Need help diagnosing/repairing my Sega MD

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dougal

dougal

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Ok brief story:

I had this Sega MD MK1 for years and years and about a year ago decided to mod it (successfully) to have a region switch and a 50hz/60hz switch.

Anyhoo, last weekend I had the not so brilliant idea of undoing what I did in order to do a neater job. Stupidly but unknowingly I was desoldering the old wires (that i had soldered to mod it) whilst the system was plugged in AND switched on :double

Now I KNOW i did a neat job removing the wires and nothing looks shorted or damaged on the board BUT the MegaDrive is NOT POWERING ON. No display and no Power light.

The Power Supply is fine, tested on a Master System and another Megadrive. And there is power getting into the system because:

The heat sink (where the two 7805 regulators are attached to) is getting REALLY hot quickly and one of the CAPS is getting rather hot too.

So bottom line: No picture and no power light but getting HOT.

Everything on the board looks fine, no shorts, no messy solder that i can see.

I have a bunch of brand new 7805 regulators. Should i try replacing them one at a time and see if it fixes it ?

Is there anything else i should look out for ?

Please please, anyone technical, i would really appreciate any help offered which might get this poor MK1 Sega Megadrive back to life. It deserves better and when it was alive was appreciated especially whilst playing SF II Championship Edition SE.

Thanks in advance :thumbsup:
 
7805 regulators are usually quite rugged with over-current and over-temperature protection, so they don't die often. If one of them gets really hot, it could indicate a short on the secondary side.

With the device off :), measure the resistance from the output of the regulators to Gnd. It should go to infinity, or at least a high value.
 
demolition said:
7805 regulators are usually quite rugged with over-current and over-temperature protection, so they don't die often. If one of them gets really hot, it could indicate a short on the secondary side.

With the device off :), measure the resistance from the output of the regulators to Gnd. It should go to infinity, or at least a high value.
Click to expand...

And if it goes to infinity or a high value, what sort of value + should i look for ?

Would that mean the regulator is ok ? I did test the voltages and they are both giving 5 volts from 1 of the 3 pins.
 
With high value, I mean 100+ ohm. If you see <10 ohm then there probably is a short of a kind.
You also mentioned that there is a cap which becomes hot, and they normally don't at least for decoupling. Which voltage do you see across it? If it is 5V, then this cap could be the cause of the problem if is has started leaking current, and the regulators are struggling to keep up.
 
demolition said:
With high value, I mean 100+ ohm. If you see <10 ohm then there probably is a short of a kind.
You also mentioned that there is a cap which becomes hot, and they normally don't at least for decoupling. Which voltage do you see across it? If it is 5V, then this cap could be the cause of the problem if is has started leaking current, and the regulators are struggling to keep up.
Click to expand...

Ok, I'm at work now but when I go home i'll test the regulator and check the voltage on the CAP and post the results.

Thank you so much for your help. Maybe with your help there is a chance the Megadrive will live once again :)
 
Were you using a potential-free soldering station or a cheap "plug it in wall iron". If it was the cheap version, you have most likely fried something (or several things) on the PCB, which created shorts inside some IC. Cheap irons aren't isolated, so you were probably pushing some volts on to the wire you touched.

Bryce.
 
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Bryce said:
Were you using a potential-free soldering station or a cheap "plug it in wall iron". If it was the cheap version, you have most likely fried something (or several things) on the PCB, which created shorts inside some IC. Cheap irons aren't isolated, so you were probably pushing some volts on to the wire you touched.

Bryce.
Click to expand...

Cheap plug in the wall iron.... Possible effect from MD being switched on and using the cheap'o iron ?
 
Exactly: Ground circuit of the PCB was really grounded, Iron supplied a voltage (of unknown value) and it took the route of least resistance to earth (probably through an IC). Not good I'm afraid. Are any ICs (other than the 7805) on the PCB getting hot?

Bryce.
 
The hot CAP is giving 0.08 volts when the system is on. It is C53 and rated at 10v 100uf. It gets very hot very quickly. Does not look leaked or bulging.

---------- Post added at 19:13 ---------- Previous post was at 19:08 ----------
9apyja4y.jpg


---------- Post added at 19:13 ---------- Previous post was at 19:13 ----------

Is this the right setting to read the ohm's ?

---------- Post added at 19:22 ---------- Previous post was at 19:13 ----------

If so the results are strange. For ground I'm using the heat sink. When I test the pins I get a result and it quickly hues back to 1. Results are not always the same.

On one 7805 I get 160-200 on 1 pin and about 130-150 on the other.

On the other one, one pin gives anything from 35 to 170 and the other seems to start - for half a second then goes up to anything from 30 to 140 and then goes down very very low.

---------- Post added at 19:23 ---------- Previous post was at 19:22 ----------

The IC's don't seem to be getting hot
 
Sounds like there's a serious short somewhere on the board. Whatever's getting hot is what's shorted. The spurious readings are normal, it's because some capacitor is discharging through the meter.

Bryce.
 
dougal said:
The hot CAP is giving 0.08 volts when the system is on. It is C53 and rated at 10v 100uf. It gets very hot very quickly. Does not look leaked or bulging.
Click to expand...
As Bryce writes, the capacitor will charge/discharge to the meter, so when you measure the resistance you have to wait until the reading has settled, and that will be the result.
To me it sounds like that cap has died and from the specs I'm guessing it's a decoupling cap which should have 5V across it. I don't have the schematic for it though, but I would start by replacing that cap with a new one, preferably a low ESR type from a known quality brand (Panasonic, Nichicon, Sanyo, Rubycon etc.).
 
As long as it is 100uf the voltage does not matter if it is 10v or more right ?
 
I replaced the hot cap (i only had a 100uf 25v) but same result.

The new CAP is still getting hot fast, the MD is still dead :(
 
Could the MD be dead forever ?

---------- Post added at 20:00 ---------- Previous post was at 18:17 ----------

utavu4yd.jpg


---------- Post added at 20:01 ---------- Previous post was at 20:00 ----------

Look !!!!! I don't know what I did but its working !! Must have been a short

---------- Post added at 20:11 ---------- Previous post was at 20:01 ----------

Now only problem is its stuck in 60Hz which is fine but a few games won't work at 60Hz such as Street Fighter II special championship edition which says its for PAL & SECAM.. LOL

I know its just a case of shorting JP3... But for now bugger that i', just happy it works and cant be bothered for now to pull it apart again
 
Just fitted a 50hz/60hz switch... Works like a dream. The short was on one of the pins related to the 50Hz/60hz and languages... JP3 i think was shorting with JP2
 
Well we can consider this one solved!

Thanks guys for the help... I am very happy... :)
 
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