Power failed, R60 extreme hot :(

[Crazyboss]

Hi There.

I got a problem with my MTX. I found out that the R60 (its just under the PAL-board) is extremely hot after power on for maybe 5 seconds.

It also seems like I only have around 4v out of the 7805. I wonder if it could be the tip that failed?

Around the area, with the Regulators (next to the Modulator), have a spell of burned-electronic.

Any suggestions?

[AndyKey]

Change the power transistor.
Top right of motherboard.
I had this problem and this fixed it for me.

Note, I also changed the regulator, but it was the power transistor that fixed it.

{{{ Andy

[Dave]

Andy said of his original problem . . . . . .

"And as if to prove my point, I've only just realised my really hot resistor is actually R62"

[1024MAK]

Do you mean R60, a 1k ohm normal 1/4W resistor that feeds the -5V line (check this supply voltage), or R62, a 10 ohm (normally coloured green) larger power resistor that feeds the 7805 voltage regulator?

See picture:-

R60 and R62 resistors.JPG (188.9 KiB) Viewed 21828 times

R60, the 1k ohm resistor, is only likely to overheat if one of the 4116 video DRAM chips fails.

R62, the 10 ohm resistor, controls the transistor (Q4, TIP2955) that boosts the current for the +5V line (helping take the load off the 7805 regulator chip). You should get approx 0.6V to 0.7V across this resistor (measure the voltage between the two legs of the resistor) if Q4 is working correctly (and the +5V line is +5V +/- 5%).

If Q4 has died and the 7805 is trying to supply all the current, the voltage across R62 will be much higher (much greater than 0.7V).

If you replace Q4 or have a problem with the +5V supply (or a problem is suspected), I recommend removing fuse FS1 (3.15A) and measuring the current (power down first!). I tested two MTX's and one was drawing 860mA, the other was drawing 932mA.

Hope this helps

Mark

Edited to correct me saying R60 when I meant R62

[Dave]

And having "stolen" a copy of Mark's photo

I have created a page about this problem here :-

http://primrosebank.net/computers/mtx/r ... _power.htm

[Crazyboss]

Hi.

Before replace the tip2955 i got around 7v between the legs at r62.

After replace, i get the same, so its another problem. I get around 20v in at 7812, and 12v out, so it seems to be right.

I get only 6v in at 7805 by the way, and only 3v out, could it be a faulty 7805?

[Dave]

what are the Mains PSU output voltrages?

follow Mark's tip (not TIP!), remove the fuse, and measure the current being drawn in the 5V circuit
(remove the fuse with the power OFF and disconnect any add-ons, REMEMOrizer, RS232, RAM boards etc)

[1024MAK]

Hi.

Before replace the tip2955 i got around 7v between the legs at r62.

After replace, i get the same, so its another problem. I get around 20v in at 7812, and 12v out, so it seems to be right.

I get only 6v in at 7805 by the way, and only 3v out, could it be a faulty 7805?

So are you getting about 13V at FS1? In my MTX I get about 11.2V at FS1.

With it switched off, test using the resistance range for continuity between R62 to one side of FS1 and to Q4 TIP2955 emitter terminal/leg. Then do the same from the other side of R62 to Q4 TIP2955 base terminal/leg. Finally test between Q4 TIP2955 collector terminal/leg and the +5V power rail / 7805 output terminal.

The Q4 TIP2955 should turn on when the emitter is approx 0.6V to 0.7V greater than the Q4 TIP2955 base terminal. The current flowing into the emitter terminal then leaves the transistor via the collector terminal. If Q4 transistor turns off, this current flow stops.

if a 7805 voltage regulator is starved of input voltage, the output voltage will drop, as it requires approx 2.5V across it to correctly regulate the output. As Q4 is not helping, too much current is flowing through R62 causing a large voltage (7V approx) to appear across it, leaving too little voltage for the regulator to work with.

Mark

Edit: Here is a picture of the pin-out


Note that the centre pin (pin 2, the collector) is also internally electrically connected to the heat-sink tab, and therefore must be insulated from the heat-sink. Test that this is true by testing using a meter on the resistance range. Otherwise this will be a short circuit and could damage it (or if you are lucky the fuse will blow and save it).

[Crazyboss]

Tried with another PSU same result

[1024MAK]

See edited post above.