VERY Dim video output from Amiga 4000

[Castellen]

My guess, if I were someone else, would be that capacitors were going down the swaney, and providing too low voltage.

I don't know why nearly *every* computer issue is somehow attributed to capacitors, and they've got nothing to do with 'boosting' any voltage.  In the Amiga, most of the electrolytic capacitors are used for AC decoupling on DC supply rails, which essentially means they'll remove some of the noise on DC lines.  You'll find the computer will mostly run without the electrolytic capacitors fitted.

Anyway, I'd agree with the suggestion that it's likely to be related to an issue around the video DAC area.  I had a similar discussion around 'too dark RGB video' in this thread: https://forum.amiga.org/index.php?topic=76335
This was A1200 specifically, though the A4000 circuit is similar.  Check the DAC Vref is close to 1.2V to begin with.

If you get stuck, you can compare with another machine, or I've probably got some handwritten servicing notes I can scan as a point of reference.

[Castellen]

The too dark image is the result of the analogue levels on the R, G, B outputs being too low, and there's usually three factors involved:
1. Incorrect DAC reference voltage, though you've confirmed this is correct at 1.2V

2. If C460C or C460D has become resistive, which can happen, it will be pulling COMP (DAC pin 40) lower than expected, my service notes (attached) show this typically measures 1.7 - 2.5VDC

3. Some kind of impedance mismatch with your monitor, resulting in the R, G, B levels being excessively low, though that's unlikely given the monitor works correctly with the A500


It's not common for the DAC ICs to just 'go dim'.  They typically either work, or they don't.  And it's rare for these ICs to fail.  I've been repairing Amigas for around 30 years and I don't recall seeing a single DAC failure.

I'd suggest using my notes attached for reference, or compare with another A4000.  Pay attention to the DC level on DAC pin 40 to begin with, try removing C460C (100nF) on the bottom side and see if that makes any difference.  You may also find that heating the ceramic capacitors with hot air can temporarily 'fix' the resistive condition, which usually returns once it cools.

You could also check you have video data on all 24 digital inputs to the DAC, though that doesn't match the fault description.

[Castellen]

I measured the voltage coming out of these two, and its showing 3.9v, which seems way above the 1.5 - 2.5v your sheet indicates is correct.

It's probably OK, I would have noted those values while working on some working systems, and it's likely there will be some variation between different machines.  If you see the datasheet you referenced, you'll see the voltage on COMP is a function of the reference voltage and the 'full scale adjust' input.

And you'll see it is saying the outputs are in the 20-30mV range...

I'm curious about how you're measuring the RGB output signal.  It's a complex waveform (see page 7 of your referenced datasheet), not a static DC voltage.  It's also important to note that the RGB outputs of the DAC are current sources (see the explanation on page 8), meaning that the voltages observed will depend on the impedance presented to the RGB outputs.  What that means is that unless the monitor with your SCART cable terminates each of these three signals with 75 Ohms, the voltage won't be correct, which could explain your issue.

As a point of reference, I've measured one of the RGB outputs of an A500 at the 23-way connector, with the signal driving a 1084S test monitor, correctly displaying the RGB colour bars test screen on Amiga Test Kit at the normal/correct intensity.  A correctly working A4000 will show the same waveform.  See attached, which shows three complete horizontal lines.  Given the large proportion of the white level, the three lines shown likely correspond to the horizontal white bars displayed on the test image.  The scope is showing 500mV/division.  You'll note the 'blank level' is at 500mV, while the full scale 'white level' is around 1.6V, or 600mV above the 'blank level'.


This might be a good time to check the unknowns around your SCART cable if there's any doubt with that, else you may be going down the rabbit hole of fixing something that isn't broken.  Another idea is to connect the A4000 to a known good RGB test monitor such as a 1084S using the standard RGB cable, which will isolate your issue to the A4000, or to your monitor/cable.