BennVenn Screen trouble then smoke after power component upgrade!

[Rossoe]

So my poor MkII has had some troubles - and I'm fearing I've damaged something serious! I need some help confirming what I should try next.

 

First the unit was sent for recapping - came back and screen faded immediately on boot up, 

Thought it was screen issue, so bought a BennVenn replacement, along with new mosfet (Q12) installed those, but screen still faded as soon it turned on.

Inspecting board I noticed 3 caps placed with incorrect polarity! so I changed them out myself for new one's. 

Screen still did the same however I could see slightly more this time, but as soon as I turned brightness wheel it faded to black - much like video below of a user with similar issue.

 

I also noticed another user with same issue:

I decided to replace all the power components as highlighted below, after changing the diode, transistors and resister nothing changed, after switching the mosfet I got a garbled screen for a second and some crackling noise from speaker when powering on! I switched off, then re-seated cart and powered on again and noticed a tiny bit of smoke - I did not see where it was from, so immediately switched off and now I'm nervous what to do with it! I've had a look at all components and can't see any burnt area's, I have not opened the metal plate over the main chips. My main worry is if I've blown Mikey and Suzy.

I'm not sure how to check Diode in circuit but using Diode mode on multimeter - positive on Anode shows 0.26v, whilst positive on cathode shows 0.649v

 

 

[Rossoe]

Ah damn it - I just checked the power adapter and it's reading 14.5v at 6.8a using multimeter!  I'm assuming that's what might have fried something on my Lynx?

Any chance the zener diode may have offered some protection? and is there a risk that the BennVenn is toast as well? I opted for the diode to be installed for power circuit protection on that as well.

 

[EmOneGarand]

You seem be missing a part if that second image is current:

I believe its an Inductor part of the backlight circuit

Edited September 18, 2022 by EmOneGarand

[Rossoe]

Ah yes, that's a part that is required to be removed as part of the BennVenn screen install.

[+Stephen Moss]

11 hours ago, Rossoe said:

Ah damn it - I just checked the power adapter and it's reading 14.5v at 6.8a using multimeter!

If 6.8a means 6.8 Amps I don;t believe you could pull that much current out of it, and even if you could you would only read a couple of volts out if that, so I am not entrily sure how you measured that.

As for the voltage it is an unregulated power supply, so it is not unusualy to read an output voltage of around 12-14V when the lynx is off. Only when the Lynx is on and drawing the correct amount of current will the output voltage drop down to 9V.

14 hours ago, Rossoe said:

I've had a look at all components and can't see any burnt area's

Try smelling the PCB, what ever component failed may still smell burnt, if you do smell something it will only give an approximate area of the fault but at least it will narrow down what may be damaged and a closer look with a magnifying glass may revea more that with the naked eye.

 

As for what to try a common approach is to use a 5V source connected across C41. Perferable a source with a settable current limit would be used, although you couls use something like a USB  charger, just make sure it is one you don't want in case the fault with the Lynx damages it. That bypasses the internal power circuitry, so it then then works the issues lies with the power supply circuitry, if not the issue lies elsewhere.

 

But before you do that I would remove D13 and check it out of circuit as it may just be the image (some sort of reflection) but it looks like there may be a crack or something (silver band) across the middle of the body which could indicate it has failed as the body colour should be solid orange all the way along.

Considering that you indicated whoever you sent it to could not get the caps in the corrcect way round, prehaps the did the same with the Zenner or used and under rated one. 

[Rossoe]

That's a relief re the power supply, I'm complete beginner re electronics - so didn't know how to test it appropriately! what is the best way to test a power adapter before plugging it into Lynx?

 

I think that silver line on diode is there on another one I have spare as well (see pic below)

 

I'll attempt that 5v across C41

[+-^CrossBow^-]

@Rossoe Does your lynx have the copper sheet on the other side that covers over part of the solder side of the board? If so, was the tape or other insulating material used to cover the solder points of the cap that is replaced within the RF shielding properly? When I replace that cap out, I clip the leads pretty close to the PCB. When I solder it back in, I add additional solder after clipping the leads so make those solder joints rounded and smooth before putting the tape back on it. I usually follow that up with 1 or two small pieces of kapton tape as well before putting the copper sheet back into place. The rounding of the soldering is to make sure that there aren't any sharp points from the leads that could find their way to poking through the tape and making contact with that copper sheet covering everything thing.

 

Not saying that is what happened, but I could see that causing some problems.

 

The only thing that will  happen if the diode is installed backwards, is that the lynx will fail to power on at all because the diode being in the wrong direction, is doing its job and not allowing the current to pass through.

 

[+Stephen Moss]

20 hours ago, Rossoe said:

I think that silver line on diode is there on another one I have spare as well (see pic below)

As it appears in an unused on maybe that is just the way the are.

 

17 hours ago, -^CrossBow^- said:

The only thing that will  happen if the diode is installed backwards, is that the lynx will fail to power on at all because the diode being in the wrong direction, is doing its job and not allowing the current to pass through.

Remember D13 is a 4.3V Zenner and Zenner's conduct in both directions, one direction they Zenner, the other they act as a normal diode.

Consequently, I could be wrong but I think that if it is inserted the wrong way round the Lynx probably would not function as the main circuity will only be getting a supply of around 1.5-2V, but if that is enough for the Oscillator driving Q12 to run then the resulting voltage (and thus power dissipation) across Q12 and L15 will be about double what it should be which they may not be able to withstand. 

[Rossoe]

18 hours ago, -^CrossBow^- said:

@Rossoe Does your lynx have the copper sheet on the other side that covers over part of the solder side of the board? If so, was the tape or other insulating material used to cover the solder points of the cap that is replaced within the RF shielding properly? When I replace that cap out, I clip the leads pretty close to the PCB. When I solder it back in, I add additional solder after clipping the leads so make those solder joints rounded and smooth before putting the tape back on it. I usually follow that up with 1 or two small pieces of kapton tape as well before putting the copper sheet back into place. The rounding of the soldering is to make sure that there aren't any sharp points from the leads that could find their way to poking through the tape and making contact with that copper sheet covering everything thing.

 

Not saying that is what happened, but I could see that causing some problems.

 

The only thing that will  happen if the diode is installed backwards, is that the lynx will fail to power on at all because the diode being in the wrong direction, is doing its job and not allowing the current to pass through.

 

Mine does have the copper sheet over the entire back, some seem to only have a small section around the cart slot, I had not thought to cover the caps at the back! the copper sheet seems to have a plastic backing, but I guess there is always a risk that it could puncture through with sharp legs.

 

Zener is definitely the right way round as I made sure the black stripe is facing the metal shield as per instructions.

[+-^CrossBow^-]

@Stephen Moss I was only stating that installing it backwards, will simply result in the Lynx not power on. I've done it myself a few times when I wasn't paying close enough attention or in instances he band might not be as visible as other lots I've gotten in. 

 

So installing it backwards won't damage anything and again, just results in the Lynx not turning on because as you said, it doesn't allow enough to kick it on when installed backwards

 

 

[Makara]

Smoke is never a good sign and indcates something is getting extremely hot, usually from a short. If you already checked all caps are correctly polarised, one option is to spray IPA over the board, then power it up. You will see the IPA evaporate quickly where the heat is coming from.

 

I would try recapping it again, maybe the incorrectly polarised caps have been damaged, or damaged some of the other caps. The fading screen issue looks like a capacitor problem TBH.