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HP 71 (JD177 - D8901A) problem

 
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venomous_poison
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PostPosted: Tue Nov 27, 2007 1:11 pm    Post subject: HP 71 (JD177 - D8901A) problem Reply with quote

I have a monitor HP, chassis Jean JD176. It exchanged the trasistor output horizontal which was in short for a new (equivalent model).
When the circuit is no load, or without the deflectors,
without the flyback cable and without the circuit tube connected, the source is usually turn on.
When connects any of these charges, the source is turn on and turn off.
I remove the the flyback cable and close it from the mesh tube, and when the source turn on there is a good source spark, ending when the source turn off.
Someone could help me.
Tks!
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Thu Nov 29, 2007 11:37 am    Post subject: Reply with quote

You should NEVER try to run a CRT monitor or tv with the deflection coils disconnected, also you should NEVER try to draw sparks from the EHT cable to ground.
Both those practices will surely cause further damage in the horizontal deflection or other parts of the monitor.

When checking the EHT, use instead a DMM and a proper high voltage probe.

If you have done a thorough check of the horizontal deflection circuitry (measure all power semiconductors, look for burnt resistors, bad solder joints, traces of flashover, swollen capacitors) but not found any obvious faults, check the flyback transformer. A good way of checking it is to take it out of the circuit and measure the inductance and Q factor of the primary winding (the one connected from the positive power supply to the collector/drain of the line output transistor). Then compare the results with a transformer from a similar monitor, not necessarily exactly the same model. A bad transformer will show a much lower inductance and Q factor. Also, the inductance value of a bad transformer will change a lot when measirung at different frequencies (for example, 1 kHz and 10 kHz) while the inductance changes are well below 20 % for a good part.
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