Help with repair

[1024MAK]

Transistor heat sink insulations 85.jpg (103.02 KiB) Viewed 9703 times

Mark

[Retrocosm]

Ok thanks Mark, going by your picture if I put my meter on the metal plate in the foreground and the tab there is a connection, is that the short?

Sorry for all the hand holding, I'm learning on the job as it were.

Charlie

[Martin A]

I'm powering down after 10 seconds or so as the board starts to smell hot.

Have you tried touching the chips on the board, to see if any are running hot?

[Dave]

The picture I posted was only meant to show the insulated "washers" (not many suitable pictures thrown up by a quick search).

Mark

There was a great photo on this web page ......

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

Click on the 5th photo down to zoom in.

[Retrocosm]

If I remove the nut and bolt from the TIP2955 and lift it upwards so it's completely clear of the heatsink and put the meter on the heatsink and the tab of the TIP2955 there is still a connection.

Charlie

[1024MAK]

If I remove the nut and bolt from the TIP2955 and lift it upwards so it's completely clear of the heatsink and put the meter on the heatsink and the tab of the TIP2955 there is still a connection.

Good that at least you know that it is not the insulation for the TIP2955 that is the cause. Bad news is of course that the short circuit is else where.
Have you de-soldered one leg of the zener diode ZD2 yet?

Mark

[Retrocosm]

Hi Mark, no I haven't had a chance yet, hopefully this week.

Charlie

[Retrocosm]

I attempted to de-solder a leg of the zener diode ZD2. While I was heating the solder and applying light upward pressure on the diode it broke in half. Given how easily it came apart I think it was pretty much already broken.

Charlie

[1024MAK]

I attempted to de-solder a leg of the zener diode ZD2. While I was heating the solder and applying light upward pressure on the diode it broke in half. Given how easily it came apart I think it was pretty much already broken.

Charlie

Sounds like it. Alas, it does make it hard to test the diode to see if it had gone short circuit. But you can still test the resistance of the +5V supply to 0V to see if you no longer (hopefully) have a short circuit.

If the short circuit has gone, then that indicates that the cause of the short was zener diode ZD2. However, zener diodes normally only go short circuit when too much current flows through them and they overheat. This should not happen in normal use, as the zener voltage is higher than the +5V supply voltage (ZD2 is rated at 5.6V and the +5V supply should never go above 5.25V).

The purpose of ZD2 is to provide some limited protection to the logic chips, CPU etc in the event that the components that make up the +5V regulation circuitry fail in such a way that the output voltage goes above 5.6V

Ideally, it would be nice to disconnect the output of the +5V regulation circuitry from the rest of the board, but this is not easy to do

The next best thing, is to de-solder the 7805 voltage regulator (Reg2) and the power transistor Q4 (TIP2955) and test them out of circuit.

More later (out of time, sorry, have to go out).

Mark

[Retrocosm]

Sounds like it. But you can still test the resistance of the +5V supply to 0V to see if you no longer (hopefully) have a short circuit.

Thanks for that info Mark, where should I put the meter to test this?

Charlie