Computron monochrome raster display - UK Vintage Radio Repair and Restoration Discussion Forum

jimmc101 · 26th Mar 2020, 4:12 pm

I'm trying to repair an IFR A-8000 spectrum analyser with an intermittent total loss of the display.
I believe the display module is the same as the A-7550 which I have the service manual for.
Access to the display module is difficult so I would like an idea where the fault is before starting to dismantle the unit. (i.e. Display module or Video Processor)

The display is Computron 115-35 DMK7 from 1987 I've attached an excerpt from the schematic and a drawing of the PCB.

I would appreciate any information on the display and in particular the type number of IC101.
I assume that the display should produce a raster in the absence of sync pulses, is that correct?

Of course if anyone has a service manual for the A-8000 then it would make my day. (I already have the operating manual.)

Jim

TonyDuell · 26th Mar 2020, 5:49 pm

Where does the signal you've labelled 'Horiz Drive' go?

The fact that there are no frequency/hold controls on the bit of circuit you've posted (as far as I can see, it's not easy to read) suggests to me that there are no vertical/horizontal oscillators there. Which means the monitor section may well need the 'sync' signals from the processor PCB to do anything.

jimmc101 · 26th Mar 2020, 7:59 pm

Thanks for the reply.

The 'Horiz drive' goes to a driver transistor which is transformer coupled to the line output stage. To my untutored eye the whole circuit looks very similar to a section of a monochrome TV of that era.

Having spent some time looking at the construction of the analyser it looks as if it is more easy to access display module than the video processor so that's my plan for tomorrow.
Construction is similar to A7550 as shown in the attachment.

Jim

McMurdo · 26th Mar 2020, 9:31 pm

many embedded monitors are like as tony has suggested. Without the sync pulses (at least, the horizontal) they often won't run at all.

A disadvantage of this is when trying to get the monitor to run when on its own. If you're guessing the frequencies, you can have the monitor take excessive current or even flash over as it runs at whatever you feed it.

jimmc101 · 26th Mar 2020, 11:24 pm

Thanks for that, I shall have to try and make up an extension cable so that I can run the display out of its housing.
It's either that or program a microprocessor to provide sync pulses.
I've found elsewhere in the manual that V sync is a 19.608kHz square wave.
I can't find V sync but it doesn't matter it's shorted out by the link across C116A . (This is a deliberate mod listed elsewhere on the schematic.)

Jim

jimmc101 · 27th Mar 2020, 9:08 pm

I managed to get it apart and found that a lot of corrosive glue had been used to hold large components and cables down.

The ribbon cable in the picture connects to the CRT base and the worst corrosion occurred on the G2 conductor, same at the other end.

IC101 remains a mystery, It really annoys me when manufacturers sand off markings.

Jim

26th Mar 2020, 4:12 pm	#1
jimmc101 Heptode Join Date: Mar 2009 Location: Portsmouth, Hampshire, UK. Posts: 674	Computron monochrome raster display I'm trying to repair an IFR A-8000 spectrum analyser with an intermittent total loss of the display. I believe the display module is the same as the A-7550 which I have the service manual for. Access to the display module is difficult so I would like an idea where the fault is before starting to dismantle the unit. (i.e. Display module or Video Processor) The display is Computron 115-35 DMK7 from 1987 I've attached an excerpt from the schematic and a drawing of the PCB. I would appreciate any information on the display and in particular the type number of IC101. I assume that the display should produce a raster in the absence of sync pulses, is that correct? Of course if anyone has a service manual for the A-8000 then it would make my day. (I already have the operating manual.) Jim Attached Thumbnails

26th Mar 2020, 5:49 pm	#2
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: Computron monochrome raster display Where does the signal you've labelled 'Horiz Drive' go? The fact that there are no frequency/hold controls on the bit of circuit you've posted (as far as I can see, it's not easy to read) suggests to me that there are no vertical/horizontal oscillators there. Which means the monitor section may well need the 'sync' signals from the processor PCB to do anything.

26th Mar 2020, 7:59 pm	#3
jimmc101 Heptode Join Date: Mar 2009 Location: Portsmouth, Hampshire, UK. Posts: 674	Re: Computron monochrome raster display Thanks for the reply. The 'Horiz drive' goes to a driver transistor which is transformer coupled to the line output stage. To my untutored eye the whole circuit looks very similar to a section of a monochrome TV of that era. Having spent some time looking at the construction of the analyser it looks as if it is more easy to access display module than the video processor so that's my plan for tomorrow. Construction is similar to A7550 as shown in the attachment. Jim Attached Thumbnails

26th Mar 2020, 9:31 pm	#4
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,271	Re: Computron monochrome raster display many embedded monitors are like as tony has suggested. Without the sync pulses (at least, the horizontal) they often won't run at all. A disadvantage of this is when trying to get the monitor to run when on its own. If you're guessing the frequencies, you can have the monitor take excessive current or even flash over as it runs at whatever you feed it. __________________ Kevin

26th Mar 2020, 11:24 pm	#5
jimmc101 Heptode Join Date: Mar 2009 Location: Portsmouth, Hampshire, UK. Posts: 674	Re: Computron monochrome raster display Thanks for that, I shall have to try and make up an extension cable so that I can run the display out of its housing. It's either that or program a microprocessor to provide sync pulses. I've found elsewhere in the manual that V sync is a 19.608kHz square wave. I can't find V sync but it doesn't matter it's shorted out by the link across C116A . (This is a deliberate mod listed elsewhere on the schematic.) Jim

27th Mar 2020, 9:08 pm	#6
jimmc101 Heptode Join Date: Mar 2009 Location: Portsmouth, Hampshire, UK. Posts: 674	Re: Computron monochrome raster display I managed to get it apart and found that a lot of corrosive glue had been used to hold large components and cables down. The ribbon cable in the picture connects to the CRT base and the worst corrosion occurred on the G2 conductor, same at the other end. IC101 remains a mystery, It really annoys me when manufacturers sand off markings. Jim Attached Thumbnails