A3000D - NTSC Screeen Issue

[Tumbleweed]

Hi all,

I have managed to damage my A3000D as a result of foolishly playing around with it - installing a TF4060. The TF4060 is not the problem. The problem is user error, I should just leave things alone.

I foolishly tried to use the TF4060s onboard IDE with a 2.5 inch CF card adapter. The TF4060 needs the INT2 mod to work. The A3000D doesnt have the INT2 mod.

In trying out the CF card adapter with a SD to CF card adapter I managed to damage the RDB on the SD card which I've been using as the hard drive for the 3000D using a SCSI2SDCard. Thinking I had a hardware issue I disassembled the 3000D and after lots of fiddling I managed to install OS3.2 (the 3000d has 3.2 Roms). All good so far.

I then tried to use NTSC Hires-Laced and thats when the problems started. Shuddering video. I switched back to PAL - no problems. Back to NTSC Hires-laced shuddering video.

Also I have a PicassoII card - the pass through from the 3000D VGA became unstable - black screen. Plugging the VGA from the monitor into the 3000D VGA - picture and all good. The PicassoII I think is ok but I have not tested it in another Amiga. My B2000 has the same card and I'll use it to test the one from the 3000D.

I have read that shuddering video can be a pain to track down and requires lots of tech I dont have (good quaility oscilloscope etc).

Is there anything simple I can try first? Ive checked jumper settings. Cleaned the board with isopropy alochol; pressed down on chips etc etc.

Tnaks in advance.

Weed

[kolla]

It seems unlikely that you have destroyed anything.
It’s not just a matter of the Amber switch being on or off?
You don’t specify whether you have tried with or without Amber active.

What monitor do you have attached to the RGB/VGA?

[Tumbleweed]

Hi Kolla,

I hope you're right - unlikely Ive damaged something. Ive tried the switch - Amber on / Amber off. Last night I changed the setting on J200 - was on pin 2-3; I changed to 1-2 - made no difference. Changing it back again - also made no difference. My thinking was to default it to NTSC - looking at:http://amiga.serveftp.net/a3000_hardwareguide/jumpers.html J200 switches between NTSC and PAL.

Just had a thought - maybe I dmanaged the pins in the VGA port. But if I had I would have though the it would also have impacted the output when set to PAL.

AGNUS is 8372A - 318069-03 [which is PAL]; DMAC 390537-02 and Ramsey 390541-04; Buster is 390537-02. The DMAC / RAMSEY combo looks to be ok based on: https://www.retroisle.com/commodore/amiga/Technical/Firmware/customchips.php

Thanks for your help.

Weed

[Tumbleweed]

Have managed to find some time to do more testing.

Symptoms:

Fuzzy / unstable video after switching to NTSC from PAL. Video is stable and clean when in PAL mode.

While cleaning the board with isopropyl alcohol (A3000D switched on) the fuzziness improved. Soaking the area round Amber removed the fuzziness entirely. I then waited for the broad to dry and switched it back on - fuzzy video again. This time I soaked the area to the left of Amber, round the resistors R484, R454 and R477) remioved the fuzziness entirely. Checking the resistors I get 10 Ohms on R484, 3.4 k on 454 and 33.1 ohms on 477. Looking at Chuky's Component Locator Chucky 1.41 R454 should be 10k. My board is a Rev 7.3. Any ideas why resistance would be less than 10k - I measure its less than half.

Bypassing R484 and/or R454 removes the fuzziness. I'll try replacing these two resistors and see what happens.

Weed

[Castellen]

Any ideas why resistance would be less than 10k - I measure its less than half.

Are you measuring this in-circuit?  In which case you're measuring a complex impedance network, or in other words, you're measuring across other components in parallel with that resistor.  Therefore you'd need to unsolder one end of the component to test it correctly.

It's unusual for a resistor to become low in value, so that wouldn't be my first guess.  But it's just a pullup resistor, the exact value in that location will make little difference to the circuit.

Bypassing R484 and/or R454 removes the fuzziness. I'll try replacing these two resistors and see what happens.

I didn't see if you mentioned if the video was OK on the 23 pin RGB port or not, my skim read may have been too hasty.  But if the 15kHz video is OK, and the 31kHz video on the VGA port has the issues as described, that sounds like an issue with the 28MHz video clock.  That is created by a phase locked loop (PLL, U481).  This circuit is fairly sensitive, and it can be affected by corrosion damage from a previously leaked RTC battery.  You'd usually remove components that are visibly corroded, clean corrosion from the PCB surface, and replace the components with new ones.

If there's not been previous corrosion damage, then the solution might involve a simple adjustment of the PLL clock phase.  With the video artefacts visible, turn VR470 (beside the flicker fixer enable/disable switch) 1-2 turns in one direction to see it if improves.  If not, turn it back to the original position, then 1-2 turns in the other direction.

Failing that, the PLL centre frequency (28.375MHz) may need adjustment, which involves making the PLL run open loop (J482 on pins 2-3), measuring either the 28.3755MHz output (J483 pin 1) or the output /2 (14.18775MHz at J483 pin 2) while adjusting VC470 for a free-running centre frequency as close as possible.  Obviously you'll need a frequency counter for this, or a scope with a frequency counter.  Then replace jumper J482 pins 1-2 to run closed loop.

A quick Google search just found this well written PLL adjustment guide:
https://www.tsb.space/knowledge-base/a3000-flicker-fixer-repair-and-adjustment/

[Tumbleweed]

Thanks Castellen.

I hooked up my 1084 to the 15KHz and it looks to be ok. There is flicker visible but not the phasing I can see on the VGA port hooked up to my LCD. There is no visible sign of corrosion from what I can see (see pics).

What sort of scope would do the job? These can be pricey and Ive looked at obtaining one previously but the price has always put me off.

Thanks.

Weed

[Tumbleweed]

Pushing down on VC470 seems to have sorted it. Switched off. Fitted my Picasso II and the pass through is working.

Weed

[Castellen]

Pushing down on VC470 seems to have sorted it. Switched off. Fitted my Picasso II and the pass through is working.

That suggests that the trimmer capacitor VC470 may be intermittent.  That would fit the symptoms, since it would affect the PLL centre frequency, which will impact the performance of the scandoubler.

Personally I'd replace VC470 to prevent the problem resurfacing, but that will require recalibrating the PLL centre frequency as described above.

What sort of scope would do the job? These can be pricey and Ive looked at obtaining one previously but the price has always put me off.

You can do a lot with a simple monochrome 60MHz dual channel digital storage scope, which are easily available on the second hand market and not expensive.  Since I rely on my test equipment for a living, I stick to professional brands such as Tektronix, HP, Agilent, Fluke, etc.  I've seen plenty of people have good success with the many Chinese brands out there, though your mileage may vary, and the user interface can be hit and miss.  Keep in mind that you'll then need to learn how to use it, though there are probably countless internet videos on the topic, if you're into that kind of thing.

Another suggestion would be to buy a simple used frequency counter.  Something that counts up to 100MHz would be fine.  You can sometimes pick them up for $20 since they're considered old-tech and no-one wants them anymore.  And in this case it won't matter if it's out of calibration.

[Tumbleweed]

Thanks Castellen.

I'll replace VC470 and also look at replacing the other compoents U481 and the OKI buffers as per the link in your other post. Second hand scopes maybe not as expensive as I thought. I found this:
https://www.ebay.co.uk/itm/326366618527?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=z6DJe-b4TpW&sssrc=2047675&ssuid=&widget_ver=artemis&media=COPY

2 Channel up to 100 Mhz.  For less than £100 roughly US$80.

Thanks for your help once again.

Weed

[Castellen]

I'll replace VC470 and also look at replacing the other compoents U481 and the OKI buffers as per the link in your other post. Second hand scopes maybe not as expensive as I thought.

I'd recommend only replacing the parts likely to be defective, just VC470 at the moment.  U481 is clearly working OK, and it's unlikely to be intermittent.  If the de-interlacer is working correctly, there's no need to replace the Oki field memories.

To help with testing, I've copied the Commodore test images for testing/calibrating the A3000 scan doubler and de-interlacer to my webserver:
http://amiga.serveftp.net/temp/A3000_video_adjust.lha

That HP 54600B scope is a good instrument at that price, it's ideal for what you'd need.  My only concern is they don't guarantee it being functional, it states "Item powers up unable to test as we do not have the knowledge to".  WTF, that seller deals in a huge range of advanced test equipment, yet they don't know how to conduct a basic functional test of a simple oscilloscope?!  That would normally ring alarm bells, though that appears to be the standard disclaimer they put on every Ebay listing.  I'd check first if they will refund/exchange if it turns out to be defective.  Or ask for a photo of the display showing a sine wave or whatever on both channels, they've got dozens of other listings for signal generators which can do exactly that.

[Tumbleweed]

Thanks Castellen. The low prices look suspiciously low vs prices from other sellers which are at least double. I'll may be seer clear. I had a l ook at some low cost Chinese ones on Amzon.

https://www.amazon.co.uk/OWON-Oscilloscope-SDS1102-Professional-Generator/dp/B0B4294BY1/ref=sr_1_23?dib=eyJ2IjoiMSJ9.nSpJY7iLmlMuSujMS8NT6xUVn7dLYzQBQGvgAHn0FNA3ycm3MZqIQ_rTLORJNm4t-QYEuGgGI5TJdCes1jnxDg0vmO6wMaiYSk4UkTZVaK-WUN0wqNvvO9b1I-ONkRnXI-6k-NyODdFeRCrn_bH4C2wfiU3QsuuNT9Ve-CtE-9HeiYKj7-aeJhZ4ckG4tydEzf4tJ-a0AxzTrrSvo6fvzF1Y_E0Dc2GG7DrmIf9po4Iuu35PRIkLAfzsnv-ecj0Cn49z1QJfkGYktBMQ0f5gOtjBhAX1ETpnBIpyxSTeA2lM26uraIo5oEnpIFznqxu8zIQPCvtY7oqaALiwX4_Bd2bbSsyIib6v2lhr71YgBeN28XLxrHPNyQ7J0XDFUKqeB4E8edxslGOlYtG4Pc4aj8c9dywyHIEnlXOf1t6imQ5GhSn2PnjchsVEY6t02Om3.tT5RrPyRgUK2wbVqGxuZijtiH9bOO15rzv-Domgvp2A&dib_tag=se&keywords=oscilloscope&qid=1738743042&sr=8-23

Also thanks for the test software. I'll have ago at running it when I can get a chance. Loosk like it will be the weekend again before I can some time to look at it properly.

Weed

[Castellen]

The low prices look suspiciously low vs prices from other sellers which are at least double. I'll may be seer clear. I had a l ook at some low cost Chinese ones on Amzon.

I suspect the HP 54600B seller is genuine given their extensive inventory and sales history.  It would help a lot if they put slightly more effort into the sales detail.  They've gone to the trouble of powering it on and taking photos, it only takes seconds more to connect a signal generator to show it's fundamentally working.

You could potentially get a good deal with that, but definitely get something in writing that they will refund/exchange if it's not working, or ask for a photo of the display with some kind of waveform on both channels; even the built-in square wave probe calibration signal is fine.


But the Chinese scope looks OK for home/hobbyist use as well, and you'll probably get a set of probes with it, which I doubt they'll include with the HP 54600B.

[Tumbleweed]

I opted for the one of Amazon in the nd. Arrived today. Ive desoldered V470 and replaced it. That said I may have used the wrong size. What is the capcitance of V470? The one I removed is blue and the one I relaced it with is white.

I hooked up the scope to pins 2-3 of J483 with and put the jumper on 2-3 on J482. I adjust V470 (plastic trimmer tool) and no matter how many turns I cant get the clock to be much more than 10 mhz. 

Think Ive got a few options:

1. Ive not set the scope up correctly. (I am a newbie - but think I ve got it set to read frequency correctly forllowing the guide that came with the unit)
2. Ive used the wrong size replacement part. I'm thinking likely as th eone I removed is 'blue' and the one I replaced it with is 'white'. Since ordering the part Ive learned that they are colour coded. White has a higher max pF (10 pF) than blue (7 pF). But thinking if its more than blue then I should still be able to set it correctly - just don't turn is to the maximum setting. 
3. Ive bodged the soldering. Checked connectivity with my multimetere and all ok. Part is fitted the right way round and matches the layout on the board.
4. Something else is wrong elsewhere on the board?

[Tumbleweed]

4. Something else is wrong elsewhere on the board?

Forgot to put the jumper (J482) back!. Once I did that - all looks to be ok.

Think that's my Miggy working.

Thanks for all your help (again) Castellen. Thats three miggy's you've helped me fix over the years (my 3000T, 4000D and now my 3000D).

Best.

Weed.

[Castellen]

Forgot to put the jumper (J482) back!. Once I did that - all looks to be ok.

The clock waveform doesn't look great, are you sure you have the probe ground connected correctly?  The easiest place is usually on the top lead of the flicker fixer enable switch, which happens to be a convenient ground connection point.

Also make sure your probe is set to x10 (for 10M Ohms impedance).  With J482 set to the open loop position, you should be able to get the PLL free-running centre frequency very close to to what it should be; doesn't need to be exact.  The range of the white trimmer capacitor is likely to be close enough if that table is correct.  Otherwise you can put the original trimmer back in to see how that performs by comparison.

No idea where you were seeing ~10MHz, you'd need to describe how and where things are connected, and what's displayed on the scope.