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Old 09 February 2018, 06:16   #1
TCP
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Amiga 500 slowdown/speedup issue

Hello,I am currently working on my Amiga 500 and encountered an issue i need help with identifying the problem .

Hardware: The Amiga 500 has a Rock Lobster Board and Kick 1.2 is a US/NTSC unit. 512 Memory expansion and A520 TV modulator. So a very vanilla machine.

Problem: It seems that the Amiga slows down and or speeds up the GFX and sound when running some programs.
E.g. the Crystal Crack cracktro from the game Moonstone.

Normal version from youtube:
[ Show youtube player ]

Thats how its running on my Amiga.(ignore the sound at the beginning and poor pic quality, i used a crappy videograbber to connect it to my PC). It runs faster when the jumping dot is on the screen, slows down when the main cracktro runs and speeds up again when the cracktro closes out with the dot sequence again (not on my video).

[ Show youtube player ]

I ran a game with the Paranoimia cracktro and the same issue (slow) no gfx errors or sound crackling...just slow.
[ Show youtube player ]

Then I loaded Shadow of the Beast (first one) and it ran perfectly fine...gfx and sounds. Played it a bit and was normal, too.

I did some work on the Amiga in the last weeks (see below for background infos on that). I cannot say if the Amiga, when i bought it years ago had the same issue or it happened recently. (due to no way of connecting it on my TV see below). I noticed that sometimes the Amiga Freezes, and reboots (soft and hard)with the red light blinking or a green screen. After i pressed on the ROM Chip, it worked normal again. Applying some pressure on the ROM chip while it was running also could make the Amiga freeze sometimes.

Thats the only thing that i could see that would cause an issue. Not sure if it also would be responsible for the speed issue i encountered.
Would appreciate help and feedback on how to troubleshoot my lil nugget.

Thank you



Background:
Bought this unit 4 years ago. Tested it after receiving and it worked..but i couldnt see a pic, cause i had no connection to my TV. I tried the Audio and it worked. I bought a TV modulator A520 from the UK, but never setup the Amiga again ti test it.

Now a few weeks back i remembered that i maybe should replace the Battery from the 512 expansion board. Opened it up and saw very light leakage around the battery. removed it, installed a kit for a new button cell. Wanted to try the Amiga, turned it on and again no picture (A520 was a PAL version duh :/ ) so instead i used the monochrome port,but the Amiga was dead. Power light was on, but the Floppy didnt made any sound.
I checked the output from the power supply and the +12 V rail was dead. Eventually located that it had a bad voltage regulator that malfunctioned when getting warm (after 20 seconds). So i replaced it and did some measurements and yup it had power, but the voltage kept going up and down a lot. Seeing its a unit build in 87, i decided to replace all the caps. In the power supply, The Amiga, the Tv Modulator and the memory Expansion (i forgot the 3 in the floppy drive ). So I replaced them all in the last days and also replaced the crystal from the A520 with a NTSC one and grounded pin 20 on the MC1377 IC and started the Amiga. Had the Rom issue again, but then it ran and with color . However then i encountered the speed issue.
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Old 09 February 2018, 11:15   #2
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My first guess would be simply it is caused by your Amiga being NTSC while most Amiga software was written with PAL machines in mind. 60 Hz vsync can screw up timings for stuff designed to run with 50 Hz. Do you know for sure how that cracktro is supposed to run on an NTSC machine?
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Old 09 February 2018, 12:28   #3
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Yep, you're running out of raster time in the main part, that's why the Crystal crack intro runs at the wrong speed on your machine. The intro misses (every other?) call to the playroutine because of the difference in screen length/raster time.. In the jumping dot part it's too fast because your framerate is 60Hz and the tune is composed for a 50Hz frame rate.

Not much you can do. You'll find many people over there have converted one of their Amigas to PAL when they got back into the Amiga in the 2010s.. :-) For a 1.2 era machine, you'll have to swap a PAL Agnus in there. Later machines have it switchable. You will also have to ditch the modulator in this case and get an RGB display + suitable cable to hook it up. If you want to stay true to the CRT experience, a Sony PVM is a good choice.
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Old 09 February 2018, 14:30   #4
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Quote:
Originally Posted by Jope View Post
Yep, you're running out of raster time in the main part, that's why the Crystal crack intro runs at the wrong speed on your machine. The intro misses (every other?) call to the playroutine because of the difference in screen length/raster time.. In the jumping dot part it's too fast because your framerate is 60Hz and the tune is composed for a 50Hz frame rate.

Not much you can do. You'll find many people over there have converted one of their Amigas to PAL when they got back into the Amiga in the 2010s.. :-) For a 1.2 era machine, you'll have to swap a PAL Agnus in there. Later machines have it switchable. You will also have to ditch the modulator in this case and get an RGB display + suitable cable to hook it up. If you want to stay true to the CRT experience, a Sony PVM is a good choice.


I’ve never fully understood the implications with the modulator.

If you were to replace the Agnus and the crystal on an NTSC Amiga motherboard, would your modulator then be outputting true PAL? Is the modulator essentially the same on all units?

Thanks.
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Old 09 February 2018, 14:46   #5
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Quote:
Originally Posted by EzdineG View Post
I’ve never fully understood the implications with the modulator.

If you were to replace the Agnus and the crystal on an NTSC Amiga motherboard, would your modulator then be outputting true PAL? Is the modulator essentially the same on all units?

Thanks.
The modulator is actually the only part here which is truly PAL or NTSC. The amiga itself outputs RGB at PAL or NTSC compatible timings.

The modulator is not quickly switchable between the two, it must be replaced.

The modulator adds the actual colour encoding on top. Not many displays support mixing colour encodings and timings.
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Old 09 February 2018, 15:58   #6
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Your Amiga is NTSC, that's the problem.
You should upgrade theAgns chip and install a PAL/NTSC switch to avoid issues (you will also need an RGB display that supports 50 and 60 Hz then, though)
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Old 09 February 2018, 17:35   #7
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Quote:
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The modulator is not quickly switchable between the two, it must be replaced.

The modulator adds the actual colour encoding on top. Not many displays support mixing colour encodings and timings.

This is why I wanted to know, thank you. I had assumed the modulator was determining its color encoding based on the clock/signal it was being fed.
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Old 09 February 2018, 17:38   #8
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Quote:
Originally Posted by Jope View Post
The amiga itself outputs RGB at PAL or NTSC compatible timings.
Nope - 60Hz mode on PAL Agnus has PAL (50Hz) horizontal frequency i.e. 15625Hz where it should be 15750Hz for NTSC as such only true NTSC Agnus will give correct timing for NTSC and vice verso.

This is valid for situation where chip-set doesn't use programmable vertical and horizontal frequency (not sure if you can precisely match HSync freq with VARBEAMEN active and master PAL oscillator).
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Old 09 February 2018, 18:10   #9
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That is accurate. Not sure what the modulator would think - rgb monitors certainly do not mind.
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Old 09 February 2018, 20:10   #10
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Awesome, thanks for all the replies. Fortunately it seems like an easy issue to diagnose.

Of course something like this is the last hurdle i stumble to bring this guy back to 100% working order, software issues. Though makes sense, as the PAL Amiga was far more popular than the US one.

Hmm so do i need the same PAL Agnus as the one that i have (MOS 8370) or are never versions also gonna be fine?
EDIT: NM just looked it up and saw i need the 8371, which is the pal version of it.

Quote:
install a PAL/NTSC switch
Are these available to buy, or are there instructions how to build one?

Thank you.
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Old 09 February 2018, 20:20   #11
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Quote:
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Are these available to buy, or are there instructions how to build one?
I might have jumped the gun there, depending on your A500 model, maybe you can't do this.

Which revision motherboard is it? If the model is right you can upgrade the Agnus and you can put the switch, plus be able to access more Chip RAM.
Jope would know and he can probably tell you with more detail (I'm a bit of an A500 noob, but I did modify my US A500 with this switch, but mine already had the Fat Agnus)

And no, nothing available to buy, you'd have to do the modification yourself.
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Old 10 February 2018, 06:23   #12
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Yah, i have the Rev 5 board. So i dont think i have the option. Already ordered a Pal Agnus and gonna see when it arrives whats next on the agenda.
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Old 10 February 2018, 09:58   #13
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A rev5 board is a bit difficult to make switchable. The old 8370/8371 has a test pin where the later 8372 has the pal/ntsc select pin. A 8372 defaults to NTSC mode in a rev5, you will have to somehow isolate the pin before installing a 8372. A 8371 will work in PAL mode, and to make it complete, you should swap the oscillator too, as Pandy noted before.

In practice with an RGB monitor, the oscillator isn't 100% necessary, but if you want to use modulators or connect the Amiga to video recording equipment and such, you will need the right frequency.
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Old 10 February 2018, 10:18   #14
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Thats sounds like the way to go. Was just browsing ebay for one,but couldnt find one with 28.37516 MHz and the right form (square ish).

Any idea what keywords i should use to find the part?
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Old 08 March 2018, 04:39   #15
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Finally managed to find a pal crystal and arrived today. So i put in the Pal crystal and Fat Agnus, made a testrun, and it worked, but i saw smoke coming from under the floppy drive.

Removed the floppy and saw it was a resistor that was burned through. Problem is that i cannot read the markings on it anymore and the Scematic i found didtn tell the Ohm walue it has. The markings of the Resistor is EM 401. Connected to the Left joy Port CN1.

Anyone know the value by chance, so i can get a new one?
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Old 08 March 2018, 10:27   #16
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I don't have the rev 5 schematics to hand, but on the ref 6 schematics, that looks like an EMI suppressor - the small 3-legged components that are placed on almost every I/O pin on the A500. This part is simply a resistor and a capacitor arranged with the resistor in series with the signal and the capacitor between the signal and ground. The values of the resistor and capacitor parts vary with the type of signal involved - it looks to me like that part is connected to pin 7 of joystick port 1. That would make sense, as that line carries +5V power and so has the current available to burn it through if it shorts. If that's the case on your board, check for a short between pin 7 and ground. If there is none, a 4.7 ohm resistor should do the job just fine, as that's what is used for that job in the A600 and 1200.
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Old 08 March 2018, 22:32   #17
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Thanks for the answer, gotta check the mouse (which was plugged in at that port) also for any shorts.
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Old 09 March 2018, 03:54   #18
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Actually found the Rev 5 schematics and it actually states there: "note: EMI 401 is loaded with 4.7 Ohm resistor" so yah,you were correct. Oddly though, i found a pic of the board and it has a 5.x Ohm resistor there. First band is green, like the burned one by me. Seems the code is Green Brown Gold Gold.....which should be 5.1. odd.

Also tested volt and current at that resistor and its 5 Volts and with a device plugged into either ports its shows 50 mA. Not sure what the normal current drain is but 50 mA shouldnt burn the resistor.
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Old 09 March 2018, 12:06   #19
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50mA sounds ok for a mouse, so that's fine. A 5.1 ohm resistor would also be fine, the precise value isn't that critical for operation. Too high and it will stop devices getting enough power, but once it's below 10 ohms you're probably fine.
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Old 09 March 2018, 19:02   #20
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Isn't it weird that it was working fine before but as soon as you changed a crystal and chip you saw the smoke?
Is the fault really at the mouse? It would have blown up before.
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