Vintage Composite Green Screen Monitor

[scommstech]

Hi
I'm trying to get the schematic for a composite green screen monitor.
This was made by BMC and is from the 1980's
The model is BM12ES.
I can't find any reference to BMC monitors but it was probably
marketed under different brand names.
If it would help I could probably get part no from the main circuit board.
Hope somebody can help.
Regards
Harry

 

[mneary]

I found that the color of the screen made no difference. Where are you located?

Is the main board powered by 12VDC? Most of them seem to have been, if they were 12 inches or smaller. Larger ones sometimes used 24VDC. I saw very few powered directly by the mains.

Where are you located?

 

[xanadunow]

If I may ask, what are the symptoms of the fult? is it in the comm's digital circuitry or in the analog part of it?

Regards,

xanadunow

 

[scommstech]

Composite monitor BM12ES

Thanks for your replys
I am in the UK. The monitor is 240V. The logic is probably 12V or 24V
Ive had a quick look at the circuit board and there seems to be only
discrete components, no ICs.
The fault is too much brightness. There is some control but it won't back off.
I am using it to display an Apple II menu and when I turn the brightness
right up the characters expand.
This would indicate low EHT, and poor regulation but without the circuit I
can't be sure of what's happening.
Hope somebody recognises the model.
Many thanks
Harry

 

[xanadunow]

Well, it is not exactly my "cup of tea" but it is fairly common to work on a piece of equipment without the diagram.

As the the monitor is still in working condition, we can assume that problem is due to deterioration of component(s), soldering joints or the combination of the above.

There should be a trimpot or two, located on the neck board of the picture tube. One of them should have the effect on the level of screen raster and the other on the focus.

[Note: a proper, isolated trimpot adjusting tool is required to do the adjustments while the monitor is operating. I do apologise for mentioning it here as I am sure you know this. I do it because of the safety, for the benefit of potential other but less experienced readers that may open this post.]

I would also examin the soldering of the whole unit, taking particular note of areas around components that dissipate the heat (like high power resistors, transistors, e.t.c). I would resolder the suspect ones and continue with examination of the PCBs, but this time - on the component side.

There could be obvious signs of deterioration of electrolitic capacitors (the sign is a bulging top of the can but not always). They seem also to deteriorate in two measurable ways, like loss of capacitance (capacitance meter is helpfull) or incresed ESR (effective series resistance). There are ESR meeters available, I have built one around 10 years ago from a kit purchased at a Dick Smith store. It is a very usefull piece of instrument and I still use it today.

As the unit is old, it is probably a good idea to replace not only suspect capacitors but the whole lot of them (electrolitics). It is not a job I particularly like but without the diagram I found it to be an effective way to solve the problem and little electrolitic capacitors are not expensive. The only real expense here is the time it takes to do it .

I am almost convinced the problem here is the overall detrioration and I sincerely hope that you will fix it.

As for the diagram, I can not be of any help to you.

Regards,

xanadunow

 

[scommstech]

Bmc Bm12es

Dear Xanadonow
You are absolutly right.
If I can't get the circuit I will probably end up doing as you suggest.
I do however prefere working from a circuit so that I can check out the rest
of the equipment.
This actual fault suggests an EHT malfunction. Age related conditions do
mostly effect electrolytic capacitors, and dry joints.
This may be the case in the EHT generation circuit. The text displayed is
normal apart from the blooming which suggests the main board power may be
good.
Its always safer to to set up the EHT as per the manual as the components
have to stand greater stresses. Later colour TVs/Monitors can be around
30KV so it can be very dangerous to mess with the pots as X rays can be
generated.
Monocrome/ black and white smallish monitors are if my memory is right about
9KV and X ray safe. This one is about 14inch.
I shall give it a bit longer, hoping for a circuit before getting the soldering
iron out.
Thanks for you interest.
Regards
Harry

 

[Nigel Goodwin]

Classic cause of 'blooming' is the EHT rectifier duff.

 

[scommstech]

BM12ES Monitor

Hi Nigel
If I could turn the brilliance down, I would suspect the EHT rectifier.
The brilliance bring wrong (too bright) suggests the tube biasing has a fault.
A circuit diagram would tell me what voltages to expect on the tube.
I was hoping that some of the members might know of a good supplier of
schematics for older equipment.
Regards
Harry

 

[Nigel Goodwin]

You don't really need a circuit diagram for the tube voltages, they will be pretty standard - what makes it too bright is the cathode voltage too low (assuming the grid is fixed around zero, as usual). Another cause would be the A1 voltage too high, they may be an O/C resistor in the ground side of the A1 control.

 

[scommstech]

Hi Nigel
You are right, it's just that I've spent the last year having to repaire dozens of different
pieces of equipment without their circuits. Ive just had enough of working blind.
In a weeks time I may have a go to fix the monitor, but it would take me minutes with a
diagram.
Regards
Harry