• Please review our updated Terms and Rules here

Commodore 1081 monitor dim image

A

Arn_K

Member
Joined
Jan 4, 2023
Messages
24
Hi everyone,

Today I purchased a very clean looking Commodore 1081 monitor, but it doesn't seem to work properly...

I tried connecting my amiga 500 to the scart connector and there does seem to be an image (kickstart hand), but it is very dim. (see attached photo)

So I tried a different scart monitor (1084s) to make sure the computer and cable are working, and both are fine.

The next thing I tried is connecting my Nintendo Wii to the 1081 monitor and then a nice clear picture appeared.

So for some reason the Wii does work with this monitor, but the Amiga doesn't, is it possibly related to the frequency of the video signals?

I'm not sure where to start with this repair. Is this a common or known issue?
 

Attachments

  • 1081 dim picture.jpg
    2.3 MB · Views: 14
My first question would be whether your lead / cable which works with the 1084 is wired for composite video only or for RGB and composite video, or for RGB + composite sync only?

The effect you have on screen looks, potentially, as though the monitor is expecting an RGB signal -only- but is being fed with a composite signal.

The bezel surrounding the screen looks a lot like the one on my Philips CM8524 which at one time I did use with an Amiga and still use with my Atari STs. My Philips has an RGB / CVBS mode select switch - does your 1081 also have that, and if it does, what happens if you change from one mode to the other?
 
It looks like the VDU must have synchronized, or you would not be seeing the faint image

The character of it indicates there is a break in the signal path and what you are seeing is just a small amount of the video signal's high frequency components on edges being coupled across by a few pF of capacitance. It could be a signal select switch in the wrong position or a broken connection. It would be easy to find tracing the signal through with the scope.
 
The 1081 does have an RGB/CVBS switch, when it is in CVBS mode I get the faint kickstart hand, and when it is in RGB mode I get a white screen with nothing on it.
The white screen is also there when the scart cable is disconnected or the Amiga is off.

Could that mean that the R, G and B signals are all stuck high, and they create white?
 
I have measured a few more things on the monitor. The R, G and B signals seem to be OK, then they go from the scart connector through a few transistors and passive components. The signals do all have lower voltages after these transistors (about 4 or 5 volts), I believe this is fine. Then the signals go into a TDA3505 video mixer IC.
Could this IC be the problem? Or am I going in the wrong direction?

PS. The color composite input is working.
 
>>> PS. The color composite input is working.

When you state this, how are you testing it?

The colour composite input is CVBS (Composite Video Baseband Signal). But, above, you state that "when it is in CVBS mode I get the faint kickstart hand".

This indicates the composite video input is both working and non-working...

SK2 is the switch that switches between CVBS and RGB. If you look in the lower-right of the Commodore 1081 schematic, you should see SK2 and the associated circuitry. This is the CVBS/RGB switch (I think). A number of voltages are switched when the switch is in the two positions. I would ensure that the correct voltages are present with the switch setting in both directions.

This does switch +12A to pin 11 of the TDA3505 to operate an internal switch; and voltages such as +11, +11A and +11B feeding the RGB input transistor buffers. If these voltages are not correct - or do not switch at all - this will cause your problem.

Also notice that there are a number of low-valued safety resistors around the CVBS/RGB voltage switches (e.g. R235, R232 etc.). Look for a resistor with two (2) parallel lines and and '!' mark inside a triangle on the schematic. Over time, these resistors can just fail - and you will not get any voltage supply to the following circuitry.

Incidentally, this is the manual I am using: http://www.zimmers.net/anonftp/pub/cbm/schematics/monitors/1081_Service_Manual.pdf

Dave
 
Last edited:
Looking at the schematic, I agree with @daver2, the thing to check is the +12A voltage on pin 11 of the 3505 IC, to check if it is changing to the correct values when the CVBS/RGB switch is deployed, this is how the two different signal sources are selected inside that IC. Those signal switches in the IC, if off, are not 100% perfect isolators and they can couple a tiny amount of signal through when off, and that would account for the faint image. And check the 11v voltages feeding the RGB transistor buffers as @daver2 says.
 
Bear in mind that Arn_K said in the initial post that while the 1081 does not work with his Amiga, it does work with his Nintendo so it would be useful to know whether the connection from the Nintendo to the SCART connector is:

-Composite only

Or

-Composite + RGB, with the composite signal in RGB mode serving as composite sync for the RGB interface.

It would also be useful to know whether the image from the Nintendo is displayed:-

-Only with the monitor set to CVBS mode,

-Only with the monitor set to RGB mode,

-With the monitor set to either of those modes.
 
Last edited:
Forgot to add. When I bought my (seemingly identical) Philips CM8524 back in around 1986, I found that it did not work in RGB mode, only in composite mode. As it was under warranty I took it back, it was repaired by the retailer's off site repair shop and came back working. As I wasn't involved I don't know what the fault or fix was, but it does show that an RGB-only fault is possible.
 
OK, I have measured some things you have mentioned, this is what I found.

-Pin 11 TDA3505, switches between ~2,1v and 0v, this is the same as on the schematic, so I think this part is working correctly.
- +11 is about 11 volts, so this should be fine
- +11A: Nothing 0 volts...
- +11B: Nothing 0 volts...

So I think there is something wrong with the 11A and 11B voltages, These were tested in RGB and CVBS mode, and both give the same results.
These voltages are based on +11 and are connected to it via R214, the side of R214 which is connected to 11A and 11B has 0 ohm between it and ground.
Could something be shorted there or is the false measurement because it is also connected to ground via a capacitor?

The Nintendo only uses a composite signal no RGB, and I only get an image then the monitor is set to CVBS mode.

When I said the composite input is working, I tested in through the composite RCA connector, not the scart connector. (Sorry for the confusion)
 
Excellent,

If you have voltage at +11 but nothing at +11A Then check:

1. R214 for being open circuit.
2. C214 being short circuit (to 0V).
3. Something on the +11A rail being short circuit (to 0V)

C214 will start off as a short circuit (0 Ohms) and then charge up if you leave your probes on there. If the resistance stays at 0 Ohms, then there is a short circuit somewhere. In this case, I would remove C214 and test it out of circuit. HOWEVER, please note the orientation of C214 BEFORE removing the device - as it is polarised.

If C214 is short circuit (when measured out of circuit) - replace it. Measure the resistance for a good 30 seconds to a minute though to see if the capacitor charges up at all...

With C214 removed from circuit, check for the resistance across where C214 was fitted, to ensure there is no residual short circuit.

Dave
 
I have removed C214, and it was shorted, Replacing it with a new 100uF cap fixed the issue :D

Thank you all so much for your help! I'm really happy to have my 1081 working again.
 

Attachments

  • 1081.jpg
    2.9 MB · Views: 8
Mine has worked ever since it was repaired back in 1986 (... I shouldn't really jinx it by saying that, should I?) but that is something worth my writing down on the circuit diagram for the future.

Well done to all concerned.
 
You must log in or register to reply here.
Back
Top