Is this the correct screen for a C104129-001 REV 2?

[oski_98]

I recently bought a rather odd Lynx II console from eBay for repair. Here's what happens when I supply 5V to C41 with a game plugged in:

• Screen is blank/dark
• Backlight is on
• Power led is solid (it doesn't flicker the way a healthy lynx does on startup)
• Audible click (sound engaging?)
• No smoke
• No overheating
• Brightness adjust has no effect
• Backlight button has no effect
• Reset has no effect
• Volume adjust has no effect

 

The board revision is " C104129-001 REV. 2". I researched this board a bit online and I have a suspicion that the behavior I am observing might be what happens when an incompatible screen from another Lynx II revision is plugged into the board. I believe someone tried to repair this board and tried to swap out the original screen with another Lynx II screen that is not compatible. Unfortunately, I don't know of a way to tell if the screen is the right one for the board. I've attached some pictures and would appreciate it if someone could help me figure out if this is the right screen for the board?

 

 

The reason why I think someone tried to repair this board:

• The rubber cleats on the back were unusually easy to peel off (usually I have to loosen them up with a hair dryer).
• The "okay" inscribed on the back of the screen in sharpie.
• There was a lot of flux residue on various parts of the board that I had to remove with IPA. Especially around the cartridge port pins, it looks like someone removed the port then added it back in (they did an OK job they just didn't bother to clean the flux away). You can see this by comparing the first (dirty) and second (clean) pictures I took of the board.

 

Anyway, I would appreciate any thoughts or ideas on how to troubleshoot this board. I think probably my next move will be to test the ICs with a logic probe. Or perhaps should I try plugging in this screen to a different revision working Lynx II?

 

Thanks in advance...

Edited December 6, 2021 by oski_98

[+karri]

even with no screen at all you should get the music if the game boots at all.

 

If it does not boot the error is deeper.

[MichelS]

Flux residue seems normal - my PAG0300 looks exactly the same. Guess the SMD parts were soldered automatically by pick and place and through-hole parts were soldered manually afterwards.

My screen assembly also has the same number on the back side - C104159. But this may also be the part number of the backlight only.

[oski_98]

Thank you for the helpful responses. I've measured some voltages on various test points and would appreciate your thoughts. First, my setup:

 

* 5V to C41

* I have a game in the ROM cart port

* I have only the speaker attached (based on input from karri that I should be able to get sound even with no screen)

 

Here are the voltages measurements using TP16 as GND.

* TP1: 0.4V

* TP12: 7.8 V

* TP15: 4.8 V

* TP19: 3.7 V

* TP21: 7.2 V

* TP22: 1.3 V

* TP24: 1.0 V

* TP25: 1.8 V

*  TP 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 17, 20, 26, 27: 0V

* Mikey pin 18: 4.7 V

* Mikey pin 57: 0.4 V

* Suzy pin 4: 0.4 V

 

Another interesting observation is that when I would test TP1 or Mikey 57 (reset) I would hear a very low but audible click when the probe touched the pad. I wanted to test more of the Mikey/Suzy pins but the pin numbers on the schematic are too fuzzy for me to read. Is there a resource with the pins clearly labeled? Are the pins the same as the Hayato chip?

 

I would appreciate any feedback on these measurements. Does anything look off? Based on some other threads I've reached out seems like TP1 should be at a higher voltage?

[oski_98]

I managed to find a schematic that I can read and realized that there is a power on reset circuit affecting TP1. Briefly, this circuit exists to delay when the IC is engaged. When power is supplied, an RC circuit charges a capacitor, and when the capacitor is charged 5V are supplied to the reset pin. When power is disconnected the capacitor discharges through the diode. Unfortunately, on my Lynx TP1 never reached a voltage higher that .4 V so what is going on?

 

I tested the 100K resistor and it measured 30K. This seems low but that wouldn't affect the capacitor charging. Voltage at the junction of the diode cathode and resistor is 4.9 V (expected), but .4 V at the junction of the diode anode, resistor, 10uF capacitor, and TP1. I don't think there's anything else affecting this circuit so either the capacitor isn't charging or there's something terribly wrong with the IC. Thoughts?

[oski_98]

I removed the 10uF capacitor, discharged it, tested it, and it read 10.60uF. I replaced it anyway on the circuit, and the voltage at TP1 still reads 0.4V. So the capacitor was not the issue, and I'm still pretty sure the voltage at this pin should not be at .4V, but I don't have an easy way to confirm this since my other Lynx II has an RF shield covering that test point. If anyone else can provide their voltage reading for TP1 that would help. There's another post around this forum that says it should be around 6V but that seems too high?

 

So I'm a bit stuck for now. On the bright side, I think I finally have an excuse to invest in an oscilloscope. I'll keep posting notes here in case it ends up helping somebody else.

[oski_98]

I did discover that the D10 diode on the board is bad. However, IIUC the schematic, this diode protects the circuit if someone hooks up batteries with the polarity reversed? It shouldn't affect anything when delivering 5V to C41? Did I get that right? Would be nice to get some confirmation on that. Or just some encouragement

 

Still waiting on the oscope, but I had some time to read more posts on this forum. Seems like this could be corrupt RAM chips?

[oski_98]

I was able to do some initial testing on the board with an oscilloscope. Here's what I'm seeing:

• The clock circuit seems to be functioning fine (picture below)
• On Mikey, I don't see any waveforms other the clock on XTAL0. The level on the pins are either held high or low.
  • A0-A7 are 0, 0, 0, 1, 0, 0, 0, 1
  • D0-D7 are 0, 0, 1, 1, 1, 1, 1, 1
  • WE=1, RAS=1, CAS=1, ROW=0, COL=1
  • ROM=1, CLK1=1, CLK2=0, CLK3=1, SYNC=0, NOEXP=1, TXD=1, RXD=1, PM2D=1, NMI=1
  • RESET=1
• On Suzy, the address lines are the same as Mikey, CART=1, RESET=1 (I'm eliding the other pins for brevity)

I found this post from @rj1307 on the forum, which suggests that no waveforms on any of the signal buses (I'm assuming signal buses == address and data lines) means the processor is damaged (Mikey in my case). I would really appreciate it if someone can confirm this. I don't know whether it's normal for the processor to also show no waveforms if the DRAM is bad. I guess I could do some research on what is normal behavior for 6502 chips in general (even though Mikey isn't exactly a 6502 chip maybe the behavior is similar).

 

[oski_98]

It's alive! For anyone that's been following either this thread (probably nobody at this point:) or my related thread on the Mikey tester. I was finally able to fix this Lynx. Ultimately, the issue was likely Suzy. But I'm going to capture what I've learned here for posterity. Maybe this never helps anybody, but you never know.

 

First let me correct a few things on this thread:

• Despite what I posted earlier in this thread here, the voltage for TP1 was never 0.4V. When I first started taking voltage readings with my F-117 digital multimeter I had it set on "AUTO-V LoZ". Briefly, this is a low impedance setting that is not appropriate for testing electronics. Don't make this mistake!
• There was also nothing wrong with D10 on this circuit. As @Stephen Moss, explains here, this diode is simply bypassed when the power jack is not plugged in. So if you plan on testing this diode, first plug in the power jack, but don't plug the adapter into the wall. I tested it again like this and it worked.
• Anyway, there's probably no point in testing the entire power circuit if you're supplying a 5VDC directly across capacitor C41 for the Lynx II (it's different for the Lynx I) and you're getting a stable 4.5-5V at TP1. Doing this effectively bypasses the entire 12V to 5V power circuit. And actually, maybe this is the way we should wire the Lynx to run in the age of abundant USB chargers.

Now on to how I fixed this Lynx. As noted in my related thread, I pulled Mikey out and built a test circuit that provided some pretty good evidence that the chip was working fine, and Suzy looked like the more likely culprit. So I replaced Suzy on the board and put the original Mikey back on. Nothing changed

 

So then I started testing other parts of the board, focusing on the components right of the power circuit, and especially on the parts of the circuit wired to the Mikey pins I had probed in my tester circuit. I found a cold solder joint at L10, but after fixing that I was still getting no joy from this Lynx II.

 

Finally, I started to doubt my previous testing of the original Mikey chip and replaced the original Mikey with a new one from Best Electronics. All my other boards are Hayato so I figured I might as well. Now here's where the learning happens, because I am very new to working with SMD components. Removing components with the hot-air rework station was easy enough, but I was having a lot of trouble putting things back on. So I started inspecting the solder joints with a gemologist loupe I happened to have and noticed that some pins weren't properly soldered to the pad. Through painstaking trial and error I had to fix this, and I also started testing every connection to the pad with a continuity tester by putting one lead at the top of each pin and another on the pad, then moving the lead on the pin to the next pin, then moving the lead on the pad on the next pad, and so on to test the connection to the pad, and to ensure that there were no accidental solder joints between pads. After spending several hours ensuring everything was fine I fired the lynx back up and still no joy But the pain and suffering of fixing the soldering joints on the new Mikey chip gave me some doubt that maybe the joints on the Suzy chip I replaced had issues as well. So I looked at those with the loupe, tested them with the continuity tester, and indeed I found some bad joints. So I fixed those then plugged in the Lynx and it came alive

 

So at this point I feel the weight of the evidence points to the Suzy chip being bad, but since I replaced the Mikey chip I can't be 100% sure. What I can say with 100% certainty though is that a Mikey chip that is in good working order can seem dead on the address and data bus lines if it's not hooked up to Suzy properly. However, it seems like you will always see waveforms on XTAL0, X16M, AUDOUT HI and AUDOUT LO from a working Mikey chip. What I'm curious about now is whether a legitimately toasted Mikey would ever show waveforms at all on those specific pins (I would guess no because chips fail catastrophically), because that would be an high confidence test for whether Mikey is working when troubleshooting the Lynx. If anyone has any dead Mikeys they want me to test I'd be happy to do that to answer the question.

 

Finally, I guess I need to figure out a better way to solder PLCC SMD chips. I'm thinking I made a mistake when I didn't use solder wick to remove all of the solder from the pads after removing the components that were originally there. Perhaps spreading some flux over the pins then using the hot-air station to heat up the components would have re-seated the components properly? I would appreciate any feedback from folk that have more experience in this area. Thanks again.

[+karri]

Interesting read. And congrats for fixing your Lynx!

[oski_98]

I realized I never posted a happy ending picture to this thread, so here it is.

 

And a peek at the guts after replacing caps, rebuilt power circuit, BennVenn LCD, and added USB port.

 

I also replaced the speaker and polished the screen to remove some minor scratches. I don't have pictures after that work but here's the new speaker that worked out quite well.

 

 

And that's all for now!