Digital piano volume control... how hard can it be?

[jarl]

I have been looking for a digital piano for a while, and last week the opportunity finally showed up on craigslist: a Technics PX7 which "only" had a problem with the volume. The piano produces a very light sound, and doesn't change when moving the volume control. "How hard can it be ?", I thought... Well... Let's say I just remembered how I hated analog electronics back in the day

One of the images shows the guts of this thing. The volume control is on the "control-L" board. Next to the linear pot controlling the volume, there's a M5218L op-amp. When I touch some of the op-amp pins with my oscilloscope's probe there's a loud click from the speakers, but when I move the slider there isn't any obvious change on voltages on any pin. Also, when pressing the keys, there doesn't seem to be any audio signal present on any pin. My conclusion -for what is worth- is that this op-amp is being used only to control the gain of the main amp -not sure it's common practice, though-. There are two more PCBs I have no cue about their function, for a grand total of 10 PCBs on a single device.

The diagram show the configuration of the parts related to the slider (from observing the PCB). However, the cables coming out from this board go to the "NR" board, from there to the "main" board, and somehow (not sure yet how) to the "amp" board.

The amp board contains, among other things: one Toshiba TA7273P audio amplifier, one 7815 and one Panasonic B941 (the three on the same heatsink), and two Panasonic B945 on a separate heatsink.

I hope this is enough information for someone among you to be able to make a guess of what may be wrong. Among other things, I would love to understand the idea behind the circuit. Thanks!

[jarl]

Re: Digital piano volume control... how hard can it be?

What $2000 could get you 25 years ago...

[jarl]

Re: Digital piano volume control... how hard can it be?

More information:
- Connected the keyboard's "line out" to an external amp -> No sound on the external amp
- Connected an MP3 player to the "line in" on the keyboard -> Very loud sound through the keyboard's speakers, no volume control (I'm not sure the main volume control should control this, though)

So... main amplifier seems to be fine, something else is fried?

[jarl]

Re: Digital piano volume control... how hard can it be?

Uhm... the problem has been identified, but it doesn't make much sense...

Somehow, the pins connecting to both swipes were "open", meaning there wasn't any electrical connection to the swipe tracks. I don't know if it was due to a manufacturing problem or what, but I have never heard of both swipes on a potentiometer going 100% disconnected without any obvious mechanical damage. The other four terminals (connected to the resistive track) were fine.

In any case, that was the issue. I used conductive paint -the kind used to fix the rear window defogger in cars- to "connect" the tracks to the pins, and the thing seems to be working, at least for now. I still need to replace the potentiometer for a new one, though, but it's proving hard to find. Anyone around here has -or knows where to find- a dual 5K slide potentiometer, audio taper, with a 30mm throw?

BTW: I'm still interested in understanding how they use the 5218 to control the volume on the main amp. If someone has the schematics of one of this things (or better yet, the service manual) it would be great.

[Betz7of11]

Re: Digital piano volume control... how hard can it be?

Jarl, I have the same problem with my new (old) techniques, I have the blue prints but still not sure what you did with the connector paint and on what pins. I have the Exact problem with mine. Thanks irene

[jarl]

Re: Digital piano volume control... how hard can it be?

Hello Irene,
It has been a while, but if I remember correctly the potentiometer had one pair of conductive "rails" per channel. For each channel, one of them is a variable resistor, and has one pin connected to each end. The other rail should be connected to just one pin, but there was no continuity. I know Iused conductive paint to retouch the rail, but I can't remember if there was a broken path somewhere I could see.
If are interested in playing the piano and don't care much about cosmetic issues, you may be able to replace the potentiometer with a rotary one.
I gave the piano to a friend and he in turn gave him to a friend of him, so I don't know if the pot failed again.