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

[+karri]

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.

[Level42]

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 October 25, 2017 by Level42

[tzok]

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 November 9, 2017 by tzok

[rockpin]

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!

[haightc]

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.

[tzok]

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

[cowdog360]

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
https://www.digikey.com/product-detail/en/alpha-omega-semiconductor-inc/AOD514/785-1357-1-ND/3060919

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!

[displaced]

Hi,

 

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)

[Bagpuss]

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.com/web/p/zener-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.

 

Thanks,

 

Andy.

[Bagpuss]

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.

[rmzalbar]

I have a question about the power circuit. Looking at the schematic, it looks like:

 

ZD13 zener diode fails short: ~0.7 volts, switched, between the rails.

ZD13 zener diode fails open: 9v, unswitched, between the rails

Q12 mosfet fails short: 9v unswitched

Q12 mosfet fails open: 0v unswitched

 

Is that correct? I'm trying to estimate the chances of fail-deadly. I'm thinking of leaving the whole thing alone, but adding crowbar overvolt protection, if necessary. I looked at doing a 5v regulator mod, but those have a parasitic power loss when the console is off, can still fail deadly, and I intend to stay with 9v batteries.

[tzok]

More or less correct. As I said before - original voltage regulating circuit is ok (actually this is a switch mode power supply - SMPS), if you want to add additional protection just put a 5.2V transil in parallel to the output of this circuit, and preferably also a fuse somewhere before the transil or the whole regulator.

Edited April 26, 2019 by tzok

[Rickee]

Hi, does anyone have stock of the 1N5991B zener diode and the correct mosfet? I cant find stocks in australia and console 5 and mowser both end up with postal charges around 30 aud, which seems excessive for a padded envelope.

 

thanks!

[bdoi]

7 hours ago, Rickee said:

Hi, does anyone have stock of the 1N5991B zener diode and the correct mosfet? I cant find stocks in australia and console 5 and mowser both end up with postal charges around 30 aud, which seems excessive for a padded envelope.

 

thanks!

I have some left if you want, PM me

[Ben M]

Hi, I have recently purchased a lynx 1 with Mcwill lcd, vga out and the recom 5v mod. I’m considering returning the power system back to standard. Could someone tell me how complex a task this would be and what parts I would need? Any help would be very much appreciated.

[rj1307]

I would do it for free, but shipping to Poland and back will cost a lot.  
If you can solder SMD components, I can send you the parts you need. All you had to do was take a photo of the motherboard.

[Ben M]

That’s a very kind offer thank you! I’m pretty good at soldering but mostly it’s putting together micro fpv drones or radio control related things so not much experience with smd components. I’d certainly give it a go though, maybe get some practice in first! Did you get the pics I Sent you the other day? 

[rj1307]

19 minutes ago, Ben M said:

Did you get the pics I Sent you the other day? 

I can not find.

[Ben M]

14 hours ago, rj1307 said:

I can not find.

I’ve sent the pics again to your email contact on paypal, let me know if you don’t receive. Thanks for your help with this.

[+-^CrossBow^-]

17 hours ago, rj1307 said:

I can not find.

I also sent you the specifics for this +5 Recom DC-DC power mod in the thread here in the AA forums through PM the other night just in case.

 

[rj1307]

Yes, both messages have arrived.
@ Ben M, tomorrow I will look carefully at the pictures and find out what is missing. I'm going to work a night shift soon.

[Ben M]

On 2/24/2020 at 4:58 PM, rj1307 said:

Yes, both messages have arrived.
@ Ben M, tomorrow I will look carefully at the pictures and find out what is missing. I'm going to work a night shift soon.

I’ve now fitted all the components you kindly supplied except q9 and r73 which are supposed to be removed for the McWill screen. I tried to test for 5v at vcc and ground before refitting the Mcwill screen with game cart in but I’m only getting 0.6v pressing the on button makes no difference. I did notice that d20 on my lynx has been replaced with a solid link but I’m not sure if that would make any difference? Hopefully I haven’t damaged any components with my novice soldering! 

[Ben M]

On 3/14/2020 at 7:28 PM, Ben M said:

I’ve now fitted all the components you kindly supplied except q9 and r73 which are supposed to be removed for the McWill screen. I tried to test for 5v at vcc and ground before refitting the Mcwill screen with game cart in but I’m only getting 0.6v pressing the on button makes no difference. I did notice that d20 on my lynx has been replaced with a solid link but I’m not sure if that would make any difference? Hopefully I haven’t damaged any components with my novice soldering! 

All sorted now! I somehow managed to solder L15 coil in wrong. All back together and working perfectly. Big thanks to Rafał for all his help. 

[sjk]

I want to replace the diode and mosfet in my Lynx I as preventative maintenance, but I'm not able to find them available here in Australia. Does anyone have spares that they could send over? ?

[+remowilliams]

On 6/5/2017 at 6:48 AM, karri said:

The MOSFET's were more expensive 757-TK2P60DTE16L1NQ.

 

I ordered these same MOSFETs (exact part number) from Mouser a couple weeks ago to finally get around to doing the power supply refresh in my two main Lynxes (a Lynx I, and a Lynx  II both with original McWill LCD mods) Both Lynxes are in fine working shape with no issues.

 

I replaced the zeners with 1N5991Bs and then dropped in the MOSFETs,  the Lynx II (first up) seemed to be working fine (quick basic power test) so I moved on to the Lynx I.

 

While leaving the Lynx I on the bench for extended powered on testing for a bit I thought I smelled something odd.   Sure enough the MOSFET was running insanely hot, with a thermocouple MOSFET surface temp was hitting 130C and still climbing.  The board was so hot that the heat from the other side of the case seemed like it might melt the rubber button pad.

 

Going back to do extended power on testing in the Lynx II the MOSFET also seems to be generating the same massive amount of heat, but the board and location is able to sink it immensely better.  Surface MOSFET temps were still running away.

 

Dropping the Lynx I's original MOSFET back in restored normal operation temps.   I ordered the Lynx tune up parts from console5 which I think are these MOSFETs and they are installed (in both Lynxes) and working fine at around room temperature.