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 am
Nem 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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grantspain
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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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grantspain
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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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