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Old 15th Oct 2021, 10:51 am   #1
pudwink
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Default Can anyone help a vintage computer with a CRT issue.

Hi all
I was wondering if any of you vintage CRT guys could help me with a problem I have with an 1970s computer.
I have a Sharp MZ-80K from the late 70s which has a built in CRT display now the guys onthe vintage computer side have helped me get it going but they have suggested I post here to get the CRT experts to help.
When the computer is switched on it is fine then after a min or so the top of the screen starts pulling and gets worse and worse but If I turn it off for about 30 secs and turn it back on the screen is fine then it starts pulling again after a short time.
I will include the service manual which might help one person did sugest it might be
It's a monitor issue. The flyback transformer isn't settling down properly after a VSYNC. The computer part is working correctly and the display is correct, only mistimed until it settles down.

Check the caps and diodes in the monitor scan control section.
But I am not very good at reading schematics so if anyone could help me I would be very grateful.
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Old 15th Oct 2021, 2:37 pm   #2
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Default Re: Can anyone help a vintage computer with a CRT issue.

Hi

I would start by checking the PSU decoupling C's around the display section C2004, C2005 etc. also any other C on the 12v rail. Looks like a problem following vertical flyback.

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Old 15th Oct 2021, 8:44 pm   #3
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Default Re: Can anyone help a vintage computer with a CRT issue.

C2016 coil coupling cap can lead a hard life on these types of circuit.
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Old 15th Oct 2021, 9:08 pm   #4
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Default Re: Can anyone help a vintage computer with a CRT issue.

I've just looked at the circuit diagram and it seems this monitor uses composite sync and has an actual line timebase, not just direct drive of the line output transistor as so many terminals and computers with built-in monitors used.

Simple things first: does a tweak of the horizontal hold control (L2001) make a difference?

Then, is the sync signal healthy at test points 4 and 5 on the monitor diagram?

The tearing at the top of the screen is something I've seen a lot when the vertical sync pulse is just one long pulse with no horizontal sync pulses during it. Have a look at the signals round IC26. The XOR gate connected to pins 8/9/10 combines horizontal and vertical sync pulses. There should be horizontal sync pulses (probably about 5us long every 64us) at pin 10, vertical ones (probably about 200us long every 16ms) at pin 9, and the two combined at pin 8. Critically, at pin 8 the vertical sync pulse should have "gaps" in it where horizontal sync pulses occur. A similar circuit is used in lots of arcade games, and I've seen faulty XOR gates mean that the horizontal sync pulses disappear during the vertical sync pulse. This confuses the horizontal timebase in the monitor.

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Old 15th Oct 2021, 11:09 pm   #5
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Default Re: Can anyone help a vintage computer with a CRT issue.

Using the horizontal control does nothing for the pulling at the top of the screen.
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Old 16th Oct 2021, 8:08 am   #6
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Default Re: Can anyone help a vintage computer with a CRT issue.

The problem could be caused by almost any of the electrolytic capacitors smoothing the 12 V supply.

I’d start with C2051 and C2021 that smooth the supply to the horizontal oscillator.

Either replace the capacitors or, if there is space, test by piggy backing new capacitors across the old.

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Old 17th Oct 2021, 11:14 am   #7
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Default Re: Can anyone help a vintage computer with a CRT issue.

I will order some capacitors this week and give it a go.
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Old 2nd Nov 2021, 8:55 pm   #8
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Default Re: Can anyone help a vintage computer with a CRT issue.

Hi all
Sorry not got back sooner but today I took the CRT board out and replaced 12 of the capacitors as I had them I thought I might as well replace them which was C2004,C2051,C2047,C2021,C2049,C2038,C2016,C2005,C2 050,C2045,C2012,C2015.
Now I know they are not all to do with the area we have issue with but I thought while the board was out I would do it.
Put it all back together and still the same problem which was a big let down there is a couple of caps I did not have so not replaced them so dont know if sods law they are the ones at fault or it is something else at fault.
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Old 5th Nov 2021, 1:33 pm   #9
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Default Re: Can anyone help a vintage computer with a CRT issue.

On a positive note, some possible causes of the fault have been eliminated which takes you a step forward. It is still worth checking the supply as suggested in the previous thread post #293 ‘scoping the +12V supply to see if it is dipping or sagging every time the display sets off on a new scan down the screen. It should be steady 12V, maybe with a slight sawtooth shape to it but without any big dips or downward spikes.’

Check that the 5 V supply is within ±0.25 V of nominal. Also use an oscilloscope to check for ripple.

Look for anything visible like a poor connection.

Post #4 in this thread has good suggestions. The waveforms are in the Service Manual but post #4 of this thread has a more detailed explanation of the SYNCH waveform at IC26 pin 8 (waveform 4 on the display section diagram).

Good luck.

David
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Old 11th Nov 2021, 4:21 pm   #10
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Default Re: Can anyone help a vintage computer with a CRT issue.

Hi all
Going to replace the rest of the capacitors just for piece of mind before I try other things I have removed a 6.8uf 25v cap from C2017 but it has no negative markings on it and its the only cap that does not have a + marking onthe board.
Can anyone tell me if I replace this with a normal capacitor and what about the polarity.
Many Thanks
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Old 11th Nov 2021, 4:39 pm   #11
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Default Re: Can anyone help a vintage computer with a CRT issue.

I think I can make out 'NP' in the 4th photo. That normally means 'non polarised', it's a special type of electrolytic capacitor that can be fitted either way round and which will stand AC without problems. Do not replace it with a normal elecrolytic capacitor.
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Old 11th Nov 2021, 4:53 pm   #12
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Default Re: Can anyone help a vintage computer with a CRT issue.

There are a few standard ways of describing these, sometimes they are described as

'Bipolar'
or
'Nonpolar'
or
'Non-polar'
or
'Non-polarised'
or
'Non-polarized'

A search on the usual auction sites for '6.8uF' + one of the above terms should get you a few hits. As Tony said, do NOT put a normal, polarised 6.8uF in this position.

Although the original part is 25V there would be no problem with putting a 35V / 50V / 100V in there as long as it physically fits. Modern electrolytics of a given value / voltage are somewhat smaller than their 1980s equivalents, so if you want something that looks about the same as the original part, go up a voltage or two.
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Old 12th Nov 2021, 2:39 am   #13
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Default Re: Can anyone help a vintage computer with a CRT issue.

It is even possible to fit a stack of SMD parts between a pair of bits of tinned copper wire.
Those NP caps are not the best for reliability.
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Old 12th Nov 2021, 7:46 pm   #14
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Default Re: Can anyone help a vintage computer with a CRT issue.

When selecting ceramic capacitors (SMD) you would need to be very careful to choose the right ones. I'd prefer a polypropylene foil capacitor of a pulse rated type or maybe even one intended for loudspeaker filter applications.
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Old 12th Nov 2021, 9:25 pm   #15
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Default Re: Can anyone help a vintage computer with a CRT issue.

This monitor uses flywheel synchronisation. The filter capacitor for this is C2025. If that loses capacitance, line pulling is the likely result.

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Old 12th Nov 2021, 11:58 pm   #16
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Default Re: Can anyone help a vintage computer with a CRT issue.

C2025? Unluckily, one of the few which pudwink hasn't yet replaced (post #8).
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Old 15th Nov 2021, 12:47 am   #17
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Default Re: Can anyone help a vintage computer with a CRT issue.

C2017 is the line coil coupling capacitor, non polarised or 'bipolar'. It will be a decent quality one, don't use the cheap ones sold for audio crossovers as it might vent. When these failed in industrial monitors i used to fit a film capacitor.
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