Sick A600 with partially working Diagrom Serial Outpu

[psoma]

Hi,

Working on an A600 with an yellow/orange screen on KS 2.05 - I plugged in DiagROM and I'm getting interesting results over the serial (red/green flashing screen I should add):

So it's kinda working, but I'm getting invalid characters substituted in with good ones, and my understanding is the count at the start should read 1,2,4,8,16,32,64,128,240,15,170 and 85 so I seem to be losing a data bit somewhere... just don't know where to look exactly. Any thoughts?

Cheers!

F`rb`fd bdfnrd thhr tdxt v`r bhn`rx ntlbdrr: 0, 2, 4, 8, 06, 22, 64, 028, 240, 04, 060 `nd 84
Tn hdlp xnt fhnd bhtdrrnrr tn p`tl`. Nnv rt`rthnf nnrl`l rt`rttptdxt dtb
Z0l@lhf` Dh`fRNL V0.2 , 02,@tf,08 , Bx Jnhn (Bhtbjx.Thd F`nf( Hdrtdll
Tdrthnf hf rdrh`l lnnpb`bj`d`ptdr hr hnrt`lldd: <> NNT DDTDBTDD
, P`r`lldl Bndd $ff , Rt`rt nf RNL, BPT Rddlr rnldvh`t `lhvd
Bhdbjhnf rt`ttr nf lntrdbtttnnr fnr dhffdrdnt rt`rttpr:
Rdt `ll Hntdrrtpt dn`bldbhtr (HNTDN@ $dff08`( tn Dhr`bldd: Dnnd
Rdt `ll Hntdrrtpt rdptdrtbhtr (HNTRDP $dff08b( tn Dhr`bldd: Dnnd
Rdt `ll DL@ dn`bldbhtr (DL@BNN $dff086( tn Dhr`bldd: Dnnd
Tdrthnf hf NVL hr vnrjhnf: Z22lNJZ0l
, P`r`lldl Bndd $fd , Tdrt TDR.LDR lhnd
, Tdrt nf vrhthnf vnrd $FFFF tn $400 Z20lF@HLDDZ0l
, Tdrt nf vrhthnf vnrd $00FF tn $400 Z20lF@HLDDZ0l
, Tdrt nf vrhthnf vnrd $FF00 tn $400 Z22lNJZ0l
, Tdrt nf vrhthnf vnrd $0000 tn $400 Z22lNJZ0l
, Tdrt nf vrhthnf bxtd (dvdn( $ff tn $400 Z22lNJZ0l
, Tdrt nf vrhthnf bxtd (ndd( $ff tn $400 Z20lF@HLDDZ0l
, P`r`lldl Bndd $fd , Rt`rt nf bhhpldlddtdbthnn
@ddr $00000400

[psoma]

Update: I also tested out and have continuity on all the RGA and DRD pins to/from Paula.

[Daedalus]

You're missing bit zero by the looks of things. That could be caused by board damage, or failure of a buffer chip, or failure of any major chip including the CPU. Probably in that order of likelihood too. Start by checking the continuity of D0 and DRD0 to each chip, then check that like for shorts to ground. After that, you'll probably need a logic probe at the least to diagnose further, or preferably an oscilloscope.

Edit: Just saw your update - so DRD should be intact anyway. Check the D lines too though from the CPU to the buffer chip U22.

[psoma]

Cool, thanks mate - have packed up for the night but will check those tomorrow!

[psoma]

Alrighty... buzzed everything out. Continuity is good.

But! DRD0 shorts to ground. That would explain missing the least significant bit in the Diagrom number sequence.

Traces look good... I'm guessing this could also be caused by a fault in any one of the components connected to DRD0?

Cheers.

[psoma]

By cutting traces I've isolated it to Agnus... getting a 3 Ohm short to ground. With Agnus DRD0 out of the equation:

"Garbage before this text was binary numbers: 1, 2, 4, 8, 16, 32, 64, 128, 240, 15, 170 and 85"

So the rest looks good but obviously I'm not getting any sensible video output with a data line down.

Anything that can be done here or is it a case of "replace Agnus"?

Cheers.

[psoma]

Incredibly, I have 2 x 600s here and BOTH have this 3Ohm short to ground issue on pin DRD0!!

The second one also has a VCC leak to ground causing memory to blow up, so it's in a world of pain... the first one only seems to have this DRD0 short to ground issue.

Weird.

[Daedalus]

Hmmm, potentially that's the issue alright. How closely did you isolate the traces? Because it could also be a PCB issue with corrosion in a via or a nearby pad causing the short.

[psoma]

Yeah, was hoping that was the case, but I lifted the pin off the board and in complete isolation from any traces it still measured 3.3ohm.

There's still the possibility that a combination of shorts on other pins result in a short to ground on DRD0, but none of the other pins are shorting to ground, so without power running through it, I'm highly doubtful