Author Topic: 10mhz motherboard issue (Read 3844 times)

Thegadgeteer · « **on:** February 12, 2019, 11:37:13 AM »

Hello folks,

Newbie here, please forgive, and feel free to correct me if I breach protocol. I've scanned the existing topics, but didn't find anything relative to my problem. I have a slant top S-Plus that I got from a friend who got it in a non-working condition. There was a blown diode in the mb (D3) that my friend attempted (and failed) to repair. When he gave it to me, it sat in my shop for about 6 years when I finally pulled it out, and replaced the diode.

It has worked great for a year when one day I got error code 12, mb battery voltage low. No biggie. Ordered a new battery, replaced it without incident, inserted the mb back into the machine where it seemed to work fine, with one noticeable difference; the sound was about half the volume of what it was pre-repair. After about 5 minutes it just froze and didn't do a thing. I opened it and smelled that hot component smell, and discovered that that diode was hot as a firecracker.

If I let it cool, it works for a few then freezes again. Gonna replace the diode again and see if that does it. Any ideas, suggestions, comments would be really appreciated.

Dave

Stayouttadabunker · « **Reply #1 on:** February 12, 2019, 01:19:24 PM »

I have to ask...is the grey stripe around the [D3] diode facing away from the GND?
[C45] cap not blown?
Check the In4002 [CR11] as well...
I'm pretty sure something connected to the [D3] is affected.

Thegadgeteer · « **Reply #2 on:** February 12, 2019, 06:50:37 PM »

Thx for the thoughts, Stayouttadabunker. Yeah, it was right.

The cap and the In4002 are fine, physically. I can check the capacitor fpt capacitance, but how does one check the In4002? Is it an SCR? Actually, it was hot, too. But, considering the heat sink on it, I assumed that was normal.

Stayouttadabunker · « **Reply #3 on:** February 12, 2019, 07:02:25 PM »

Quote from: Thegadgeteer on February 12, 2019, 06:50:37 PM

Thx for the thoughts, Stayouttadabunker. Yeah, it was right.

The cap and the In4002 are fine, physically. I can check the capacitor fpt capacitance, but how does one check the In4002? Is it an SCR? Actually, it was hot, too. But, considering the heat sink on it, I assumed that was normal.

The [U47] triac is what has the heat sink attached on top of it.
I was talking about the black In4002 power diode [CR11] that's laying horizontally flat on the board....lol

Try to link this topic to a member named Badbaud....he's really good with circuitry on MPU's & board repair.
Me? I barely know the difference between a resistor and a triac...haha
I only know what they do & which way they're supposed to point to.
Definitely something's heating up and going out of specs.
If you inbox me an email addy, I can try to get you some MPU schematics if you want?

Thegadgeteer · « **Reply #4 on:** February 13, 2019, 09:16:24 AM »

Ah, ok. (I would insert blushing emoji here, if there was one) Me too. I can muddle my way through, but I am an electronics amateur, at best. Really haven't done much since the days when you could still walk in to Radio Shack and buy resistors, diodes, and capacitors off the shelf. Man, do I miss that place.

Thx again. I'll try to do those things.

Badbaud · « **Reply #5 on:** February 13, 2019, 04:01:45 PM »

The 1N4002 is a silicon diode. 1 Amp current rating. It passes the 5V logic voltage from the large metal can regulator to the logic circuitry.

If it is getting hot check the Tranzorb diode that connects to the cathode side of the 4002.
It is a larger diode that shorts to protect the 5V logic voltage from going to high.
But, sometimes those Tranzorbs short just for the heck of it.

Stayouttadabunker · « **Reply #6 on:** February 13, 2019, 05:02:36 PM »

How does a dummy like me using a digital Ideal multimeter check to see if that diode has the right specs Badbaud?
I tried 6 different S+ MPU's.
All the MPU's I'm have on the floor next to me are 10MHz with no battery or SP & SS chips installed.

I'm not sure which setting I should put the dial on my multimeter at to try and check that 1N4002 diode.
In the photo below, I set the dial to 20k and put my probes on the [D3] diode and I get this reading...>>>

[D3] = 1.14 AMPS?? or Ohms?
I'm not sure what I SHOULD be getting...maybe I should just mail them to you - I'll pay for all the damages! haha

Photo below is me checking the [D3] diode.
Click on photo to enlarge if you want...>>>

Amechanic · « **Reply #7 on:** February 13, 2019, 05:41:56 PM »

I believe that the setting between the 200 & F on your meter is to check diodes. You should get a reading between 400-600 when you test with your meters lead in one direction on the diode, and nothing when you reverse the leads.

Stayouttadabunker · « **Reply #8 on:** February 13, 2019, 06:12:20 PM »

Quote from: Amechanic on February 13, 2019, 05:41:56 PM

I believe that the setting between the 200 & F on your meter is to check diodes. You should get a reading between 400-600 when you test with your meters lead in one direction on the diode, and nothing when you reverse the leads.

uhh...isn't the "F" for Fahrenheit, while "C" is Celsius when using heat probes on the multimeter sir?

The symbol setting right of the 200 ohms is the "tone" for when I'm checking continuity.

I'm getting a reading of 1.14 ohms in either direction at the moment though when setting the dial to 20k.

Badbaud · « **Reply #9 on:** February 13, 2019, 06:29:10 PM »

Between the "F" and the "200" on the scale is a diode check.
You can remove one leg of the 1N4002 from the board and check it both ways, put your leads red to anode, black to cathode, and you should read around .500, which is also the forward voltage drop of the diode.
Then swap your leads, red to cathode and black to anode and the reading should be much higher or "OL", no reading at all.
If you get a beeeeeep then the diode is shorted.
Next put your red lead on the cathode of the 1N4002 and your black lead on the "-" of the battery (ground), if you get a beeeeeep that means the Tranzorb is shorted.
PM me if you want to send your board(s) to me for a thorough diagnostic and repair.

Amechanic · « **Reply #10 on:** February 13, 2019, 07:16:46 PM »

This is my multi meter and the setting I use to check diodes.

Stayouttadabunker · « **Reply #11 on:** February 13, 2019, 07:30:59 PM »

Okay...I was just leaving but you made me pull out my soldering gun....

Let me apologize to Amechanic for the "F" and "C' confusion.
I think he meant for me to put my multimeter dial on the setting BETWEEN the "F" and "200" settings.
I have always use the "))))" setting on my multimeter to check for continuity...haha

Anyway, I soldered in a battery holder and put in a new coin battery.
I then unsoldered the cathode, GND or "-" side of the [D3] diode and discovered it's an 1N6373 diode instead of a 1N4002.

Posted below are 4 photos of the readings I get from [D3] when the probes are moved.
No beeping occurred.
Multimeter was showing resistance readings of the diode?

Click on photos to enlarge if needed to see my probe placements.
I tried to follow your instructions...let me know if I have the probes in the wrong positions?...>>>

Amechanic · « **Reply #12 on:** February 13, 2019, 08:00:23 PM »

By looking at your first two pics, I think that diodes good.

Stayouttadabunker · « **Reply #13 on:** February 13, 2019, 08:05:14 PM »

Quote from: Amechanic on February 13, 2019, 08:00:23 PM

By looking at your first two pics, I think that diodes good.

I agree Amechanic!

Thanks!

Now the Gadgeteer needs to check his diode on his board!

Badbaud · « **Reply #14 on:** February 13, 2019, 08:13:41 PM »

1N6373 is the Tranzorb (Transient Absorb) which prevents over voltage on the 5V logic circuit.
They can read fine with no voltage on them but if bad will short out.
Their purpose is to latch on if there is a over voltage transient on the 5 Volts, those are normally very brief and the Tranzorb will short the 5V briefly.
They fail because over time they accumulate so many transient protection events that they die and short out from excessive use.
It is like a self resetting electronic fuse.
The numbers you are reading on the meter in diode check mode is actually the forward voltage rating for the diode. All silicon diodes have a slight voltage drop when current passes through them.
Usually around .5V, germanium diodes will read about .3V, Schotty diodes cas be even lower.

It is a internal resistance that causes a voltage drop which the meter can read.
If you diode check a LED the reading you get is the voltage needed to light up the LED. A standard red LED is about 1.5V, some ultra bright LED's can be as high as 2.80V.
To determine what resistor value to put in series with a LED to light it up you take the supply voltage and subtract the forward voltage of the LED then assuming the forward current the LED needs is 20 mA.
R=E (source voltage minus forward voltage) /I will give you the resistor value then P=I(sq) x R to figure the wattage of the resistor.
Back to the Tranzorb, if you don't have a board tester you can solder 2 long wires to it and hook to your volt meter then plug in the board and power up.
Watch the Tranzorb voltage and see if it drops to a very low voltage after power has been on for an hour or probably less.

Thegadgeteer · « **Reply #15 on:** February 13, 2019, 08:37:11 PM »

Hello Badbaud, thx for lending your expertise. Actually, it's the Tranzorb diode that I replaced a year ago that is getting hot now after I replaced the battery. I replaced the Tranzorb again, to no avail. It still works fine for 5 minutes, then the machine freezes.

Stayouttadabunker · « **Reply #16 on:** February 13, 2019, 09:02:59 PM »

Quote from: Thegadgeteer on February 13, 2019, 08:37:11 PM

Hello Badbaud, thx for lending your expertise. Actually, it's the Tranzorb diode that I replaced a year ago that is getting hot now after I replaced the battery. I replaced the Tranzorb again, to no avail. It still works fine for 5 minutes, then the machine freezes.

haha! It doesn't matter - it might be a bad one...time to pull out the multimeter and do what I did~

I wanna learn how to check the rest of the IC components around that area as well.
I find myself googling how to check this and how to check that.
I try to teach myself but it's pretty awesome when you have guys like Badbaud & Amechanic as well as others to help us all out here on NLG.

If you get a little overwhelmed on how much is really needed to get that board working well again, it would be best to just pack it up and ship it out to Badbaud.
He will check everything on that MPU and let you know whether or not it's "saveable" (couldn't think of a better word at the moment! lol)

Badbaud · « **Reply #17 on:** February 13, 2019, 09:21:52 PM »

Ya, that over voltage could be AC ripple caused by a shorted power input diode or a faulty filter cap, or a 5V regulator that is shorted input to output or floating high.

Thegadgeteer · « **Reply #18 on:** February 15, 2019, 10:04:27 AM »

Ok, after tracing and probing around, the Tranzorb is good, the cr11 and cr12 caps are good, the IN4002 is good, but after pulling the C27 cap and checking capacitance, the numbers climb but go to OL. The same thing the meter does when you short the probes together. Is it bad, or is it possible that 6800μf is just out of range for my Fluke fc902 meter?

Thegadgeteer · « **Reply #19 on:** February 15, 2019, 10:11:37 AM »

If I test it under the resistance setting, I get full continuity one way, then (reversing the probes) momentary continuity followed by rising resistance.

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Author Topic: 10mhz motherboard issue (Read 3844 times)