Problem with pretty simple SCR-dimmer

[fezder]

Hey,

At first i thought this would be easy repair, at least it didnt look complicated, and the construction is same as normal SCR dimmer circuit.
So, there is DIAC (DB3) that triggers triac. Triac had small burnt mark, i checked it and it was indeed faulty, S/C to be specific. Fuse was intact however. Nothing else seemed to be faulty, so i changed it with new one.

I didn't have exact equilavent, but i do have BT136's (broken was BTB04) which seems to be pretty close equilavent. After changing it, well as you may guess, it didn't go like in movies.

Now, i checked DIAC in case it's faulty just to make sure. I compared it to other brand-new working DB3's, and their waveforms are same, so it seems intact also.

Now, the funny thing is, i get the waveform i'm excepting OUT circuit, but not in circuit. The waveform i'm looking is DIAC's trigger pulse.

Any help with this supposedly easy circuit is appreciated, i'm feeling pretty, well dump now.

And if someone wonders, i run this circuit via isolation transformer, and 'scope is grounded

 

[tunedwolf]

Circuit dependant, but I would be expecting to see a sawtooth waveform from the trigger Diac. Your problem however, is most likely the fact that the BTB04 is a low gate current triac (10mA) and the BT136 isn't (35mA). I wouldn't expect that the waveform from the trigger Diac is going to look correct when trying to drive the additional demand from the BT136

 

[fezder]

Oops, seems i looked datasheet wrong. Thanks for reply. I'll see if i can fetch correct one.

Any suggestion what would be correct replacement?

and other wild question, which might be poking with stick around bee hive, is that triac's gate resistor is 30ohms, and as this new one requires more current to gate, could this be solved simply by reducing gate resistors value? Im pretty noob with triac topology...(as with sometimes with anything hehe...)

 

[tunedwolf]

The best replacement would be the original part type that the circuit was designed for, obviously, however you could dimension the other circuit values to fire the triac you have now, if you can't obtain a suitable replacement. Just lowering the gate resistor value is not going to cut it, you need to make sure that the trigger circuit can actually provide the current to trigger and hold the triac in the on state. The BT136 is also available as a sensitive gate, 4Q type (it depends on the letter after the voltage rating). Maybe that would be easier for you to obtain locally? The sensitive gate type BT136 is much closer to the original triac in terms of spec and should work in your circuit without modification. From memory, I think the sensitive ones were types D & E

 

[fezder]

i had a feeling resistor might not be enought to make it work. i'll see local markets to see what are to offer

 

[fezder]

i managed to find btb04 from TME, arrived in couple days, very fast delivery via DHL.
But, so far no luck, same result as before

 

[tunedwolf]

I suppose it's possible that the Diac bit the dust by trying to drive the BT136 or the original faulty one. Dimmer's aren't particularly difficult to repair, but like most things, methodical testing is usually what gets you a positive result

Make sure that you have voltage across the Triac main terminals, MT1 & MT2. Check continuity across all tracks etc.
If all else fails, lift one end of every resistor and make sure they are all within tolerance. If you have a cap meter, remove them and make sure they are ok, and if you don't have one, then replace them one by one starting with the trigger capacitor on the Diac. The values won't be particularly critical so replace them with the closest one in value you have that has the same or better rated voltage.

 

[Carlos Ferreira]

I usually would check, without question, the components near by the BTB04.
In almost every occasion, one component damaged is likely to be damaged by another one, or to damage another one when broken. (I think you will know that)

It would be nice to have a picture of this board, since I am just talking without knowing exactly how this components are used for

 

[fezder]

seems i troubled forum with silly thread, as the ''customer'' wanted it back. probadly got tired of waiting, i dunno....
But, when i did have it, i ckecked, diac out circuit, and measured fuses/resistors etc. only thing i didn't touch were pcb tracks.
too bad i don't have this thing anymore, would have been great opportunity for learning, as triac dimmers are new to me, but i do understand how they work, at least i think so.
But, thanks for your time guys!

 

[Nigel Goodwin]

Historically quite a few 'early' colour TV's used thyristor (SCR) PSU's - it was VERY common for either the SCR or DIAC (or both) to fail, and we used to keep decent quantities of both in stock for that reason - best idea was to simply replace both, and then look further if it didn't cure it

 

[tunedwolf]

That's a shame Fezder, I would have liked to have seen this through to conclusion. However, it does present an opportunity for you to create a mini article around building one that does use a BT136

Nigel, I used to have one particular Normende portable that ping ponged all the time for this. It happened often enough that it had its own component drawers! First sniff of a surge or spike on the mains and the crowbars would fire, taking out the Thyristors, Diacs and fuse. I got to the point where I could almost predict when it was coming back in based on how many times I seen the lights dim the previous night. I eventually persuaded the lady owner to replace the set, and she did, bought a shiney new TX9...something about out the fire comes to mind

 

[Nigel Goodwin]

I eventually persuaded the lady owner to replace the set, and she did, bought a shiney new TX9...something about out the fire comes to mind

TX9 were pretty good sets, although the early versions had their own severe PSU problems with crowbars and self destruction - happily they later redesigned the TX9 to use an IC (TDA4600?) based PSU.

 

[tunedwolf]

Yup, the ubiquitous TDA4600. Unfortunately it was the mark 1 that she purchased
Couldn't have been a better match. I seem to remember it lasting her about 9 months before I finally felt sorry for her and gave her a wee Tatung

 

[Nigel Goodwin]

Yup, the ubiquitous TDA4600. Unfortunately it was the mark 1 that she purchased
Couldn't have been a better match. I seem to remember it lasting her about 9 months before I finally felt sorry for her and gave her a wee Tatung

I loved Tatung

 

[tvtech]

I loved Tatung

Now I am totally lost....

As we are behind the rest of the World here in South Africa....please tell me what "Tatung" is

Have I missed something that might still arrive on our shores

Regards,
tvtech

 

[fezder]

i'm curious too, to be honest ...

 

[tvtech]

i'm curious too, to be honest ...

Could be a wee vibrator of some sort to help the customer out..nah ....that is why guessing is bad

TDA4600 rings a bell though. Locally made sets used them. Still lost here....

?????

Regards,
tvtech

 

[tunedwolf]

Tatung was a Taiwanese company making small TV sets back then. They bought over Decca here in the UK in the late 70's/ early 80's as I remember, and started producing some of the most reliable little TV's on the market. Great company to deal with too, technical info was always readily available and their engineering staff could only be described as excellent. I sold loads of their small portable TV's and quite a few of their 20" sets back then. About the only thing that ever went wrong with them was the TDA4600 power supply start up capacitors and the venerable 180K start up resistor. The larger ones used to kill a LOPTX on a rare occasion and was often caused by dry solder joints on the little line driver transformer. Fantastic sets to work on though. Takes me back to nostalgic times where customers actually wanted their gear repaired as opposed to dumping it in landfill because the remote handset isn't working like they do nowadays

I think the Tatung brand is still going, but I have no idea if it is still the same company or not. Companies and brands changed hands almost weekly during the 80's and 90's

 

[tvtech]

^^^^

Thanks tunedwolf

I learned something today too. Rep for you.

Regards,
tvtech

 

[fezder]

umm, could you clarify bit? What's the deal with 180k resistor? pretty big value, and how come LOPTX's die?