CRT computer monitor dies with loud crack

[Andre Majorel]

Last time I powered it, my Iiyama HA202-DT made a cracking
sound, similar to the sounds CRTs make when powered on, but
longer, louder and ominous. The screen is completely black and,
when fed a video signal, the monitor keeps "resetting" itself
every few seconds (the same "click" it makes when changing video
modes).

I suspect something in the high voltage circuit is blown but see
no obvious sign of damage. Any ideas ?

Thanks in advance.

 

[[email protected]]

It sounds like the switching power supply is detecting an over-current
situation and is shutting down, then trying to restart itself again. I
would check the diodes in the secondary of the power supply and all
large transistors and diodes in the horizontal/high-voltage section.
John

 

[Skype_man]

Possibly flyback arcing, & shorted the output. Don't see
that unit around here. Sky.

 

[Andre Majorel]

It sounds like the switching power supply is detecting an over-current
situation and is shutting down, then trying to restart itself again. I
would check the diodes in the secondary of the power supply and all
large transistors and diodes in the horizontal/high-voltage section.

Thanks. The rectifier bridge is rather well hidden but I checked
the semis on the left side. There is a big transistor (Toshiba
2SC5570, horizontal deflection output) with a dead short between
pins 1 and 3.

I don't have the schematics to check, but a short between the
base and the emitter seems suspect to me. Not that I'm an
expert, mind you.

 

[Sam Goldwasser]

Thanks. The rectifier bridge is rather well hidden but I checked
the semis on the left side. There is a big transistor (Toshiba
2SC5570, horizontal deflection output) with a dead short between
pins 1 and 3.

I don't have the schematics to check, but a short between the
base and the emitter seems suspect to me. Not that I'm an
expert, mind you.

B-E short may be OK if it's driven by a transformer. It's C-E short
that's guaranteed dead.

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[Charles Schuler]

B-E short may be OK if it's driven by a transformer. It's C-E short
that's guaranteed dead.

Be sure to understand that Sam is talking about in-circuit ohmmeter testing.
Out of circuit, B-E short is fatal.

 

[Andre Majorel]

Be sure to understand that Sam is talking about in-circuit
ohmmeter testing. Out of circuit, B-E short is fatal.

I'm not an EE, but I had figured that much. Thanks. If I
can't get the schematics, I guess I'll have to desolder the
transistor to make sure.

The 2SC5570 is spec'd for voltages in the kV range ; that makes
it a good candidate for arcing. There were long "whiskers" of
dust on the leads. Could an arc have developed that way ?
Perhaps the replacement could use some heat shrink tubing on its
leads...

 

[Sam Goldwasser]

I'm not an EE, but I had figured that much. Thanks. If I
can't get the schematics, I guess I'll have to desolder the
transistor to make sure.

The 2SC5570 is spec'd for voltages in the kV range ; that makes
it a good candidate for arcing. There were long "whiskers" of
dust on the leads. Could an arc have developed that way ?
Perhaps the replacement could use some heat shrink tubing on its
leads...

It may be the horizontal output transistor. If it is shorted, the next
question is whether it deid on its own or due to some external cause.

Sure, dust can result in a short circuit but could be elsewhere. If it
really made a loud noise as it died, there may be visible evidence somewhere.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

 

[Asimov]

"Charles Schuler" bravely wrote to "All" (13 Nov 05 16:56:59)
--- on the heady topic of "Re: CRT computer monitor dies with loud crack"

B-E short may be OK if it's driven by a transformer. It's C-E short
that's guaranteed dead.

CS> Be sure to understand that Sam is talking about in-circuit ohmmeter
CS> testing. Out of circuit, B-E short is fatal.

Most modern H-output transistors virtually have a near B-E short
because of an internal damping resistor between 10 and 100 ohms. At
least it will seem like a short on the 20K & 2K dmm range.

A*s*i*m*o*v

.... You may be a tech if you're entertained by a 6-pack and sparking HV.

 

[Andre Majorel]

"Charles Schuler" bravely wrote to "All" (13 Nov 05 16:56:59)
--- on the heady topic of "Re: CRT computer monitor dies with loud crack"



CS> Be sure to understand that Sam is talking about in-circuit ohmmeter
CS> testing. Out of circuit, B-E short is fatal.

Most modern H-output transistors virtually have a near B-E short
because of an internal damping resistor between 10 and 100 ohms. At
least it will seem like a short on the 20K & 2K dmm range.

The DMM shows 1.1 ohm. Sounds high for a short, but it's the
same figure if I put both probes on the same lead. :->

 

[James Sweet]

The DMM shows 1.1 ohm. Sounds high for a short, but it's the
same figure if I put both probes on the same lead. :->

It's probably the secondary of the drive transformer causing that,
remove the transistor and check it out of circuit.