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
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.
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.
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:
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.
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.
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.
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.
(http://www.vintagehofner.co.uk/britamps/watkins/schematics/100poweramp2a.jpeg)
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.
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.
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.
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...
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..
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.
Cheers guys!
Quote from: J
@ noddy: it would be nice to have some gut shot of your Torque, never seen one inside.
/quote]
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.
@ JMF, Thanks for the correction. Where would we be without the sharp eyes of the experienced. :dbtu:
As comparo
Woohoo,, Just found this pis of board, I did take pics as well but forgot I had them. :duh
Yes obviously a little different layout but looks close to same.
Note the Ribbon cables they should be banned as they don't take kindly to much movement. A real pain to reconnect as very little trace is present and these often lift with too much heat.
Re the big Yellow disc on the output might need explanation.
I don't trust the current limit stuff I find the poly switches are a god send and have tested these many times on My Amps which have NO Short Circuit protection and not once have I blown any active devices. Heck one simple part replaces a whole lot of fiddly bits which takes up real estate.
These PTC's (Poly Thermal Switches) can also be used to save speaker from over current.
The HiFi folks don't like them claiming all sorts of sonic impairment but for R&R guitar perfect.
I just had a thought???
We now have 2 pics for comparison.
Mine has the inductor on output,,, Hum?
Could the unit be going into supersonic Oscillation and be the cause of the initial problem? surely the zobel network (which seems to be there) should stop that?
Oh yeah,
I'd be interested in comment from someone who knows which way pnp transistors point,, :P
Regarding that lone VAS (The voltage Driver or Voltage Amplifier Stage, I think is the term?)
There is often no heat sink on that transistor yet it's actually the only device that see's the Full Rail to Rail voltage?
On some Amps I've messed with I notice that it can run warmer that those 2 predriver.
just wondering? Headscratch?
Phil.
Interesting to compare both.
One looks "more Pro" being fiberglass, silkscreened parts, fully tinned below, driver heatsinks, etc. ... although it lacks short protection and output inductor.
The Ozzy one has these, but no parts silkscreen and cheaper phenolic material.
Maybe Torque wanted to cut costs down and either got the lowest bidder or made them by themselves.
The basic schematic is probably the same ... what else?.
I have used output inductors in about 30% of my amps and none on the others.
Never found a difference.
I think layout and grounding is the *big* factor referring to stability, all others help.
Zobels *yes* are indispensable.
Hi again. I have an update... I *was* searching for shorts on this board... but I found two transistors (would be Q1 & Q2 on this schematic although they are numbered differently on mine...) that, when removed measure "open" all round with my meter on the ohms setting... however using the diode/capacitor setting they read 'open' (3volts) one way and 0.7volts the other between the three legs. Now, which should I go with? Are they suspect, would you say?
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.
Transistors are always measured in the diode setting, that's what it's for.
Google how to measure transistors with a multimeter, as not to repeat the same time and again.
Good luck.
Yes, of course... my meter doesn't have a dedicated transistor tester, as some do. And I never really got out of the habit I developed on my old analogue meter which only had an ohms range... anyway here's an update - as of this afternoon the amp is fixed!
But I think there is more to this than meets (or met) the eye!
All I did was to remove those two transistors... then replace them, possibly swapping them around!? I also lifted all three zeners and checked those again...remember this is all treble checking as I have gone over and over this amp already. So I ran up the variac again - and there were zero volts on the output..! I monitored both rails and it all behaved beatifully. I next attached the speaker... no hum! So it seems to have 'fixed itself'.
Now remember, this is an amp I already fixed once and after a couple of days use it blew again. I am expecting it to do the same as I haven't actually found anything definitely wrong. Maybe I 'disturbed' something - I might never know....
My next step will be to reattach the preamp section ribbon cable and feed some audio through it... but this begs another question... what would the recommended procedure be here?
Soak test it all via the bulb just in case the problem rears it's head again? Tap around (lightly) and see if I can 'force' the fault to reappear? I am going to have to put all my faith into this bulb to protect it should this be an intermittent problem...hehe!
Cheers,
Tom
Quote from: J M Fahey on April 20, 2012, 09:55:54 PM
Transistors are always measured in the diode setting, that's what it's for.
Google how to measure transistors with a multimeter, as not to repeat the same time and again.
Good luck.
Yes, all analog meters had a single resistance setting for everything. Oh well.
Believe it or not, I sometimes still use my old analog (needle) meter , both to keep trained with it, and because some things are best shown by a wiggling needle than by rapidly changing numbers.
Well I think my Torque amp is now fixed... just on soak for a few days... and no issues yet, touch wood. I am attaching my revised circuit diagram - I slightly edited the one posted by (phatt) Phil, making it a circuit of the one I have here. A couple of changes... removed the protection circuitry, added a few components and moved one 'trace'... please let me know if it "looks about right", it's quite confusing as you probably know, trying to follow the path!
It might be useful to someone with this same model combo...!?
Thanks for all the help and advice.
Cheers
Tom
Quote from: J M Fahey on April 22, 2012, 10:28:45 AM
Yes, all analog meters had a single resistance setting for everything. Oh well.
Believe it or not, I sometimes still use my old analog (needle) meter , both to keep trained with it, and because some things are best shown by a wiggling needle than by rapidly changing numbers.
Hi Noddy,
Well done you worked that out well :tu:
just need to flip Q5,, which was my booboo. :-[
Others here will know more than myself ,, but I'd be a bit worried about that *220R* off the base on Q2.
In my real world testing Destruction of perfectly working transistors. :duh
I did realize that if that is set around 1k it seems to stay safe but depends on circuit/voltages and much other stuff which is mostly beyond my ability to understand let alone explain clearly.
One quick Q?
what is the DC voltage at the base of Q2?
Circuits like this are a very good learning tool for understanding DC coupled stages as they are very basic and in my experience can often sound better for guitar work.
From what I've understood (while blowing things up) the idea is to keep the base of Q1, Q2 and the speaker out should be all close to Zero Volts.
Phil.
Hi Phil,
Thanks for the reply and the comments. My redrawn cct does indeed have Q5's emitter amended! Also I'm pretty sure that the resistor is 220ohm... and as for the voltage on the base of Q2... well the amp is now all reassembled and I am loathed to open her up, in case I disturb something again. I was fixing this amp for a friend and have told them it's all done! Success...etc... Hahaha!
I wish you'd have asked these questions a little earlier, I could have checked it all for you. It has been quite a learning curve for me... and I have now also built a Variac/Light Bulb station which I shall be using more in future, I get the feeling...;c)
Can anyone else comment... about the 220ohm... or even what volts there 'should' be on Q2's base...?
Cheers
Tom
Quote from: phatt on April 24, 2012, 09:52:39 AM
Hi Noddy,
Well done you worked that out well :tu:
just need to flip Q5,, which was my booboo. :-[
Others here will know more than myself ,, but I'd be a bit worried about that *220R* off the base on Q2.
In my real world testing Destruction of perfectly working transistors. :duh
I did realize that if that is set around 1k it seems to stay safe but depends on circuit/voltages and much other stuff which is mostly beyond my ability to understand let alone explain clearly.
One quick Q?
what is the DC voltage at the base of Q2?
Circuits like this are a very good learning tool for understanding DC coupled stages as they are very basic and in my experience can often sound better for guitar work.
From what I've understood (while blowing things up) the idea is to keep the base of Q1, Q2 and the speaker out should be all close to Zero Volts.
Phil.
1) The voltage on Q1's base will be a few mV DC, because its base current flows through R2 to ground.
The voltage on Q2's base *should* be the same, because it also flows through a 10K resistor to ground : R7 .
And where is that ground connection?: through the speaker.
I didn't even mention its value because 4 or 8 ohms are *nothing* compared to 10000.
In fact it won't be *exactly* the same because Q1 and Q2 do not have the exact same current gain, but anyway they will be only a few mV from each other.
So , in a nutshell, Q2's base will also be a few mV (less than 100mV ) from ground.
Exactly what Phatt said from his considerable experience, only I added the *why*.
2) those 220 ohms shown are necessary so the amp has gain, otherwise you would need to hit it with over 20V RMS.
The AC (signal) gain is= (R7/220)+1=(10000/220)+1=46.5 ... call it 50.
Which in practical use means , since it can put around 20V (20000 mV) into that speaker, it will need to be driven with = 20000mV/50=400mV .
Easy for any preamp, even a 9V battery powered one.
The 47uF capacitor in series with the 220 ohm resistor blocks DC, so the amp has AC gain around 50, but DC gain of 1 , which is good because otherwise any DC at the input would produce a speaker burning disastrously high DC voltage at the output.
This amp uses a classic design, time tested and used by the Millions , basically for one reason: it works ... and very well.
Long live SS !!!!!! :dbtu: :dbtu: :dbtu: :dbtu: :dbtu: <3)
Quote from: noddyspuncture on April 24, 2012, 05:13:57 PM
Hi Phil,
Thanks for the reply and the comments. My redrawn cct does indeed have Q5's emitter amended! Also I'm pretty sure that the resistor is 220ohm... and as for the voltage on the base of Q2... well the amp is now all reassembled and I am loathed to open her up, in case I disturb something again. I was fixing this amp for a friend and have told them it's all done! Success...etc... Hahaha!
I wish you'd have asked these questions a little earlier, I could have checked it all for you. It has been quite a learning curve for me... and I have now also built a Variac/Light Bulb station which I shall be using more in future, I get the feeling...;c)
Can anyone else comment... about the 220ohm... or even what volts there 'should' be on Q2's base...?
Cheers
Tom
Quote from: phatt on April 24, 2012, 09:52:39 AM
Hi Noddy,
Well done you worked that out well :tu:
just need to flip Q5,, which was my booboo. :-[
Others here will know more than myself ,, but I'd be a bit worried about that *220R* off the base on Q2.
In my real world testing Destruction of perfectly working transistors. :duh
I did realize that if that is set around 1k it seems to stay safe but depends on circuit/voltages and much other stuff which is mostly beyond my ability to understand let alone explain clearly.
One quick Q?
what is the DC voltage at the base of Q2?
Circuits like this are a very good learning tool for understanding DC coupled stages as they are very basic and in my experience can often sound better for guitar work.
From what I've understood (while blowing things up) the idea is to keep the base of Q1, Q2 and the speaker out should be all close to Zero Volts.
Phil.
Well excuse me while I go bang my head against a wall :loco
:-[ I've now taken the prize as I've achieved boobooing my own booboo,, a double booboo :lmao: :lmao:
Yes you did fix the upside down transistor :tu:
Phil.
Quote from: J M Fahey on April 24, 2012, 11:54:22 PM
1) The voltage on Q1's base will be a few mV DC, because its base current flows through R2 to ground.
The voltage on Q2's base *should* be the same, because it also flows through a 10K resistor to ground : R7 .
And where is that ground connection?: through the speaker.
I didn't even mention its value because 4 or 8 ohms are *nothing* compared to 10000.
In fact it won't be *exactly* the same because Q1 and Q2 do not have the exact same current gain, but anyway they will be only a few mV from each other.
So , in a nutshell, Q2's base will also be a few mV (less than 100mV ) from ground.
Exactly what Phatt said from his considerable experience, only I added the *why*.
2) those 220 ohms shown are necessary so the amp has gain, otherwise you would need to hit it with over 20V RMS.
The AC (signal) gain is= (R7/220)+1=(10000/220)+1=46.5 ... call it 50.
Which in practical use means , since it can put around 20V (20000 mV) into that speaker, it will need to be driven with = 20000mV/50=400mV .
Easy for any preamp, even a 9V battery powered one.
The 47uF capacitor in series with the 220 ohm resistor blocks DC, so the amp has AC gain around 50, but DC gain of 1 , which is good because otherwise any DC at the input would produce a speaker burning disastrously high DC voltage at the output.
This amp uses a classic design, time tested and used by the Millions , basically for one reason: it works ... and very well.
Long live SS !!!!!! :dbtu: :dbtu: :dbtu: :dbtu: :dbtu: <3)
Arrh,, Mr Fahey you are very kind about my experience :lmao:
Thanks for the maths as it does help explain all those transistors that sacrificed there guts to help an Amateur learn.
Re discrete Pwr Amps,, I have one of my first ever attempts at hand drawn PCB somewhere in the shed and the schemo from a Electronics mag, So I'll try and find it,, might be an interesting exercise to analyze the merits or lack off.
Oh Yes the first one did actually work but the second one blew instantly.
Oh Well 50 % success for a beginner was not bad ay?
Cheers, Phil
That's what the lamp limiter was invented for ;)
Thanks guys... glad I joined this forum, you're a great bunch of friendly and knowledgeable chaps. Who'd have thought this little problem amp of mine (well, my mates...) could have provided this much debate...;c)
Looking forward to my next problem/project...haha!
Stay good!
Tom
Quote from: phatt on April 25, 2012, 10:09:53 AM
Quote from: J M Fahey on April 24, 2012, 11:54:22 PM
1) The voltage on Q1's base will be a few mV DC, because its base current flows through R2 to ground.
The voltage on Q2's base *should* be the same, because it also flows through a 10K resistor to ground : R7 .
And where is that ground connection?: through the speaker.
I didn't even mention its value because 4 or 8 ohms are *nothing* compared to 10000.
In fact it won't be *exactly* the same because Q1 and Q2 do not have the exact same current gain, but anyway they will be only a few mV from each other.
So , in a nutshell, Q2's base will also be a few mV (less than 100mV ) from ground.
Exactly what Phatt said from his considerable experience, only I added the *why*.
2) those 220 ohms shown are necessary so the amp has gain, otherwise you would need to hit it with over 20V RMS.
The AC (signal) gain is= (R7/220)+1=(10000/220)+1=46.5 ... call it 50.
Which in practical use means , since it can put around 20V (20000 mV) into that speaker, it will need to be driven with = 20000mV/50=400mV .
Easy for any preamp, even a 9V battery powered one.
The 47uF capacitor in series with the 220 ohm resistor blocks DC, so the amp has AC gain around 50, but DC gain of 1 , which is good because otherwise any DC at the input would produce a speaker burning disastrously high DC voltage at the output.
This amp uses a classic design, time tested and used by the Millions , basically for one reason: it works ... and very well.
Long live SS !!!!!! :dbtu: :dbtu: :dbtu: :dbtu: :dbtu: <3)
Arrh,, Mr Fahey you are very kind about my experience :lmao:
Thanks for the maths as it does help explain all those transistors that sacrificed there guts to help an Amateur learn.
Re discrete Pwr Amps,, I have one of my first ever attempts at hand drawn PCB somewhere in the shed and the schemo from a Electronics mag, So I'll try and find it,, might be an interesting exercise to analyze the merits or lack off.
Oh Yes the first one did actually work but the second one blew instantly.
Oh Well 50 % success for a beginner was not bad ay?
Cheers, Phil