? Or 40VAC between the two hot ones?

You say you find 120VAC getting TO the transformer, but it could have an open primary. Power off and unplug from the wall. Looks like you have a small board for incoming 120VAC mains. On the end of the board I see a two wire connection to the transformer. I assume that is where you founf 120VAC getting to. Now with power removed, set your meter to resistance and see if the transformer shows some lowish value or if it is opan. Open means bad.

Thank you Enzo for that information.

I found that there is about 21vAC between either output wire and the center one and about 42vAC across the two outer wires, which I assume means that the transformer is working properly. I found that the power LED light is on when the power is switched ON but the rest of the Keyboard is dead. Do you or anyone else have any ideas as to what I could check next? If you need any other schematic diagram, I can upload that particular section (it probably wouldn't be a good idea to upload the entire service manual since it is over 6 Mb). Thanks again.

Nick

[Enzo]

Oh? In your first post you mentioned no lights, now you got lights?

OK, tranny works, move on to the +/-15v supplies. They seem to use simple three-leg regulator ICs, both working? Then there are isolated supplies, Q5, Q7 etc. Those OK?

Right below those I see a power-on mute circuit (PMUT), your thing could be stuck in mute mode.

Do the headphones work?

[NickFil]

Oh? In your first post you mentioned no lights, now you got lights?

OK, tranny works, move on to the +/-15v supplies. They seem to use simple three-leg regulator ICs, both working? Then there are isolated supplies, Q5, Q7 etc. Those OK?

Right below those I see a power-on mute circuit (PMUT), your thing could be stuck in mute mode.

Do the headphones work?

Hi Enzo:

Sorry about the first post. I did not realize that this keyboard has a small LED light that indicated Main power ON. The rest of the keyboard is dead. I'm not sure if I tested this right, but I put my meter on the outer legs of Q5 and Q7, and I get the indicated voltages (+15 and -15 respectively). I would assume this means that the regulator ICs are doing their job in providing the appropriate voltage and that Q5 and Q7 are working. Please correct me if I'm wrong. Also, the headphones do not work. Any other suggestions?

Nick

P.S. I appreciate you walking me through this problem.

[Jazz P Bass]

There is a boat load of stuff to check.
This is not a simple keyboard.
O/k, the + & - 15 volt regulators are up & running.
The 5 volt regulator also must be functioning.
Beyond the power supply:
Try the Aux in jack.
This will bypass most of the preamp circuit.
Also try the line out to an amplifier.
This should bypass the internal amplifier.
You need to narrow down the failure mode.

[NickFil]

There is a boat load of stuff to check.
This is not a simple keyboard.
O/k, the + & - 15 volt regulators are up & running.
The 5 volt regulator also must be functioning.
Beyond the power supply:
Try the Aux in jack.
This will bypass most of the preamp circuit.
Also try the line out to an amplifier.
This should bypass the internal amplifier.
You need to narrow down the failure mode.

Thanks Jazz P Bass for your input:

I checked for 5 volts at the main board, and I found it at D12, on one side of the diode. I would assume that the 5v regulator is, therefore, working properly. I hooked up my guitar to the Aux In, and got nothing. I hooked up the Piano to an amplifier via the Line Out jack, and got nothing. Do you have any other suggestions? Thanks again.

Nick

[Jazz P Bass]

How proficient are you at signal tracing?
I would prove that IC 20 is accepting signal.
From the keyboard or from Aux In.
If it is, then prove that it is coming out.
Line out appears to be independant of the power amp circuit.
Likewise the headphone circuit.
IC 20 feeds all three.

[NickFil]

How proficient are you at signal tracing?
I would prove that IC 20 is accepting signal.
From the keyboard or from Aux In.
If it is, then prove that it is coming out.
Line out appears to be independant of the power amp circuit.
Likewise the headphone circuit.
IC 20 feeds all three.

I have limited electronics knowledge and experience. I really don't know how I would accomplish signal tracing. Is there a device used to do this? If there is, I'm sure I can learn how to use it. If you can recommend a testing device or method, I will try to do it. Thanks for your help.

Nick

P.S. I looked at the schematic, and was able to follow the path to and from IC 20. Some of what you said made some sense to me.

I have also attached the entire Service Manual with all schematics in PDF format.

[Jazz P Bass]

You can use any device that gives off a signal for troubleshooting.
A CD player, your guitar.
Look up the datasheet for IC 20.
You can use a volt meter set on volts ac (one probe at chassis ground, the other at test point)
With a signal in Aux In you should see the signal coming out of IC 20 output pin.
If not, it may be the fault.

[NickFil]

You can use any device that gives off a signal for troubleshooting.
A CD player, your guitar.
Look up the datasheet for IC 20.
You can use a volt meter set on volts ac (one probe at chassis ground, the other at test point)
With a signal in Aux In you should see the signal coming out of IC 20 output pin.
If not, it may be the fault.

I may be totally wrong in my assumption that, although there may or may not be signal at IC 20, could this be the problem which renders the entire keyboard dead - no lights or display at control panel? Would a faulty circuit at Aux In be the cause for the entire power failure of the Keyboard? Wouldn't the problem be lack of voltage, amperage, etc. at the circuit boards rather than audio signal? Please correct me if I'm wrong, as I don't claim to know much about electronics. Thank you.

Nick

[Jazz P Bass]

Missed the fact that you have nothing on the display or LED's.
There are a number of DC voltages on the power supply.(voltage & a letter in a triangle)
You must verify that each & every one is good.

[NickFil]

Missed the fact that you have nothing on the display or LED's.
There are a number of DC voltages on the power supply.(voltage & a letter in a triangle)
You must verify that each & every one is good.

I located and checked most of the indicated voltages. A few I could not find, such as the +15 A to the left of Q3 and the -15 A to the left of Q4. I also could not locate the +15 A just above and a little to the right of D8 (two resistors in serial). I also could not find the two +5 D voltages at C81 and C94. Could these be the two black capacitors that had no voltage on either leg shown in the photo I have attached? The two wafer (brown) disc capacitors (I think?) shown in the photo have no voltage on either leg also.

I will continue to look for the voltages I could not find. I appreciate your help.

Nick

P.S. In the photo attached, the square IC5 gets quite hot. I don't know if that is any indication of anything.

[NickFil]

I located and checked most of the indicated voltages. A few I could not find, such as the +15 A to the left of Q3 and the -15 A to the left of Q4. I also could not locate the +15 A just above and a little to the right of D8 (two resistors in serial). I also could not find the two +5 D voltages at C81 and C94. Could these be the two black capacitors that had no voltage on either leg shown in the photo I have attached? The two wafer (brown) disc capacitors (I think?) shown in the photo have no voltage on either leg also.

I will continue to look for the voltages I could not find. I appreciate your help.

Nick

P.S. In the photo attached, the square IC5 gets quite hot. I don't know if that is any indication of anything.

I have finally figured out and located all the voltages, and have found that they are as indicated. Do you have any ideas of what is next? Thanks once again!

Nick

[Jazz P Bass]

The 'next' item is to prove that the voltages are reaching the display board.
Looks like +5A & +5B.
The gate array ic is what turns on a few of the led's.(schem Page 24)
I do not know how warm IC5 'should' get.
It is the 'tone generator' IC.
If that is bad, you are out of luck.