A520 not working on A500 with a bad mod

[spawnerbr]

Hi all.
Got an Amiga 500 recently that on the bottom of the PCB has a mod but after some research I can't find any information about the purpose of this mod. First thing that I though was a PAL/NTSC mod. Fixed. But the Amiga works on NTSC video mode. To play PAL games I have to use PAL Booter or Degrader.

The Amiga is working perfectly using a RGB cable to a VGA monitor, but I tried to use an A520 and no video output. Tried 3 A520 (2 PAL and 1 NTSC). No image output. So, i got pictures from other Rev 5 boards that I have and checked that this mod is a cut from Aguns IC and some kind of Bridge to skip one pin. I put the pic here. So, I back everything to check if this is affecting the A520, but same thing. No video output on all A520. Only on mono output (from the board) or using a RGB cable.

I have 2 doubts:

1) What is the purpose of this mod, if this is not to set a full time the Amiga on PAL? The Agnus 8375 318069-17 on my Rev 5 looks a PAL Agnus, but the computer is not on PAL.

2) How check what can be the problem that is not generating video output on A520. All 3 are working perfectly on other Amigas.

Thanks!

[r.cade]

I would make sure none of the voltages are missing... It outputs -12V, 12V and 5V on pins 21-23 and also make sure you have the clock signals on 1,2. None of those are used on regular RGB monitor, but genlocks and A520 may use them.

[pandy71]

A520 may be faulty based on your description... firstly follow r.cade suggestions and verify voltages (preferably in A520 after hooking to Amiga so you will be sure that everything works OK from connectivity perspective - not sure if there is A520 schematics available somewhere).

Seem A520 require +5V and +12V to operate, analog part use +12V where digital part use +5V - it may be issue with MC1377 IC - this middle 70's design, if i recall correctly quite warm, perhaps with tendency to overheat.

Oscilloscope will be very handy for such issues...

[spawnerbr]

Originally Posted by pandy71

A520 may be faulty based on your description... firstly follow r.cade suggestions and verify voltages (preferably in A520 after hooking to Amiga so you will be sure that everything works OK from connectivity perspective - not sure if there is A520 schematics available somewhere).

Seem A520 require +5V and +12V to operate, analog part use +12V where digital part use +5V - it may be issue with MC1377 IC - this middle 70's design, if i recall correctly quite warm, perhaps with tendency to overheat.

Oscilloscope will be very handy for such issues...

All 3 A520 are working. I described that. Thanks

[spawnerbr]

Originally Posted by r.cade

I would make sure none of the voltages are missing... It outputs -12V, 12V and 5V on pins 21-23 and also make sure you have the clock signals on 1,2. None of those are used on regular RGB monitor, but genlocks and A520 may use them.

Checked and the voltages are -12V, -1 and +5V. So the pin 22 is giving me wrong voltage. Checked the voltagem before the EMI404 and still has -12V, -1 and +5V. So the problem is before...

[spawnerbr]

PROBLEM SOLVED.
Started a test with AmigaTestKit disk and no sound. On this time I'm using the mono video output from the 500. So I move the power connector and sound. While hearing the sound, I connected the A520 and video! Move a little the power connector and no video on A520. The problem is the power connector! Thanks all!

[spawnerbr]

Cold joint solder Connector > PCB.

[pandy71]

Originally Posted by spawnerbr

All 3 A520 are working. I described that. Thanks

True - nevertheless issue was related to power supply - it seem that not only A520 didn't worked correctly in your case.