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MX7000 CRT - No Power

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flex
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flex posted on Fri, Jan 11 2019 12:13 AM

Hi Guys,

I am new here!

I picked up this lovely CRT recently for next to nothing but it wouldn't switch on and I could see no red standby light at all. My plan is to bring this beauty back to life for use in a retro gaming setup.

I spent some time doing lots of research into the common faults and found this old thread Workshop How to rebuild a MX4000 MX6000 MX7000 and AV9000 which has been extremely useful.

I tested the components on the main board and found a failed TR33 (BU2508DF) transistor. I couldn't find any dry joints or anything like that but i replaced the following components in the circuit.

TR33, C100, C101, C102, C103, C104, C105, C106, & D82.

So after replacing them.... Success! Red standby light came up and the TV switched on. I was feeling quite proud of myself at this point but alas it wasn't to last.... 5 minutes later it switched itself off and went back to standby Crying

The TV now has the red standby light on but wont come out of standby the green power led lights up for a second and then goes off again. However I have just noticed that If i leave the TV unplugged for a bit it will turn on again for a minute or so before dying again.

I am sending out a cry for help for anyone who has experience with these models that might be able to help me!

 

P.S. Sorry for the duplicate thread, i did post this in the General forum but never got any replies, I have only just realized I should have posted it here but couldn't find a way to move it.

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solderon29
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These are sophisticated telly's which have a "powerfail" self check system at start up.All the data comms and supplies are checked,and if found to be below par,the set will revert to standby to protect itself.the manual details how these systems can be checked,but it's worth checking a few "usual suspects" before getting too deep into faultfinding.

The components you have replaced already are in that list,but also it's worth checking C12,which is a blue cased 6n8  capacitor sited near to the main transformer.It's very common for this cap to become dry jointed,and the symptoms of this are as you describe.

I would replace C29 and C31 in the power supply too.

Regards,

Nick

flex
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flex replied on Fri, Jan 11 2019 7:00 PM

Excellent thanks... I will check these and report back!

flex
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flex replied on Wed, Jan 16 2019 5:58 PM

Ok so I checked those capacitors removed them and re soldered them to the board, none of them looked suspect or had dry joints and capacitance test with the meter shows them to be in normal tolerance. C12 i believe to be OK bu I will order replacements for all of these anyway.

The fault has now progressed though, the TV no longer actually switches on, it comes out of standby but no picture and I was hearing a SHHHHHHHH sound for a few seconds followed by a bang and then back to standby mode.

I had read somewhere to check for arcing in the dark at the bottom of the board so thought I would check this while being extremely careful and keeping a safe distance etc. to see if I could tell exactly where it was coming from.

What I could see was there was a couple of big sparks (a few seconds apart with that shhhhhh sound) that looked might be coming from around the high voltage transformer area although I cant be sure exactly where, I did manage to film this from a distance but the footage is a bit jerky.

The TV still has the red standby light and TR33 seems to check out ok with the meter still and I ordered double of all the parts I have already replaced as they was cheap enough, so yeah bit of a head scratcher locating the actual source of the issue. I read that sparks could be due to a faulty transformer, so wonder if this might be worth replacing?

Any ideas?

solderon29
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That does seem rather sinister.It's possible that the casing of the deflection/eht transformer is cracked,but it's unusual.You need to take GREAT care here,in handling the transformer,as the final anode connection of the crt retains a charge voltage even when the set is unpowered.

It's also unlikely that you will be able to source another transformer other than a salvaged item.

Are you confident that the soldering of C102 and the printed circuit around it is sound?Arcing here will cause hissing and burning,and cause the set to shut down too.

Proceed with caution.You may need to consider taking the tv to a service centre with experience of crt tv's,to be on the safe side?

Nick

 

flex
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flex replied on Sun, Feb 3 2019 10:38 PM

I did find some dry joints on R2 (Thermistor for Degauss) and C2 seemed a bit wobbly so re-soldered these but was still a no go therefore I came to the conclusion after much investigation and swapping capacitors that it was likely to be the high voltage transformer and I was not able to find a replacement for it.

Not all was lost though I picked up another MX7000 (SW1.0) in excellent condition with the motorized stand for about £40... It had been sitting for 5 years unused but powered up fine at the sellers house.

When I got it home after cleaning off some dust from the inside (it is an amazingly clean looked after set even on the inside) it did switch off after about 20 mins and went back to standby mode but I could tell the issue was no where near as sinister and it had a different error code (PF = Powerfail) and in my investigations I ended up mixing the mainboard from this set with the other boards from the original one and now have a fully working set which has stayed on for hours straight... and whats nice is I am able to enter service mode with the remote etc. This must have something to do with the AV switching board that came from the original SW4.7 set as it wasn't possible without shorting the pins in its original configuration.

I have given the other set to an electronics guy i met recently (friend of a friend) to get a 2nd opinion on it to see if it is indeed the high voltage transformer or if it is still repairable but either way I am happy!

solderon29
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Good new's thanks for the update and pleased to hear that you are now enjoying an MX7000.

Nick

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