VOGONS


Reply 20 of 39, by Eleanor1967

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Since neither one of those parts are expensive I just bought the transistors I posted before (I really should read what I buy, they are TIP142, not TIP127 as before. Also they are way bigger.) and a LP2951 chip. I just soldered the new transistor in and I now get the usual 4,9X volts when the board is jumpered for 5V and 4,5x volt when the board is jumpered for 3.3 and 4 which is atleast less than before.

Is this a sign that the transistor was at fault? And that if I would by a TIP127 it should work at 3.3 as it is supposed to? I compared the datasheets but I just don't know if the difference between these transistors can be blamed for the now lower wrong voltage.

Reply 21 of 39, by Eleanor1967

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I finally got around to playing with the board again. Installing the TIP127 did the trick for me, I now get a nice 3,4x Volt again, 4 and 5 Volt setting work as well. I but since my guilt wasn't fully paid off after hurting the poor board so bad, I also soldered the him 3rd missing jumper, which wasn't there from the factory for the FSB selection on, which now allows selecting FSB higher than 40 up to 66. Fun times benchmarking CPUs with different clock and FSB speeds are ahead 😀

Thanks for all the help, I would not have done it without it.

Reply 22 of 39, by Auzner

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Cool, glad it worked out. That larger mosfet had different switching (slower) and current (higher) characteristics than what the circuit was designed for. So the motherboard resistors supporting its functions were not selected for that part. Good job though on keeping at it with the soldering iron to achieve what you wanted. It's interesting that in their day, these parts were evolved up to where a system builder did not need to solder anything, but with their age it's come full circle. Back then you'd probably RMA the motherboard and they'd exchange it for shipping cost.

Reply 23 of 39, by gerwin

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gdjacobs wrote:

The pass transistor is the device in the TO-220 package (standing up with the metal tab). In the simplest sense, it provides (in the context of this circuit) a controllable resistance based on the input from the 2951 regulator, like an electronically controlled rheostat. Based on your drawing, I'm assuming we're dealing with the base or gate (i.e. the control input) on the right most pin (generally referred to as pin 1) which is typical for TO-220 transistors. If the impedance across the other two pins in either direction is less than 10s or 100s of kOhms, the device has likely failed short, should be removed from circuit (and tested to confirm another part in the circuit hasn't failed), and replaced if confirmed to be bad.

Through my own fault (CPU orientation) I fried the VRM functionality of a LS486E Rev C1. At 5 Volt CPU voltage the board functions OK. What components are likely to be defective? The TIP127A PNP Transistor (TO-220 package) or that tiny LP2951CM (SMD SOIC-8 package?) How can I find out with just a multimeter? I measure a fluctuating 0 to 1 Volt on the back of the TIP127A. Maybe start by replacing the TIP127A with a TIP125 and see how it goes?

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Reply 24 of 39, by gdjacobs

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Not sure I understand the symptoms as described. Is the gate voltage for the TIP127A fluctuating?

All hail the Great Capacitor Brand Finder

Reply 26 of 39, by gdjacobs

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I'm guessing the VRM is fairly similar to the LP2951 linear regulator circuit in the application notes, only using a PNP transistor instead of a MOSFET. Test Vbe and Vce to see if the TIP127A isn't failed short, failed open, and that the gate is in decent shape. If the PNP seems okay, it'll be in the passives or the regulator control IC.

All hail the Great Capacitor Brand Finder

Reply 27 of 39, by gerwin

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gdjacobs wrote:

I'm guessing the VRM is fairly similar to the LP2951 linear regulator circuit in the application notes, only using a PNP transistor instead of a MOSFET. Test Vbe and Vce to see if the TIP127A isn't failed short, failed open, and that the gate is in decent shape. If the PNP seems okay, it'll be in the passives or the regulator control IC.

Thanks for the suggestions! I am not that knowledgable to say wheter or not a PNP transistor is in shape, not yet. Attached some of the measurements I did: Voltages while running and resistances with the board powered off.
The solder at the capacitor near the 8 ohm resistors looks odd, and how come over the big resistor is Zero Ohm?

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Reply 28 of 39, by gdjacobs

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For reference, I expect the regulator will be similar in structure to the example from the application notes above, potentially with some differences in the passives and potentially with the error signal strapped down.

Looking at the measurements, the base appears to be connected as expected, but Veb is way high. I suspect that TIP127 might be caca. It should be in hard saturation with those voltages, but Vce is -3.5V where in saturation it should be -0.5V max.

All hail the Great Capacitor Brand Finder

Reply 30 of 39, by gerwin

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I replaced the TIP127A this evening. Put a TIP127 in its spot. This one: TIP127 TO-220 Bipolar Single Transistor (BJT), Darlington, PNP, 100V, 5A, 65W 60.
Also replaced the big green resistor with a 5 Ohm one.

The motherboard is back in business with 3 Volt CPUs! 😀
It supplies 3,40V Vcc as it should. So glad I don't have to replace the tiny LP2951.
Wonder if the LS486E motherboard designers anticipated the typical CPU orientation problems, and chose this VRM setup as to be easily repairable after an overload.

About the big green resistor: Can someone with this board please measure it for me? I think it says Yellow, blue, gold, silver = 4,6 Ohm 10%. But the one that was removed from this board measures around 0,5 Ohm?

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Reply 32 of 39, by Intel486dx33

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I have a couple Lucky Star LS-486 motherboards. I tried to get one working once and it did work for a while and then failed with keyboard lock out.

Does anyone know how to fix this problem?
Does anyone know the pin-out layout for the keyboard lock jumpers on the motherboard ?

Reply 33 of 39, by quicknick

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gerwin wrote:

I replaced the TIP127A this evening. Put a TIP127 in its spot.

Can anyone clarify for good what type of transistor does this board use for its VRM? Some time ago I was admonished for suggesting (for this exact board) a standard PNP instead of a Darlington. But the board originally had a TIP127A installed, and it seems that it's not a Darlington at all. Anyway, glad that it's working regardless of the transistor type 😀

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Reply 34 of 39, by gerwin

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I noticed that datasheet and the absence of "darlington" as well. But TIP127A is rather obscure, ebay has zero results on that term, whereas TIP127 gives 122 results. Actually even TIP127 is not available at for example Conrad.nl.
But I am not very knowledgeable about transistor variants, and I read somewhere on vogons that TIP125/126/127 would be OK, and just went ahead. Regardless, my measurements of 3,40 Volt Vcc (default) and 3,60 Volt (change of jumper 13, resistor modded from 4V default) are what they are. The system is running with an Am5x86 133MHz (FSB 33MHz), no problems.

Reminder: "About the big green resistor: Can someone with this board please measure it for me? I think it says Yellow, blue, gold, silver = 4,6 Ohm 10%. But the one that was removed from this board measures around 0,5 Ohm?"

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Reply 36 of 39, by Roman555

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gerwin wrote:

Reminder: "About the big green resistor: Can someone with this board please measure it for me? I think it says Yellow, blue, gold, silver = 4,6 Ohm 10%. But the one that was removed from this board measures around 0,5 Ohm?"

It's not a resistor. It's an inductor. You can see a reference designator "L4" near it.
I think it is a filter for power supply of the PLL.
So "Yellow, blue (violet), gold, silver" = 4,7mkH 10%. And its resistance for direct current is near 0 Ohm. I've just measured.

Reply 37 of 39, by Deunan

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gerwin wrote:

I noticed that datasheet and the absence of "darlington" as well. But TIP127A is rather obscure

It is indeed, it seems to be a rather unique single PNP with high gain (for a 10A power transistor) and very low saturation voltage. The closest thing I found that should work is D45H8G: https://eu.mouser.com/ProductDetail/ON-Semico … lqVlK0ZEBkmM%3D
D45H11G will work as well but you don't need that 80V max voltage. The only problem is the docs give twice higher saturation voltage of 1V (but at 8A instead of 6A) so it might run hotter - I think even a small, simple U-shaped heatsink would help with that.

Reply 38 of 39, by gerwin

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Roman555;
Thank you for correcting that matter of the inductor disguised as resistor. I will put the inductor back on the board.

Deunan;
Thanks for recommending a better replacement for TIP127A. So I guess I will try out that D45H8G then.
Earlier I already put a slightly bigger U-Shaped heatsink on the TIP127, but the space is very limited for such on this motherboard.

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Reply 39 of 39, by gerwin

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Finally installed the D45H8G as the voltage regulator chip.

Measured Vcc:
3V jumper setting gives 3,40V (both with and without a CPU)
4V jumper setting gives 4,14V (only tested without CPU, this is WITHOUT modded resistor)
4V jumper setting gives 3,56V (only tested without CPU, this is WITH modded resistor R33)

Practically the same as the TIP127 situation some posts earlier. That is a good thing.

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