Which video mod for composite?

[Dukes909]

So I disconnected all three connections to the RF modulator and hooked Mark's design (on a breadboard) up to the T1000. Now I have no video at all - booooo . What should the voltage on the center pin of the composite out be reading? I am getting around +0.12V. I don't see anything wrong in my design that is different from Mark's other than using a 100ohm resistor for R3. Any other ideas? Also, I am getting +2.7V on the pin 16 of the ULA if that helps.

[1024MAK]

Yes, you can pick up the +5V connection from the +5V supply to the modulator.

The video feed to the modulator needs to be disconnected. And instead connected to the transistor buffer circuit.

The third wire to the modulator comes from the channel change selection switch, so is not needed anymore.

Because of the capacitors, there should be no DC voltage on the output (signal / centre pin of the phono socket).

At the junction of R3 and C3 you should get nearly +5V.

Is the transistor connected the correct way round?

Mark

[Dukes909]

Yes, you can pick up the +5V connection from the +5V supply to the modulator.

The video feed to the modulator needs to be disconnected. And instead connected to the transistor buffer circuit.

The third wire to the modulator comes from the channel change selection switch, so is not needed anymore.

Because of the capacitors, there should be no DC voltage on the output (signal / centre pin of the phono socket).

At the junction of R3 and C3 you should get nearly +5V.

Is the transistor connected the correct way round?

Mark

I have the transistor in correctly and have even tried a second one. I am getting +2.71V from the ULA pin 16 that is going to R1. At intersection of R3 and C3 I am getting +4.98V. Does the value of R3 matter a great deal? I have tried 47ohm and 100ohm with no success.

[dessony]

1024MAK,

I tried my best to help Dukes909 with his computer. I recognized that you know the history of ULA better. I also supplied your video transistor buffer amplifier circuit with a stripboard to this gentleman. I knew about a printed circuit board based on your VTBAC.

Thank you very much in advance.

Cheers,
DesSony

[Dukes909]

1024MAK,

I tried my best to help Dukes909 with his computer.
DesSony

I appreciate your help earlier; I just happened to have a 2n2222 transistor onhand but not the NTE123AP one so I made Mark's on a breadboard. Evidently I have something incorrect and will take it apart and try again. It seems like I should see something on the screen!
Cheers
Dukester

[1024MAK]

At the junction of R1 / Q1 base measure this voltage. I’ll refer to this as the ‘base’ voltage.

Then at the junction of Q1 emitter / R2 / C1 / C2 measure this voltage. I’ll refer to this as the ‘emitter’ voltage.

This emitter voltage should be about 0.65V less than the base voltage. If this is the case, then the transistor is being switched on.

If you have not already disassembled the circuit, maybe if you post up a photo, we may be able to spot the problem.

The value of R3 is not critical. But values greater than 150Ω are not recommended.

Mark

[Dukes909]

At the junction of R1 / Q1 base measure this voltage. I’ll refer to this as the ‘base’ voltage.

Then at the junction of Q1 emitter / R2 / C1 / C2 measure this voltage. I’ll refer to this as the ‘emitter’ voltage.

This emitter voltage should be about 0.65V less than the base voltage. If this is the case, then the transistor is being switched on.

If you have not already disassembled the circuit, maybe if you post up a photo, we may be able to spot the problem.

The value of R3 is not critical. But values greater than 150Ω are not recommended.

Mark

Thank you. Here is what I measured:
base voltage: +2.72V
emitter voltage: +2.34V

Transistor is a 2N2222
My phone camera doesn't do well at close ups but here are a couple of pictures:

The white wire comes from ULA pin 16
The top left red wire is +5V from T-1000
The top right blue wire is 0V from T-1000

The red and blue wires in lower right go to RCA jack (unshown). Negative lead of C1 goes to the same rail (where the red RCA lead is plugged in) as the the other capacitor although the angle doesn't show it in pictures.
Cheers
Dukester

[Aurelbois]

I think you have a 100K ohm resistor connected to the Emiter instead of a 100 ohm resistor.

[Dukes909]

I think you have a 100K ohm resistor connected to the Emiter instead of a 100 ohm resistor.

Well spotted!! Who put those in the wrong bin? Cheers for spotting that.

Changing to a 100 ohm, I get
emitter voltage: +1.95V

I do get a picture now, but it is considerably worse than what I started with on the existing RF modulator as the display is not stable(see video link):
https://imgur.com/RyThYpZ

Image:

Cheers
Dukester

[1024MAK]

Lack of stability indicates that the TV is not happy with the sync part of the signal.

You could try adding one or two silicon signal diodes (such as 1N4148 or1N914) between the input from the ULA and resistor R1. The anode (a) end goes towards the ULA. The cathode (k) goes towards resistor R1. If one does not work, try fitting two in series.

Alternatively try fitting a 100Ω resistor between the phono centre (signal) pin and 0V/GND.

The weird colours are due to the colour decoder getting confused and misinterpreting the signal. I’m not sure what to suggest here, as this is much less common in PAL land.

Mark