Midibox MB6582 flashing LCD backlight ** FIXED **

[Syntax]

Hi all,

Besides my analog input and feedback wiring my MB6582 is operational. Just recently I noticed the LCD backlight starts flashing at random when applying power to the unit. Just wondering if a bad T1 transistor is causing this??

-Frank

Edited May 26, 2010 by Syntax

[clem!]

Hi Frank,

LCD backlight starts flashing at random

if the LCD isn´t used I would first try to use another PSU - some LCD-Backlights need more power than someone suggests.

Best regards,

clem!

Edited January 8, 2010 by clem!

[Syntax]

Hi Frank,

if the LCD isn´t used I would first try to use another PSU - some LCD-Backlights need more power than someone suggests.

Best regards,

clem!

Just have the regular C64 PSU which should work fine I think. I could however unmount some SIDs to keep down the overall power need.

[Syntax]

Did some testing with only one set of SID's. This makes no difference the backlight still keeps flashing. I'm not sure its the power supply as at first the display worked fine. Does somebody has any suggestions for solving this? Last thing to test for me is to replace T1.

[Syntax]

Had some other business lately, but for now I'm want to sove this LCD flickering flashing issue. As suggested before, this could be PSU related. When fully loaded (8 SIDS and LCD) I measuring approx, 4,5 VDC on j3. Is this critical?

The display I'm using is a black/with Displaytech Ltd LCD MODULE 204A SERIES with following specs for the backlight.

For Supply voltage VF (V) Typical 3.4 and Max 3.5

For Supply current IF (mA) Typical 20 and Max 25

Any help in this is highly appreciated

[Wilba]

It sounds like this display is like the ones I have, they only have a single LED on the side, and so they don't need the maximum current supplied by the standard MIDIbox Core backlight circuit (and thus also MB-6582 base PCB), which is around 280mA or more.

I recommend inserting an 82K resistor in series with R4 (the 1K) or replacing it with machine pins (like you probably used for the SID filter capacitors as recommended by the build guide) so you can try different values. This will reduce the maximum current supplied to the LCD backlight. I'm surprised the LED hasn't burned out already.

Perhaps I'm totally wrong. What colour is this display? Does it look like it is uniformly illuminated or is it brighter on one side? If it's a negative white display (or anything other than yellow-green), it's probably edge-lit (single LED). The typical current (25mA) suggests only a single LED.

This doesn't quite explain the flickering though... if it was a current supply issue, i.e. too much current draw by SIDs + LCD, then it would stop flickering when there was only one SID.

What's the J3 measurement with only one SID? 4.5V DC is too low, it should be much closer to 5V (like 4.9V or more), so there might be a current supply issue at play here too, so you can confirm that by measuring J3 voltage with different loads, like one to eight SIDs installed.

It can't hurt to try replacing T1 and checking the solder joints in that area, in case a poor solder joint causes this intermittent flicker... but also, if it's only happening on power on and not later, that's suggesting the load is too much on power on and 5V supply is unstable but then becomes stable later. So you'll have to consider both possibilities (bad solder joints/T1 vs. current supply).

[Syntax]

Perhaps I'm totally wrong. What colour is this display? Does it look like it is uniformly illuminated or is it brighter on one side? If it's a negative white display (or anything other than yellow-green), it's probably edge-lit (single LED). The typical current (25mA) suggests only a single LED.

I'm using a white text on black background LCD. And so far as I can see its edge-lit

What's the J3 measurement with only one SID? 4.5V DC is too low, it should be much closer to 5V (like 4.9V or more), so there might be a current supply issue at play here too, so you can confirm that by measuring J3 voltage with different loads, like one to eight SIDs installed.

I can remember during first testing the voltage was near 5VDC. So probably the PSU can handle the load. I'm having a C64 PSU model 'brick' which should deliver 1.7A on the 5VDC line. On the other hand I used bright LED's. To have an acceptable brightness I had to use 7,5K resistors

It can't hurt to try replacing T1 and checking the solder joints in that area, in case a poor solder joint causes this intermittent flicker... but also, if it's only happening on power on and not later, that's suggesting the load is too much on power on and 5V supply is unstable but then becomes stable later. So you'll have to consider both possibilities (bad solder joints/T1 vs. current supply).

I did replace T1 with no results, so I assume this circuit is OK

If I get my hands on some resistors I start trying different values. I'll keep you informed!. Thanks for now.

[Syntax]

It sounds like this display is like the ones I have, they only have a single LED on the side, and so they don't need the maximum current supplied by the standard MIDIbox Core backlight circuit (and thus also MB-6582 base PCB), which is around 280mA or more.

I recommend inserting an 82K resistor in series with R4 (the 1K) or replacing it with machine pins (like you probably used for the SID filter capacitors as recommended by the build guide) so you can try different values. This will reduce the maximum current supplied to the LCD backlight. I'm surprised the LED hasn't burned out already.

Well I replaced the resistor with a 120K value and the display is perfect. Hopefully the LED did not suffer a lot and will last forever!

The PSU voltage still worries a little. I just start using the thing and see if it will give any problems.

Thx Wilba keep up the good work!

[Wilba]

The good thing about these Displaytech LCDs is that it uses a single 3mm LED. If it ever dies, you can remove the bezel, cut the LED leads and desolder the remainder of them in the pads, twist the LED to break the glue holding it into the clear acrylic diffuser, then replace with another 3mm white LED. Replacing the bezel can be tricky, you need to get correct alignment between the LCD and the PCB via some conductive rubber things. If it's wrong, you get garbage on the display.

I just thought I'd mention it for future reference... some people would just replace the whole LCD, but personally I like to fix things when it's possible.

[Syntax]

The good thing about these Displaytech LCDs is that it uses a single 3mm LED. If it ever dies, you can remove the bezel, cut the LED leads and desolder the remainder of them in the pads, twist the LED to break the glue holding it into the clear acrylic diffuser, then replace with another 3mm white LED. Replacing the bezel can be tricky, you need to get correct alignment between the LCD and the PCB via some conductive rubber things. If it's wrong, you get garbage on the display.

I just thought I'd mention it for future reference... some people would just replace the whole LCD, but personally I like to fix things when it's possible.

Will keep it in mind :)