7 Mhz issue and diskdrive issue

By Sky_hawk

Champion (267)

Sky_hawk's picture

22-10-2014, 20:09

I've got a 8235/00 with DOS2, 7Mhz and 1MB RAM and 64KB SRAM (custom) in it.
When I turn on the 7Mhz, the video (RGB) is distorded very slightly, but still annoying, it kind of looks like a very small vertical sinus distortion any ideas how to solve this?

I also has a DS-drive that is 5volt only, where the original SS-drive was 5+12v. if the head-position motor turns on, the screen is even more distorted, by kind of in the same way.

Any ideas are welcome.

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By Manuel

Ascended (19471)

Manuel's picture

22-10-2014, 22:13

Wasn't this due to a too big load on the 5V? I've heard about it before. I guess you need to fix up the PSU to support more stable (and more power on) 5V. Probably was described on the forum earlier.

By Zilogger

Resident (39)

Zilogger's picture

26-10-2014, 15:52

The distortion when using the FDD is a common thing when build a PC FDD (5 Volt only)
in a VG 8235/20 and a NMS8245 without using an extra 7805 to Power Supply the FDD only.

By Sky_hawk

Champion (267)

Sky_hawk's picture

01-11-2014, 22:33

I've tried a 7805, and it works ok, but it causes a lot of heat (obviously, because it has to dissipate (12-5)*1A = ~7 watts at full disk activity) and there is very little room for a cooling fin inside a 8235/45.

I solved this using a DC-DC converter,


It is an incredibly cheap and very effective device. It basically converts any input voltage between 4.5 and 28 volt to a adjustable output voltage. I have set it to 5 volt and it works like a charm! I used a scope to monitor the output voltage during heavy disk usage, it has a ripple of < 100mv which if fine for a diskdrive.

Anyway, no more nasty interference in the video anymore, I'm very happy with this result...

By Angelo Sanna

Resident (40)

Angelo Sanna's picture

21-03-2023, 23:55

Manuel wrote:

Wasn't this due to a too big load on the 5V? I've heard about it before. I guess you need to fix up the PSU to support more stable (and more power on) 5V. Probably was described on the forum earlier.

Hi Manual Bilderbeek. Not because of the load, but the problem is that the motor of the heads give some unwanted noise on the 5 volts line, known as EMI spikes (Electronic Magnetic Interference).

Everything described else about that is based on hobbyist experiences, not from professional work experience. That is no problem, so that’s why I react. The question you could ask is: why and what is it that caused a problem?

So, the documents about "power supply modification" by removing an coil for the 8235/45 is only to cover the symptom, but not the solution. The solution lies in the modification of the disk drive. Like create a “spark killer” on the head-motor. Because of the quick and cheap design at that time of the PC disk drives, and also because the drives where meant to be mounted in a PC (which motherboard and power supply have better filtering), it is what it is.

The coil in the power supply is to filter LF/HF unwanted signals, from and back to the electricity network (AC volts 110/110V Japan, The States, Asia, 230V EU/240 UK, etc.).

And maybe you won’t notice anything, but still it sits there with an good reason: to prevent unwanted LF/HF noises in your device or outside your device. Never remove such things, because you or someone else think it is not needed. So that document about this modification is not legit, it is prevent something but not an solution.

The same goes for the "power supply repair modification document 8250/55/80” by changing only the bridge rectifier for a bigger one. """Bigger diodes is the solution!!""

No. Not needed if you take away the core of the problem, which is the capacitor after the bridge in the power supply, the 22.000 uF.

Why? I will explain you: Because the bridge/rectifier diodes gets heated of the higher current that is flowing through, because it is loading the big 22.000 uF capacitor. That costs much more current. The diodes is dissipate the heat then…. There you go.

It is a design fault in the power supply, because if you change the 22.000 uF for a 3300 uF, you have solved the core of the problem.

Get rid of the riple on the DC line is the combination of caps and a good filter before and after an regulator (which you can find in the NMS8245 power supply. Like I describe above: removing the coil to “prevent” unwanted noise in the picture is not the solution). The coils in the NMS 8245 power supply are also described in the manufacturer STR2005 documentation.

The manufacturer and designer of the NMS 8245 power supply have listen to that document. Also, remind the last MSX2 design from Philips was the NMS8245 which was a good one, like the good sound circuit in it as example.

Bigger diodes to make the power supply more to handle? No. Because the limit is the 5 volts regulator, that can handle a max. current of 2 or 3 Amps (peak). If you want to use, let say, two slot expanders with 2x4 = 8 devices on it, please use an external power supply for your expander. Same goes for all extra drives (HDD, FDD, etc.).

A statement about some of the technical documents on sites like the File Hunter (no offence to no one, it is still my opinion): In electronics there are rules. You can’t break them.

The documents made by Albert Buurmeijer, Kees Folst and Hans Oranje are legit and professional ones, they know what they doing and talk about. These are known names. You can trust such documents.

Because I’m often addressed in the past for being rude because of a strong opinion I will not mention which documents are not. But you can fill in the blanks. And it does not make sense to mention, everyone does his thing and a penny in the pocket. But the problem is that these documents are not valid and can cause more problems like the conversion to MSX2+ document of the NMS8245. I will say something about this below this reaction. Stay tuned.

These are hobbyists documents. I know, it comes from a good MSX heart and enthusiast hobbyists, and it is not so hard to get on the right track. So that’s why someone must ring the bell. I will do that. No problem. Because it is easy to give an opinion, I have also worked with this problems and worked around to find solutions. I will share my experiences and all this with the whole MSX scene. So everyone can benefit.

I have read some reactions on MRC from people having problems with sync on some newer monitors. On CRT there are not much problems. But they are. And the solution is not to prevent, but to solve.

The rule for building an power supply like this one in our MSX is that you take about 2000 uF per Ampere, after the rectifier.

So if you want to handle 1.5 Amps, you need the minimum of 3000 uF. It is not an standard capacitor value, so you get 3300 uF (like 1000/2200/4700/6300 etc.). The formula to calculate this is: Δ U = I load / fC. And so on for more current like 5 or 6 amps. But that is for the old fashion power supply from the 80's with 7805 ~ 7824V... And all regulators between it.

For SMPS (Switched Mode Power Supply) there are other rules, it is a completely other technique with a Mos-fet that is switched on a high frequency like 100 kHz or so (and that is named after it).

If you look at the power supply of the NMS8245 for example, it have also an 3300 uF capacitor after the bridge for take out the DC rimple on the input of the STR 2005, which is the 5 volts regulator. And on the output there is also a cap and filter. In the NMS 8250/55/80 it is the Si-3052 regulator for the 5 volts.

If you take a look in the manufacturer specifications of both regulators (STR 2005 and the Si-3052), you will never find such a big cap of 22.000 uF, because it is not needed. It causes trouble, like in the 8250/55/80. It causes a higher current flowing through the bridge rectifier and dissipate much more heat.

Ok, now that last thing about the NMS8245 MSX2+ conversion on the internet, as above described:

More people have trouble on LCD/LED screens. Sync problems after conversion. On CRT the problem is not visible. The solution for the csync signal problem that comes from pin 6 on the V9958 is to follow the Omega schematics and or the MSX2+ service manual like the Panasonic/Matsushita.

If you do that, the problem is solved. Why? Because the difference between V9938 / V9958, on the V9958 it is an output, not in/out tri logic.

You have to remove some resistors and transistors that is lying in the way of the csync. From pin 6 csync output from the V9958 VDP.

If you have an oscilloscope, you can see why that is, and the difference between the signals. The solution is not to prevent, like: put a resistor and cap to prevent a bad sync., something I have read on this forum. But to follow the electronic rules. The rules were followed by Matsushita, which is based on the Yamaha V9958 documentation, so we will follow that.

The documents from Hans Oranje about converting the NMS8280 to MSX2+ is a good one, but also the old document from the MCM at the beginning of the '90s, where they mentioned the resistors on the pins which are not used. You can find them also in the Omega circuit diagram. Follow that, and there will be no problem because in those days MCM had several writers of the technical articles with known names. At that time there was some more quality, and no YouTube with technical tutorials where everyone could be a star... (of course there are good ones!).

But to mention again a name of known MSX technicians, Albert Buurmeijer was one of them.

In the 8245 conversion document the pins on the V9958 are directly to 5V or GND. Please do not do this, but put always an resistor of minimum 10K or higher to prevent the VDP is dissipate more heat. It was also described in the MCM article from the 90's, and in the Omega circuit which is based on known service manuals. That is why I have some trouble with this not legit documents from nowadays. The idea is good, but…. it could be done better.

I think it is time for a sort of quality control with some known name from the scene in technical way to get a stamp of approval on such documents. But that would be to offended to hobbyists. These are the days, everyone have the right to publish such documents. So a good healthy electronic mind of thinking is needed to prevent such faults.

So, this was my long time contribution for now, because I react not every day, so much to do in live, see you all next time! Wink

By sdsnatcher73

Prophet (3967)

sdsnatcher73's picture

22-03-2023, 07:06

Thanks for explaining this very clearly. I am certain it will help some open minded enthusiasts!