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Lynx Power Circuit Upgrade/Repair - Better than 5V Mod

34 replies to this topic

#26 karri OFFLINE  


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Posted Tue Jun 27, 2017 11:51 AM

Thanks for the info.


I knew about this when I ordered the zeners and chose one with identical specs. It seems to provide identical results as well.

My Lynx outputted 4.8 V before the mod and 4.8 V after the mod. So the error is somewhere else.

#27 Level42 OFFLINE  


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Posted Tue Oct 24, 2017 11:57 PM

I've recently been looking at Atari 8 bit computer power supply schematics.

From the ones we have the schematics/can see the inner parts I know the following:

There are two types of PSU's Atari used:

1) dead easy conventional PSU, the kind we learned to design at electronics school in the 80's: transformer-> Diode Bridge->filter cap->7805

2) "better" set up with transformer-> Diode Bridge->filter cap->"advanced" linear power regulator IC that controls a transistor (FET?) through which the current to the computer flows.

There are some types of A8 PSU's that are completely sealed in and is impossible to know what's inside....one of these types is know to go bad (usually known as the Ingot) which i am pretty sure has a number 2) set up

The number 1) set up can provide 1A .....maybe 1,5A if they used an advanced version of the 7805. I have measured (with a calibrated Fluke DVM that a standard Atari 800XL uses about 0,85A while running a game cartridge. That's pretty close to the 1A max. so hopefully they used the 1,5A 7805 which gives enough "head room". Don't forget devices draw a much higher current for a moment when they are powered up.

The number 2) will allow more power (usually 2A, some even more) because the current is not flowing through the power regulator but through the transistor that is controlled by the regulator.

HOWEVER......just like with the Lynxes MOSFET: when the transistor fails it can fail in two ways....it can go open, but usually they short which then makes the full unregulated 10V or so pass through to the computer and at least kill the RAMs in it....maybe more.

Now, from my 10+ years in arcade PCBs work, 30+ years in electronics and from what I've read about 78XX regulators in general, is that when they fail (which isn't all that often) they die a peaceful, meaning that about always they will put out a lower, safe voltage then they should. Most common way they go is that they show their specced voltage when unloaded but "collapse" when there is a load.

So despite being technically a better design, the nr 2) set up is actually more risky.

Why tell you all this ? Well...despite all it's flaws mentioned above (inefficient, drains batteries even when powered off), the 7805 solution is maybe the safest one to go with..........

Edited by Level42, Tue Oct 24, 2017 11:59 PM.

#28 tzok OFFLINE  


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Posted Thu Nov 9, 2017 8:42 AM

I've recently get into my hands a dead Lynx II. After reading a few topics here, on Atari Age, I've decided to try to bypass Q12, as suggested in many threads. Authors of these posts suggested that this is a safe way to check the console. So I did it and only a backlight and power LED turned on... then i"ve decided to find and study a schematic diagram of the console. After doing that I've realized that bypassing the Q12 was a huge mistake, that probably definitely killed my console. On the schematic there is a 50 Ohm resistor between earphone socket ground and the main ground - but that is not true in my console... why it is so important. Many people tell here that Q12 is just a power switch and has nothing to do with a voltage regulator - this is completely wrong!!! Q12 together with a CD4069 are crucial parts of the voltage regulating circuit. This regulator also acts as a power switch, but in normal operation most of the voltage drop is done on the Q12. D13 and R74 are just a voltage reference source for this regulator, and D13 together with Q8 are kind of an over-voltage protection circuit. However if you short earphone socket ground terminal with negative battery terminal you effectively short Q12 source and drain, feeding full 9V (actually even 13V as original power adapter is unregulated) onto the over-voltage protection circuit. Some voltage will be lost on inductors (L15 and L12/L20) and on the wire you use, but if you use a thick wire, or even worse - directly bypass Q12, by shorting its D ans S pins, the protecting circuit may not withstand the current and break resulting in completely burning all circuits inside the console.


So, if you ever decide to bypass the Q12 MOSFET in your LYNX II, make sure you use 5V regulated supply instead of the original 9V unregulated one, and have no batteries in the console, or else you are risking a complete and irreversible damage to your console.


If you wish to protect your LYNX for the future, don't go into any 5V mods, just add a 5.2V transil diode (e.g. PESD5Z5.0.115) in parallel with C41.

Edited by tzok, Thu Nov 9, 2017 8:56 AM.

#29 rockpin OFFLINE  


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Posted Thu Dec 21, 2017 12:29 PM

I wanted to replace the zener diode on a couple lynxes, but I ran into a problem. Cannot tell which side of the Lynx diode is the cathode. I've got a diode with 3 color bands (green, white and brown/gold). Any ideas? Thanks!

#30 haightc OFFLINE  


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Posted Thu Dec 21, 2017 3:21 PM

I as I recall it's on the left, but you can check this easily enough using a multi-meter. 

If you have already remove the diode you can just check continuity between the pad and the testpoint.   I was to say TP15, but check the power diagram on the forum to make sure.

#31 tzok OFFLINE  


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Posted Thu Dec 21, 2017 3:23 PM

Not every 4V3 Zener will fit. Band (cathode) goes towards TP19 / Q8 Base.

#32 cowdog360 OFFLINE  



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Posted Wed Aug 8, 2018 4:39 PM

Interesting, I like the crowbar diode across C41. Will have to expriment with that one.  And yes, Q11/12 is pretty critical.  Looks like the older one I was sugesting is gone now.  I've starting using AOD514's 

The nice thing about this MOSFET is that the RDS(on) is lower, and the on resistance is much lower (about 46X!), meaning less heat, lower battery usage, more efficient!

#33 displaced OFFLINE  


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Posted Wed Sep 12, 2018 4:04 AM



The info here was vital to help me bring my Lynx 1's board voltage down from ~7.8V to a more normal 5.1V so I could fit a McWill screen.


I ended up using the original model zener diode from mouser.co.uk (although shipping from the US was not exactly cheap, since there was no UK stock).


I couldn't find any of the mentioned MOSFETs -- they were all out-of-production.  However, the Farnell website suggested a substitute, which I fitted and it's working fine:


IRLR024NPBF -  MOSFET Transistor, N Channel, 17 A, 55 V, 65 mohm, 10 V, 2 V


(although even this part is available only until stock is exhausted)

#34 Bagpuss OFFLINE  


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Posted Fri Mar 15, 2019 3:10 PM

Just been working on my Lynx 2 prior to installing a McWill screen, and I've come across a bit of an issue with the power supply. 


I've recapped the whole board, and also replaced the MOSFET and diode. 

I used the IRLR024NPBF mentioned in the post by displaced above, as I couldn't source the original in the UK easily. 

For the diode, then I used: https://uk.rs-online...diodes/5443569/

which RS insisted was a drop in replacement for the original diode. 


However, when I now measure the board voltage between the GND on the removed transformer for the screen, and the +ve leg of C23, I'm only seeing 3.9V. 

My initial thought is that the diode isn't quite right. However, I'm wondering if there might have been a PSU issue with the original components, as the screen had stopped working (backlight did work). 


Would really appreciate some assistance in further troubleshooting. 





#35 Bagpuss OFFLINE  


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Posted Sat Mar 16, 2019 6:44 AM

Have worked it out. Was measuring at the wrong point on the board. User error. Doh!

Getting 4.89V where it matters, so all good. Looks like the rectifier and diode combo were good, after all. 

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