Amiga 1200 no Video Out put

[Pior]

I have an amiga 1200, it is a rev. 2B motherboard with 3.0 roms stock and all caps were changed and no leakage or damage to the board. I am using a good power supply which works on my other 1200 and 600 just fine.
The issue I have is It boots up fine and floppy drive & harddrive read but I do not have any Composite or RGB output. I have a 50Hz VSYNC signal but nothing coming out from the the U30 Video DAC from pins 25-AG, 26-AR, or 27-AB. I check the three with a volt meter all three are 0V. I did check the 5V & GND pins to the chips all are good as well. I thought maybe the Video DAC was bad and replaced with a new one but same issue no change. I did check all the traces for continuity from the U30 Video-DAC and all checked out good as well.
I put the DaigRoms 1.2 in and used Ser cable to my PC with Putty since I was not getting any video out of my 1200 and performed all the test everything seem to check out good on the board.

* Another thing I notices is if I put an expansion 4mb card on it the power light goes from dim to bright but the Floppy drive or Harddrive does not respond, seams to work find with out it
* Also one time the kick start screen came up but was very dark I could hardly see it was so faint, but never got it again.

Any help/direction would be great

 

[miggymad]

Might be either a dry joint or bad cap.

 

[Pior]

All the solder joints look good and have good continuities. Which cap do you think could be bad. I did replace all the can service mount caps and threw mount caps

 

[Watson]

The problem could also be a broken E263R Ferrite Bead.

 

[Pior]

I just check that one as well and I get 5V on both sides of the Ferrite as well. That one is good as well.

 

[miggymad]

Check the following:

• R405 4.7 ohm resistor
• R406 4.7 ohm resistor
• 8362 (U4)
• 74HC245 (U40-U41)

Any chance a stray solder blob is the cause? Check the areas around your cap replacements carefully.

The main concern seems to be the loss of power when you plug in the 4mb expansion. IMHO the power circuits should be high up on the list for further scrutiny

 

[Pior]

miggymad your last reply, is the info you noted for an Amiga 1200 rev 2B motherboard. I can not find any of this on my 1200 board or on the PCB explorer

 

[miggymad]

Oppsie my bad! Copied from wrong schematic -too many windows open

Check the chips at locations U43, U45, U46 & U47

 

[Pior]

OK Thanks, what should I be looking for, Voltage?

 

[Pior]

I check all four U43, U45, U46 & U47 chips on Voltage at all pins, then compared it with my good working 1200 with good video, all pins on all four were the same readings no difference on any f the pins. I would guess they are all working properly Any other directions

 

[miggymad]

Test U49 with your multimeter - should be in the region of 5v.

If considerably less (nearer 1v) then temporarily connect pins 1 and 2 and see if that solves the problem. Make sure EVERYTHING is disconnected/removed before attempting this - floppy drive, 4mb expansion etc. - Bare machine only, just power, video cable and keyboard (LEDs if you wish to check that too).

If you see the Kickstart screen when power is applied then there is your area of concern. It's a previously recognised issue with 2A boards where the component has died unexpectedly and needs replacing. The only problem is the part is no longer made - however, the equivalent TC54VC43 (CMOS open drain) works just as good.

See here for more information - this is also a problem for some CD32 boards as well apparently.

Datasheet for the replacement component is here

Following the diagram here shows if U49 is faulty then there is only enough power for the expansion port to initialise, then go no further resulting in the dreaded black screen.

 

[Pior]

Ok Just checked U49 and it piers to be working correctly. I am getting the 5V to all resets and the machine will boot fine but with out video and with the DiagRoms it show all is good working condition using the Serial cable to my PC using Putty. Main thing is No video output it acts like I am not getting the signal to the DAC chip for the RGB signal....

 

[miggymad]

I'm at a loss here too. If everything checks out as you say, and it was working fine before the recap then the only thing I can suggest now is that you check each new cap and the surrounding areas carefully again.

 

[Pior]

It did not work prior to recap So I did recap and same results It looking like D215 maybe missing

 

[cpiace64]

hi, on pin compare on dac i have 2,40/2,60V

check r216 and c216

 

[comraider]

It looking like D215 maybe missing

This is common on many boards, it's not the cause of having no signal. The result of D215 not being populated is seeing some ripples in the video output when disk access is occurring.

Just grasping at straws here, have you tried the digital RGB out lines to see if you get any signal there?

 

[Pior]

That is what it was D215 was missing I installed it and the resister on the back side and now perfect video...

Thanks A1K site helped on this fix

 

[Watson]