TV keeps blowing horizontal output transistor
Matt Jusinski mntr stnd
output transistor. This does not happen when I disconnect the
drive from the driver transistor. The excessive current I measured
through the transistor all passes through the flyback transformer,
which ohms out ok with the specs in the sams photofact for the set.
I checked all the caps, resistors, etc. and everything ohms out ok.
The caps check out to about the right values. If I were in a shop
I would try another flyback, but I am not.
Can anyone give me some ideas. I have spent a lot of time, bought a
lot of fuses and transistors, but can't catch the bad part. I have
been using a variac and an isolation transformer. The drive waveforms
scope out perfect, but if I connect the output, all hell breaks loose.
Matt J.
Sam Goldwasser
> Matt J.
No great words of wisdom - it sounds like you are doing the right thing.
In addition to the Variac, you should put a 100 W lightbulb in series
with the line. Even better would be to put a 50 ohm or so power resistor
in series with the collector to the horizontal output transisotr (HOT).
Just maybe, this will save a few of your transistors.
Does it die immediately when you connect the flyback? Have you tried
disconnecting the outputs from the flyback? If not, try that. You can
just unsolder the flyback and connect jumper wires to the primary. It
works. In this case, definitely run with the lightbulb since sometimes
total supply regulation depends on one of the flyback outputs.
I assume that you have checked the DC collector voltage and that is fine.
What about the damper diode? Is it built into the HOT or separate?
If there were an excess load on one of the flyback outputs I would not
expect an instant death. However, an internal short could do this.
Maybe even open caps across the HOT.
If you have a high speed current probe or if there is an emitter resistor
that you can clip your scope across, check out the current waveform
through the HOT - it should be a more or less linear ramp starting
from 0 when the HOT switches on. The maximum of the ramp should be
much less than the current rating of the transistor - I would guess
an amp or 2. If it is really high or non-linear then there is almost
certainly an shorted output or internal short in the flyback.
Just be super careful in there.
--- sam
Douglas Wargo
in it! I've been in your position many,many times (not wanting to accept
an expensive solution) and I almost
always wind up wishing I"d have popped for the FBT to begin
with and saved myself the wasted time. Good luck.
-
WARGOD HXZ...@prodigy.com
Mark Kinsler
there's a technique for testing flyback transformers that involves
ringing the circuit with a pulse waveform and watching the ringing on the
oscilloscope. If it damps down too quickly you've got a coil or
capacitor problem.
Also, a lesson from audio work: If the HOT driver is direct coupled
and a transistor up the line is open or shorted, the weird voltages can
finish off output transistors. It happens in audio, so maybe it's worth
checking those drivers and pre-drivers (if such exist) in your horizontal
output circuit.
Mark Kinsler
Sam Goldwasser
> I know from nothing about television repair, but it seems to me that
> there's a technique for testing flyback transformers that involves
> ringing the circuit with a pulse waveform and watching the ringing on the
> oscilloscope. If it damps down too quickly you've got a coil or
> capacitor problem.
I don't know what you mean by a capacitor problem if you are testing an
flyback on the bench. But yes, this is a valid technique. It will reveal
shorted windings (though not open ones but these can be ohmed out.) It may
not be enough of a stress test to find all problems, however.
Here is one way:
From: a...@siva.bris.ac.uk (PDP11 Hacker .....) (with some of my comments.)
1) Connect a capacitor across one winding of the suspect device (this works for
flybacks, chopper transformers, motors, mains transformers, etc). Hope for a
resonant frequency of a few kHz. (I would use the winding connected to the
HOT.)
2) Apply a pulse waveform to the parallel resonant circuit. In 1960, most
'scopes had a 'sync out' on the timebase that provided a few 10's of volts at
enough current for this. A circuit in 'Television' magazine a couple of years
ago used a BU508, a 12V psu, and a small oscillator built from a 4011 chip.
3) Look at the waveform across the resonant circuit with a 'scope. A good unit
will give a nicely decaying oscillation, of a few 10's of cycles. If there's a
shorted turn _anywhere_ in the device, the oscillations will be seriously
damped, and you'd be lucky to see 2 complete cycles. Experience/comparison with
a known good device will tell you what to expect. (Compare with a similar
good flyback to get an idea of what it should look like.)
The circuit in Television Magazine (I think it was sometime in 1993) is quite
simple, and cheap to build. Of course you need a 'scope to use with it, but
then that instrument is useful for other purposes.
I have an alternative method for testing flybacks using a 12 V chopper
for drive. A good flyback will (1) minimally load the driver circuit and
(2) put out several KV on the CRT anode connector. A flyback with shorted
windings will heavily load the driver and have little or no output voltage.
> Also, a lesson from audio work: If the HOT driver is direct coupled
> and a transistor up the line is open or shorted, the weird voltages can
> finish off output transistors. It happens in audio, so maybe it's worth
> checking those drivers and pre-drivers (if such exist) in your horizontal
> output circuit.
> Mark Kinsler
The driver is almost never direct coupled. There is uaully a driver
transformer of 6:1 or so ratio between the HOT and the previous stage
so this is usually not a problem.
Frank Vickers
limit the current through the hori output. If the flyback runs hot then
it is most likely bad even though it checks good with ohm readings. Most
likley the voltage trippler which is intergrated into the flyback is bad.
There is no way to ckeck it that I know of. If the light burns real
bright you might have a short in one of the secondary voltage supplies
off the flyback, most likley the vert out put chip. Those chips are known
offenders.
Good luck
Wayne Vickers
TNB...@PRODIGY.COM
SkiDiveMan
the market.
Chuck Veazey
Pittsburgh, PA
SkiDiveMan
is a primary to secondary short.
Ever hear of the LC 75 Sencore Z Meter. It is a fine piece of test
equipment for this type of work.
Douglas Wargo
shorted windings. However the reason most TV shops don't bother is that
ringing will not tell you if the diodes
in modern flybacks are shorted or leaking or breaking down under a load.
This is the most common reason modern flybacks fail, so the large
investment in the Sencore ringer isn't
justifiable.
-
WARGOD HXZ...@prodigy.com
A.R. Duell
>I know from nothing about television repair, but it seems to me that
>there's a technique for testing flyback transformers that involves
>ringing the circuit with a pulse waveform and watching the ringing on the
>oscilloscope. If it damps down too quickly you've got a coil or
>capacitor problem.
There is such a technique - I use it all the time, and it works very
well. What you do is to connect a known-good capacitor across one of the
windings (often the primary winding that was connected to the HOT), and
apply a pulse waveform, either from the timebase output on the 'scope, or
from a special pulse generator - there was a design in Television in 1993
which used a BU508 transistor and a 4011 NAND gate chip. That design
works very well.
BTW, as you're using a known-good capacitor, then too much damping
implies there is a shorted turn somewhere on that core - not necessarily
in the winding you're testing.
> Also, a lesson from audio work: If the HOT driver is direct coupled
>and a transistor up the line is open or shorted, the weird voltages can
>finish off output transistors. It happens in audio, so maybe it's worth
>checking those drivers and pre-drivers (if such exist) in your horizontal
>output circuit.
Almost all horizontal deflection systems transformer-couple the base of
the HOT. Does anyone know why?
> Mark Kinsler
-tony
Mark Kinsler
Thanks. Now I feel better for not having bought one of these things...
M Kinsler
Sam Goldwasser
One (probably secondary) reason is that this provides one of
the isolation barriers between a line-connected HOT and flyback primary and
the signal circuits of the TV.
A more important rational is that a transformer is nice easy way of
impedance matching the horizontal driver circuit (100s to 1000s of ohms)
to the few ohm input impendance of the horizontal output transistor
base which requires upwards of several amps for proper drive. A typical
driver transformer may be in the 5-10:1 turns ratio representing 25-100:1
impedance ratio.
A byproduct of all this is that it is almost impossible for a faulty
driver stage to kill the HOT.
--- sam
A.R. Duell
>>Ringing the windings of the flyback will indeed tell you if you've got
>>shorted windings. However the reason most TV shops don't bother is that
>>ringing will not tell you if the diodes
>>in modern flybacks are shorted or leaking or breaking down under a load.
Absolutely correct, and although I use my (home-made) ringing tester, I've
been caught that way once. But I do enough older repairs (and repairs on
monochrome monitors where the only diode is in series with the output,
and can thus be isolated by lifting the anode cap) that it's still a very
useful instrument to have
>>This is the most common reason modern flybacks fail, so the large
>>investment in the Sencore ringer isn't
>>justifiable.
1) The homebrew tester (the Television Magazine design) costs < 1 flyback
transformer, not including the scope. I suspect most of the bits are in
the average junk box, anyway.
2) It tests chopper transformers as well. I repair computer power
supplies, and knowing the transformer has not got shorted turns is a
great help. Useful on those DEC chokes in the H74* series power modules
as well :-)
>Thanks. Now I feel better for not having bought one of these things...
Well, I'm very glad I built mine.
> M Kinsler
-tony