Horizontal O/P Transistor (TV)

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

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.

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

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.

--- sam | Sci.Electronics.Repair FAQ:

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

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

Regards, H.

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

That's what I think.

--- sam | Sci.Electronics.Repair FAQ:

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.

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, 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

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

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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