520STM vertical lines yellow and grey, no hi res

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russellnash
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Re: 520STM vertical lines yellow and grey, no hi res

Post by russellnash »

The error(s) is address 8 bits 0 to 7. How do you know where to look on the board for such errors?
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frank.lukas
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Re: 520STM vertical lines yellow and grey, no hi res

Post by frank.lukas »

When MDATA 0-7 are completely wrong and it is a 260/520ST may be U23 or/and U27 are faulty. If it is a STFM U58 or/and U68 can be faulty.
russellnash
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Re: 520STM vertical lines yellow and grey, no hi res

Post by russellnash »

Thanks Frank. I had a look at the schematic. I do not know what G EN means on pin 11 of U23 and U22 but no voltage appears there. These are connected to pin 9 Latch on the MMU, no Voltage appears there either.
russellnash
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Re: 520STM vertical lines yellow and grey, no hi res

Post by russellnash »

I understand little about the problem. D0 to D7 show errors i.e they are TTL high which means 5V when they should be low 0V? Where should they be 0V, at the MMU or CPU or Shifter or everywhere? I have checked continuity between MMU to U23, U27, Shifter, CPU and RAM. No breaks in the circuit. The D0 to D7 all have 5V on them from wherever I test them. I have read the Field Manual but it doesn't explain what these error messages from the cartridge mean. It says that 8 bytes of ROM are mapped into FC000-7 at adresses 0-7. does that mean TTL low i.e. 0V? No idea.
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Re: 520STM vertical lines yellow and grey, no hi res

Post by exxos »

Have you checked ras,cas from the MMU to ram ? Check uds,lds from CPU to MMU ..

Your ram could be simply duff. They don't take much abuse and high prone to failure on STMs.
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russellnash
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Re: 520STM vertical lines yellow and grey, no hi res

Post by russellnash »

I have been reading a lot of information, firstly from the Service manual and then from RJ's topic on the Mega ST.Ras, Cas from MMU to Ram have continuity but they do not pulse when I turn on, or ever, which I believe is what you can normally expect, and therefore they are shown as TTL high in the error message. UDS and IDS from CPU to MMU also have continuity. 8 Data lines have the same problem. How likely is that all eight are broken? I tried my MMU 4mb Adapter which although I have problems with it is known to boot, as a way of trying other RAM and the results are identical. When the cartridge is inserted I get a red screen with white vertical lines. Red is the normal error screen as I understand it. One thing that happens is that pin 17 Halt on the CPU swaps to 0V for a while and then back again and repeats this process. BERR stays high during this the whole time from the start. I have tested the Reset circuit and seems to work fine. I don't know how sensitive the CPU is but being as I know that the original power supply was giving out too many volts maybe it was damaged?
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Re: 520STM vertical lines yellow and grey, no hi res

Post by russellnash »

Found something. I could smell bit of burning which I didn't notice before. RAM chips U38, 34 and 30 become so hot I can't touch them for more than a couple of seconds. Pins 2 and 14, D1 and D0 on the circuit diagram have 0.1V on them, but not the others. The other chips are also cool. I guess that means they are defective and that it is the cause of the boot problem rather than a symptom. I assume the the STFM RAM 0111 would be suitable as a replacement?
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Re: 520STM vertical lines yellow and grey, no hi res

Post by exxos »

Yep #0111 is for STM. sounds like a good start to replace all the ram and put them in sockets. I would also suggest replacing the large cap on the motherboard as the PSUs are pretty bad and could be killing stuff.
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Re: 520STM vertical lines yellow and grey, no hi res

Post by rubber_jonnie »

Using a multimeter to measure voltages on data lines (Or any TTL state) is really quite impractical, because they will change state far faster than your multimeter can detect reliably, unless it's something like /HALT or /RESET on the 68000, which will be low or 0v when active and high, 5v, when inactive. The only time you might be able to get a solid 5v/0v reading is if a bit is stuck at 1 or 0 and the state simply doesn't change.

You won't be able to detect this transition with a multimeter. A logic probe (or analyser) is cheap on eBay, £10 or so and can can give you a LOT of information about the TTL states.

So when you hit reset, /RESET & /HALT will drop to 0v briefly when they becomeactive, and should return to 5v when the machine starts to boot.

You really need as a minimum a logic probe that can detect these state changes, or if you can stretch to it (Impractical if you just have one machine to fix) an oscilloscope.

In my experience of a similar problem on a Mega, the problem was caused by broken data lines between the CPU and MMU, and my logic probe showed that the data lines were in an indeterminate state.

If you have hot chips, then they are likely shorted internally and dead, and you should replace them.

I saw you'd checked continuity of the data lines to the MMU, but have you gone through the RAM banks and checked continuity of the address and data lines through the whole set of RAM chips?

Do a bank at a time and make sure they are connected through the whole bank and back to the MMU and CPU.

You should also check the CAS/RAS lines but be sure to understand the whole column/row layout before thinking continuity is broken.
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DoG
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Re: 520STM vertical lines yellow and grey, no hi res

Post by DoG »

I would go on Frank's suggestion and check the controller chip (between mmu and ram) since 8 bits in a row is faulty. A bit strange if 8 ram chips is broken side by side
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