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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 22nd Mar 2018, 12:55 am   #1
qsilver
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Default Microvitec Cub 1431 Power but no picture.

Came to the screen one day and powered it up and I've got nothing except the Red neon indicator and the sound of the degaus. I don't have any static on the screen at power up like you can sometimes get.

I thought this was a series 3 screen but after taking a closer look its a series 4 1431 MS4F.

My questions are, how safe is it to work on a CRT as I understand these use EHT.

How can I safely discharge the 22kV as I've seen no specific way to do it other than put a screw driver under the anode cap attached to an earth and that sounds a bit crude and dangerous to me.

Is this a common problem or does it point to a dead tube?
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Old 23rd Mar 2018, 4:20 pm   #2
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Default Re: Microvitec Cub 1431 Power but no picture.

If you're not used to working with HV and CRTs then probably best not to attempt anything, but here is the service manual that has some troubleshooting:
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Old 23rd Mar 2018, 5:02 pm   #3
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Default Re: Microvitec Cub 1431 Power but no picture.

There are lots of things which can go wrong with a CRT monitor, and the tube is very unlikely to be the cause of the problem. Generally there's no need to discharge the EHT because there's no way to come in to contact with it unless you deliberately disconnect the anode cap from the CRT, which is probably not necessary.

To find out why the monitor isn't working, you need to do some basic voltage checks to see whether the power supply and timebases are working. These do involve measuring voltages up to several hundred volts, which can be dangerous. How confident are you working with electronics, and can you use a multimeter?

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Old 23rd Mar 2018, 6:45 pm   #4
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Default Re: Microvitec Cub 1431 Power but no picture.

I thought it was unlikely that the tube was dead, thanks for the information about the EHT. I feel a little safer knowing it is not just "Out in the open".

I've worked on a lot of amplifiers and some valve amps. My multi meters go up to 600 or 1000VDC. The highest voltage on a valve amp I've worked on would be in the region of 400V.

I appreciate this is high voltage and I do want to proceed cautiously if it isn't an overly complex fault.

Thanks for the manual - I had already looked at that one but this doesn't match the schematics and looks to be a Series 4 and not a Series 3. There is only one fuse present and the layout is completely different.

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Old 24th Mar 2018, 4:55 pm   #5
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Default Re: Microvitec Cub 1431 Power but no picture.

Hi

Could this https://www.google.co.uk/url?sa=t&rc...DD0GGBF_vBqSl5 be the correct manual? It's a pdf from the stardot.org.uk website.

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Old 25th Mar 2018, 10:10 pm   #6
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Default Re: Microvitec Cub 1431 Power but no picture.

That looks to be the same manual as the previous one posted and covers the Series 3...

I've taken a look on the internet this weekend and I think I've found one that does match mine on another site. I'll try to upload it here...

It's just a little too big to upload so I'll provide a link below.

Series 4/5 Service Manual

This looks to be it.... it's the Cub 1431/MS4F.
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Old 26th Mar 2018, 12:42 am   #7
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Default Re: Microvitec Cub 1431 Power but no picture.

I used to see hundreds of these. The TDA4600-based power supply in the Series 4 MS4F was unreliable and they later returned to their previous discrete design. Check for HT (115V) at D10 cathode. If absent, provided there is no short on the HT rail the PSU is probably blown up. With R18 and R227 lifted at one end, lack of HT is conclusive diagnosis.

As with many SMPSUs, it is necessary to change all faulty parts at once, otherwise powering back up when only some parts have been changed, can destroy the replacements. When changing the TDA4600 and TR1, check everything nearby in the non-isolated area. I used to change about 12 components as a matter of course such as those in the base drive path of TR1, without bothering to test, and they usually worked first time.

IIRC, another common problem that could kill both line and frame was a solid short on C306, the reservoir cap for the 24V flyback supply to the frame output. In this case there will be some volts on the HT but no EHT. The TDA3651 was also a fairly common casualty.

As mentioned above there is not usually any need to discharge the EHT, so long as you are not disconnecting either the EHT lead or the tube dag ground. But note that unlike most other Cub chassis, this model uses a diode-split line output transformer that will remain charged for weeks, rather than discharging in seconds through the focus pot.
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Old 28th Mar 2018, 8:29 am   #8
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Default Re: Microvitec Cub 1431 Power but no picture.

Out of curiosity is it an ex education unit?

I probably disposed of 100 or more CUBS from one of the Carlisle Schools back in the 2000's...

Along with countless BBC-B's and Archi...

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Old 28th Mar 2018, 11:15 pm   #9
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Default Re: Microvitec Cub 1431 Power but no picture.

Smashing monitors and tough as old boots. If it's led a hard life then capacitors going low or resistors going high are the main culprits. The cubs also suffered from bad joints in all the usual places such as power components and pcb headers, SMPSU and LOPT inductors.

The siemens TDA4600 series psu's are luckily easy to fix if necessary. If there's a short on the secondary they usually whistle painfully with a slowly decreasing tone when powered off.

The toshiba crts last forever, (did they use others too, I forget?) I've had them with clear text burned solidly into the phosphors but still capable of a bright sharp picture.

Some of the older transluscent poppy connectors had a habit of pinging off as you discharged the final anode so mind it doesn't fall across a bare arm.....
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Old 5th Apr 2018, 1:53 pm   #10
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Default Re: Microvitec Cub 1431 Power but no picture.

Thanks for the replies, wanting to try and get back to this. What votlage am I looking for on D10? Just 115VDC? The reason I ask is my DMM goes up to 600V/1000V so I don't want to run the risk of damaging it if it could float quite high. Should I just clip a lead to the diode and measure?

Might sound obvious but I have not worked on one of these before and so I want to be as cautious as possible.

I have noticed some ROE capacitors in there that look reminiscent of those in a Cyrus amplifier which always went bad. I'm not sure which circuit they are on yet but I wonder...

As far as where this came from, I'm not sure where it originally was but it is possible it was from a school...

This CRT looks to be a Mitsubishi actually, reminded me of my Grandfathers old Mitsubishi TV with a flat screen over the CRT screen. I always remember it as having the best picture of all the TVs I was used to.

Nothing inside looks particularly hot or scorched so unless they did a great job in there with heatsinking, it probably hasn't led a very hard life but still could be joints I suppose.

There's now screeching or anything when it is powered on. You hear the degaus and thats it. No high pitched noises (I can usually here them) and no static feeling on the screen. Just the neon bulb.
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Old 5th Apr 2018, 7:40 pm   #11
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Default Re: Microvitec Cub 1431 Power but no picture.

Assuming the manual linked to in post #6 is the correct one, it's possible that the startup resistor (according to the manual, R6, 15K/5W) has gone open circuit.

This provides the control chip with initial power to get it going. After that, the control circuit runs from a supply provided from the transformer pin 1, rectified and smoothed by D7 / C15.

Also check R19, and R11, the 2 * 82K resistors in series.

A word of warning: Before attempting these resistance measurements, check that the input smoothing capacitors (4 * 100uF, high voltage) are discharged and if they are not, discharge each of them in turn with a resistor (say 47K) held in insulated pliers.
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Old 5th Apr 2018, 10:15 pm   #12
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Default Re: Microvitec Cub 1431 Power but no picture.

Yes, check the DC volts, it won't be much higher than it says in the manual and certainly not in excess of your 600V range!!

If you're in any way unsure of what you're doing then don't put your fingers inside with the mains plug in the wall and double check you've removed it before touching anything.

The mk4 in particular were used in the pottery industry on spray dryers which essentially were machines that generated a lot of fine mineral dust. It was not uncommon to see the bottom part of the casing and the entire circuit board submerged in this grey powder where the EHT had been acting as a sort of air purifier and hoovering up cupfuls of the stuff. Always got 'em going again! Nice easy Friday afternoon job.

If the main PSU is not working then check the small electrolytics around the SMPSU chip and in particular the base drive capacitor. Then check the high value resistors in the same circuit as per previous advice including R12. The set HT pot can also fail so worth replacing, it can go noisy and cause jitter or potentially a SMPSU blowup.
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Old 7th Apr 2018, 12:44 am   #13
qsilver
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Default Re: Microvitec Cub 1431 Power but no picture.

Thank you again for the really helpful insight. I've been reading up on SMPSU's recently so is the 5W resistor you mention the "bootstrap" resistor that kick starts the SMPSU perhaps?

Would you recommend removing the circuit board and checking the underside? Would that help the measurement of the capacitors and resistors too? I don't think it would be easy to check the capacitors for charge without removing it.

Does that sound like a sensible first step?
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Old 7th Apr 2018, 1:41 am   #14
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Default Re: Microvitec Cub 1431 Power but no picture.

Certainly you will find it difficult to check any components unless you access the track side of the PCB. I don't know the physical layout of these but in general, looking at the unit from the rear you should be able to free the PCB, slide it back towards you a couple of inches and then swing it outwards and upwards taking care not to let it hit the tube neck. If it has front mounted controls you may have to take the knobs off before sliding the PCB out. Some metal cased monitors may have a bottom hatch or overall cover which can be removed to gain access to the underside of the PCB without removing it.
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Old 8th Apr 2018, 12:43 am   #15
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Default Re: Microvitec Cub 1431 Power but no picture.

Quote:
Originally Posted by SiriusHardware View Post

Also check R19, and R11, the 2 * 82K resistors in series.
I would replace these resistors as they can go high, use high voltage metal film resistors capable of handling at least 350V like these
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Old 9th Apr 2018, 5:16 pm   #16
qsilver
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Default Re: Microvitec Cub 1431 Power but no picture.

It doesn't look like I can access the bottom of the PCB without unplugging everything... which may mean I do have to discharge the EHT?

I measured the DC voltage on the cathode of D10 and it measured 23V so I imagine that means something is leaking or open?

When I powered it up, I did hear a quick rising high pitched squeal for a second and then it stops which I don't remember hearing the last time I tried to power it up.
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Old 9th Apr 2018, 5:24 pm   #17
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Default Re: Microvitec Cub 1431 Power but no picture.

Forgot to add some pictures so here we are.

I also noticed some dirt and a little oxide on two trimmers on the CRT board...
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Old 9th Apr 2018, 6:59 pm   #18
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Default Re: Microvitec Cub 1431 Power but no picture.

That sounds like the PSU is OK but being loaded down by the line output. Before spending time on anything else, check for a short on C306, the 24V flyback supply reservoir. You can test from above the board between the field shift link or the end of R313 and ground. Those orange ROE caps are a liability and it is worth changing that and C308 too.
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Old 10th Apr 2018, 1:06 am   #19
qsilver
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Default Re: Microvitec Cub 1431 Power but no picture.

I thought the ROE's would be a liability.... I could just change all the capacitors eventually...

I'll check for a short on the capacitor. I do have a capacitor tester if that will work on the same place?

I had a read up on the service manual and it looks like in order to remove the board, I will need to discharge the EHT etc and then remove the board.

Also, after I've energised the circuits, how long does it take for the HT to discharge or is there an easy place I can measure for it and discharge if needed?

Looking at the schematic, how would a short on C306 load down the 115V supply? Maybe its a dumb question...?
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Old 16th Apr 2018, 4:37 pm   #20
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Default Re: Microvitec Cub 1431 Power but no picture.

I'm having trouble accessing R313 to measure resistance to ground on it. It's right next to the EHT heatsink and LOPT and I don't have a probe thin enough to get between them. Which link is field shift link?
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