Hello,


- measuring short between base - emitter in circuit
- out of circuit transistor measuring 45 ohms in both directions (base -
emitter)
- measuring short without transistor in circuit

I assume the transistor is gone and I don't think there should be a
short without the transistor in the circuit.

Can anybody confirm this for me ?

Thanks !!

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"˜†" " " wrote in message
.. .
Hello,


- measuring short between base - emitter in circuit
- out of circuit transistor measuring 45 ohms in both
directions (base - emitter)
- measuring short without transistor in circuit

I assume the transistor is gone and I don't think there
should be a short without the transistor in the circuit.

Can anybody confirm this for me ?

Thanks !!
Some horizontal output transistors have an internal B-E
resistor of 30 ohms or so and some also have an internal C-E
diode with the cathode connected to the C pin.

David

"?" " " wrote in message
.. .
Hello,


- measuring short between base - emitter in circuit
- out of circuit transistor measuring 45 ohms in both directions (base -
emitter)
- measuring short without transistor in circuit

I assume the transistor is gone and I don't think there should be a short
without the transistor in the circuit.

Can anybody confirm this for me ?

Thanks !!

Some HO transistors have an integral B/E damping resistor which will give a
faulty reading on the DMM diode-check function, if the transistor is
directly driven by the secondary of the horizontal driver transformer (check
it actually has one!) - this will read as S/C on the DMM.

A description of the symptoms that led you to look at the HO transistor
would make it easier for the folks on this group to help you, meanwhile
there are a number of other things you could examine. A visual inspection
sometimes hits the jackpot, such as failed flyback tuning caps - square or
dipped types often bulge while disc ceramic ones usually crack and show a
burn mark. There is often a pair of series diodes in parallel with the C/E
which are to do with width modulation - these are prone to failure in some
makes/models, you could also look at any other diodes connected to the LO
transformer which serve scan derived supplies.

Suggest getting a known new (good) device and check it, comparing the
readings you get with the original with an operational device. Then check
the rest of the circuit to determine what may have blown the original.


"ian field" wrote in message
...

"?" " " wrote in message
.. .
Hello,


- measuring short between base - emitter in circuit
- out of circuit transistor measuring 45 ohms in both directions (base -
emitter)
- measuring short without transistor in circuit

I assume the transistor is gone and I don't think there should be a short
without the transistor in the circuit.

Can anybody confirm this for me ?

Thanks !!

Some HO transistors have an integral B/E damping resistor which will give
a faulty reading on the DMM diode-check function, if the transistor is
directly driven by the secondary of the horizontal driver transformer
(check it actually has one!) - this will read as S/C on the DMM.

A description of the symptoms that led you to look at the HO transistor
would make it easier for the folks on this group to help you, meanwhile
there are a number of other things you could examine. A visual inspection
sometimes hits the jackpot, such as failed flyback tuning caps - square or
dipped types often bulge while disc ceramic ones usually crack and show a
burn mark. There is often a pair of series diodes in parallel with the C/E
which are to do with width modulation - these are prone to failure in some
makes/models, you could also look at any other diodes connected to the LO
transformer which serve scan derived supplies.

þÿ& " " writes:

Hello,

- measuring short between base - emitter in circuit
- out of circuit transistor measuring 45 ohms in both directions (base
-emitter)

As others have noted, this is proably normal. Internal ~50 ohm resistor
between B and E.

- measuring short without transistor in circuit

You mean between the PCB traces? The HOT is probably driven directly by a
small transformer ("Horizontal Drive Transformer") whose output winding is
very near a short circuit and also normal.

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Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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On Tue, 10 Jul 2007 19:04:22 -0400, ? " " wrote:

- out of circuit transistor measuring 45 ohms in both directions (base -
emitter)

Sounds bad. Should depend on polarity. What voltage does
your meter put out in Ohms range?

- measuring short without transistor in circuit

Follow the circuit from base to ground. You may find
a transformer in there which would explain low resistance.

Regards,
H.

Heinz Schmitz writes:

On Tue, 10 Jul 2007 19:04:22 -0400, ? " " wrote:

- out of circuit transistor measuring 45 ohms in both directions (base -
emitter)

Sounds bad. Should depend on polarity. What voltage does
your meter put out in Ohms range?

Probably OK. If it has a built-in damper diode, will have ~50 ohm internal
B-E resistor.

- measuring short without transistor in circuit

Follow the circuit from base to ground. You may find
a transformer in there which would explain low resistance.

That's what I think.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

"AJ" wrote in message
...
Suggest getting a known new (good) device and check it, comparing the
readings you get with the original with an operational device. Then check
the rest of the circuit to determine what may have blown the original.

One strategy many engineers use is to cut the B+ track to the HO transformer
primary and bridge it with a 60W bulb, if the lamp lights before the new
transistor is fitted then there is a short in the windings - or possibly any
width modulation diodes that may be connected to the circuit.

If the lamp does not light then it is possible to fit the transistor,
usually the lamp will limit the current and protect the new transistor (but
not always!) this often allows the stage to be partially powered without
damage and allowing symptoms to be observed.

Sam Goldwasser wrote:

- out of circuit transistor measuring 45 ohms in both directions (base -
emitter)

Sounds bad. Should depend on polarity. What voltage does
your meter put out in Ohms range?

Probably OK. If it has a built-in damper diode, will have ~50 ohm internal
B-E resistor.

Sam, I just took a new 2SC4924 (npn HOT in some CRT monitor).

This had R(BE) 14 Ohm vs infintiy (*) with an ohm meter putting out
2.5 Volts in the x1 range.
(*) meter connected one way vs the other

Setting a resistor with 42 Ohms parallel to BE, I measured
13.5 Ohms vs 42 Ohms.

That is, even if there is an internal base-emitter resistor of ~50
ohms, it is still possible to check by resistance measurement
wether the BE junction is ok.

Regards,
H.

Thank you all !!!!!!!!!!!

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