NMS 8245 black screen repair

Expert (83)

16-11-2017, 22:21

I have this dead NMS8245 , and tested the following using oscilloscope

- Clock signal ok on Z80, exists on VDP pin 64 (21 Mhz) but seems a bit flaw and instable, it shows 1,20 vpp and 1.68V max is this normal ?

- Address bus signal , seems ok on all bits, it's squared and correct in aplitude

- Data Bus, it's surely flaw , can you suggest the cause ?

If clock on VDP is not ok, I think I'm going to replace the quarz, but if it's ok I would go and remove the rams, I can then plug a 64k expansion to see if ram is the problem, would this be a good starting point ? What next ?

Edit: Checked the EPROM using minipro , it reads ok

By gdx

Enlighted (6438)

17-11-2017, 10:53

It seems that the battery was removed because it sank. Check the tracks and components around this location. The liquid seems to have done damage.

By marcoexo

Expert (83)

17-11-2017, 12:17

Yes I removed the battery, I scratched tracks around it, the scratched part is gnd, I don't see any other damaged track I'll check more anyway

By marcoexo

Expert (83)

27-11-2017, 00:31

Made a little progress on this, and found a missing signal on rom enable... It was caused by the little jumper setting labeled TST1 , to disable rom for servicing, connection to the board was faulty so I repaired with small wire at destination.

By marcoexo

Expert (83)

27-11-2017, 00:54

...Now I have all data and address lines correctly flowing pulses, but still black screen.
VDP seems to receive the signals data, address and clock, and provides RAS and CAS for VRAM , but CSR (VDPCR) , CSW (VDPCW) all stays fixed high and I don't see any /VDPINT generated, also don't have any /IORQ on cpu (stays high)

There's absence of any write to VRAM, R-/W signal stays high also, I believe this should suggest a broken VDP ? (of course R/G/B signals stay all at fixed voltage , no signal pulses are generated)

edit : R-/W signal from vdp to vram is always high, suggesting vdp never runs, i believe that this signal should always show activity, right ?

By Wierzbowsky

Guardian (3649)

27-11-2017, 09:35

Is RST set correctly after startup on both CPU and VDP?

By marcoexo

Expert (83)

27-11-2017, 10:47

Yes just double checked on vdp and cpu pins, rst line stays normally high and goes low briefly when I press reset button.

By marcoexo

Expert (83)

27-11-2017, 12:28

one consideration, if both CSR and CSW show no activity, does it mean that the VDP is not invoked at all at boot phase ? This would mean the problem is not VDP I guess...

By marcoexo

Expert (83)

03-12-2017, 15:51

Going on with repair... I have found that when removing the Eprom, turning on or resetting causes the relais on the board make the typical click, but it doesn't when eprom is inserted back. Tried to reprogram a 27c512 but no luck, still does the same.
Something is certainly going on between cpu and other components when I turn it on. I have clock signals and data, but I still have bad waveforms on D1, D3, D6 (not perfectly squared signals)
Still no INT between CPU and VDP.

Can someone explain why do I heard a relais click only when no eprom is inserted Shouldn't it be just a hardware click does the relais depend on boot sequence ?
I read somewhere I could change the eprom and use a basis diagnostic one, what rom file can I use on this machine ?
Thanks for helping

By Pentarou

Hero (563)

05-12-2017, 19:03

Today my 8235, for some unknown reason, decided by itself to change the text color and have it the same as the background color... I spent a few hours trying to find why it was showing the boot logo but not the basic prompt

Anyway, while I had the machine opened I tested a few things: The relay will click without an EPROM/ROM, so that isn't strange.
Seeing as you had corrosion in the RPC (clock) area, I remembered reading somewhere that a MSX2 will not boot if it can't access the clock chip and so I tried to short its address lines. When I had them shorted the MSX would NOT boot (black screen, no CSR/CSW) but there was activity on the bus lines. Did you check that IC?

By Grauw

Ascended (10821)

05-12-2017, 19:50

Did you try typing color 15,4,4 (blind)? If that helps, type “set screen” after and you should be good again. Maybe replace the battery…