Lynx II & BennVenn LCD - Audio & Garbled Video

[DocSchlock]

I have a Lynx II that outputs garbled video but the audio is OK. Thinking it was the original LCD, I bought a BennVenn Rev6 and am getting the same garbled video with it.  It would appear this is not an LCD problem but a video signal problem and am wondering if anyone has suggestions on what else I could check.

 

• The motherboard is labeled as C104342-001 REV .1 [3]
• The capacitors, power mosfet and diode have already been replaced.
• Thinking it could be a bad ribbon cable socket, I direct wired the BennVenn but got the same garbled video result

 

 

[oski_98]

Do you have an oscilloscope or logic probe? You can test the pins and compare them to the measurements I posted here.

[+-^CrossBow^-]

Did you remove the RF shielding or did it come that way?

 

[DocSchlock]

-^CrossBow^-: I removed the RF shielding.

 

I have access to an oscilloscope but am no expert. Here are measurements for the pins that the BennVenn LCD uses:

 

VCC - 6.22V DC
RES - 6.11V (avg), 59.52 Hz
CLKA1 - 4.6V (avg), 6.4vpp, 2.083 MHz
CLKA2 - 4.6V (avg), 6.5vpp, 2.083 MHz
CLKA3 - 4.6V (avg), 6.6vpp, 2.083 MHz
CL2 - 188mv (avg) 5.9vpp, 3.145 Khz
DL0, DL1, DL2, DL3 - Logic signal (hard to capture)

Measurements were taken between the LCD ground pin (pin 26) and the corresponding pins above.

 

Is the measured voltage of 6.22V too high for the 5V pin? Not sure what would cause that or if that's indicative of another issue.

5V Pin

 

CLKA1

 

CLKA2

 

CLKA3

 

CL2

 

RESET

Edited February 27, 2022 by DocSchlock
Updated Image format

[BennVenn]

6.22 is too high, its bordering damage to Mike/Suzy/Dram if it hasn't already.

 

Your CLKA1,2,3, RES signals look good. CL2 looks normal but the amplitude of them doesn't - could be an artifact from your scope though or the higher than normal voltages being clipped somewhere. 

 

If that LCD kit of ours has the zener installed, it will definitely be shunting some current which will make a mess of some of the waveforms. I suspect it isn't fitted though if you're seeing 6.5v.

 

I'd look into your 5v issues, or power it from usb during testing. The DRAM is pretty sensitive to over voltage but you wouldn't be getting game audio if that was cooked so probably not that.

 

The white band at the bottom of my lcd indicates my board is getting all its signals, and the timings are OK. Fix the 5v and hope that Suzy is ok

[oski_98]

As @BennVenn suggested I would recommend supplying 5V directly to C41. After you sort out the LCD issues, consider either replacing Q12, Q7, Q8, and D13 (there are kits available for this) or buy a USB port, or breakout board, and consider always supplying power through USB.