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Old 01 April 2017, 11:25   #1
PARALAX
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Exclamation A600 - No Blue Colour on RGB

I need some help! I got an A600 several days ago with wrong colours on RGB-Port due to missing blue. I already replaced all 47 and 68 Ω resistors on the front side on the board behind the RGB-Port and even the both 74HTC244 IC's but no change. Then I interchange the capacitors for the direct RGB output to ensure that they are not broken. After resoldering I find out that the right capacitor (see picture) obvioulsy don't get any signal. If I solder the other ones seperately I get a red or green picture but the right one gives me a black screen only instead of blue, no matter which capacitor I use. So the problem can't come from the capacitors itself.

Soldering all capacitors as normal only gives me a yellow-brown boot screen as shown in the second picture. I also measure all the connections between the RGB-Port pins and the capacitors with a voltage and continuity tester and they are all okay without interruption, even the blue one. The blue signal must be lost somewhere before capacitors pads but I couldn't find out where. I also measured all the resistors on the front and back side of the RGB-Port and they all have the correct values (but I couldn't measure all the capacitors with my multimeter due to missing function).

Could any one help me how to trace the signal and find out the reason why no blue signal is coming to the capacitors, even when both 74HTC244 IC's are working? I've run out of ideas currently but I want to give up the board yet completely.
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Old 01 April 2017, 12:18   #2
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Just making a coffee and some toast - loading up the a600 schematics - brb =)
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Old 01 April 2017, 12:30   #3
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@PARALAX

Firstly my friend, we need to test the colour of of the composite out - if this is working fine then we can isolate a lot of variables - if not then we will have to look at a little further back along the chain.
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Old 01 April 2017, 12:44   #4
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@Thread

Unlike the A1200 the A600 doesn't use a Video DAC (Digital to Analogue Converter) instead it uses a similar method to the Video Hybrid component (essentially an intensity resistor ladder) that is in the A500/A2000 system.

To produce Analogue Red,Green and Blue it uses the CX1145 IC to drive out the AR,AG and AB respectively.

The output limitations on the Video Port are decoupling with a ferrite bead and a 47pf (pico Farad) for ripple reduction/attenuation.

So what could this problem be - well I suspect you will find this issue is related to low VIO+ (bad capacitor(s)) or the CX1145 having a pad fault or a fault in it's self since you have replaced the 74'244's

Last edited by Zetr0; 01 April 2017 at 13:28.
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Old 01 April 2017, 13:01   #5
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Quote:
Originally Posted by Zetr0 View Post
@PARALAX

Firstly my friend, we need to test the colour of of the composite out - if this is working fine then we can isolate a lot of variables - if not then we will have to look at a little further back along the chain.
Sorry, I forgot to mention that. The Composite-Output is fine - no lost colours. So I think that there is no issue with the Denise-Chip but maybe one of the other (blue) resistors or capacitors around the 74HCT244 or anywhere on the back side of the board? I don't know...

A PCB-Explorer like with A1200-Boards could help but I only have a circuit diagram here from V 1.5-Boards (I have a V 1.3 Board here). So I can't examine all the traces in detail.

Last edited by PARALAX; 01 April 2017 at 13:09.
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Old 01 April 2017, 13:22   #6
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@PARALAX

Hello my friend, firstly this is good news - secondly you need to change the thread heading to A600 not A1200 as the video processing is vastly different.

Firstly it should be said that on the Amiga A600 the CXA1145 is responsible for making the Analogue Red, Green and Blue used in the Amiga Video output - this is not the case with the A1200.

Since you are working on an A600 here its important to use the right schematic -



I would test for continuity from pin 21 of the CX1145 to -

R231C(1k) then check its resistance
This is a constriction from output to reduce voltage

R232C(33k) (check resistance)
This is a pull down to ground to balance output

Q233(2n3904)
A transistor to open the output gate for the voltage to flow VOUT controlls this and you will need to test for VOUT when the system is live - check the VOUT of both pin 23 (RED OUT) and pin 22(GREEN OUT).

R233C(150 Ohm)
This is a pull down to ground to balance output

R234C( 75 Ohm)
This is an in line resistor for reducing voltage
--- its entirely possible that some capacitor leak may of damaged the pad (and or) track from PIN 21 (Blue-Out) from the CXA1145 - so perhaps an inspection - continuity test and solder re flow might help here.

Good luck =)
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Old 02 April 2017, 14:20   #7
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A1200 or A600? Anyway, both are quite susceptible to damage to one or more of the RGB lines from capacitor leakage to the left of the IDE port. I've seen it on several Amigas now, so that's what I'd put my money on. Not a huge deal to repair, but might need some fine soldering work i the area to rebuild damaged traces.
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Old 03 April 2017, 09:58   #8
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Quote:
Originally Posted by Zetr0 View Post
--- its entirely possible that some capacitor leak may of damaged the pad (and or) track from PIN 21 (Blue-Out) from the CXA1145 - so perhaps an inspection - continuity test and solder re flow might help here.
That's what I think as well. I recapped this board some days ago and even on the side around Pin 13-24 I noted some heavy verdigris due to leaked 10 μF capacitors but trying to resolder this side of the chip didn't help.

Here are my results of the resistor tests:

R231C: 0,991 kΩ
R232C: 11,8
kΩ (instead of 33)

Board Backside:

R233C:
150,1 kΩ
R234C: 74,9

So at least R231C, R233C and R234C should be okay but there is no connection from one of them to Pin 21 of the CXA1145M. Instead I get a connection from Pin 23 to the 1 kΩ resistor. Also I'm a bit confused of the low value of R232C. There are no labels on the board for this resistors, maybe due to lack of space? Did I measure the right ones (see picture)?

Also I couldn't figure out how to test the transistor and voltage on Q233. As I'm not so familiar with circuit diagrams like with the PCB-Explorer on A1200, could you mark the both measure points where I have to test the voltage please? You can do it with the pic attached in this post.

Would it be possible to contact you via skype or elsewhere for better help in case that I have no luck here?
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Last edited by PARALAX; 03 April 2017 at 10:07.
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Old 03 April 2017, 10:52   #9
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@PARALAX

Hello my friend, I don't want to waste any of your time, and trying to pull a working signal from that IC would be - so sadly you are going to have to remove the CXA1145 and the Q233 transistor (2n3904) -

I would also humbly suggest using an organic acid - like lemon juice to neutralize the base that has built up in that area - once that is done than clean with isopropyl alcohol.

here are some notes

Code:
Inspection - 

 1) Track below C460 : looks like its damaged (cut)
 2) Replace Q233 (2n3904)
 3) Replace CXA1134M 
 4) Resistors noted in RED as R231C and R232C are in fact R231A and R232A*
*R231C and R232C : if memory serves are on the underside of the motherboard.

There is a lot of corrosion on this board my friend, it will make it hard to test voltages and resistances with it still there.

You might be able to save the CXA1145M encoder once you remove it from the board and give it a good clean - do you have a sonic cleaner? you can get these from eBay for about 10 Euros and they are well worth it for cleaning small PCB's and components.

Alternately you could replace the components

CXA1145M Sony encoder
Q233 : 2n3904 - SOT 23

You should be able to get both of these components from local(ish) sellers for under 10 Euros =)

So I would at this point instead of trying to get it to work with what you have - remove the CXA1145 and Q233 transistor - clean up the area with an organic acid to neutralize the base (lemon juice or vinegar) "leave for about 10 minutes on the PCB" then clean with IPA (use a tooth brush in gentle motion).

Recently I have been watching RetroGameModz YouTube channel recently and I think this particular video is relevant on how to deal with corrosion
[ Show youtube player ]

Last edited by Zetr0; 03 April 2017 at 11:05.
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Old 03 April 2017, 17:59   #10
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I had the same problem with a A600 recently ,there was no Red so I pulled out my cheapo logic probe and went around the components checking for the RGB signals and I found it within a few minutes ,mine was the Q231 transistor it was bad, changed it with one from a donor A600 and its all working again.

The good thing with a logic probe is you could see if the CXA1145M is outputing the RGB signals like Zetro said and go from there.
I would swap out the Q233 and try it anyway as it seems these SOTS are prone to go bad.
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Old 03 April 2017, 22:50   #11
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Today I found a loose connection somewhere around the video output after reflowing the CXA1145M. After this I even get a perfect output with blue colour from RGB but it was unstable. So I did remove, clean and resolder the CXA1145M. Also I cleaned the pads and everything under the chip with isopropyl alcohol from the leaked capacitor. Same with the three transistors Q231-Q233. Then I discovered interruptions from Pad 20-22 to the circuit lines which I resoldered as well. So normally all connections should have been reconstituted. But instead I still get not blue output and now the blue output is even missing on Composite which was working before. I don't get it!
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Old 04 April 2017, 00:17   #12
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Problem solved! Seems that there was a bad connection on Pin 4 (Blue Input) at the CXA1145M. After reflowing the chip everything works again with full colours and without slack joint. Thank you, guys! Another Amiga saved and I think the owner will get lucky low.
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Old 04 April 2017, 11:08   #13
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Well done PARALAX!

did you take any pictures of your finished work?
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Old 04 April 2017, 11:09   #14
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@Mods

Could we change the title to "A600 - No Blue Colour on RGB" instead of the current one as this is relative to the A600 not the A1200

Cheers guys!
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Old 04 April 2017, 20:32   #15
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Quote:
Originally Posted by Zetr0 View Post
did you take any pictures of your finished work?
Sure!

Vorher = Before
Nachher = Afterwards
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Last edited by PARALAX; 04 April 2017 at 20:38.
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Old 04 April 2017, 20:38   #16
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Excellent!

That looks a lot better
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