Roland E-20 LCD contrast

[frisc0]

Hi,

I recently picked up a Roland E-20 for cheap. In great condition, just had to replace 1 tactile switch and now in perfect working condition, however: the LCD screen is rather 'dim'. Only if you look at the lcd screen from a sharp angle, the characters are clearly readable. I could not find a service manual for this synth/keyboard, so not sure if the contrast can be adjusted. There is no knob for this on the device itself. There is a potmeter next to the connector to the lcd board, but I'm not sure if this is connected (could not measure a direct connection from the potmeter to the lcd connector), see attached photo. I do not dare to change the potmeter without knowing for sure what it does.

Does anyone have the service manual available as .pdf? Or know if this potmeter indeed is changing contrast?

Thanks.

[ataradov]

Measure both sides of the resistor. If one is ground and the other is power supply, it is almost guaranteed that it is the contrast pot. And even if not, I don't think there is anything so sensitive that you can't return it back to where it was.

Also, it is close to pin 3, which is the contrast on a standard 14-pin (no backlight) Hitachi interface. Pin 1 would be ground and pin 2 would be VCC. So all pins of the resistor should be connected to pins 1, 2 and 3 of the connector.

EDIT: although it is reversed, and pin 1 is on the other side. But the visible traces to the controller show that it is a standard Hitachi interface, so check what goes to the pin 3 of the connector.

[inse]

As simple as that: mark the wiper position with a sharp pen and then find out whether it has influence on the contrast.
If not, return to marked position.

[Audiorepair]

Schematic attatched.

[inse]

If you are not getting the contrast set satisfyingly, you might consider swapping the LCD module by a STN type.
The HD44780 chip gives a hint that a standardized character matrix display has been used.
In the old days only the gray mode TN LCDs have been available with rather poor contrast ratio.
Nowadays everyone has switched to STN types with high contrast (black/blue on green background).
Even compatible OLEDs may be available if you prefer.

[frisc0]

Thanks all for thinking along and providing this valuable info. Looking at the schematics I think it's safe to assume the pot is for the contrast. I'll mark the position and try it, thanks again!

I want to keep this device original, so no lcd swap if not necessary.

Btw, @Audiorepair: do you have the full service manual (.pdf) for the E-20? Can't seem to find it anywhere...

I was wondering how the CPU board communicated with the synthesizer board (the same board as in the famous MT-32/MT-100! see attached image) but now it's clear it's communicating via the Tx/Rx lines on the 8-pin connector. Probably just TTL MIDI signals since the MT-32 works with only the synthesizer board?

[Renate]

Look at the corrosion/flux on the display board towards the bottom there.
It could be leaking your contrast voltage. Time to clean it.

[frisc0]

It worked! The potmeter was indeed for the lcd contrast. I could turn it up with the result as shown in the attached pictures. Also cleaned the pcb but that did not have any noticeable effect, but never harms...

Thanks all for helping out!

And if anyone has the full service manual for this synth, please upload!

[Audiorepair]

It's not possible to upload large files here.

CK32 board schematic attached.

[frisc0]

Thanks AudioRepair! However, this seems to be another board than what is in my E-20, which actually says MT-32/MT-100 instead of CK-32 and has different connections (only 1 connector from the CPU board including power lines), so maybe a different version of the E-20?

I would love to receive the full service manual for the E-20, maybe you can upload it to the synfo.nl website? They host a lot of service manuals already and would gladly receive this one as well. Hope that works, thanks in advance!

From synfo.nl:

Thanks to all who are helping to make this site grow,
including those who contributed hi-q scans of their own.
Quality scans of pre-2000 electronic gear are welcome at info@synfo.nl