Suddenly no video on a MS2930

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grantspain
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Re: Suddenly no video on a MS2930

Post by grantspain »

turn your screen volts down to min and see if it stays on
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nem
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Re: Suddenly no video on a MS2930

Post by nem »

Ioncannon wrote: May 3rd, 2021, 4:02 amNem can a necked tube even produce an image?
I don't see how it could. So you're in the clear for that.
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Re: Suddenly no video on a MS2930

Post by Ioncannon »

grantspain wrote: May 3rd, 2021, 8:20 am turn your screen volts down to min and see if it stays on
Will try again to double confirm, but I had tried that already. IIRC, at the furthest counter-clockwise position (left), the flash stops. If I nudge it a bit to the right it returns, but no screen changes. I kept moving it to the right but I didn't see any raster. Didn't want to keep going too far in case it overvolted or something.

I also measure at the right fuse and TP3 (they are both connected) for B+. Was at 174v.
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Re: Suddenly no video on a MS2930

Post by Ioncannon »

Hm am I checking B+ correctly?

Negative lead to the chassis heatsink while positive touches TP6 near the right fuse. I saw the voltage stabilize at around 172v. Turned up B+ to 180v exactly with no video source plugged in.

The reading is strange though as B+ spikes to 200v+ when the screen turns on until stabilizing at 180v. Could this be what is triggering the protection circuit or is my meter screwing up?
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Re: Suddenly no video on a MS2930

Post by grantspain »

you are reading correctly but the reading you are getting is odd, as soon as b+ goes over 180v then the protection circuit kicks in but yours is then stable wheras in general the b+ would shut off
my theory could be hv goes too high
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Re: Suddenly no video on a MS2930

Post by Ioncannon »

If the protection circuit kicks in what exactly happens? Does B+ shut off or just the HV? Or all power lines (IE: Heater voltage)? I still hear a whine even after the screen goes dark.

Hmm I wonder if the bipolar caps might be an issue. The ones in the cap kit had a much higher ESR. Will also check around the power supply to see if there are any issues. Any other place I should check?
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Re: Suddenly no video on a MS2930

Post by grantspain »

normally the b+ goes to zero but knowing how the flyback behaves if in fault is the question here- do you have a bad flyback or do you have a bad hv protection circuit, is the eht going above 29kv?
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Re: Suddenly no video on a MS2930

Post by Ioncannon »

Waiting on a better multimeter to use with this probe. Will report back by end of week. Interestingly the fuse doesn't blow so that's at least good.

Ground clip should be clipped to the cage/metal chassis (surrounding the tube, not the board) when probing, right? What I mean is the same place as when discharging?
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Re: Suddenly no video on a MS2930

Post by Ioncannon »

Ok Grantspain, finally probed the EHT. Seems it is going over the protection limit with readings of 29-32kV appearing and immediately discharging. Contrast and screen pots are low.

Here is a video of the values. Probe was grounded to the dag spring:

https://youtube.com/shorts/_nLdikXLTUM

I also tested B+ again with this new meter and no longer see any over volting or fluctuation there so my guess is it was just a cheap meter. It either shows 180.02v at the fuse or dips a bit at the test point near it. Still, even with the HV shutting off it still shows 180v. Reading through the MS-2931 manual (since they are similar) I wonder if it's the X-Ray protect triggering, which may not shut off B+? The manual says it will engage protection at 31.5kV which is in line with the max reading the meter is seeing.

Still arcs once with the yoke and the tube rear when powered on, probably due to the over volting? Hope my earlier screw up didn't break the yoke. Get normal resistance readings but have no way to test inductance.

Edit: Reading the "High Voltage Block Circuit" section..... Q701 seems to turn the voltage on and off to stabilize the high voltage. Could be stuck open? Or U701 is fried and not controlling it right.
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Re: Suddenly no video on a MS2930

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I would like to see the arc, it could be tube or yoke itself
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Re: Suddenly no video on a MS2930

Post by Ioncannon »

Hard to get on camera but it seems to be between the edge of the yoke front and surface of the tube. Across the gap made by the rubber stoppers. One tiny bolt when it first turns on. It's so small and faint the camera can't pick it up unless it's dark.
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Re: Suddenly no video on a MS2930

Post by grantspain »

is the crt earth good to the neck card
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Re: Suddenly no video on a MS2930

Post by Ioncannon »

It is connected. Could do a continuity test to make sure it's a good connection.
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Re: Suddenly no video on a MS2930

Post by grantspain »

there is a hv level pot on this chassis, adjust that down and see what happens
i doubt the tube is to air otherwise you would have funky lightshow in the neck, i can't see it being the yoke otherwise you would not have got full deflection, it could be flyback as i know these flybacks fail just like this but normally you get a very loud crack on power up
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Re: Suddenly no video on a MS2930

Post by Ioncannon »

Will give it a try. The pot is still siliconed shut so surprised it wavered. Actually there is a ton of rust on the middle leg, maybe this pot failed? Wonder if it is acting as if voltage was cranked 100%.

Yeah no loud cracks or anything like that. It sounds like the HV circuit can't stabilize the voltage and XRay protect is saving the CRT from destruction.

Just wanna say thanks for the tips, I think we are getting close to the issue. Before touching the pot I'll double check the 701-705 series of diodes and such to see if anything is busted.
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Re: Suddenly no video on a MS2930

Post by nem »

Ioncannon wrote: May 9th, 2021, 11:18 pmActually there is a ton of rust on the middle leg, maybe this pot failed? Wonder if it is acting as if voltage was cranked 100%.
Measure the resistance between the outer legs and or wiper (depending on how it's configured).
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Re: Suddenly no video on a MS2930

Post by Ioncannon »

Yep, it's above the original 47 kOhm rating but probably within it's tolerance. The pot was actually stuck but a bit of pressure and moving it back and forth freed it up. Getting a range from 46 Ohms to 50 kOhms between the wiper and left pin (from the back) when I move it from fully clockwise (right) to counter-clockwise (left). Can't measure the other pin as it's connected with the wiper. The original position from factory was 24 kOhm.

Tonight I'll move it all the way to the left and slowly ramp it up watching the HV. Shouldn't be a problem with starting from a low voltage position, right? Manual says to start in the center but don't see how under volting would cause issues.

I also tested all the 70X diodes around the area and it looks fine. Q701 MOSFET doesn't have any shorts either so seems good. Could just be all the minute cap changes and rusting pot shifted the HV upwards.
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Re: Suddenly no video on a MS2930

Post by Ioncannon »

Well that killed the excitement.

HV-Adj has no effect on the voltage. Same effect even with it set all the way left, voltage spike and then shutdown.

I didn't try adjusting it all the way to the right in case I risked blowing the tube (the adjustment could be backwards though?).
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Re: Suddenly no video on a MS2930

Post by grantspain »

it could be the flyback, these model flybacks are well known for problems
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Re: Suddenly no video on a MS2930

Post by Ioncannon »

Hope not since the current "new" flybacks seem to be.... not the best?

Seems I am stumped at this point though. Still want to test the MOSFET but don't have the DC power supply to properly open/close the gate. Reading the schematic I don't see how a bad flyback would cause this issue as the voltage level seems to be more controlled by switching that MOSFET on and off for certain periods to "set" the voltage.

The again, if the MOSFET was broken would Xray Protect even work? Hm. I did read someone elses' post that a bad component can "blind" the U701 chip from the current voltage letting it raise uncontrolled.

Random Question; is static on the back side of the tube normal?
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