UK Vintage Radio Repair and Restoration Discussion Forum

UK Vintage Radio Repair and Restoration Discussion Forum (
-   Vintage Computers (
-   -   Sharp MZ-80K help needed on repair (

pudwink 12th Oct 2021 10:41 am

Re: Sharp MZ-80K help needed on repair
Had a look this morning and resoldered some of the pins still having issues even the slightest touch on the 8255 and it effects it just wondering if it's worth changing the socket again for a turned pin one.
But Most of the keyboard works most of the time and it has not crashed since so brings me back to the screen pulling any thoughts on what might be causing it something must be breaking down when it gets warm then fine once it cools down.

SiriusHardware 12th Oct 2021 11:28 am

Re: Sharp MZ-80K help needed on repair
If the socket and its connections are OK it should not matter whether it is a turned pin one or a conventional one. If you are unlucky you have somehow managed to spread / open up the pin 1 contacts in the socket, but replacement of the socket would fix that.

The one potential drawback with turned pin sockets is that they are very, very difficult to remove from double sided PCBs like this without causing damage so if you do fit a turned pin type be absolutely sure that you will never want to remove the socket again.

Your actual problem may be on the PCB - an intermitted connection between the pin 1 pad and its associated track, or as suggested earlier the pin 1 pin of the socket may be folded underneath the socket and not through the hole as it should be - although if it was, the pin 1 pad on the underside would be suspiciously flat with no pin sticking out through it.

There is 'a thing' called freezer spray which is designed for finding thermally progressive faults, you let the fault appear and then go around spraying suspect components - when you hit the right one, bing, the fault disappears so you know that's the guilty component. That's the theory anyway.

Have a good look at the waveforms on IC26 pins 8, 9 and 10 and also at the video waveform on IC3 pin 2. Those waveforms should be clean and sharp with good vertical edges, no rounded corners at any point. Then let the fault appear and have a look at the waveforms again. See anything different?

pudwink 12th Oct 2021 1:02 pm

Re: Sharp MZ-80K help needed on repair
4 Attachment(s)
scoped the pins but they never changed when the fault appears.

SiriusHardware 12th Oct 2021 4:04 pm

Re: Sharp MZ-80K help needed on repair
Images 1 and 3 didn't really catch what we are looking for, maybe try again on those ones, they are likely to stay mainly in one state and only pulse to the other state for a very short time, so try to catch a whole one of those pulses on each of those pins.

Are the controls, especially the 'H-Hold' controls exposed on the rear of the monitor, in other words can you adjust them from outside? If so try adjusting 'H-Hold' gently up and down from its present setting when you have the 'bent top' problem on the display. If turning it left or right a bit cures the bent top line, leave it like that. If it makes no difference return it to exactly where it was.

pudwink 12th Oct 2021 7:01 pm

Re: Sharp MZ-80K help needed on repair
With pictures 1 and 3 there are no wave formed but the two lines do show pulses when I alter the time/div.

SiriusHardware 12th Oct 2021 7:05 pm

Re: Sharp MZ-80K help needed on repair
Sorry, I think maybe my terminology leaves something to be desired - for me a digital 'waveform' is any signal which has a mixture of high parts and low parts in it, not necessarily 50-50 high-low like the ones you saw on the address lines in the NOP test.

The pulses you see (when you alter the time / div) need to look nice and sharp and square, not buckled or rounded.

What about the H-Hold control on the CRT display, can you get to that to try adjusting it a little?

pudwink 13th Oct 2021 9:43 am

Re: Sharp MZ-80K help needed on repair
H-Hold control moves around but will not correct the pulling of the text.

SiriusHardware 13th Oct 2021 7:51 pm

Re: Sharp MZ-80K help needed on repair
Sorry to hear that, would have been a nice simple remedy had it worked. Another two other possible scenarios -

-Thermal (time related) fault in the CRT monitor circuit
-Thermal (time related) fault on the horizontal sync generation circuit.

If you don't have any freezer spray you could try letting the machine cool right down (maybe overnight) so that the 'bent top' fault is not present, then go over the mainboard with a hairdryer set to a fairly hot temperature to see if that makes the fault appear sooner than it normally would. I would start with IC8 and move on to the other ICs in the 'Video Generator' section of the circuit diagram (PDF page 34 / manual page 32).

What about that problem with the 8255 pin 1, did you sort that? If there is a problem with that pin somewhere under the 8255 there could be problems with other pins which are related to the other problems you keep having. If you sort that problem out you could end up fixing or at least finding the reason for some of the other problems as well.

Mark1960 13th Oct 2021 10:09 pm

Re: Sharp MZ-80K help needed on repair
This might be a question for the CRT TV guys, is the deflection voltage for the monitor higher at the top of the screen and having some limiting effect in the CRT circuits? Can degraded capacitors in the Crt circuits or power supply have this effect?

pudwink 14th Oct 2021 7:31 pm

Re: Sharp MZ-80K help needed on repair
Asked someone in the sharp Facebook group who said this.
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.

Mark1960 14th Oct 2021 8:56 pm

Re: Sharp MZ-80K help needed on repair
Make sure the monitor circuits are clean of dust that tends to accumulate on the HT circuits before you do anything else in that area. Take care as they can hold charges at high voltage even after being powered off for quite some time. An insulated paintbrush is useful for removing the dust.

pudwink 14th Oct 2021 9:36 pm

Re: Sharp MZ-80K help needed on repair
I am not very good at reading schematics so if anyone can point me in the right area of the board I need to be checking capacitors I would be very grateful.

SiriusHardware 14th Oct 2021 10:21 pm

Re: Sharp MZ-80K help needed on repair
This forum has probably the highest density of CRT circuit experts anywhere in the world but most of them probably don't read this (computers) section and I don't count myself as anything more than an interested amateur when it comes to CRT stuff.

You could try a separate thread in 'Vintage TV and Video' with the heading 'Sharp MZ-80K CRT monitor fault', post a couple of images of the fault condition and and point them to PDF page 42 / Manual page 40 of the service manual (Monitor circuit diagram) and PDF page 41 / Manual page 39 (PCB layout of monitor). Also the monitor block diagram, PDF page 19 / Manual page 17. For any CRT gurus who do happen to read this, the service manual which includes the monitor circuit details was posted in #3 of this thread.

Given such good service information and good images of the fault condition I'm sure someone in the TV section will be able to point you in the right direction.

All I can tell you is that the upper left and top of the circuit is the video section, upper right is the audio circuit, the middle-left section is the sync separator, the centre middle to centre right section is the vertical deflection / scan circuit and the bottom section going across horizontally is the horizontal oscillator, drive and line output stage. If the fault is that the scanned picture is full height but the top line is bent sideways I think you are looking at a problem in the lower third of the diagram, if the top of the picture is compressed (all the scan lines at the top squeezed into a smaller space) then more likely the middle (vertical scan) section.

From your point of view it's good that the monitor is powered from 12V so there aren't any mains voltages backed up by unlimited current lurking in there and waiting to kill you, but the monitor circuit itself does generate some high voltages especially the voltage (Kilovolts) fed to the tube via the thick rubber lead and cap going to the tube, and there are also voltages of well over 100V in the area around the line output transformer / tube neck.

Don't forget the simple stuff as well, try 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.

Don't just plonk the scope probe on any random point in a monitor like this, if you accidentally hit one of the high voltage points you could potentially damage the scope.

With care you could also use freezer spray to try to isolate this fault as long as you kept it away from the high voltage areas.

All times are GMT. The time now is 5:50 am.

Powered by vBulletin®
Copyright ©2000 - 2022, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.