VERY Dim video output from Amiga 4000

[Retronaut]

Before making this video, please watch (this section) as you'll get an idea of the issue....

Amiga 4000: Is the Fastlane Z3 the best home for a BlueSCSIv2 Upgrade?

When I filmed that, I had to post process it A LOT to increase its brightness to acceptable levels for this video. I had assumed that the issue was to do with the RGB to SCART cable (unlikely) or that my trusty SAMSUNG LED with SCART was on its way out (more likely).

However, today, I tested this by plugging in my A500, and the video looks bright and clear, as I remember it. So, A4000 has a problem  .

Calling on the ancient wisdom of the Amiga collective. Would anyone know what this is a symptom of?

My guess, if I were someone else, would be that capacitors were going down the swaney, and providing too low voltage. However, I had this machine re-capped just last year. So its very unlikely this is the issue. However, the recap was because of some leakage in the video area, however, I was lucky in that it seemed to be early days. I have cleaned that area, so Im surprised if its on-going corrosion.

Anyway, over to you guys. Wondering if Christ Edwards lurks here, as I would assume he might recognise the issue...

[Boing-ball]

Before making this video, please watch (this section) as you'll get an idea of the issue....

Amiga 4000: Is the Fastlane Z3 the best home for a BlueSCSIv2 Upgrade?

When I filmed that, I had to post process it A LOT to increase its brightness to acceptable levels for this video. I had assumed that the issue was to do with the RGB to SCART cable (unlikely) or that my trusty SAMSUNG LED with SCART was on its way out (more likely).

However, today, I tested this by plugging in my A500, and the video looks bright and clear, as I remember it. So, A4000 has a problem  .

Calling on the ancient wisdom of the Amiga collective. Would anyone know what this is a symptom of?

My guess, if I were someone else, would be that capacitors were going down the swaney, and providing too low voltage. However, I had this machine re-capped just last year. So its very unlikely this is the issue. However, the recap was because of some leakage in the video area, however, I was lucky in that it seemed to be early days. I have cleaned that area, so Im surprised if its on-going corrosion.

Anyway, over to you guys. Wondering if Christ Edwards lurks here, as I would assume he might recognise the issue...

Could be the Video DAC.

Worth a look here:

https://www.amigawiki.org/doku.php?id=en:parts:video_dac

It maybe a corroded trace from the Caps leakage.

[Retronaut]

Ill take a look

This ship is sinking, glub glub

Thing is though, regarding a trace. I understand that the RGB to SCART is analogue, so I would expect to see glitching, or maybe one channel being dark, and the others ok. But this is more uniformly dark on all 3. The picture is good, but just at 15% of normal brighness.

Wish I could upload an image to show, but no mechanism to do so :/

[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.

[Retronaut]

If you read my post, it says, if I were someone else. i.e. I was anticipating others telling me that it is a capacitor issue.

But in my case, I know its nothing to do with capacitors, as they are all new. Now, could it be failing resistors? Im not sure, as I am not an electronics student.

As you say, its certainly got something to do with the DAC you might assume.

Ill take a look at it, and see if anything sticks out. I do hope its NOT the chip itself, as it will be a tricky replacement,  but lets see.

[Boing-ball]

If you read my post, it says, if I were someone else. i.e. I was anticipating others telling me that it is a capacitor issue.

But in my case, I know its nothing to do with capacitors, as they are all new. Now, could it be failing resistors? Im not sure, as I am not an electronics student.

As you say, its certainly got something to do with the DAC you might assume.

Ill take a look at it, and see if anything sticks out. I do hope its NOT the chip itself, as it will be a tricky replacement,  but lets see.

If it is the Chip and you don’t want to attempt the replacement. Then give Ian a shout at mutant-caterpillar based in Anglesey North Wales.

[Retronaut]

I think I might be able to manage the DAC replacement, though there are some things I need to find out first.

TBH, though it would be nice to practice it on a similar chip on a broken board. Does anyone have a similar board, junk that they could send me (id pay postage)

In other news

So I and a friend (electronics hobbyist) had a pour over the schematics, and the DACs spec sheet and here is what was found.

• The DAC requires a 5v supply to work, I measured this and its 4.84v, so it seems fine.
• The DAC requires a reference 1.2V supply, which comes from a near-by chip, I measured this, and its 1.23V, so it appears fine.
• The DAC has a compensation signal, and thats supplied by (I assume) a 580ohm resistor, so I guess if that was failing it would cause issue. Measured resistor, seems ok.
• The chip has 3 pins for RGB, I measured these and they are 5mV when the machine is booting (black screen) and then jumps to 30mV when displaying Workbench.

The 30mV value is the exact same as the pins on the DB23 monitor socket. So its clear there is no issue between the DAC and the socket.
(later I will measure the same outputs on my Amiga 500, to check 30mV is not correct)

It also appears the DAC is getting all the right inputs as well.
So it appears that the DAC is getting the correct digital signals in, but it is not then multiplying these values out, to the correct voltages?

So, what do you guys think? To me now it could be 2 things.

• Dry solder joint on DAC somewhere, causing odd behaviour.
• The DAC is dying.

Some things I could try

• Heating the DAC, in isolation, to see if its possible a chip failure. I understand warming chips up can sometimes temp fix issues. Showing it IS the chip on its way out
  
• Re-flow the pins for the DAC, its possible a dry solder joint is causing this.
• Replace the DAC :/

[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.

[Retronaut]

Thanks Castellen, an update.. I noticed that if I pressed just to the side of the DAC (between it and U211), the image got a little brighter, though it required quite some pressure.

So in case of a dry joint, I reflowed the pins on that side of the DAC and also on U211. Also the jumpers near there, just in case. No change.

Its lat here now, but I'll give your Capacitor info a look tm. It makes sense that if C460c went wonky, then it would modify the brightness. Me and my friend discussed this. But I did not know the expected voltage out of it, and I did not have a method to measure it in circuit so...

I'll try the heating first. Fingers crossed its this, a pretty simple fix if it is!

[Retronaut]

I tried heating the C460D and C460C Capacitors and it made no difference.

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.

I also checked the SPEC sheet for the DAC, and unless I am mistaken, I THINK the RGB signals ARE going out in spec, have a look at its SPEC doc
https://www.analog.com/media/en/technical-documentation/data-sheets/adv7120.pdf

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

This being the case, I am begining to wonder if my RGB to SCART cable is the issue. Maybe it working on the A500 is a false positive. Because it APPEARS the RGB output is correct.

I wondering, if anyone else has an A4000 out there that can measure its RGB pins and see what they output when a light grey Workbench is displayed.

[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.

[Retronaut]

I came to a similar conclusion earlier today, it appears NOTHING appears out of scope as much as Id expect to see the dimness I am observing. So, maybe my cable has a fault, or the monitor ONLY when combines i.e. THIS RGB to SCART cable, with THIS Samsung monitor.

I DO own a 1084s, and a cable for it, that was previously tested as good.
So why have I not tested it so far? Because Im having a VERY bad back for going on 4 weeks now
and the 1084s is upstairs, and quite heavy.

Looks like Ill have to grit my teeth and bring it down to test. As you say, I fear this is a rabbit hole and in reality the Amiga 4000 is fine.

Lets see!

[Retronaut]

Well, I noticed today that when the machine was left on for a while during testing, say 20 minutes plus, the screen would brighten.
In fact, on a couple of occasions, it actually got back to normal brightness, or very close, actually usable.

So I measured the 3 variables, that could alter the brightness on the DAC and of the 3 it was the FS ADJUST, pin 42 which varied.
According to your sheet, this should be approx 1.236 volts but it was starting for me at 75mV and over time would rise to 250mV.

So I THINK the resistor which drives this may have issues.
To test this, I used a heat gun to warm it whilst on, and it visibly brightened as I warmed it.

So I ordered a direct replacement for it, at 562ohm, though I had to get a 2mm long one, as 3mm ones seem to be very-rare these days.
To go along with it, I also ordered a large 1k potentiometer, and a couple of surface mount ones as options.

If the 562ohm replacement does not work, is a little too dim still say, I may try the potentiometer instead and adjust it to see if there is a different sweet spot.

I also ordered some 100nF and 10nF ceramic capacitors, just in case the COMP capacitors have issues as well, though I don't think they do.
This should all be here Friday, so hopefully I can test this all on Friday/Weekend.