Marconi Mark 8 Camera, PSU questions

[austvarchive]

Well, part of the ongoing (6+ years) attempt to get one of my marconi mark 8 color camera chains operational, a friend came by to help get it working.

So far, the CCU appears to work fine (all diagnostic boxes appear on screen).

BUT, the camera PSU module which is known for its issues had failed, blowing the fuse each time. The PSU module was repaired, but the problem now is that the camera line fuse is still blowing, EVEN without the module in place.

The voltage was tested at the system PSU at the fuse and it measured 280v, where as according to the manual, the camera should be supplied with 180v, so we wondered if this was the cause of the blowing fuse rather than a fault in the camera.

If this was the case, why would the voltage be so high, or is this normal and the voltage gets changed somewhere else down the line?

We dont have the other volume of the manual with the diagrams for the main psu to know.

So, i wondered if anyone familiar with these cameras would know what the possible problem was and the next course of action as without further info we are rather stumped..

[BrianSummers]

There are any number of things that could be wrong, 280 volts would definitely upset the camera PSU, so the main psu needs fixing first.

The main ht supply comes from the transformer and bridge rectifier in a conventional fashion. This is then fed to the regulator, a simple chopper SCR, control is on the PCB and SCR on the heatsink below. Use a light bulb as the load and get it about right before inserting the camera PSU, It may be the main PSU will run without the camera PSU present, but you may have to fool the regulator to run without the feed back from the camera. There is a dc link round the camera cable involving the camera on-off switch. When working correctly the start up sequence is as follows:-

Mains on, main psu should settle at a bit less than 180 volts, camera psu fires up and the camera returns a signal to the main psu so that the voltage that reaches the camera is 180 volts. The point here is the main PSU should supply a lowish output until it gets the feedback signal to compensate for loss in the cable.

Check for dead silicon in the main PSU PCB and the SCR on the Heatsink.

The camera PSU is another kettle of fish entirely, again it is a inverter type. Most of the troubles are due to overheating of the main transistors. Check and replace the diodes in the base circuit. they MUST be fast types. 1N4448 more on this later.

[austvarchive]

Thanls for your reply Brian, it seems i have sorted out the problems so far by working out there are now issues with the main power supply unit rather its still the camera psu module that has issues.

I have found out there is supposed to be an extender board slotted in near the module in the camera head, however in all of mine it is missing and there are none to be found in the spare boards box!

I wondered if you or any other collectors out there who have one of these cameras had a spare one available?


BTW: I found out the camera head i am working on is: serial 371

[XDCAM]

Is it Board No 1A11 . I was thinking it would be possible to get a blanks made .

[austvarchive]

I've no idea of the number? infact i dont know what the extension card looks like!

Do you have one in your camera head in the "spare slot" (i think it depends on the build model too!)

how would you make a blank?, you'd need both the male and female connectors from somewhere

what other equipment used those sockets, did it have a name?

[XDCAM]

We need to find a Board (No Componets on it ).Then you take a picture of it too make a negative .Then you get a Blank board made .Main problem is finding a Board .You Never know what somebody saved !

[austvarchive]

I assume yours is missing too?

[XDCAM]

I've got more Fuse wire than track .I think most boards are the same .

[austvarchive]

i dont know what that board is

still no further with getting this camera working...

[XDCAM]

That Should be the Board your missing 11A1 in the Head Power Supply .Its used in the A/B Models . As you can see , it does not have many components on it .

[austvarchive]

no, that is not the missing extender board, that is one of the little boards that slot into the head psu module, what i am missing is the extender board which should be slotted in somewhere next to the entire head module itself, it would be a bare board with a male and female connector, the idea is to plug the head module into it and it operate but be outside the body of the camera so u can work on it.

[XDCAM]

Thats the Board we are looking for ? Never seen a Power Supply Ex Board .

[austvarchive]

No, its not the one

yes seems they are very hard to find!

[XDCAM]

What model have you got ,As I have No Empty slots in my heads A/B

[austvarchive]

the B model and the one before it that has just the marconi badge