Resurrecting an MTX500.

[stephen_usher]

It’s easy enough to create an eye by looping a bit of bared wire and soldering it, without letting the hole fill with solder

I found a PDU earthing wire at work with an eye at the end. It's a bit of an overkill in terms of wire thickness but it's fitted.

[stephen_usher]

I've tried piggy-backing another 74LS08 on the top of 7E and it SEEMS to be better, as in I can run chess... for a while... until something warms up. Then it becomes unstable.

I'll swap that chip anyway as it's the last one in the whole memory addressing sub-system I've not changed.

[Martin A]

Silly question, did you replace the TIP2955 in the power regulation section ?

(20 pages of thread is a lot to re-read, so I'll just ask instead )

[stephen_usher]

Silly question, did you replace the TIP2955 in the power regulation section ?

(20 pages of thread is a lot to re-read, so I'll just ask instead )

Nope. I'm guessing that might have to be the next thing to try given that I've replaced all the digital components which relate to the memory access, but I've run out of time now and I'm having to pack everything away. I'll probably next have time to look at this in September.

From the 'scoping the +5V line looks stable and good, if very slightly high, voltage.

Replacing the 74LS08 hasn't made any difference after all. When the machine is cold and first starts up it does work properly for a few minutes but then shows the problem.

So, as you hint, the next thing to do will probably be to overhaul the power sub-system, replacing the regulator and the electrolytic capacitors.

P.S. If this year's Memofest is in Cambridge I'll bring the machine along. Cambridge is only a couple of hours away so doable as a day trip.

[1024MAK]

If the +5V rail is within spec (4.75V to 5.25V) and is clean and noise and ripple free, the power supply section is not your problem. Also, if the +5V supply was bad, it is very unlikely that the CPU would manage to run anything.

If the fault appears to be heat sensitive, try using a can of freezer spray (or air duster used upside down). Use cardboard to shield circuitry that you don’t want to be affected, then spray on the chips you suspect.

Mark

[stephen_usher]

If the +5V rail is within spec (4.75V to 5.25V) and is clean and noise and ripple free, the power supply section is not your problem. Also, if the +5V supply was bad, it is very unlikely that the CPU would manage to run anything.

If the fault appears to be heat sensitive, try using a can of freezer spray (or air duster used upside down). Use cardboard to shield circuitry that you don’t want to be affected, then spray on the chips you suspect.

Mark

I've got some freezer spray. I'm not sure which chips to suspect given that they're all brand new.

It's easier to list the chips I've not replaced THIS WEEK...

B: 2, 8
D: 9
E: 1, 4, 5, 6, 8, 9
VDP and 7 RAM chips
J: 10

IMG_1462.jpg (1.53 MiB) Viewed 16853 times

[Dave]

I see what you mean about the new earth strap's size!

The one on my cooker is the same size

If you are "spray happy" - you might as well do C5 and C6 too while you're at it

[1024MAK]

Sorry, I should have been clearer. Start with one, any one. Shield the rest. Wait until the symptoms appear. Spray that chip to cool it a bit (you don’t need to actually freeze it). Direct the spray at the centre of the plastic package (the dead centre is where the actual silicon chip is). See if it has had any affect, reset the machine or power cycle if needed.

If no joy, move on to the next chip...

Obviously, with a lot of new chips, it makes sense to start with any originally fitted chips... Especially the video DRAM chips and VDP.

Mark

[stephen_usher]

I've now tried this and thought that I'd found it, SN74LS27N, 7A as I had CP/M operating for the longest time I've been able to, if I kept the chip cold.

So, I've swapped it for another of the brand new ones. It's certainly a bit better, it now takes 2-3 minutes rather than 1 minute to stop working now. Bad batch of chips?!

None of the chips other than the CPU and VDP (+RAM) get even noticeably warm to the touch.

[1024MAK]

Do the suspect chips work in another circuit? [Maybe breadboard a simple test circuit].
If yes, it could be something that feeds the suspect chip supplying a marginal input signal, or something that the suspect chip feeds, having a damaged input stage and hence once the 74LS27 warms up to nearer it’s normal operating temperature, the output is no longer able to drive a fault input stage properly.

You could also try ‘scoping the 74LS27 inputs and outputs.

Mark