ST Rev B troubleshoot no video output

[Steve]

Hi guys,

I'd like some assistance in diagnosing this STM I got recently. It arrived faulty and I did all the basics, clean and re-seat chips and such.

Symptoms are:
When turned on, VGA monitor 'comes alive' as though it detects sync, but no display output. An interesting thing I noticed: On my ST>VGA adapter it has a little orange LED which is *always* lit, even if the machine is turned off. But - on this model it is not lit at all. I'm not sure what voltage this little LED is powered by, but this symptom could be an explanation for the lack of output?

What I have inspected:
* 68000 - HALT is TTL HIGH
* 68000 - BERR is HIGH
* 68000 - CLK is 8Mhz
* no hot chips (to touch) just warm
* Swapped Shifter for a known-good one

Any thoughts appreciated!

Thanks,

[exxos]

Have you tried RF ? VGA adapter may need 12V ?

[stephen_usher]

A) Do you have diag cartridge and something to monitor the serial port output?
B) Do you have an oscilloscope so that you can watch the address and data bus (and possibly ROM select line from the MMU) when you switch on?

Without those items you might as well read tea leaves.

A mostly dead machine like that could be almost anything which will stop the CPU reading the ROM and initialising the shifter. MMU, RAM with stuck high output, bad ROM, bad traces, anything!

[PhilC]

@Steve just a hunch, what does it do when you select mono mode?

[Steve]

Have you tried RF ? VGA adapter may need 12V ?

Doesn't have a modulator, but the VGA adapter works on my other ST so it should be fine.

[Steve]

@Steve just a hunch, what does it do when you select mono mode?

No difference in mono mode - I might swap the TOS roms with known working ones next.

[Steve]

A) Do you have diag cartridge and something to monitor the serial port output?
B) Do you have an oscilloscope so that you can watch the address and data bus (and possibly ROM select line from the MMU) when you switch on?

Without those items you might as well read tea leaves.

A mostly dead machine like that could be almost anything which will stop the CPU reading the ROM and initialising the shifter. MMU, RAM with stuck high output, bad ROM, bad traces, anything!

a) Yes I do I'll definitely use it for this
B) Yes I've got all the tools these days, but it'll be the first time I use them for proper diagnosis so please don't assume I know how to do certain things yet =)

[PhilC]

@Steve just a hunch, what does it do when you select mono mode?

No difference in mono mode - I might swap the TOS roms with known working ones next.

I had a theory you might have had the same issue as me. Have a look at the diodes and transistors around the shifter with your scope, half of the diodes were failing.

[Steve]

Thx @PhilC I'll check! So basically just make sure the voltage across the base/collectors/emitters is similar across the board?

[PhilC]

@Steve I can't remember exactly now but remember transistors are used as amplifiers as well as switches. On my board I had a black screen but vga sync.

Because halt is high it sounds like something is going on.so the diag cart will prove that.

Check the rgb circuit, the diodes and pullups etc.

Reason I asked about mono mode is that my vga adaptor does not work correctly as there is no 12v on the monitor port and to get it to go to low resolution I had to break a connection on the port but can't remember which one yet.

Also if your board has a -20 mmu or glue they aren't directly compatible with one from an STFM or at least mine wasn't.