Question about testing Hor O/P

[peterk]

I'd like some feedback as to some methods to test Horizontal output stage.
After changing all faulty components in the power supply I have B+ working with a 100W lamp and the horiz trans removed. When this transistor is soldered back in, the power supply blows.

1. The trans measures ok with a diode test across EBC
2. The flyback transformer tests ok with a project type LOPT tester
3. The Horiz yoke is still connected to the Collector.

If I remove the yoke and LOPT from the circuit then the horiz collector has no load, so is there a method that I can use like maybe a 10W res across the C-E (or is this not advised) or some other method. I'd like to isolate fully all windings and then test B+

I think that it may also be possible that the horiz trans breaks down due to high voltage but yet test ok with a meter, am I correct?

I'd appreciate input from more experienced technicians than I am, thanks.

[Guest]

The Horizontal transformer does many things. If you have the schematic, you could remove all of the diodes and try it again but how do you know your Horizontal Output Transistor is Ok? Disregarding your horizontal coils, how does the input winding of your Horizontal Output Transformer measure??
If a secondary supply was faulty maybe you would hear a squeeling from the powersupply, If it 'blows' then maybe you should test the HOT and HOTransformer???
The HOT base is fed by a coupling(isolation) transformer, what is the waveshape like here? There is a hold down capacitor across the c-e of the HOT. Maybe there is a diode across here aswell, or is it inside the transistor? There are resistors and diodes inside this transistor are you sure it is Ok?

[peterk]

thanks but this doesn't answer my question.....what sort of load do I need to replace the yoke and LOPT to test with HOT connected to take the 900V pulses, (inductive, resistive or something else), someone must have a trick I guess.

[Guest]

I tried to post last night but it would not take it. this is what I wrote, I hope it explains:-
The B+ is the output from the chopper transformer which feeds the HOT and eventually the line transformer. If you are lucky there may be a B+ marking on the circuitboard and maybe even a post or PC pin to measure from. Make sure you measure from the correct ground, as there are several grounds in a set. (90 - 160Vdc) If it is lower than specified there may be a short on one of the secondaries. Do the heaters glow near the tube socket?
Measure the resistance collector to emitter of the HOT, if it is low both ways remove it and measure it again. (remember there are resistors and maybe a diode across the pins)
Hardwiring is wiring with hard wires. (soldered or screwed each end)
I am not sure, anything will do I guess, as long as it can withstand the B+ voltage. The voltage you are talking about is produced by the resonant circuit, the Deflection coils across the HOT c-e (capacitor coupled) the capacitor across the HOT c-e and the LOPT in series with the HOT. Therefore you will not be testing at full voltage.
If it is a 'ring' tester you are referring to make sure you test the correct connections on the LOPT. Ring test the deflection coils also, HOT c-e. Test the diode and capacitor across the HOT c-e for shorts. HOT's are hard to test with a multimeter.
Your globe should do as long as you disconnect the deflection coils and the LOPT.
The usual method is to put the globe in series with the B+, preferably after any large capacitor, if there is a short it will glow bright and limit the current allowing you a few seconds to measure waveshapes.

[peterk]

I have done all these DC tests and results are good yet when I connect the HOT collector and switch on the P.S. trans blows. The only thing that can draw full current with the horiz pulses present is the LOPT and Horiz yoke that is why I am curious to replace these with some sort of load so can this be done or am I only dreaming.
Diode testers, LOPT ring testers and multimeters are only 9V dc test instruments and can only find faults in 80% of cases. The real stress to the cct comes when kV's are applied. You mention the globe across B+ in place of the coils, a lamp is only a resistance whose value changes with the amount of current, I think that you are only referring to DC testing, B+ is only 130V DC. With the collector connected are you saying that a 100W globe will handle the 900V pulses which means that the globe will be strobing.
An LOPT is a resistive and inductive load to the Horizontal o/p whereby magnetic flux is created in the coil by the rising and falling horiz pulses thereby a lamp will not behave in this way.
So my request still stands, is there a suitable load IN PLACE of the LOPT and H Yoke to enable the HOT to be tested in circuit?

[Guest]

Not really. The LOPT, deflection and capacitors form a resonant circuit which produce the high voltage you speak of. If you remove all of them and place a globe instead in circuit, it will be subject to B+ only. I did not suggest this, you did. This will test your HOT if you are not comfortable with measuring it with a meter. Maybe you need to replace the HOT, if it is expensive use a general replacement but only for a few seconds. What is the waveshape like on the base of the HOT, something bad here can destroy the HOT within a second. There is a coupling transformer and a resistor here that you need to test, maybe you need a 9V battery to get it to run? (from the chip)
A ring test is not a 9V test. It is a 'ring' test, it tests for shorted turns. Did you try putting a turn around your LOPT core and shorting it?
Maybe you can hear a squeal for some seconds before your powersupply transistor blows and this would suggest one of your LOPT secondary supplies may be shorted? Why don't you meter them all first then maybe disconnect them? (remove one end of the diode)
You are right about not being able to test the HV section. I have had LOPT's that test the same as the new one with all tests but only the new one will work. None of them blew the powersupply though.
The globe in series with the B+ stops you blowing expensive silicon devices while you search for shorts. A 100W or 150W globe should be enough to let your T.V. startup if it is Ok. (depending on the T.V. size) If the globe glows brightly you still have a short!
(the globe should come on and die to a dull glow as the T.V. starts up)
LOOK FOR SHORTS.



What is actually blowing?

[HughIreland]

Greetings all...
I have been just reading this thread with interest . I also just discovered this forum and websites . So this may be closing the barn door after the horse is gone .
One thing I like to do after chasing all bad caps , ring test, flyback secondary diodes , power supply , cold joints etc. is put up board on bench without chassis . Just a suitable sub for yoke (I just measure the rough dc resistance to be close to actual yoke) no tube ( I have seen the occasional shorted tube) and put the free end of anode into a CERAMIC coffee cup with or without measuring probe and fire set up . I am always happy to hear that hissing sound of high voltage . I recently had to toss a young set in the trash because the BONDED yoke was the cause of the symptoms you describe(dead short) but it sounds like you have eliminated that .
You say that power supply blows, do I infer that the HOT survived ? What blew in the power supply ?
Some of these switching supplies have a feedback system from flyback to modify current/voltage , check that they are intact . Schematics ar not always needed . If this is a copmputer monitor , a lot of them upon start up run through the high scan rate briefly and look for feed back . The reason I mention it is because I had one that all looked good but would squeek like it was about to run and then destroy HOT . Turned out to be a bad cap in the switchable tuning circuit behind the HOT for the different scan modes .

I hope you have (had?) success
Hugh Ireland
Hugh's Electronics and Computers

[Guest]

Welcome to the Board, Hugh. It is correct that the HOT doesn't appear to blow when the set is powered up. It is a 2SD2499 which is 6A/50W which seems to test ok with a multimeter diode test across BCE. The secondary winding o/p of P/S supplying B+ blows the fusible resistor feeding the trans 2SC2335 (which is 7A/40W) and this goes s/c also. Being that it is rated at a higher rating than the HOT it surprises me that it blows and the HOT seems to be OK, therefore my suspicions lean towards the HOT being faulty when it is fully conducting. A 100W lamp being fed by B+ with the HOT out of cct is showing a glow that isn't too bright therefore the SMPS is ok untill the supposedly good HOT is reconnected and lamp removed then it's all kapput again.
Looking at the cct the SMPS o/p trans is in series to the Primary of LOPT then in series with the Collector of HOT, that is why I thought that I'd like to replace the LOPT Primary with some sort of load with similar characteristics thus isolating any Secondary effects upon the Horiz stage. I tried to pull apart an old LOPT to get to the primary but it's all full of resin.
I have previously had a set that also tested HOT ok and I replaced it and works ok but now I'd like to prove this in a technical fashion rather than just trial and error as it appears that most people do.
PS..this is just a hobby for me so there is no urgency on my part although it is someone elses CTV and I don't want to blow up more than I need to by experimenting

[HughIreland]

Ahh I get it now,
I would like to scope the base signal while lamp load is on and measure frequency . load the base with a 47 ohm 1 watt to ground (emitter) and hook on scope, failing that do you have a multimeter with freq. function? But frequncy would not appear to be your problem since HOT survives , I would look for another source for high current drain .
Some sets do not start vertical until horiz. feedback , maybe vert out is s/c? and most vert. circuit use scan-derived voltage to operate . Although they usually have a flameproof resister for protection but diode can short and feed the filter all the AC it can handle, looking like a short.

As to a substitute load for primary any old inductor will do , even a hand wound coil on an open frame core . If you have an inductance meter just get close in inductance with say, an SMPS transformer , it would handle the B+ and we don't care what the secondaries do as long as they are not connected . That would at least resonate with cap(s) and produce the high potential flyback pulse you wanted(do not measure wtih scope or meter!)
You could just put a resister in place of primary but that would not produce the flyback pulse . Having said that , I don't believe you have a bad flyback or horiz. yoke bad . The lamp test has the B+ at the yoke most likely, unless it has to be undone mechanically to connect lamp . A bad flyback will (not always) destroy the HOT .

Can you disconnect the vertical yoke for test ? but that would cost another Transistor most likely , pull the vert.IC intstead? I am curious how the set performed prior to it's death . Was the vertical folded over or nonlinear? was there a storm at the time . A bench supply to provide your own B+ would be nice , then you could monitor/regulate the current . That brings to mind a time when I had no test equip. A 1 ohm power resister and a cheap volt meter made a good current meter for higher current ,but, most modern meters have a 10 amp ac/dc one built in . You could also use a current limiting power resister from the existing power supply to keep it running long enough to look for hot spots .

A common problem I am ecountering on a lot of SMPS is the timing caps in and around the primary . A 2.2 uf at 200 volt is often the culprit. When a load is demanded the transistor may be driven into over conduction . You probably do not have an ESR meter so this means bridging suspect caps with good ones or outright shotgunning the circuits(replacing all caps in area) However you say the destroyed trans. is in series with primary of flyback . What controls it's base current I wonder.

That variable bench supply would be handy because you can connect a function generator to base of HOT with square wave at right freq. and bring supply up slow . There are suggested test circuits in datasheets for transistors sometimes(but you have that in the chassis of the set already)The rest of the set does not need to be running to produce high voltage . If you don't have access to test equip. to get your empirical results I would use the power resistor in place of 2SC2335 (which shows in my book as a 12 amp 100 watt switch) this transistor may be the modulator for E-W correction which would point back to vertical problems maybe.

I hope my rambling helped some
Later
Hugh

[peterk]

There's a few good ideas here so I'll give them a try, thanks.
With regards to the caps, yes, I've tested them with a ESR meter and changed all the bad ones. The rectified o/p of the SMPS feeds a 2SA1013 which turns on the one I mentioned previously 2SC2335 and once turned on it passes full current through C_E to the HOT.
I'll post in a couple of days.