MS2931 going into protection

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chibishin
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MS2931 going into protection

Postby chibishin » January 29th, 2017, 7:27 pm

As the title states, I have an MS2931 going into shutdown immediately after power up.

No neck glow and no HV. Potting compound on H.Drive coil had leaked, but I removed the HOT, coil, and FBT and cleaned that up. With FBT and F701 removed, and HOT/coil installed, I've wired a lamp holder to the metal frame and PSU side of F701; 75W bulb does light up for a second or two.

HOT tests fine out of circuit.
R511 replaced since it had come into contact with the potting compound.
B+ is steady around 183VDC.
Vertical IC was replaced, showing 15VDC on pin 7, and 15VDC on pin 4; pins 5/6 show nothing (not sure if this is normal, absent an input signal), but I do read -15VDC from T901.
U921/U922/U602 are outputting steady 12VDC/5VDC/9VDC.
U951 pin 6 and U351 pin 26 both stay low; VCC reads 12VDC and 5VDC, respectively.
U901 pin 10 reads about 26VDC, and pin 7 reads 5V after shut down.
Heater reads around -10 to -11VDC, and even after 30 minutes the heater circuit hasn't drained. not sure if this is normal.
Haven't pulled Q901 and checked that, but I figured if all of my output voltages were good I shouldn't need to?

Not sure what to check, now. Any suggestions?
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mufunyo
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Re: MS2931 going into protection

Postby mufunyo » January 29th, 2017, 10:51 pm

Have you run the tube through a tube tester?
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chibishin
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Re: MS2931 going into protection

Postby chibishin » January 30th, 2017, 1:03 am

the tube is fine, I have another 2931 that works just fine on it.

the chassis goes into shutdown on my test bench not connected to the tube.
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Re: MS2931 going into protection

Postby mufunyo » January 30th, 2017, 1:50 am

Alright, I'll leave it to the experts then :awe:
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grantspain
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Re: MS2931 going into protection

Postby grantspain » January 30th, 2017, 2:22 pm

have you check q701 and u701
chibishin
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Re: MS2931 going into protection

Postby chibishin » January 30th, 2017, 3:37 pm

grantspain wrote:have you check q701 and u701


I'll pull q701 when I get home tonight and check it.

what am I looking for on u701?
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Re: MS2931 going into protection

Postby grantspain » January 30th, 2017, 3:54 pm

q701 is something to do with hv regulation and controls the hv protection circuit i believe
i had that on one chassis once cause me a similar fault

the fact you have solid psu voltages and look to have an ok deflection circuit points towards the issue being hv regulation-but these are complex chassis so nothing is straightforwards with these
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Re: MS2931 going into protection

Postby chibishin » January 30th, 2017, 4:30 pm

grantspain wrote:q701 is something to do with hv regulation and controls the hv protection circuit i believe
i had that on one chassis once cause me a similar fault

the fact you have solid psu voltages and look to have an ok deflection circuit points towards the issue being hv regulation-but these are complex chassis so nothing is straightforwards with these


do you recall if F701 was blown on that chassis? mine is still good, but was just curious.

I was wondering if I should see the trigger pulse on pin 1 of u701 or is it shutting down before it reaches that point?
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Re: MS2931 going into protection

Postby grantspain » January 30th, 2017, 4:49 pm

no fuse was fine on my chassis,i am not sure what signal you would expect to see on the ic
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Re: MS2931 going into protection

Postby chibishin » January 31st, 2017, 7:31 pm

q701 might be bad but I'm not sure. - on source, + on gate and move it to drain briefly shows a reading but then it goes out of range almost immediately. I don't have a variable PSU so I can't apply a higher voltage than what can be supplied by my DMM.
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MS2931 going into protection

Postby Rafaelsic » March 7th, 2017, 10:00 am

I know the feeling. I was sure that it was going to be a hard to find wiring problem the first time, and was quite surprised to find out it was a 30 fix.
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Re: MS2931 going into protection

Postby chibishin » March 7th, 2017, 1:02 pm

Rafaelsic wrote:I know the feeling. I was sure that it was going to be a hard to find wiring problem the first time, and was quite surprised to find out it was a 30 fix.


what?

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