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by 1024MAK
15 Jun 2019 15:54
Forum: GENERAL DISCUSSIONS
Topic: MTX 512 (debugging thread)
Replies: 31
Views: 370

Re: MTX 512 (debugging thread)

4116 DRAM chips contain 16384 memory locations. But only one bit wide. Hence in the data sheet it says the chip is a 16384 bit memory. But we need the data width to be 8 bits wide (one byte), so eight DRAM chips are needed, each DRAM chip supplies one bit of the eight bit byte. Total memory will the...
by 1024MAK
15 Jun 2019 15:30
Forum: GENERAL DISCUSSIONS
Topic: MTX 512 (debugging thread)
Replies: 31
Views: 370

Re: MTX 512 (debugging thread)

Piggy backing only works sometimes, and not very often. So I think it is not a very reliable method of testing. The problem is that DRAM chips are very complex devices and there are countless ways that they can fail. If the original fitted chip decides to output a logic low at the same time that the...
by 1024MAK
19 May 2019 16:05
Forum: HARDWARE
Topic: Resurrecting an MTX500.
Replies: 219
Views: 4659

Re: Resurrecting an MTX500.

If the resistor is getting hot, yep, a new TIP2955 is needed.

Mark
by 1024MAK
19 May 2019 15:08
Forum: HARDWARE
Topic: Resurrecting an MTX500.
Replies: 219
Views: 4659

Re: Resurrecting an MTX500.

Sometimes the people copying / writing the descriptions and specifications on the suppliers web site / catalogues don’t fully understand the datasheet. And sometimes the datasheets are misleading. The voltages and currents of PNP transistors are opposite compared to NPN transistors, so are normally ...
by 1024MAK
18 May 2019 23:56
Forum: HARDWARE
Topic: Resurrecting an MTX500.
Replies: 219
Views: 4659

Re: Resurrecting an MTX500.

CMOS devices designed to work with 5V TTL systems should be okay with logic one/high levels of 2V and greater. So all the Zilog Z80 series CMOS devices, 74HCTxxx chips and all compatible EPROM, EEPROM and SRAM chips. However with modern EEPROM and SRAM chips, it’s best to double check with the data ...
by 1024MAK
18 May 2019 17:19
Forum: HARDWARE
Topic: Resurrecting an MTX500.
Replies: 219
Views: 4659

Re: Resurrecting an MTX500.

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’...
by 1024MAK
18 May 2019 16:16
Forum: HARDWARE
Topic: Resurrecting an MTX500.
Replies: 219
Views: 4659

Re: Resurrecting an MTX500.

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...
by 1024MAK
18 May 2019 15:10
Forum: HARDWARE
Topic: Resurrecting an MTX500.
Replies: 219
Views: 4659

Re: Resurrecting an MTX500.

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 ...
by 1024MAK
17 May 2019 00:35
Forum: HARDWARE
Topic: Resurrecting an MTX500.
Replies: 219
Views: 4659

Re: Resurrecting an MTX500.

The MTX is not connected to mains earth. If the double insulated TV is causing problems due to the internal Y type suppression capacitor leaking current, connect the MTX case to mains earth.

Mark
by 1024MAK
16 May 2019 17:42
Forum: HARDWARE
Topic: Resurrecting an MTX500.
Replies: 219
Views: 4659

Re: Resurrecting an MTX500.

Don’t forget that the RAM bank switching needs the I/O system to be working okay. Although if BASIC is working, the bank switching for the ROMs must be working okay. Before changing anything, I suggest a short machine code program be run that can actually test if the correct RAM banks can be switche...