Need help fixing A4000

[Mr_Vanos]

Hi all... long time lurker here!

A couple years ago I acquired an A4000 with fairly typical battery acid damage from the clock battery, and I set about to fix it shortly afterwards. Frustrated, I put it away and now I have the bug to fix it again. The symptoms I've got are: black screen... that's it. The black screen does have sync at both 50Hz and 60Hz rates depending on how the PAL/NTSC jumper is set on the Alice. I have done a bit of probing around the board but as this is the first AGA based Amiga I've ever tried to repair (and also only the second A4000 I've ever seen... the first was in a Commodore dealer in Denver when it was first introduced), I'm mostly stumbling around to find a solution.

The board I've got has a Super Buster -11 on it, KS 3.0 ROMs, I have a WarpEngine clocked at 28MHz with a 25MHz '040 on it, 2MB SIMM in the chip RAM slot, nothing in the fast RAM slots (and given the damage to the board, I'm doubtful fast RAM will work). The Warp Engine has four SIMMs on it, so it does have some local bus RAM apart from the chip RAM.

I get a black screen regardless of whether or not chip RAM is installed (if I read various guides right, a green screen can be expected if the chip RAM is missing). I've checked the power rails from the power supply and they seem to be all good including the power good signal, I see the system attempting to shift things out of the clock port shift register... data lines on the CPU expansion port wiggle for a few seconds and stop (presumably filling up cache RAM and then stopping). I've recapped most of the board, but no luck thus far.

I was hoping that there existed some ROMable diagnostics I could burn and replace the Kickstart with to help aid in this, but AFAIK none exist. It even seems like ROMwack was removed from later KS > 1.3 which would have been somewhat helpful.

I do have a couple of brand-new CIA chips. Will a bad CIA exhibit this behavior? If so, I'd be inclined to throw those on the board and give it a whirl. Ideally I'd like to have a spare CPU board (A3640 or A3630) to test with since I have no idea if the warp engine is even good.

I have access to SMT soldering equipment at work, oscilloscope, and EEPROM/PAL/GAL burner at work.

Any pointers from veteran A4000 fixers is greatly appreciated.

[Mr_Vanos]

Hey sorry about your problems. Did you try another cpu board? When you power the unit up press the Caps Lock repeatedly. Tell me if they light stays lit after like 7 hits.. this diagnoses cpu.. Even with a bad CIA it should boot afaik. I wouldnt just willy nilly change cias unless you know thats the culprit.  And no there is no "romable diagnostic" roms.

I'm still in the process of looking for a spare CPU board to swap in. I put a feeler out on the marketplace forum to see if anyone had one.

As for the keyboard thing, I did read about that as well. I don't have a Commodore Amiga keyboard, instead I've got a Northgate Omnikey/101. It is configured for Amiga mode, but I've never seen it work with any Amiga. The 4000 is all I have apart from a broken A1200 motherboard too.

EDIT: To answer your question though, pressing the caps lock key repeatedly does nothing.

[Mr_Vanos]

There are diagnostic ROMs available for the A4000, I forget who wrote the software.  The French chap 'Cosmos' was trying to sell copies at one point.  I suspect they would generally not be useful as the hardware needs to be mostly functional to run the software from ROM of course.  The diagnostics appeared to perform some fairly basic functional tests on system I/O, video, etc.  The same kind of thing you'd be able to fault find with a blind monkey and an oscilloscope.

I developed my own diagnostic hardware and software for the A4000 a few years back to aid with many repairs.  It runs custom software from ROM which communicates over the system data and address bus to a piece of hardware that sends progress codes to the serial port of another Amiga that decodes and displays the information to show where the software fails to communicate with sections of hardware.  Works well and have used it many times for the repair of 'dead' boards.  Unfortunately it would be far too much time and effort to turn into a commercial product, given the extremely specialised use and therefore low sales volume.  And unfortunately I lack the time to write enough usable documentation for anyone to build a usable reproduction of the hardware, which is reasonably complex.


As for fault finding the said A4000... yes, certainly the first step is testing with a known good CPU board, even an A3630 is fine.  Of course make sure that the two clock source jumpers are set correctly according to the CPU board.  Check the ROMs aren't in backwards or the two ICs are swapped or anything stupid.  Check ROMs don't have any bent/broken pins.

Bad CIAs can certainly cause boot failure as described, but then again so can a failure of just about every other bit of custom silicon on the board.  CIA failure is unlikely if you know the history of the machine and nothing silly has been plugged into the Centronics port.

Check the status of the various reset lines, all of which are active low.  i.e. the system will only run when _reset = 5V.

Check that all of the various system clocks are present.  Check carefully for corroded/open circuit vias around the real time clock area.  If certain input lines are floating, the hardware can sit in an undetermined state.  Had an interesting fault recently where battery corrosion damaged the mouse data shift register U975 which caused the mouse data line to be floating, which caused Lisa/U450 not to generate the 14MHz clock, which resulted in Alice/U211 not generating the 7MHz clock which resulted in Gary/U150 holding the system in reset state.

Check the RTC/U178 is not getting warm.  When it gets badly damaged from corrosion it can go into a state where it just draws loads of current and causes a bus hangup and therefore a boot failure.  Remove U177 (RTC latch) if there's any signs of corrosion around the RTC circuit, this can cause an address bus hangup if damaged.  The system will boot normally without U177 and/or U178 fitted.

And yes, you should get an instant green screen with no chip memory fitted.  The hardware needs to have all reset lines inactive (5V) and clocks present for this to work.

In general, there's much that could go wrong.  I wrote a basic bit of fault finding info for A3000 non-booting issues here years ago that may help a little more.

Hope that helps.

All good information... thanks. I'll check out your guide. Yeah, an A3630 or 3640 would be useful, if anything to just determine if everything else is good. Back when I tested it before it appeared that all the power rails, resets, and clocks were working. Those are usually the first things I check when I'm looking at a dead board (I troubleshoot and fix a variety of hardware for fun, from arcade game boards to dead computers).  I think I even checked the shift register you're referring to with an O-scope to make sure the data was good coming out of it. Nothing's hot on the board, so I don't think anything is shorting out.

I figured it would be nice if there was a simple ROM that would just twiddle some I/O or do a serial port console for reading/writing memory locations.

[Mr_Vanos]

Magnetic: I just tried to PM you and your mailbox is full.

[Mr_Vanos]

Bad CIAs can certainly cause boot failure as described, but then again so can a failure of just about every other bit of custom silicon on the board.  CIA failure is unlikely if you know the history of the machine and nothing silly has been plugged into the Centronics port.

I know next to nothing about the history of the machine except that it came from NASA via a government surplus auction. It had a NASA asset tag on the front when I purchased it. My guess is it may have seen use at NASA TV since it was fitted with a Video Toaster 4000. Marshall Space Flight Center is nearby and I believe NASA TV is run at same.

[Mr_Vanos]

Try plugging mouse to mouse port and then to joystick port and see if you get any video signal. If possible also try plugging mouse and joystick simultaneously or even better two mouses.

This may sound like a joke but helped me to repair one A4000 Rev B after first trying to find the faulty part myself and with the great help from Castellen.

My A4000 Rev B symptoms:
Since I only got black screen with no video signal. I never bothered to plug in mouse or joystick, but when I did they closed the circuit somehow and gave classic "Insert disk image of Kickstart 3.1". Then I held both mouse buttons to get to Early Startup Menu and notices mouse buttons worked but mouse pointer did not. This gave me boots to change U975 and U976 which I had not changed yet, because they looked ok with no leakage around them.

After changing U975 and U976 A4000 Rev B started to operate normally! Either one of these troublemakers managed to not only prevent boot but also to kill video signal too which fooled me to think there was some bigger problems so before finding the fault I had already PLCC socketed and changed both CIAs, Paula, Super Buster and U460 (video DAC?).

Changing CIAs is very often suggested but believe when Castellen suggested is that that they require someone using Parallel or Serial ports to accidentally break them. Some decades ago I broke A500 CIA by hot unplugging laplink or was it null modem cable. After seeing broken U975 and U976 I am not than eager to hot plugging or unplugging mouse or joysticks either. People plug and unplug them like USB ports for some 10 years which may cause damage eventually. If you disagree feel free to pass on your arguments.

I tried plugging a mouse into the mouse port and attempting to bring up the early boot menu. That didn't work. I did monitor the serial data out of U975/U976 and I believe they are working correctly given the set of inputs they have going to them, so I don't think that's the problem in my case. I am tempted to throw the CIAs on there, just because I have them. I have a couple of PLCC sockets to fit as well, so it's not a big deal for me to do. Couldn't hurt to try.

[Mr_Vanos]

Given the state of how bad A4000's are in the wild, it seems like some simple rommable diagnostic software would be a god-send.

[Mr_Vanos]

I always start at the psu. Are all the voltages good?
Check the motherboard connector.the yellow +5 pin is known to corrode as if its been hot and not make contact. sometimes the motherboard female pins spread also causing a no/bad contact problem.

Assuming they are ok,i check all the jumpers. Its a good idea to move the motherboard jumpers on their pins as i have had tarnished ones not making good contact in the past. Verify they are all in proper default positions.the INT/EXT motherboard jumpers should be set INT on both for the warpengine..

Check the jumpers on the warp engine and make sure you have a scsi hd hooked to it with proper termination.from memory it will still boot just fine w/o a hd,but i want to be sure i tell you right and a drive present is a safe bet. Jumpers are here: http://amiga.resource.cx/exp/warpengine

One wrong jumper somewhere can cause no boot and ruin your day.

Pay particular attention to ram and if possible stick 4 good simms preferably of the same size in the warp engine and set the jumpers accordingly.(ram only need be in 2 proper sockets technically,but using 4 eliminates any question of which sockets to use).

install the warp engine(be sure its fully seated) and power up.Keep in mind if no ide drives and floppy are present,3.1 roms delay boot a long time.

if this still is not working,you might try swapping the 3x roms from socket to socket,they may just be in the wrong sockets.verify they are A4000 roms, i have had 4000's where people installed A1200 roms etc.

 look at the battery area for corrosion on nearby chips for damaged traces.

if that is ok, it could be caps leaking. i have fixed a good few boards that had traces eaten that run under some caps.removing caps and checking for continuity on traces that run under is a good idea before installing new ones. they are notorious around the audio circuit.

Still no go? If it helps i am in texas and could be willing to take a look at it as a last resort. time is short for me always but i will try and help.

meanwhile it would be a good idea to have a known good kbd, the suggested flashing caps key check is useful. you can adapt a 2000/3000 kbd with a common pc din to ps2 adapter if you have one.

Good luck


Mech

Excellent info. Now that you mention it, I think that the jumpers on the motherboard you mentioned are set to EXT instead of INT. I'll move those and try again. The ROMs were swapped and I swapped them to the right position, so that was definitely a problem before, but still no go. Still, I'll try the jumpers again tonight.

[Mr_Vanos]

Yes a dead CIA can zombie the system. In many cases you'll notice one running much hotter that the other. Seen it a couple of times. The more typical CIA failure lets the system boot and just keeps the floppy from working. Did any of the battery damage make it over to the sim sockets... even a little? I've seen that cause rot under sockets also leading to problems.

Plaz

Oh yeah, the acid got as far as the fast ram sockets... so there's definitely concern there. I have RAM installed on the Warp Engine and the chip RAM but no on board fast RAM.

[Mr_Vanos]

Agreed, that would be incredibly helpful in the right situation.  Unfortunately the UART (the bit that makes the serial port work) lives in Paula.  And for Paula to work, it first needs valid clocks that are divided and generated by both Alice and Lisa, so the bulk of the video sub-system needs to be working.  And Paula lives on the other half of the Amiga sub-system on the chip memory bus side.  And to talk to that, the data bridge needs to be completely working in both directions (Bridgette in the A4000).  And to talk to that, the address decoder (Gary) needs to be working.  And to talk to that, the entire CPU subsystem needs to be working.  And... hey that's most of the computer already.

Well, my idea would be to bypass most of those components. If you could use just a timer and GPIO in a CIA, you could twiddle some pins on the CIA, and perhaps even bit bang a serial interface with a clip on RS-232 line driver.

The other way is to replace various bits with known good bits - also known as the 'shotgun' approach.  The problems there are that you need a source of known good bits, and you need to make sure all of your soldering work is 100% else you'll introduce new problems which simply compond with the original one.  Also, this doesn't help at all in finding PCB related faults, which are fortunately rare except in case of battery and capacitor corrosion - unfortunately which are common in the A4000.

That is a bit of a problem as I don't have a lot of known good spares. The CIAs I have are brand new, the mouse is brand new, but I don't have an Amiga branded keyboard (the northgate one does work on a PC in PC mode), no extra CPU card. I have an A1200 board that I could steal an Alice and I guess Lisa, but that board I've never seen work (and has its own PCB damage to where someone other than me did a bang-up "repair" of the CIAs and tore a bunch of pads off the board).

But seriously, if there's been any form of battery corrosion, start with removing both U177 and U178 - the computer doesn't need them to boot.  Replace both U975 and U976 and verify the integrity of all connections to these shift registers (primarily the clock and data lines) are all correct.  It's a bit of a blind stab in the dark, but this area is a very common cause of non-booting faults in the event of battery corrosion.  Even if everything visually "looks" OK.

Also check the analogue bias voltage is around +2.4V on U400/Paula pins 33 and 34.  Capacitor corrosion does bad things to the op-amp U402 which can ruin the bias in the entire analogue stage which can sometimes cause Paula to do some very unusual things.

Good to know. I'll give that a shot.

[Mr_Vanos]

Checked the Paula analog bias voltages as requested... they are +2.31V and +2.34V respectively.

I'm going to remove the RTC and U177 as suggested, and also some of the SIMM sockets for the fast ram (two for sure, maybe three), since there is some corrosion under there. Once I do that, I think I'm going to go back to basics and check the typical stuff.. power supplies good, resets, clocks, and also check the serial data from the io shift registers to see if they are any good.

I may also check that the keyboard data makes any sense too, just to see if the aftermarket keyboard is sending Amiga scancodes.