Mystery video clamp.

[Argus25]

I have a 14 inch monochrome video monitor that I have no schematic or manual on. So I have been patiently going through it copying the circuit from the pcb's. I have not been able to power it yet because of some power supply issues.

Most of the circuitry seems pretty conventional. I have attached the video stage circuit and the amplifier that drives the CRT's cathode. It uses MC1545 amplifier IC's as gain blocks and for contrast control and has a differential video input. A horizontal blanking (retrace) pulse is introduced to one of them and therefore would add to the video signal. (Though it might only be a small signal because the monitor has a full H & V blanking signal from a special purpose blanking amplifier added to the CRT's grid circuit)

The thing about it is the video signal clamp or DC restorer system which clamps the DC level after the coupling capacitor C16. It appears that the trailing edge of H sync is differentiated and fed to Q1, that feeds Q5 which operates a diode switch based on CR6,7,8 & 9, The output of this drives the clamp transistor Q6 which discharges C16 to the 5.1V rail.
I drew in a few likely pulses.

But the interesting thing is, the diode switch can only be active on positive going video as the diode switch is also driven (& gated) by Q12, which is fed from the video output.

I have never seen this configuration before, has anyone ever seen it published in a book or in another monitor ?

What would the advantage of this arrangement be compared to a standard black level clamp ?

I hope the attached circuit is readable.

[Maarten]

It never hurts to mention the brand and model and/or markings on the PCB when asking about a particular circuit. Someone might have a schematic or specific knowledge or even find relevant patents.

The clamp arrangement itself is puzzeling me a bit as well. I'll sleep on it.

[Argus25]

Originally Posted by Maarten

It never hurts to mention the brand and model and/or markings on the PCB

Unfortunately this is what looks like a custom built monitor for some sort of avionics imaging application, but I'm sure from the design of the deflection oscillators and video stages it takes a standard American format analog video signal. There is no documentation of any kind available for it, which is why I have had to draw out the circuitry by hand.

One other interesting thing about the clamp is the heavy looking filtering on the base drive to the clamp transistor, making it appear as though they wanted to get the clamp transistor into an average state of conduction, rather than just at intervals in the H blanking period which would be the customary way to do it.

[Maarten]

There are bound to be some markings or code numbers on the PCB, but it might still not be possible to identify the set by those.

Instead of - or additional to a clamp, could this be some sort of gating or switching circuit? I haven't quite figured it out yet, but by chance I found a similar circuit in the service manual of the Philips PM5193 waveform generator.

[Argus25]

Originally Posted by Maarten

Instead of - or additional to a clamp, could this be some sort of gating or switching circuit? I haven't quite figured it out yet, but by chance I found a similar circuit in the service manual of the Philips PM5193 waveform generator.

Yes it is that too, the diodes are reverse biased by the DC conditions, until when the trailing edge of H sync comes along and also the video signal driving the CRT cathode has a positive polarity, as it does in the sync and blanking interval (since it appears a blanking signal is added), then the 4 diodes conduct, which effectively shorts out the 10k resistor and turns on the clamp transistor Q6. The drive level to Q6 then becomes determined by the video signal output level which is the interesting or intriguing part.

Normally to clamp say at black level, the clamp transistor (that charges or discharges the coupling capacitor) is just turned on in the back porch area after H sync and there is no feedback of any kind from the video amp's output to the clamp circuit.

But there is the 0.47uF capacitor that stores charge in the base circuit of Q6, so I doubt if Q6 actually turns off quickly and it would appear from the value of Q6's base resistor to be conducting for over 40uS.

It might be one of those cases where I will have to get it powered and put the scope on the CRT drive, and vary the contrast, to figure out exactly how it works. I can't read the values on a number of the components either. The monitor runs from 400Hz 120V three phase AC so I'm building an inverter to run it.

It appears that the intention behind this circuit is to stabilize the DC axis of the video signal, so that changes in contrast level (video amplitude) do not affect what would be the black level of the signal. But it is a more sophisticated circuit than I have ever seen for the application before and I think there must be something very clever about it.

One other oddity is that the monitor uses a UJT as the vertical scan oscillator with sync introduced at B2.

(I'll double check the value of the 0.47uF just in case I got that wrong)

[Argus25]

Maarten,

I finally got the set powered. The circuit is an astonishingly well performing black level clamp which serves the purpose of a DC restorer, but better because it works on the black level, not the sync tips. The differential pulses out of Q5 occur at black level in the back porch, just after H sync. These activate the bridge diode system.

I looked at the output to the cathode of the CRT with the scope set on DC. The black level video voltage is rock solid, regardless of the picture content from black to white. So it does appear to be an outstanding design. Though its not a common configuration.