MM-9M 10″ portable 800-line vintage B&W CRT monitor

I love B&W CRT displays, for they bring back old memories of the long-lost past, of the first time I ever watched a television, and of my first programming exercise in QuickBASIC. So when I was a given the opportunity to inherit several MM-9M 10″ B/W CRT monitors, I could not resist it and bought them all.

At a first glance, the MM-9M is just like any other B&W monitor, except with two BNC sockets at the back, one for video input and the other for a pass-through output. Despite the INPUT/OUTPUT labels, the sockets are interconnected and can be used interchangeably. There is also a switch to select between 50 Ohm and 75 Ohm input impedance and also trimmers for focus and vertical size adjustment. The unit accepts 100-240V, 50/60Hz and can auto-switch between PAL and NTSC. At the front, there are also knobs for vertical size, brightness, contrast adjustments as well as a power switch:

Of note, there are several other similar models made by the same company. The MM-9 and MM-9M has a single video channel with no integrated speaker. The MM-9A supports mono audio while the MM-9S supports dual video input. The same case design is shared across all 10″ models. In models with an integrated speaker, the vertical size knob at the front is replaced with the volume knob. There are also MM-12, MM-15 and MM-20 with 12″, 15″ and 20″ screen size respectively.

This is a partial scan of the user manual, with the specifications and some usage instructions. I did not scan the rest of the pages which contained (very detailed!) device dimensions, some generic advisories (e.g. do not use outdoor) and the Chinese translation. The manual did not contain any circuit schematics.

The monitor works well with a DVD player using a composite to BNC adapter, showing the DVD menu of Mission Top Secret, a TV series I used to watch back in the 1990s:

This is how the same DVD menu looks like in VLC:

Looking closely, you will notice several artifacts known as chroma dots over certain areas of the pictures where colours are expected, for example in the words ACHTUNG (German for Caution), which was originally yellow. Apparently, the MM-9M, despite being manufactured in the 2000s, does not attempt to filter out colour information (for example via a bandstop filter), causing these dots to be displayed in the picture, due to the colour subcarrier (4.43MHz for PAL or 3.58MHz for NTSC) interfering with the video demodulation process. These artifacts are much less visible on my 5″ portable B&W TV, which presumably has such a filter implemented:

This is a close-up comparison of the movie title Achtung Streng Geheim! (German) or Mission Top Secret (English), with and without chroma dots:

This is Norton Commander 5.0 in all its glory, despite being swamped with chroma dots from the default blue background:

The same NC 5.0 screen looks somewhat better on my 5″ B&W monitor without these artifacts:

If we set NC to display in black and white from Commands > Configuration > Screen, chroma dots will disappear from the MM-9M display. Video will be crisp and text will be totally legible:

This is NC 5.0 on a Toshiba 14AF43 14″ CRT TV via the composite output. The presence of NTSC colour and CRT shadow mask gives the display that kind of fuzzy look and makes it hard to read even 80×25 text, despite the much larger screen size. Without the shadow mask, even a 5″ CRT would be able to display legible 80×25 text without much fuss.

The MM-9M has no issues with 80×50 EGA mode, since it supports 800 scan lines, as opposed to 480 scan lines for 14″ CRT TV or even 240 scan lines for a Sony Trinitron of the same size. Here the MM-9M is displaying Turbo Pascal 7.0’s README, using NC 5.0 in EGA mode:

On my 5″ CRT TV using the same video signal, text will be too small and fuzzy to read. It goes without saying that the quality will be much worse on my 14″ Toshiba CRT in this mode, thanks to the shadow mask.

By connecting the monitor to the Y output of an HDMI to component converter, I was able to get a crisp display of Microsoft Word in 800×600 resolution, adapted to NTSC (480i at 60Hz). There were obviously no chroma dots this time and even texts at small font sizes were perfectly readable:

This is the same screen in PAL (576i at 50Hz). The OSD incorrectly labelled the input source as 1080p. It was actually just 800×600:

To ensure the highest possible quality when taking photos of CRT, I use Open Camera, which allows me to adjust shutter speed to 1/50 (for PAL) or 1/60 (for NTSC), something not possible with the built-in camera app:

Despite the 800-line resolution, the MM-9M can only handle 480i and 576i on its BNC input. A distorted display will result if you use higher resolutions. For this reason, it is not possible to use most cheap Amazon HDMI to component converters, which always output 720p. My converter however has a button to adjust resolution and works great with the MM-9M:

This is what you will see if you hook the TV to the Y output of a 720p component source:

I took a trip down the memory lane and hooked the above 720p source to my GW Instek GOS-6103 100MHz analog oscilloscope. In TV-H trigger mode, I measured a 62.8KHz horizontal sync frequency, four times the expected value of 15.7KHz for a composite video source:

TV-V (sync pulse) trigger mode worked well too:

It was much harder to get a nice display of the 62.8KHz horizontal sync signal on the Rigol, despite all the advanced trigger settings. I needed to set memory depth to 24M (instead of just Auto) in Acquire menu, otherwise the waveform would be crude due to lack of samples:

This is the 15.7KHz horizontal sync frequency of a typical composite signal:

The following photo demonstrates the operation of the scope’s delayed time base circuit, zooming into the colour burst signal:

As with most analog oscilloscopes, in delayed time base mode, the cursor can no longer be used. We can still measure the frequency of the colour burst using the main time base, showing an approximate value of 3.57MHz, close enough to 3.58MHz for NTSC:

A more precise measurement can be done on my Rigol DS1054, showing a value of 3.597MHz:

On the Rigol, you can also select TV system (PAL/NTSC/SECAM) and whether to sync on all lines or only odd or even lines. Still I love the Instek more, as the green phosphor display brings back nostalgia.

If you want to capture the same screenshots, try to set your video output to a uniformed colour. Use textbackground(blue) followed by clrscr in Turbo Pascal to set the background to blue. Otherwise, it might be hard to get a stable display of the colour burst signal, as some parts of the screen might not be in colour.

When the picture is completely white, the composite output no longer contains a colour subcarrier. The upper portion of the waveform becomes nearly flat, since every line now carries the same uniform intensity:

Eventually I connected the MM-9M to my DVD player’s Y output. This way, I got a pure greyscale signal, without any colour subcarrier and obviously no chroma dots. With a Tecsun Q3 radio acting as the speaker, I can now enjoy old movies with audio on the MM-9M:

A single MM-9M unit in the batch did not work out of the box. The power LED did not light up when power was applied. Total current consumption was less than 5mA on my multimeter. The fuse on the PCB was not blown and there were no obvious PCB damages, suggesting some other issues with the power supply section. This unit will be an interesting repair attempt for my next post. Stay tuned!

See also
Vintage oscilloscopes to the test: Hitachi VC-6025, GW-INSTEK GOS-6103 and Kenwood CS-5275