Odd Lynx 1 power issue?

[+-^CrossBow^-]

Got a lynx 1 I've been working on pretty much all day at this point. Here is the basic issue with it:

 

- Only works when input voltage is 8.8 - 9v from the battery or AC input section. If you go above 9.2v - 9.5v the Lynx will essential lock up and the current draw on my bench supply rockets to about 2A? 

- If you use the AC adapter (Measuring 14.5v without load), the lynx will not come up and. After a few seconds one of the RAM chips will be scorching hot along with the fet.

 

Here is what I've done:

 - Replaced the ZD1 zener twice in case the first one wasn't working that I put in

 - Replaced the mosfet 2x in case the one I installed wasn't working right

 - Replaced both 3906 at Q4 and Q13. Q13 replaced twice because the first one developed a short internally when I first tested it from AC power.

 - Verified continuity with all the power stage components and with U6. No shorts found or breaks in the traces.

 

I'm really stumped on this one. It will work with +5 sent straight to C39 to bypass the power stage and will read +4.87v at TP15

With the bench supply at exactly 9v the lynx comes up but I'm getting a reading of 8.3 - 8.5v from TP15 which, seems awefully high? 

 

It seems this Lynx is incapable of regulating its voltage properly. What else be sides the zener, fet, and 3906s is there that would control any of the voltage regulation?

 

 

[joeatari1]

Seems weird for me to be offering you suggestions but it seems like there is a short or poor ground somewhere in the power regulation.  If it powers on with 5v from the bench but output stage is high when applying 9v then it must be in the power supply.  Is there a corroded or broken ground?  Anytime there is an electronic gremlin in a cars electronics (mostly what I work with) it is usually a poor ground.

[+-^CrossBow^-]

Well, there is a bit of an update though no real answers. This morning I spent over 4 hours working on it going through the schematics etc. They all pointed to U6 in some way. So I replaced it from a spare lynx 2 along with replacing the zener again. Once I did that it was actually powering on and I could increase the supply on my bench PSU without the Lynx going into over volt and pulling down massive currents. In fact it was hovering at just under 500mA while operating and I felt pretty good about things. Starting putting it all back together and then powered it from the AC adapter, and this time it wouldn't power on at all? So I broke it all down and tried again. Nothing, it appears to just be totally dead at this point. It will have to go on a back burner and be replaced with another working but needs recapped Lynx model 1 mainboard. 

 

Perhaps when I've got some spare time I can come back to it and try and troubleshoot it some more.

 

 

[+Stephen Moss]

On 5/23/2021 at 4:25 AM, -^CrossBow^- said:

I'm really stumped on this one. It will work with +5 sent straight to C39 to bypass the power stage and will read +4.87v at TP15

With the bench supply at exactly 9v the lynx comes up but I'm getting a reading of 8.3 - 8.5v from TP15 which, seems awefully high? 

I am sure you would of had the Com lead of you multi-meter at appropriate point to be measuring 5V and not 9V and so the fact you are measuring 9V suggests that the power GND and Single point ground are shorted.

Assuming that is the issue and it is not caused by the horrid headphone ground hack or a solder splash my first suspects would be Q11 but as you have already changed it and several other associated components twice I would suggest looking it at short of L6, but I personally would be looking at problem with the switching Oscillator (U6, Q13, C37 & R54) because if that is not oscillating it would give the impression of a short as the resistance of L6 and Q11 would be negligible.

Unfortunately, I will not have access to my Lynx or any equipment to provide any oscillator frequency or modulation depth measurements for comparison until September. 

 

If you have an oscilloscope to hand take a look at pins 1, 2, 3, 4, 5, 6, 8 & 9 of U6 and see if they appear to be oscillating or are static at either 9V or 0V (depending on the pin being probed) with the GND of the Oscilloscope to main ground (left of Q11 in the schematic). Otherwise the voltage reading you will get if it is oscillating with a multi-meter will be determined the frequency of you your multi-meter sampling but if there is no oscillation again you should get fixed readings of either 0V or 9V.

 

 

[+-^CrossBow^-]

@Stephen Moss This lynx has the power inverter section removed for the backlight and LCD section as it was supposed to get an LCD upgrade into it. So I've been taking my voltage measurements from the GND and VCC points where that board used to be attached.

 

Yesterday I replaced out U6 from a spare Lynx 2 as I mentioned and that at least allowed me to run it off the bench supply set to 9v with those points I mentioned above reading at about 4.86v which is exactly where I would have expected it to be. Increasing the voltage on the bench supply to about 12v the reading stayed the same 4.84 - 4.87v. Near as I can tell the battery wire leads on the Lynx 1 are straight to the same traces that the AC adapter is plugged into so I've always used that in the past to feed the lynx 1 to simulate battey use etc.

 

In probing U6 I don't recall which pins right now, but I was actually getting readings of 0v or -9v using the GND off where the high voltage section used to be located. But I can probe those pins using the main gnd.

 

 

[+Stephen Moss]

17 hours ago, -^CrossBow^- said:

This lynx has the power inverter section removed for the backlight and LCD section as it was supposed to get an LCD upgrade into it. So I've been taking my voltage measurements from the GND and VCC points where that board used to be attached.

I am assuming that means that the LCD & Backlight are not currently getting any power, which could explain the previously mentioned current draw of 500mA (missed that earlier) when I would expect almost twice that going from the AC adaptor specs. If you were powering it from the AC adaptor like that then it is likely it was running off around 12 volts instead of 9 which could explain why things were heating up when using it.

17 hours ago, -^CrossBow^- said:

So I've been taking my voltage measurements from the GND and VCC points where that board used to be attached.

Unless there is a need to do otherwise I personally prefer to measure all voltages referenced to the main supply GND as it is a known fixed voltage.

You can use the single point ground as ground for measurements of the "5V" sections of the circuit provided you are certain the power stage is operating correctly otherwise it is an unreliable and variable reference point.

[+-^CrossBow^-]

The GND point that I use is next to L6 and is part of the main ground. The +5 is all over the lynx 1 mainboard. In fact it is way more difficult to find other locations tied to gnd on the lynx than +5 as even the copper foil and RF shield is on the +5 ?! 

 

Anyway, I gave up on that lynx last night. Couldn't find any shorts from the AC/battery side to the power stage at all. It did have quite a bit of corrosion around U6 in places. So much that I removed U6 to clean it up and then soldered it back down before swapping it out and had it semi working until the AC adapter was supplied.

 

And yes, you are correct, without the backlight tube inverter power board in place, the Lynx draws half the current it would normally require.