My Problem Amplifier And Symptoms...

[noddyspuncture]

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

[J M Fahey]

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.

[noddyspuncture]

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

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.

[joecool85]

The lightbulb limiter will keep them from blowing by limiting voltage AND current in the circuit.  I use mine for all amps with unknown issues.   :tu:

[phatt]

Yep Joe beat me to it, lol. I'll post it anyway.

If the power transistors are already blown (open circuit) ,, then no current can flow and bulb will only glow while the transformer and stuff charges. then it will barely glow, indicating little current is flowing.

The idea is that if the circuit is dodgy and goes unstable then the *current limiting light bulb* will dump a lot of current which instantly drops the voltage stopping it blowing up expensive parts.
Over current is what kills the Actives.

Bare in mind these are nearly always Direct coupled units and if one thing goes it often takes down other actives faster than you can swipe your credit card.

Just go make a light bulb unit,, they save lotsa money and tears.
I should know as I built many discrete test power amps and I lost count of the amount of actives I blew up just trying to work out How they function.

If you have big voltage on the speaker output (this is the hum) you have deeper problems.

If it's the classic LTP (Long Tailed Pair) front end then it might help if you can understand that the bases of those first two transistors are key to the design. They have to be kept at close to Zero VDC.
The FB resistor helps stabilize this and keeps the Speaker out close to Zero as well.
Though some DC offset is usually present,, maybe ~100mV.
Though I've measured some old working Amps with half a volt on the output.
Some designs are obviously more robust than others.

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.

[noddyspuncture]

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

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.

[phatt]

Arrh yes,,, A1000,
That was the label on this one also..
To late in my part of the world but I'll try and dig for the schematic I drew up tomorrow.
As it's likely the very same circuit.
Phil.

[J M Fahey]

Never seen a Torque in my bench but I know they are (or were) very popular reasonably priced UK amps.
They can't have used any NASA type circuits, they must have been quite conventional.
Many of those were derived from the so called "RCA 70W amp", modded to have some extra power.
Just an example fom about the same vintage: the WEM SS 100W power amp.

They were pioneers in the UK and many others basically copied them, with small variations.
Yours might be basically this, even with some parts removed to lower cost.

[phatt]

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.

[noddyspuncture]

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

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.

[Tann-74]

Just a quick look over this amp schematic... it look's the same as a laney or carlsbro amp..

same ltp pair with bootstrap cc in the vas stage..  the only thing a miss is the zobel 10 ohm and 100n cap on the o/p end..

straight forward amp to solve...

[Tann-74]

Also the tail pair can be mpsa92 matching them give's performance and lower's offset voltage..

the driver can be BD139.. also the current limiter transistors and components can be removed for pre-fight checks.. large offset voltages can happen if the tail transistor legs are fitted in the wrong way round when using different replacements.. make sure the legs are fitted the right way in...

I would agree about different ranges of offset voltages.. but these amps should be about 100mv or so.

o/p pair can even be 2n3773's etc etc..

these amps can be tested on supply rails as low as +- 15 volts to 47+- dc..

[J M Fahey]

Yes, it is a classic design, used by thousands. Good  :tu:
A couple values may vary a little, does not change the mode of operation.
Dear Phatt, thanks a lot for taking care of tracing and now posting it.
The drawing looks fine, with only a very minor drawing correctiom: Q5 should be drawn with its emitter "upstairs" and the collector "downstairs" .
And no, please don't make jokes about Australia being "upside down" ... I live in the South too .

@ noddy: it would be nice to have some gut shot of your Torque, never seen one inside.

[noddyspuncture]

Cheers guys!

/quote]

[J M Fahey]

Hey !! , it looks *very* well made, as good as any Peavey/Laney/Marshall amplifier.
I already knew it was unjustly despised, specially because of its affordable price and because "nobody famous used it in a Platinum record".
So what ??
Thanks for posting.
PS: it lacks the short protection parts (which you can see in the Phatt and Wem schematic), typical of combo amps (or powered monitors) where there is only an internal speaker.