Amiga 500+ Black Video Question

[ricsne2010]

This is for all those Amiga Master Minds....

I have a PAL 500+.... typical battery leakage..... I removed the battery, cleaned the board, check traces using the Amiga Explorer tool online.... pulled and replaced the sockets for the GARY, PAULA, and DENISE Chips, and verified the PPLC socket for the FAT AGNUS was clean.... the GARY chip had corrosion on most of the pins that faced the battery....

Here is what I have verified....

1) Using an O-Scope, verified the 68000 is getting power, is getting a reset on PIN 15 (low to high) and seems to be waiting for ACK
2) FAT AGNUS gets reset....
3) X1 crystal is putting out the correct mhz for the PAL machine on PIN 3
4) the 68000 is getting 7mhz signal

And yet.... after all of this.... I still get a black screen..... the power supply is a brand new after market re-make specific for the Amiga 500/500+ from "The Amiga Store" online....

I have also checked the R408 (I believe that is the 150 OHM resistor just prior to the composite output RCA jack) and even replaced it when I initially measured it (while still attached to the board.... duh big no no .... since it was reading 97 ohms while on the board but when I pulled it, it read 150)

I have also replaced a burned Resistor for the Joystick Port 1 - which should have no effect...

The board is populated with Kicktart 2.04 from Commodore (the chip that came with the system when released)

I have removed the keyboard and the floppy drive.... when the keyboard was connected, the CAPS LOCK key does NOT flash at all.... at any time including when I press it after power on.... the Green LED comes on, but never goes into "BRIGHT GREEN" mode ....


So..... since the technical drawings and technical manual I have don't really spell out specifics for me to understand and troubleshoot this further.... I am asking if anyone might have some ideas on how I can test the AGNUS chip with my o-scope as well as the HIRES DENISE chip....

Thanks in advance all!

(and for those playing along at home, I also have another commodore thread about my PET 4016.... which has taken a back seat for a bit while I clear some space on my work table LOL but I promise I will be getting back to it soon)

Cheers
Rich

 

[C128 Guy]

Get yourself a diag rom.

 

[ricsne2010]

Yeah I think it might be helpful... I will burn a diag rom and see what its doing.... (I am not 100% sure if the ROM is actually at a state where its read based on the documentation that I have about the power on and startup sequence....)

 

[Crashedfiesta]

Not sure if I'll be any help but I've fixed a few A500+ in my time.

Black screen is a symptom of a bad CPU. This may sound stupid, but is it in the correct way around? One of the first boards I fixed I puzzled over for two days before realising I'd put the CPU in upside down...

If Gary had corrosion on the pins facing the battery then I'd assume that U10, U11, U12 and U13 would have caught some of that action too. I'd seriously consider replacing those chips and checking all the traces in the vicinity too. I'd also check all the components just under Gary (battery side). I had one board which wouldn't boot and it turned out that the pad of one of the resistors was disconnected from its track by a crack of about 1/10th of a millimeter. One big blob of solder later and it booted fine... (see this link here).

 

[ricsne2010]

Im planning to build a "free running" test plate for the 68000 to ensure its functional... I have 3 of these processors, none of them get "hot" but do warm up... I have tested to ensure the 7mhz signal is gettin to the correct pin on the 68000 and that the reset goes from low to high using the o-scope....

Corrosion was for sure something that I have been battling - I have removed the battery completely (I didnt cut it out, I removed it from the board by reheating the solder joints and cleaned the pads for a new battery to be installed) - I have run contiunity traces on most all of the lines from the battery and surrounding circuits to ensure the traces are good... so far I have been lucky as I have not had to repair any yet.... as for the GARY chip, I removed it from the board, removed the socket that it was in and replaced the socket with a new one... I then cleaned the pins on the GARY as best I could (using hydrocloric acid on a q-tip, along with acetone and then vinegar and then again with some IPA... but I feel as if the corrosion might have seeped into the chip itself.. I have installed a replacement but both the old chip and new are cold to the touch.... they are getting voltage and also see the reset signal....

I suspect my AGNUS is probably bad.... it warms up, but does not get hot, it also sends out the 7mhz signal to the CPU 68000, and gets the reset signal...

I just came across some docs from commodore's field engineering / repair technican manuals that have some wave forms that should appear on the 68000 and other various custom chips... when I get a free minute later this week I will put them to the test to see what I get...

Also, to respond to the "C128 Guy" - I did get a diag ROM, but as I suspected, it never gets activated since there isn't a DTACK coming from the AGNUS via the CPU.... but I did pick one up and have the serial port connected to my laptop to see what other issues might be going on (once I get past this step)

Until then, here are some images to look at