Amiga 1200 rev 1A audio fix

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Attention all owners of Amiga 1200 rev 1A!
In the audio circuit on the A1200 rev 1A motherboard, the gain of the OP-amps is set very high. In fact, it is set so high that portions of the swing of the audio is reverse biasing the polarized AC coupling caps C324 and C334. Also, the high output coming out from the RCA-jacks at the back of the A1200 rev 1A might drive some amplifiers into distortion.
The reason for this is that the feedback resistors R321 and R331 going to the inverting inputs of the OP-amps are of a too high value, which makes the OP-amps have a really high gain resulting in an audio output with a very high amplitude. The value of these resistors is 1.5 kOhms. Commodore recitified this mistake in later revisions of the motherboard. In revision 1D4, we can find 680 Ohms resistors in there instead, which sets the gain of the OP-amps to a more decent level.
This is all being demonstrated in this video.
Revision 2B and 1D3 are now also confirmed to have the 680 Ohm resistors on the motherboard. If you find another revision other than 1A that has the 1.5 kOhm resistors on the board, please let us know in the comment section.
The motherboard in this video is the very same one as the one that had a color problem earlier, which I featured in another video. Feel free to have a look at that repair as well. The video can be found here:
• Amiga 1200 color probl...

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20 фев 2013

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Комментарии : 50

@thereasonforyou7048 9 лет назад

This are the most construtive videos about COMMODORE AMIGA computers ! and an amazing tribute to Commodore this uploader it´s really a friend of ours. KEEP ON DOING more videos ! thank you

@AlenMarkov 6 лет назад

Again a fantastic job - pure fun watching this kind of videos! Thanks for all your effort to video-document it so well for all of us. Absolutely love it.

@vierspartan117 4 года назад

A pleasure seeing a professional at work

@mark12358 7 лет назад

A professional, in-depht, well done Job! I'm waiting for a video about the restoring of your Amiga 3000 :) cheers!

@huhummmmmmm 3 года назад

Old video, still gold :D Thanks!

@moorgott666 11 лет назад

Thank you very much for your interesting videos. After watching all of your repair videos where you have replaced all those capacitors I was worried about my A1200 (also rev 2B like gulgulostost has) and looked at the capacitors there, they all look very fine and the motherboard looks like new without any traces of leaked capacitor electrolyte. Regrettably I have no instruments to measure but the A1200 works very fine so I do not have to worry at the moment I think.

@mehmetpinarci1456 11 лет назад

Great video , very clean work on re resisting :)

@SuperRnine 11 лет назад

I just saw that the video I demanded is already posted on you channel even before this one. :D When I think about it now it makes sense :D.

@yorgle 11 лет назад

excellent sleuthing! If i ever get an A1200, i certainly will know what to check to fix the audio!

@RetroGameModz 11 лет назад

Thanks for the info. I haven't come across a rev 2B yet, so I haven't been able to check that for myself. However, I can confirm that rev 1D3 also has the 680 Ohms resistors in there. I think it's likely that rev 1A is the only one that has the higher value of resistors on the board.

@brunorbf 8 лет назад

My Rev 1B had 1.5 K Ohm resistors. I've replaced them for two 680 ohm 1/4w carbon resistors, I only had smd in 0805 size (1/8w). It definitely improved the audio quality

@SammYLightfooD 4 года назад

My Rev. 1B board has 470 Ohm resistors. The miggy had a previous owner but I don't think these resistors have been changed from how the solder joints/components look like.

@brunorbf 4 года назад

@@SammYLightfooD Maybe they used what they had, during production, and you can find variations in a given revision. My board was busted when I bought it, the caps had leaked all over the place, and the PCMCIA slot was vandalized. LF347 was dead, due to the corrosion. I used through hole caps then. Maybe I will recap it with smd ones this year (panasonic polymer).

@TheDrunkenPL 6 лет назад

You are freaking amazing man!

@RetroComputers 11 лет назад

looking forward to next video

@RetroGameModz 11 лет назад

I guess you're right about that. It makes sense to believe you are. But then of course, the main point of this video was not to talk about AC coupling considerations, but rather to give people an idea of that the OP-amps are configured with a really high gain on the rev 1A board. I have for a long time thought that it makes sense that a cap is not reversed bias as long as the input and output are swinging in phase. But all the info on the net I've read has fooled(?) me into believing opposite.

@bachaplegic 4 года назад

If only I had this info 28 years ago,

@RetroGameModz 11 лет назад

Thanks. I could not resist on using flux while doing it. :)

@Wallygjs 3 года назад

I would have had to straighten that audio jack that's twisted to the right, it bugged me every time I saw it in this video. Other than that great video and troubleshooting!

@gulgulostost 11 лет назад

nice fix/infomercial! Just checked my own board (rev 2B) for those resistor values, and it seems C= have spotted the mishap already in this revision as my board has those 681 resistor-values

@SuperRnine 11 лет назад

I DEMAND a video about troubleshooting and replacing the video chip that you mentioned in the begining of the video !!!! :D:D And yes great video as always. And one thing comes to mind when I watch your videos: Albert Einstein: If you can't explain it simply, you don't understand it well enough.

@kevinharrison4909 Год назад

The 1A was an advanced engineering prototypr that came out in September 1992. The A1200 didn't launch until October 1992 in the UK. Oh well at least 1A has a full 44 pin clock header which was either cut down by half or non existent on later models. What's the possibility to do a pencil mod to cut back these resistors without having to fiddle about with SMT workstations possibly damaging the board in the process.

@RetroGameModz 11 лет назад

I don't think you need to worry about the caps on your A1200 for now. I had to replace a lot of capacitors during the A4000 repair, but it seems like the A4000 is notorious for having bad caps on the motherboard according to the Amiga forums etc. But I have never heard or experienced the same thing about A1200. So if your A1200 works fine and the solder joints close to the caps all look nice and shiny, I don't think you need to worry much for the time being.

@goyroyni 10 лет назад

1D.1 also has the 680 Ohms resistors (so the quite rare 1D.2 most likely has them too).

@andrewprettyquick2070 6 месяцев назад

i thought the bias resistor was open.. and it actually was a good diagnosis

@RetroGameModz 11 лет назад

I have no idea either why a manufacturer of electrolytic caps would write something like that, but I'm absolutely sure that I've seen info on the topic in a PDF-file on general guidelines for electrolytics. Regardsless of whether that is true, I cannot see any reason for _not_ biasing the input enough to have the input swing always staying positive. I mean, what happens if, say, the audio source is playing back really low frequencies and the cap has a low impedance path to GND on the other side?

@DropDeadFrederick 7 лет назад

Looks like Commodore did a lot of tweaking of the A1200 board over time actually. It's either that or because of how cash strapped they were, Commodore resorted to sticking whatever parts they could on the boards to get them working so they could sell them. For the audio amp, on my rev 1D board I got 470 ohm resistors. I'm in the middle of recapping my board and I'm seriously tempted to replace my 470 ohm resistors with 680 ohm resistors (and add a 1uf cap to smooth out the power for the composite video). I distinctly remember my Amiga's audio distorting, and ironically while recapping my board, I noticed that the two caps that were leaking the most were the caps on the audio output (which necessitated me doing a pad repair). Oh yeah, my 1D board apparently has some other parts that others don't. For example, my board came with a 10uf cap at position C460, two extra bodge wires and SMD resistors coming off of the main bodge wire that leads to the Budgie chip, another bodge wire on the video DAC, instead of the inductor you have near the power connector I have four separate ferrite beads, at position Z222 I have what looks like a varistor (at least that's what I think it is), and mine also had a FREAKING HUGE solder blob sitting right on top of one side of R163 (it was over 1/4" in size and came from the factory like that. I'm amazed that it didn't short anything out). :D

@RetroGameModz 11 лет назад

I wouldn't really want to refer to the audio output on A1200 rev 1A as being "faulty". I would rather want to say "incorrectly configured, and in need of tweeking". :) But nevertheless, I think you're totally right in that audiophiles with a not so much forgiving amplifier must have gone nuts when they got a distorted output like that. Det verkar konstigt att RU-vid inte vill ta emot ditt inlägg. Men jag ska kolla på ditt videoklipp så får vi se om jag kan förstå mig på problemet lite mer.

@dancaini 5 лет назад

Hi,i have a A600 motherboard rev1 and have a low volume audio is possibile have the same problem the A1200 rev1? and the size of resistor is 1206??

@kyriostade7193 2 года назад

I didn't have any issues with distorted audio so i check my (unmodded) A1200 rev 1A and it has 681 resistors. Maybe they realized the issue earlier than we think

@analognexus 7 лет назад

What size smd resistor should be used ? 1206 ?

@gulgulostost 11 лет назад

They probably got a lot of complaints from audiofreaks or musicians hehe. It's quite a downer to work with something that comes from a faulty soundsource (unless you want to). BTW: Jeg veit ikke hvorfor, men youtube ville ikke ta i mot innlegg fra meg på kanalkommentar-siden i dag, men jeg la opp et lite klipp på min kanal, om problemet med å lese harddrives. Er det en PM-funksjon på youtube?

@valenrn8657 3 года назад

Luckily I have A1200 with Rev 1D4 motherboard.

@omegamsx 10 лет назад

I've just checked my rev 1B board, and the resistors say "821" and measure at about 800ohms. The line out level is higher than other devices but it doesn't seem to distort.

@RetroGameModz 9 лет назад

821 means 820 ohms (82 + one zero = 820), so getting a reading close to 800 ohms would be perfectly normal in this case. If the line level is okay for you and you don't get any distortion, then there is really no need to worry about it. The higher the value of the feedback resistor, the higher the line level output will be. That's how OP amps work. In this video, the feedback resistors were originally at 1.5 kOhms (1500 Ohms) which is quite high, and that is why the line level output at the Amiga was too high for the input stage of the audio amp in my TV. Changing the value of the feedback resistors in the Amiga to a lower value reduces the gain of the OP amps, which lowers the output amplitude of the waveform. Feedback resistors with a value of 820 Ohms is most likely not going to be a problem for any audio amplifier. However, 1.5 kOhms on the other hand could be a problem in cases where the input stage of the receiving audio amp cannot handle a swing with a high amplitude. There is quite a big difference between 820 and 1500 Ohms, so I wouldn't be worried at all about having feedback resistors of 820 Ohms in there since that is most likely going to be very well enough to keep the output voltage swing at a reasonable level in regards to the active region of the first amp stage at the receiving end of the transmission line.

@omegamsx 9 лет назад

A600 rev 1.3 board that I have here has 360 Ohm fitted for R321 and R331... Checking the A500 schematics it also mentions 360 in an otherwise identical amp/filter configuration. now.. which value is "best"?

@RetroGameModz 9 лет назад

There is really no such thing as a "best" value. It all comes down to the designers choice as he calculates and/or measures the output swing at the final OP amp stage in the audio circuit in order to set the amplitude level of the outgoing audio waveform. Too low amplitude at the output will not cause distortion at the receiving end, but you will have to turn up the volume a lot on your amplifier (TV, HiFi amp, etc) in order to compensate for the low line level that is going into it. But on the other hand, you don't want to use a too high value of feedback resistors, because then the audio output will be too high which might make your audio amplifier (TV, etc) distort the audio since either one or several stages of the amplifier at the receiving end will go into either saturation, cut-off or both during the peaks of the waveform. The feedback resistors for the OP amps in an audio preamp circuit should therefore be chosen to allow for proper output level at the back of the machine (computer, console etc) in order to not get any problems with either distortion or too low volume at the receiving end. I think that Commodore did a very poor choice when they decided to put 1.5 kOhm resistors in there as feedback resistors. I really don't know how they managed to come to the conclusion that it would be a wise choice to put such a value in there. But I have only seen such high value on the A1200 rev 1A. I haven't seen it in any other Amiga model, so this is usually not a problem and therefore nothing to worry about. If you have an A1200 motherboard of rev 1A, then you might want to have a look at the feedback resistors. If the sound is distorting on your TV when you use your A1200 and if the feedback resistors are indeed 1.5 Ohm resistors, then you know what the problem is and what to do about it. ;)

@omegamsx 9 лет назад

RetroGameModz Thanks for the elaborate replies and your excellent videos. Keep up the good work! :)

@RetroGameModz 11 лет назад

I don't think it's that easy. If this was true, it would mean that it would be totally OK to AC couple a 24Vpp voltage swing riding on only 0.5V DC bias through a polarized cap into a 0V referenced node. But doing so is not recommended. It's recommended to always keep the incoming voltage swing more positive than the voltage on the node it's being coupled into, which is 0V in this case. There has to be some truth to it when even capacitor manufactures stress this matter in their datasheets.

@zoltanh.9432 8 лет назад

Hi. What's the title of the mod music? Thx...

@RetroGameModz 8 лет назад

+Zoltan H. en.wikipedia.org/wiki/Silkworm_%28video_game%29

@greg5450 7 лет назад

Hey, I live in US and I was wondering if u would know why I would get -12 drop to below -5v while under load(computer on) on my pal A600 but my A1200NTSC is normal. Also my sound on 600 is half volume of my 1200 , way lower than it should but the 600 works very well except the low audio volume. I had also removed the RF modulator on my 600 to fit indivision out, would this affect it? Not sure how to approach it? Thanks

@RetroGameModz 11 лет назад

Indeed we're not. Not as long as it comes to Commodore hardware at least. :)

@raylyd 9 лет назад

i have a werid amgia cd32 low sound sound works i have ordered sound ic chips i done a full recap i have checked my soldering any help i need a scope any ideas on a good one :)

@RetroGameModz 9 лет назад

raylyd Is it your first scope? If so, go and get yourself an analog scope to start with. Any dual channel scope will do fine as long as it works. It doesn't have to be an expensive one. Look for one on eBay or any other similar auction site in your country. Search youtube for "EEVblog #86". I totally agree with what Dave says in that video. If you prefer a digital scope, then you might want to have a look at the Rigol DS1054Z which is probably the absolute best bang per buck scope you can get these days. However, there are times when a good old analog scope can be much easier to work with. It all depends on the circumstances. I would recommend you first to get a cheap analog scope, and then later get a DS1054Z when you can afford it.

@raylyd 9 лет назад

i allways wanted to get a scope not yet got one any you can recommend

@RetroGameModz 9 лет назад

Take a look at this video: EEVblog #498 watch?v=lZfbo-2sd1A

@raylyd 9 лет назад

thanks buddy and your videos are great hope to see more amiga videos in the future

@atatu4551 3 года назад

and what about 2B?

@MallaganVloggs 6 лет назад

Hey, you know shit ^^ I like it when people know shit. Maybe you can help me. the shit I need help with is what voltage the A1200 LEDs need, the ones for power, HDD and FDD. I keep burning LEDs. Turns out, the cable that I got with my tower for my Amiga 1200 many years ago, doesn't come with resistors, and I don't know what resistance they are on the original. I intend to put an RGB LED with the separate colors of the LED to represent the 3 different indicators of the A1200 activity. Blue being power, red being HDD and green being FDD, so that when the hard drive/dvd rom is active, the LED will go from blue to purple. When the FDD is active, it will go from blue to cyan, and when you copy between floppies and HDD it will flash white ocasionally. So, to sum it up, I need to know what voltage I need the RGB LED to be, and what ohm I need on the resistors. you think you could help me out?