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Old 25th August 2012, 01:35 PM   #1
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Default Help with Hung Chang 5506 scope

I recently got one of these. The problem with it is sometimes when i turn it on there is nothing but a bright spot in the midle of the screen that is not affected by the intensity controll. If left in this state for some time or after being trn on a few times it jumps back to normal. Could old electrolytics be the cause of this? Does anybody know where I might find a schematic or a service manual?
Thanx!
Mario.
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Old 25th August 2012, 01:49 PM   #2
Mooly is offline Mooly  United Kingdom
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Have you tried whacking the unit when it does this ? Seriously

I have a HC 5510 100Mhz scope and that has occasionally done similar, actually I suspect the trace is off the screen as mine is sometimes "blank" at power up. And the problem with mine is physical but can I find it... it goes for 12 months sometimes before it might do it.

(I've moved this to "Equipment and Tools" as it will also get more exposure here)
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Old 25th August 2012, 03:42 PM   #3
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Thanx for the reply! Yeah I've hit it and dosent seem to fix it. I just kinda stays all bright and i switch it on and of a few times and that seems to fix it. do you know where i can find schematics or a service manual?
Thanx!
Mario.
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Old 25th August 2012, 04:00 PM   #4
Mooly is offline Mooly  United Kingdom
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Have you had the top off and (carefully as there is high voltage around) had a tap around with an insulated screwdriver handle on all the PCB's just to double check.

If you have a bright spot in the centre of screen that suggests that all scanning has been lost... maybe a supply is dissappearing. There will be at least two and possible four small transistors on heatsinks that are the X and Y amplifier outputs. See if the supply to them is dissappearing. That might be a common link with the intensity not working as if the cathode of the CRT falls in voltage that would brighten the spot. Check those supplies !
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Old 25th August 2012, 06:11 PM   #5
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I've had the top of and did what you said. No change no jumping back to life. I've lockated the 4 transistors you mentioned i will test there supplies.
Thanx again.
Mario.
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Old 25th August 2012, 07:30 PM   #6
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just one more quick question. Can you tell me some ballpark figures of voltages i should be getting on these transistors. There are 2 heatsinks with 4 transistors on each, either side of the board. Without a schematic i'm just kinda quesing.
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Old 26th August 2012, 07:15 AM   #7
Mooly is offline Mooly  United Kingdom
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I would guess around 150 to 200 volts as the supply to them. These are the deflection output transistors driving the deflection plates in the CRT. Its like a long tailed differential pair with the plates driven from the two collectors As one goes up the other goes down in volts and the spot moves. Supply feeding the stage is usually around that figure.
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