Lynx II Power Circuit issue

[EvilDragon17]

Hi!

I'm having a small issue with the Lynx II Power circuit (there are tons of threads about it, but these didn't help me yet...)

 

Okay, I received a Lynx II that didn't work at all. So I gave it a full-refresh with new capacitors and also the 5V Recom Mod (which I did on multiple Lynx and Lynx II as well, so I'm fairly sure the issue doesn't have anything to do with that mod).

 

So, what's not working?

Pressing Power ON does not work.

 

What have I done to figure out the culprit?

Well, of course, the first thing I usually check in this case is the membrane and any broken traces.

The membrane is fine (and works in another Lynx II) and both Pin 6 and 7 go to TP17 and TP18.

Manually connecting TP17 and TP18 also does not power on the Lynx II.

Replacing Q8 did not fix that. Bridging the pins to check if a cartridge is inserted also doesn't help.

 

What DOES work?

I found in a thread on these boards, that connecting Pin 14 and 11 of U6 should also start up the Lynx II.

And indeed, that does work! The Lynx II starts and runs without any issues - all buttons work and powering it off does work as well.

 

So, to recap...

The power button circuit does not work. Connecting TP17 and TP18 does not start the Lynx II.

Connecting Pin 11 and 14 of U6 starts the Lynx II without any issues.

 

It CAN'T be a lot of parts now that could cause this behaviour, I guess. But I don't know which ones this could be.

The Zener diode (and most of the other diodes as well as some resistor and Q7) don't even exist anymore because the'yre not needed when you have the Recom Mod.

 

Is there anything besides U6 that could cause that issue?

 

Other idea: Could there be a workaround? Seeing that bridging Pin 11 and Pin 14 makes the unit boot, maybe I can disconnect the On-Switch from the normal connections and directly connect them to Pin 14 and 11?

That would only be a worst-case fix though, I'd rather fix what's broken, but I can't find the culprit.

 

Thanks for any help here

 

[+Stephen Moss]

On 2/25/2023 at 2:26 PM, EvilDragon17 said:

Is there anything besides U6 that could cause that issue?

As you say shorting pin 11 to the supply pins switched the Lynx on and off it would seem unlikely that U6 is e issue as the Latch comprised of U6E, U6F and associated components appears to be operating correctly.

 

If, as you sat the flex circuit is fine then I would be looking towards either an open circuit between the flex circuit and U6

On 2/25/2023 at 2:26 PM, EvilDragon17 said:

The membrane is fine (and works in another Lynx II) and both Pin 6 and 7 go to TP17 and TP18.

Manually connecting TP17 and TP18 also does not power on the Lynx II.

but did you check the connection from TP18 to Pin 13 of U6 and R69?

 

or the power on button itself.

It is possible that either it had become misaligned with the pads on the flex circuit during re-assembly or there is usually a conductive pad on those types of button (not checked it present on the lynx myself) make the circuit when pressed and that may either be worn or missing. You tried the flex circuit in another lynx but perhaps you should try swapping power button from the bad lynx in the good one and visa-versa to see what effect that has.

[EvilDragon17]

On 2/27/2023 at 10:57 AM, Stephen Moss said:

As you say shorting pin 11 to the supply pins switched the Lynx on and off it would seem unlikely that U6 is e issue as the Latch comprised of U6E, U6F and associated components appears to be operating correctly.

First of all, thanks for your help

I was just wondering, as TP18 is connected to Pin 13, not to Pin 11, so something might be broken inside U6.

 

On 2/27/2023 at 10:57 AM, Stephen Moss said:

If, as you sat the flex circuit is fine then I would be looking towards either an open circuit between the flex circuit and U6

but did you check the connection from TP18 to Pin 13 of U6 and R69?

Yes, they're all find and R69 is 100 Ohm, so all good there.

 

On 2/27/2023 at 10:57 AM, Stephen Moss said:

or the power on button itself.

It is possible that either it had become misaligned with the pads on the flex circuit during re-assembly or there is usually a conductive pad on those types of button (not checked it present on the lynx myself) make the circuit when pressed and that may either be worn or missing. You tried the flex circuit in another lynx but perhaps you should try swapping power button from the bad lynx in the good one and visa-versa to see what effect that has.

Well, it can't be the power button as the system doesn't switch on when I connect to TP18 and TP17, which should basically simulate a press of the ON button without the flex cable involved :/

[+Stephen Moss]

21 hours ago, EvilDragon17 said:

I was just wondering, as TP18 is connected to Pin 13, not to Pin 11, so something might be broken inside U6.

If connecting pin 11 to pin 14 turn the Lynx on and it stay on when you disconnect them then I cannot see how as taking pin 11high sets pin 10 low, which in turn should take pin 13 low, making pin 12 high which drives pin 11 high and it latches in the on states on as mentioned on one of the other posts that mentions it (probably one of mine).

So if there was a problem anywhere between pin 10 and pin 11, C38 should pin 11 low thereby turning the Lynx off, it is possible that an either pin 13 or 11 input is floating but still being pulled high/low capacitively rather than actively driven (open circuit track/bad solder joint).

Thus, it is not impossible there is a problem with U6 so you could replace it and see if it makes a difference but for me personally there is more evidence to indicate that it is functioning correctly than it being faulty.

 

22 hours ago, EvilDragon17 said:

Yes, they're all find and R69 is 100 Ohm, so all good there.

 I presume that was a typo and should have read 100K, otherwise something is wrong there which is likely to be a fault with U6 as failing resistors tend to go open circuit/high resistance.

 

22 hours ago, EvilDragon17 said:

Well, it can't be the power button as the system doesn't switch on when I connect to TP18 and TP17, which should basically simulate a press of the ON button without the flex cable involved 😕

 

I must admit that I have not actually had time to trace the connection from the power buttons through, but I really should at some point as the schematic seemed not quite complete to me, even before I noticed it is clearly indicated as page 1 of 2.

I know it does give the impression that Power Com and Power On should be shorted but I am not sure. I assumed that one power button would connected 9V to Power Com and the other 0V to Power Com to switch the Lynx on/off and that Power On was connected with the power LED as it is not shown on the schematic.

 

And logically, unless I am probably missing something obvious it makes no sense to me that Power On & Power Com should be connect for switching on and off as...

1. Initially Pin 13 should be high so if Power On and Power Com were connected by the On button Pin 13 would initial be pulled low by a discharged C37 causing the latch state to switch and the Lynx to power on, all seems good so far but...
2. Then the only way to flip the latch state back to Off would be put a high in Pin 13, but how would that come from C37? I would expect it to discharge through R68 and so not be able to supply a high voltage to pin 13 no longer than 0.5 seconds after power on. However, if that is how it is meant to work then you should really only need the one On/Off power button as it make no sense to use two to short the same two points together, plus the issue could then be with C37, if either it or a connection to it was open circuit it would not be able to pull Pin13 down to turn the Lynx on or the high to turn it off. 

[EvilDragon17]

Thanks for your help, I actually figured out what the problem was. It wasn't a failing part, it was a missing trace....

I'll still give some information as it might be helpful for other

 

The pins for the power On button on the flex cable connector are Pins 6 and 7.
These are directly connected without any other part inbetween to TP17 and TP18. 

So connecting TP17 and TP18 effectively simulates pressing the Power ON button. That's a good way to rule out the flex cable.

 

Another helpful hint was:

"ON Button Pressed = shorts ribbon pin 6/TP17 to ribbon pin 7/TP18 (about 100 Ohm)"
"OFF Button Pressed = shorts ribbon pin 7/TP18 to ribbon pin 14"

 

As pressing OFF did work in my case, we could basically rule out an issue with TP18 / Pin7.

Which narrows it down to TP17.

 

We know TP17 is connected directly to the Power ON button - but where else should it be connected?

 

According to the schematics I found here, TP17 should be connected to R68 and C34.

And that's what lead me the wrong way: I didn't find any C34 on my Lynx II mainboard. 

 

However, you did mention the issue could be C37 - and then I took a look at the mainboard and the schematics linked above. And in fact, it looks like what they call C34 should really be C37.

The capacity is correct and it's also connected to ground and R68. So it SHOULD be connected to TP17 as well, if the schematics are correct.

 

Well, it wasn't on my mainboard. So I did add a small wire from TP17 to C37 - and that made the power on button work again.

 

I haven't followed the trace, so I don't know where it broke - but for some reason, it did - and that simple wire fixed the unit.

 

So thanks for pointing me in the correct direction, another Lynx II is now fully restaurated and ready to be played

 

On 3/2/2023 at 11:03 AM, Stephen Moss said:

 I presume that was a typo and should have read 100K, otherwise something is wrong there which is likely to be a fault with U6 as failing resistors tend to go open circuit/high resistance.

Ah, yes, that was a typo, I missed the K on my multimeter. It is in fact 100K.

 

 

[+Stephen Moss]

12 hours ago, EvilDragon17 said:

However, you did mention the issue could be C37 - and then I took a look at the mainboard and the schematics linked above. And in fact, it looks like what they call C34 should really be C37.

Some of the component reference differ between the Lynx 1 and Lynx 2, in the Lynx 1 schematic available from here it looks like C34 & R2, but in the Lynx 2 schematic I have they are C37 & R68 respectively. Whereas the schematic you linked to appear to be using the Lynx 1 Capacitor reference and the Lynx 2 Resistor reference.

[retrosix]

You should be able to easily diagnos the issue now using the diagnostic steps here https://www.retrosix.wiki/power-circuit-atari-lynx-ii#HJuwv

 

Start from top and go down doing each test