Atari SC1224 (JVC-V2) Stereo Conversion

[+mytek]

Since I'll be using this with an 8-Bit Atari via the Sophia RGB board, and with the built-in Stereo Audio of the 1088XEL, I figured I'd post this here.

 

So here's the story...

 

I got this great looking image on screen being produced from my XEL, but quite frankly the sound from the stock speaker on the SC1224 was pretty crappy. I needed a better sound quality, and for it to be in 'STEREO' as well. All the justification I required to do this mod

 

Original stock speaker.

 

 

The original audio in the SC1224 was being produced from a AN5265 TV Sound Output Chip. For my new stereo amplifier I decided to go with a PAM 8403 break-out board with integral volume control. These are very inexpensive (as little as $1 a piece), but produce a big high quality stereo sound.

 

 

In order to mount this in such a way that the SC1224 still looked stock from the outside, I proceeded to strip all of the unneeded original amplifier parts off of the Volume/Contrast PCB and then replaced them with a 5v linear regulator (7805), some 470 uf filter caps, and the PAM 8403 board mounted with some industrial double-stick mounting tape (Scotch 411P).

 

 

Note: I filed off one side of the volume control shaft to match the original pot that was removed. The black cable on the left is a shielded dual inner wire audio coax cable used to connect to the left and right audio inputs on the DIN-13 jack, and I'm using standard small gauge speaker wire to eventually connect to the new speakers when they arrive.

 

LM7805 5 volt regulator shown mounted on the bottom side of the PCB with a few jumpers to connect up to the 14 VDC supply input (via the original 5.6 ohm inline power resistor) and the added filter caps.

 

 

For speakers, I got a pair of these coming, which are magnetically shielded and meant for video use wherever CRT's are concerned (won't skew the CRT's electron beam).

 

 

Those will get mounted inside the top part of the SC1224's cabinet where the ventilation vents presently are (one on either side). I don't intend to do any cutting of the original enclosure, and will maybe create a baffle plate that the speakers will mount to, that will itself be mounted to the underside of the enclosure with some more of that industrial double-stick mounting tape (it's very strong stuff, meant for outdoor use to hold signs that weight up to 10 lbs).

 

 

Stay tuned for more to come... Connection of audio cables, and speaker mounting.

 

BTW, if someone has a copy of the 'JVC' specific schematics for the SC1224, I would be forever grateful if you would post it in this topic

 

- Michael

 

[mattsoft]

cool project and interested to see how it turns out. Love the look of the SC1224 v.2, but the sound is pretty crappy.

[+mytek]

Ok I thought I was going crazy

 

I was testing my stereo mod on the SC1224 monitor and wasn't getting sound out of the left channel speaker. So I checked out the cable and on one end it was missing pin 5 which is designated as Audio In. So I thought it must have gotten busted off, afterall it was only missing on one end. So I bought 3 new ones from Brad at BEST. Guess what? They are also missing pin 5 on one end. So I pulled up some information on the ST Monitor jack (see below).

 

 

Sure looks like pin 5 should be the audio input to the monitor EDIT: This is the pinout when looking straight at the Monitor's jack from the rear.

 

Keep in mind I now have 4 cables all missing pin 5 on one end only. So what gives? Anyone have a clue? Keep in mind that everything else works properly, so I can't be looking at the connector backwards or something equally stupid.

 

 

EDIT: Mystery solved! Pin 5 IS NOT THE AUDIO INPUT as we are led to believe. In actuality it is Pin 1 (see excerpts from the official 520ST schematics below).

 

 

Source of Audio in on ST (note: Audio-Out is an actual output signal)

 

 

 

Connection to Video Output Jack (note: Audio-Out goes to Pin 1)

 

 

So in reality Pin 1 is the Audio input on the ST Monitor, not Pin 5. I'll have to do some re-thinking on how best to accommodate the right channel audio on the XEL's DIN-13 jack, since Pin 5 is useless, and Pin 1 is the left channel audio (I'm thinking Pin 4 would be the logical choice).

 

- Michael

Edited August 3, 2017 by mytekcontrols

[+mytek]

To add to my last post...

 

Obviously on the Atari ST Computer side of things Pin 1 will be the Audio-Out and on the ST Monitor side Pin 1 will be the Audio-In. But the fact remains that Pin 1 on both sides needs to connect so that the audio signal from the computer is passed through to the monitor. If you look at various examples across the net you'll see that people show Pin 5 as being the Audio-In for the ST monitor which it isn't.

 

 

So here's an update on the 1088XEL's DIN-13 connections, based upon my discovery of the Audio-In discrepancy, and my reassignment of the right channel Pokey output to Pin 4 (L-CH is normal non-Stereo output from Pokey). Pin 4 has no use on an SC1224 Color Monitor, since this is used by the SC124 Monochrome Monitor to signal to the ST computer to switch to Hi-Rez mode.

 

 

- Michael

Edited August 4, 2017 by mytekcontrols

[flashjazzcat]

Not sure I follow here. I always understood mono audio out was on pin 1 but we repurposed audio in (normally unused) on pin 5 as the second audio channel out for VBXE, etc. The fact pin 5 is N/C on the ST and on standard ST video cables was completely unimportant, since you still won't be able to carry stereo audio using a standard ST video cable attached to an A8 even using mono detect (since that won't be wired to an RCA audio jack or SCART audio pin either). Mono audio would work out of the box, though, using a standard ST video cable.

 

I must have missed the bulletin where mono audio was wrongly assumed to be on pin 5, but I'm still unclear as to the advantage of relocating the second channel from pin 5 to pin 4. I've been using pins 1 and 5 for all VBXE jacks and cables.

[+mytek]

Not sure I follow here. I always understood mono audio out was on pin 1 but we repurposed audio in (normally unused) on pin 5 as the second audio channel out for VBXE, etc. The fact pin 5 is N/C on the ST and on standard ST video cables was completely unimportant, since you still won't be able to carry stereo audio using a standard ST video cable attached to an A8 even using mono detect (since that won't be wired to an RCA audio jack or SCART audio pin either). Mono audio would work out of the box, though, using a standard ST video cable.

 

I must have missed the bulletin where mono audio was wrongly assumed to be on pin 5, but I'm still unclear as to the advantage of relocating the second channel from pin 5 to pin 4. I've been using pins 1 and 5 for all VBXE jacks and cables.

To explain my logic on using Pin 4 for the Right Audio channel instead of Pin 5, is actually pretty simple at least in the case of a JVC SC1224 which uses a detachable cable. Because first of all every pin 'except' Pin 5 is passed through from end to end on the 'stock' cable as supplied from Atari with these monitors. Since these cables have molded in connectors on each end, there is no simple way to make Pin 5 usable. I can't speak for the GoldStar or Samsung versions of the Sc1224 which had integrated cables, but I suspect that they likely only used whatever pins are necessary for their operation. So not having Pin 5 available for my use, meant I had two choices, either pick another pin, or cut off the end without a physical Pin 5 in place and solder in a new connector hoping that there is actually a wire going from Pin 5 on the opposite end. Obviously I picked the less messy option and opted to pick another pin that did pass it's signal all the way through, thus allowing me to use the 'stock' cable as is. Proof to what I am saying... I now have an actual 'working' system based on the Pin 4 scenario using the stock Atari cable.

 

As for my mix up on thinking that Pin 5 was the 'original' Audio-In for the ST monitor, that unfortunately was implied by the examples I came across on the net, which were specifically labeled as being for the 'monitor' side, but now it's obvious that it was actually a mislabeled example of the 'computer' side (I kid you not). This does not apply to the example I just recently posted in this topic, which was meant to describe the ports as they exist on the ST computer, and as such led me to investigate further by finding a 520ST schematic that confirmed the proper relationship and usage of all 13 pins.

 

The choice of Pin 4 was easy, because it had no physical connection on the JVC monitor side of things (same goes for Pin 8 which is designated as the RGB Control signal often used in SCART related connections, but not required for a SC1224). And of course keeping in mind that Pin 5 is not passed through on the stock detachable cable with molded in male plugs on each end.

 

And yes for all non-modified ST monitors, the original single Pokey audio output (mono sound channel) will be passed through and heard through the DIN-13 connector on the 1088XEL (assuming we use the corrected Pin 1 scenario), whereas with my SC1224 stereo audio modification (installing stereo amp and two speakers in the monitor), that will become the left channel audio instead when the XEL is set for 'STEREO' mode.

 

It is unfortunate that people choose Pin 5 to use as the right channel audio for a typical VBXE/DIN-13 installation, and was likely decided upon without consideration for converting a JVC monitor to stereo output as I have done using the stock cable it came with. No real surprise, but also not the end of the world since I can simply add a 3 terminal jumper block header to allow configuration to use either Pin 4 or 5 as the right channel audio output from the 1088XEL.

 

- Michael

[flashjazzcat]

Understood, but it was never envisaged that stereo audio would be fed through the primary connector at the monitor end. Therefore the stock cable/monitor jack was potentially good for mono audio, but more commonly (in the case of 1084S derivatives, Philips CM8833-II, SC1435, etc) the audio would break out onto two RCA jacks at the monitor end.

 

The SC1224 is kind of an unusual 'one off' case in that audio goes in via the primary connector at the monitor side.

[+mytek]

Understood, but it was never envisaged that stereo audio would be fed through the primary connector at the monitor end. Therefore the stock cable/monitor jack was potentially good for mono audio, but more commonly (in the case of 1084S derivatives, Philips CM8833-II, SC1435, etc) the audio would break out onto two RCA jacks at the monitor end.

 

The SC1224 is kind of an unusual 'one off' case in that audio goes in via the primary connector at the monitor side.

 

Although it wasn't envisioned, it does appear to work well, without any noise or cross talk problems. And since when do we let anything not envisioned stand in our way

 

After thinking about it a bit more, I decided that having both Pin 4 and Pin 5 connected as the Right Audio Channel should provide a more universal approach (no need for a configuration jumper that way).

 

 

My only concern is that in this posting which was influential in my original decision to use Pin 5 as my Left Audio input, it shows the following hook-up for VBXE.

 

The VBXE wiring scheme Candle suggested...

After which you made the following observation...

 

Not sure if it's better to put L Aud out on pin 1 for the sake of compatibility with ST monitor cables, nor can I recall exactly what was suggested there: I've been going with the method described in the graphic.

 

So unfortunately if you don't have a right channel output available (as in no 2nd Pokey chip), you'll have no sound on a stock SC1224 monitor. Any idea why Candle chose this scenario?

 

Obviously for best compatibility across all Atari RGB monitors, it would have been better to have the original Pokey audio output routed to Pin 1 in all cases (as you so aptly pointed out). But it is what it is

 

- Michael

[flashjazzcat]

I might have simply got L and R audio back to front there. I believe I wire it up with left audio on pin 1. Sorry if that was instrumental in confusing the issue.

 

Anyway, putting the signal on two pins might work out OK. Obviously I agree that we shouldn't be constrained by what's gone before, but I suppose the appealing thing about audio in was that it was arguably never used, while mono sense is used (admittedly only with mono monitors or switchable cables).

[+mytek]

I might have simply got L and R audio back to front there. I believe I wire it up with left audio on pin 1. Sorry if that was instrumental in confusing the issue.

 

Anyway, putting the signal on two pins might work out OK. Obviously I agree that we shouldn't be constrained by what's gone before, but I suppose the appealing thing about audio in was that it was arguably never used, while mono sense is used (admittedly only with mono monitors or switchable cables).

 

No problems in the long run since Pin 5 wasn't suitable for the SC1224 stereo mod anyway (due to the stock cable issues we've been discussing), no 2nd rev boards have been made, and no animals were injured in our test trials, so no harm done .

 

I believe Pin 4 is safe to use, since I doubt that anyone with a VBXE will want to be limited to monochrome video and would likely opt for a VGA connection instead if they want higher resolution.

 

Note: The Realan case I use for my 1088XEL has an extra hole that could easily be used for a real VGA connector, which would make better sense than adapting from the DIN-13 for that application.

 

- Michael

Edited August 4, 2017 by mytekcontrols

[flashjazzcat]

This is the first time I heard of a stock DIN13 to DIN13 cable socketed at both ends, and I'm wondering how many suitable Atari monitors used that arrangement (the SC1435 doesn't). I guess 99 per cent of VBXE users are running LCDs (via SCART or a 31KHz upscaler) or 1084S derivatives (which includes the SC1435), and are therefore using custom-built cables or a DB9/DB15 on the back of the machine.

 

Of course the 1088XEL is a lot more flexible when it comes to getting audio out of the machine. When it comes to legacy hardware, the most appealing thing about the DIN13 (aside from ST to 1084S/SC1435 cable compatibility when used with mono audio) is that it keeps stereo audio on a single connector, avoiding the need for a pair of RCA jacks to be added to the back of the computer.

Edited August 4, 2017 by flashjazzcat

[+mytek]

This is the first time I heard of a stock DIN13 to DIN13 cable socketed at both ends, and I'm wondering how many suitable Atari monitors used that arrangement (the SC1435 doesn't). I guess 99 per cent of VBXE users are running LCDs (via SCART or a 31KHz upscaler) or 1084S derivatives (which includes the SC1435), and are therefore using custom-built cables or a DB9/DB15 on the back of the machine.

 

The first version of the SC1224 monitor was made by JVC (3 different versions of the SC1224 were made). It uses the DIN-13 male-to-male cable (factory supplied with the monitor), and has a rear panel mounted female for it to plug into. Later when the GoldStar and Samsung versions arrived, they came with attached video cables (probably done for cost reduction reasons). The Samsung was the last model made for the ST series.

 

It's been said that the JVC had the best quality image (I don't know if that's actually true, since I currently only have the one).

 

Side by side comparison of JVC (on the left) and GoldStar (on the right).

 

 

JVC rear panel connections

 

 

GoldStar rear panel connections

 

 

Samsung version (this matched the look of the ST and XE series with the slanted button and LED)

 

 

Samsung rear panel connections

 

 

Of course the 1088XEL is a lot more flexible when it comes to getting audio out of the machine. When it comes to legacy hardware, the most appealing thing about the DIN13 (aside from ST to 1084S/SC1435 cable compatibility when used with mono audio) is that it keeps stereo audio on a single connector, avoiding the need for a pair of RCA jacks to be added to the back of the computer.

That is precisely why I wanted to convert my JVC SC1224's mono audio to stereo and take advantage of it's nearly fully populated DIN-13 male to male cable to get all the required signals into the monitor to fully support this. In this way a completely 'stock' cable would do the trick with no need to plug in any other cables

 

- Michael

[flashjazzcat]

They're good-looking monitors. I especially like the Goldstar.

[+mytek]

They're good-looking monitors. I especially like the Goldstar.

 

Yea me too Bob Woolley has one of those, which I'll be testing with as well. Curious to see if it'll work with CYSYNC feeding both the Vertical and Horizontal Sync like I did for the JVC.

 

-----------------------

 

Here's how I made the audio connections on my JVC.

 

DIN-13 connections to installed PAM 8403 stereo amplifier.

 

 

It would probably have been quite ok to leave the original PCB side connections intact on PIN 1 and 4, but I decided to take the safe approach and cut them free before attaching the left and right audio input wires going to the PAM 8403 stereo amplifier module. Nice thing about this approach, is that you don't need access to the bottom of the PCB, so less disassembly of the case is required.

 

- Michael

[gozar]

I only have one ST monitor that has a detachable cable, the JVC PS3000 (the monitor with the built in disk drive). Unfortunately, I lost the cable, and when I was looking for a replacement, I found out that Roland uses the 13-pin DIN for some of their equipment. I ordered one, but haven't had a chance to test it. I'll try to get it out and see if all 13 wires are attached.

[+mytek]

I only have one ST monitor that has a detachable cable, the JVC PS3000 (the monitor with the built in disk drive). Unfortunately, I lost the cable, and when I was looking for a replacement, I found out that Roland uses the 13-pin DIN for some of their equipment. I ordered one, but haven't had a chance to test it. I'll try to get it out and see if all 13 wires are attached.

 

I remember seeing something about people substituting the Roland cable when there was no original cable with the JVC SC1224, and apparently it works. This is the problem with having non-attached accessories with the passage of time... they tend to get lost. This happened with the monitor I bought as well, which was missing it's original cable. Initially I had kludged together a DIY one, and then later I was able to borrow one from Bob Woolley, but then later still I ended up buying a few from BEST Electronics for about $20 a piece (I figured it would be nice to have a few extras in case I come across another monitor without it's cable). I'll be curious to know how many pins are passed through from end to end on the Roland cable. Doesn't change my decision on using Pin 4 instead of Pin 5 for the stereo mod on my own monitor, but at least it provides an option to others that might attempt the same thing. And as I pointed out earlier, the XEL will now have Pin 4 and 5 wired together for the right channel, so either way will work for it.

 

Please let us know what you find out .

 

- Michael

Edited August 5, 2017 by mytekcontrols

[+mytek]

I might have simply got L and R audio back to front there. I believe I wire it up with left audio on pin 1. Sorry if that was instrumental in confusing the issue.

 

Jon just to elaborate a bit more on this... a lot of little inconsistencies occurred in the first rev of the 1088XEL design. Some of which were attributable to my lack of experience in working with certain gear in conjunction with an A8 (such as an ST Monitor via Sophia). I knew there would be problems that had slipped through the cracks, and never did I seriously think that the first rev would also be the last. It was inevitable that changes and adjustments would need to be made, with a new revision to follow. But out of all the things that could have gone wrong, the mess ups (which there were many) with the RGB-THRU connector was relatively minor in the grand scheme of things. I also didn't follow due diligence when designing this aspect, so it's no surprise that it was so screwed up in the end, that it would take multiple PCB layout adjustments to fix it. In fact the one thing I really wanted to work out of the box, was the Sophia aspect, and boy did I ever mess that up as far as signal routing from the ribbon header over to the DIN-13 was concerned (I must have been smoking something wacky that day ).

 

To show you how badly I goofed, it necessitated laying out a small adapter PCB just to correct the mis-routing problems on the V1.0 boards, and at the same time it also fixes the sync issues that became evident a bit later on (this board will not be required for the next rev of the 1088XEL).

 

 

So having a couple of audio lines swapped is very small when taken into perspective.

 

- Michael

[flashjazzcat]

Thanks Michael... I appreciate the kind words. Anything I tell you, double-check (I guess that's due diligence).

 

Will send you that BIOS sometime on Sunday, BTW.

[+mytek]

Thanks Michael... I appreciate the kind words. Anything I tell you, double-check (I guess that's due diligence).

 

Will send you that BIOS sometime on Sunday, BTW.

I'll be at VCF in Mountainview CA the whole day, and then getting home late that night, so no rush. But I certainly appreciate you making these changes, and I will give them a good test next week starting on Monday.

 

- Michael

[+mytek]

Interesting thing to note. I did an ohmmeter check on the connection points for Pin 1 and 4 going into the SC1224's PCB, and discovered that Pin 1 apparently still has something connected to it judging by a resistance reading from it to ground (Pin 4 looks to be totally floating). So although I removed all audio associated components on the separate volume/contrast PCB, there must have also been something on the main PCB as well. So when doing this stereo mod, it looks to be good practice to at least isolate Pin 1 from the main PCB as I have already done.

 

Be nice to have a schematic of the JVC monitor to refer to, but all I can find on the net is the one for the GoldStar monitor.

 

On a different topic, I was wondering if anyone knew if there was a way, as in a control, to adjust the horizontal width on the JVC monitor? I see nothing in the accessable trim pots in the back, nor in the other blue trim pots as seen with the cover removed. And the controls on the flyback only appear to control focus and the screen brightness. Reason I ask is that presently there is just a teeny tiny bit of what should be visible being cut off. In most cases this is a non-issue, but on some demos which utilize every possible square inch of the screen, stuff on the extreme edges is getting trimmed a bit.

 

- Michael

Edited August 6, 2017 by mytekcontrols

[Roydea6]

Be nice to have a schematic of the JVC monitor to refer to, but all I can find on the net is the one for the GoldStar monitor.

 

 

- Michael

How does one identify a JVC monitor Schematic.

 

SC1224_Schematic.pdf

[gozar]

Please let us know what you find out .

 

- Michael

 

 

I checked all 13 pins, and they appear to be connected straight through. I used my multimeter, which isn't very easy to do. :-)

 

So for those looking at needing a 13 pin ST cable, at least the one I bought on eBay should work. The description was 13 Pin Guitar Synth Cable - 5 FT for Roland V-Guitar GR-33 VG-88. Also, this may be an easy way to make 13-pin to 9 pin cables by ordering longer cables and cutting them in half.

[+mytek]

How does one identify a JVC monitor Schematic.

 

SC1224_Schematic.pdf

 

The schematic you posted is for the GoldStar. Here is a full service manual for that version: Atari GoldStar SC1224 RGB Monitor Service Manual.pdf

 

Since the JVC is the only version with a built-in DIN-13 jack mounted directly to the main PCB, that should be the identifier for the correct schematic. It's strange that I can't find that specific schematic and/or service manual, although I did find the User Manual: Atari JVC SC1224 (V2) RGB User Monitor Manual.pdf

 

Should I call the customer service number listed in the manual?

 

- Michael

[Brentarian]

 

Should I call the customer service number listed in the manual?

 

- Michael

 

Did you fill out the warranty card and send it in?

[Bryan]

IIRC from my ST days, the JVC did indeed have the best looking picture but didn't handle 50Hz software as well as the others (picture got a little weird at the top).