Ahhh! So you are saying it's a good idea to use a combination of Variac and Light Bulb..!?

That makes sense, I take it you would start with the Variac first, feeding your Light bulb set up from it - and then feed the equipment you are working on from the Light bulb...?

Cheers,
Tom

Quote from: DrewV on April 17, 2012, 10:57:08 PM
I'll just make a couple comments here. Regarding the level of bulb brightness, I have found that  different amps draw more or less current while "idleing" causing the bulb glow a little brighter or dimmer. Second, to prevent a situation like JM described always use a variac to bring the voltage up gently. If the amp is drawing excessive current you'll see it before you raise the V to high.

Cheers!

Thanks Phil. That looks very similar to mine - only the one I have is a tad simpler - so it's minus a few bits... which is the good way round for me! (Your schem has two extra transistors.)

There are no mods on this combo, so I don't think it originally blew because of mis-use. Now I have a schemo to work with I stand a better chance.

Thanks again
Tom

Quote from: phatt on April 18, 2012, 08:20:36 AM
Yeah I found it  :cheesy:
Don't take it as gospel but it will likely be close to what is in front of you.

From memory give or take a few brain cells the rails where around 35 VDC.
Not a rock concert level of power but a big 12 inch driver makes it OK for a fold back Amp.

My guess at why it blew??  Likely someone added a dodgy external speaker jack and did not bother to check the internal speaker was already 4 Ohm and adding another driver would definitely blow it up.

The owner who knows me well has had no problems since we fixed it  and removed the ext socket.
(over a year ago now)
Phil.

Thank you..!
Now I understand...;c)


[/quote]
True. The bulb is an auxiliary device, but your measuring instrument is the multimeter. Use it.
1) Measure both power rails, they should be somewhat lower than normal, because the bulb "eats" some "wall voltage"
Example: normal rails are +/-40V.
If you find something between 25 and 35V each, fine. Reasonable.
If you find , say, +/- 5V .... *who* is eating so much?. Abnormal.
In another Forum a guy's Aguilar 750 amplifier (over +/-70V rails) had perfect symmetical ... 1.9V rails. *Big* trouble.
2) measure DC voltage at the speaker out. You should have close to 0V there. Less than 100 mV in any case.
3) the fire test.
If your amp passes tests 1 and 2, hook the speaker and have it play some music.
You should be able to get at least 5 or 10W out of it, which is quite loud in a room.
If it passes the #3 test, only then plug it straight into the wall, without limiter.

Do not skip repair stages, or you may make an expensive mistake.
You *may* get lucky of course, but don't count on that.
[/quote]

Yes I get that... but I am still a little confused... I jumped in here  from my own thread about my faulty amp so I'll give you a scenario...

What would happen in this case:

OK, so fix a shorted output transistor fault. You put the amp on via the bulb - the lamp lit bright, so you knew there was a short. Got your meter out and found both o/p trasistors shorted. So you replace both transistors but you don't yet know if that's the fix. And, unknown to you there is "another fault" which would blow the transistors right away - normally. So, you reconnect the amp via the bulb and switch on. Now there is no short, as you've just fixed it - so what would you expect to happen here then... a dim bulb? - making it "appear" that everything is OK? Because, if so then as soon as you remove the bulb - those new transistors would blow.

And it wouldn't be a bright bulb as you've just fixed the short!

Sorry, but do you see my confusion?

Cheers,
Tom

Quote from: J M Fahey on April 17, 2012, 10:59:33 AM
You already *have* one ... it's the Lamp brightness.
Normal is dark orange, red or almost invisible; a shorted amp is bright shining as if it were connected to illuminate the room, it will usually blink in rhythm with the music.

What are your thoughts on adding an AC Ammeter to the circuit..?
Would that be useful in telling you anything extra to what you can find probing with your meter..?

Quote from: J M Fahey on March 17, 2011, 12:23:04 AM
Some images:

Thanks Phil, much appreciated.

The o/p transistors were A1000's which, after much searching as to equivalents, I replaced with MJ15003's. Both are the same by the way... NPN. Usually you get an NPN/PNP arrangement but this one's different.

OK, so the dim-bulb device would be a good idea even in this case. I'll get on it.

Cheers,
Tom

Quote from: phatt on April 17, 2012, 09:27:09 AM

Yes you will have a hard time finding schematics for Torque stuff.
I did repair a Torque Fold back a while ago might see if I can dig up the paper work.

I drew up the power stage and found it was no big deal and replaced the obscure labeled output devices with good old 2N3055 and it's now fine.
yours might be different design. 
Phil.

Thanks for the reply!

Yes I already replaced those 100ohm resistors - the two .33 ohms are fine, I checked those as well. The schematic seems unobtainable. This is a "Torque T100-3" combo. If anyone has a schematic I'd be very grateful...;c)

Also please correct me if I am wrong - but I thought a dim-bulb tester was useful in identifying an "existing short" upon switch on... but isn't my case different? I know I have a fault that will take out my o/p transistors but for the life of me cannot find any "existing short"... would a bulb save those transistors or wouldn't they just blow... and THEN light the bulb to tell me that they blew?

Cheers,
Tom

Quote from: J M Fahey on April 17, 2012, 06:26:26 AM
Yes. Normal "burnt amp" stuff.
Clearly you still have other problems.
You also blew the output transistors again.
1) search (here) and build a lamp bulb limiter.
It will protect you from burning lots of new transistors over and over.
2) disconnect your speaker until the amp is repaired.
3) replace those BE (Base to Emitter) 100 ohm resistors but also check the low value (0.33 ohms typical) wirewound emitter resistors.
They probably blew open when the transistors shorted.
4) what amp is it? Model? Maker?
Try to find its schematic and post it.
Good luck.

Hi everyone - I just joined and have a request for help please...

I am trying to fix a combo amplifier and wondered if anyone could advise me? I am normally quite competent and sucessfull in finding faults but this one has me currently stumped!

The initial symptom was blowing of fuses. I found two shorted o/p transistors - and nothing else.

So I replaced them and switched the amp on. It worked fine for a week or so - then I decided to spray the noisey volume pots - now this was on the pre-amp panel, and didn't involve removing the power amp side which I had just worked on - as the pre-amp is on the other side of the cabinet, the two panels are connected by a ribbon cable. So I sprayed the pots and replaced the pre-amp front panel.

I turned the amp on - fuse blew again..!

This time I replaced the fuse and whilst monitoring the power amp as I switched it on again I noticed two resistors 'smoking' (100 ohms between the base and emitter of both o/p transistors)

I quickly turned off (the resistors still read 100 ohms!) This time the o/p transistors weren't shorted - but obviously needed replacing (again) as they read a few hundred ohms both ways between certain legs (cannot remember which).

Again, I replaced both o/p transistors - but obviously I don't want to blow them again. And again, as before - there is nothing standing out as 'shorted' anywhere on this simple circuit. With these symptoms you would think it would be staring you in the face..!?

So next I used my Variac... as I slowly wound up the voltage, with a speaker connected and measuring the HT volts I noticed, very early on, almost just off the Variac's end-stop - I am quickly getting volts building but also a 'hum' appears on the speaker.

I suppose I have a question here - as there are no actual obvious shorts anywhere - could the driver transistors (BD537/538) be causing this even though they read fine - out of circuit - with my meter? Could they be 'leaky' or just faulty 'under load'...?

There is also another transistor (BD711) which feeds the drivers which also seems fine out of circuit on a meter...

Has anyone experienced this type of fault and could I be obn the right track?

Many thanks for any help.
Cheers,
Tom