Problem with transistors? Or something?

[Teemu_K]

Transitors pass all the voltage from collector to the base and cause a
short. Transistors are perfectly ok, they're not broken (I tested
them). The voltage is rather high but they should handle it although
they're (almost) working on their limits. (The amount voltage passed
from collector to base seems to increase while the test circuits
supply voltage is increased though.) There seems to be no faults in
the circuit build. What could cause this?

The circuit is a basic amplifier. Transistors collector connected
directly to supply voltage (65 V), no resistor. Transistor used is
BC546 complemented with BC 556 which has emitter connected to supply
voltage.

I'm amazed. What should I look for to get the circuit to operate?
Everything tells me it's supposed to work. Supply voltages are OK
without the transistors. With them the circuit shorts.

I know this is rather tricky question without describing the circuit
entirely but if someone has experience of problems or symptons like
this, (in any circuit), I'd be thankful of some hints and suggestions
of what type of fault to look for.

 

[scada]

Transitors pass all the voltage from collector to the base and cause a
short. Transistors are perfectly ok, they're not broken (I tested
them). The voltage is rather high but they should handle it although
they're (almost) working on their limits. (The amount voltage passed
from collector to base seems to increase while the test circuits
supply voltage is increased though.) There seems to be no faults in
the circuit build. What could cause this?

The circuit is a basic amplifier. Transistors collector connected
directly to supply voltage (65 V), no resistor. Transistor used is
BC546 complemented with BC 556 which has emitter connected to supply
voltage.

I'm amazed. What should I look for to get the circuit to operate?
Everything tells me it's supposed to work. Supply voltages are OK
without the transistors. With them the circuit shorts.

I know this is rather tricky question without describing the circuit
entirely but if someone has experience of problems or symptons like
this, (in any circuit), I'd be thankful of some hints and suggestions
of what type of fault to look for.

It is unclear to me what you have. You need to post a schematic. If I
understand you corectly, you are using a darlington amp with the collector
of T1 going to B+, then the emmiter of T2 going to B-! Well as soon as there
is current introduced to the base, it will short the supply!

 

[Teemu_K]

It is unclear to me what you have.

Yes I was sure it would be. Note my line "without describing the
circuit".

You need to post a schematic. If I
understand you corectly, you are using a darlington amp with the collector
of T1 going to B+, then the emmiter of T2 going to B-! Well as soon as there
is current introduced to the base, it will short the supply!

No. The circuit doesn't have negative and positive supply voltages.

It's impossible to post the schematic to everyone or to describe it in
detail (or draw it with ANSI graphics), howerer...

You, (or anyone interested), can check the circuit from

http://www.redcircuits.com

http://www.redcircuits.com/Page65.htm

to be specicic.

Its the 60W guitar amp's preamplifier's 1st and 2nd stage. First is, I
believe it is (I'm a beginner), a darlington. Second one is almost
identical without the complementary transistor. When simulated with a
software, or calculated, it works fine. In real life... no.

After the R21 the voltage should be around 59 V because the main
supply voltage in circuit I built is about 63 V, not the 60V. This
shouldn't do any harm howerer. Without transistors all the preamps
voltages seems to be ok but when transistors are put on their places
the Q1 (BC546) shorts through its collector to base. The supply
voltage decreses to about 32 V and resistors R21 and R8 (both 330 ohm)
have to handle at leat 3 W of power. Even 5W resistors heat too much.
No sound. No signal present after R1 or R2.

I really don't know what I should look for to get it working.
Everything seems fine. The transistors should be ok and I tested them.
I'm not sure whether the author of the page really built the circuit
him/herself but the poweramp operates and the preamp seems logical so
I gather it should work, and I'm mostly sure that ..I.. have made an
error. Just don't have any clue, that what it could be. No there is no
short from collector to base,.. except when the transistors are put on
their place. All the teething problems in the circuit should be fixed
by now..

BTW. I send him email. No answer. Otherwise I wouldn't be bothering
you.

Now I'm looking for the clues of what should I be looking for to get
this circuit operating. Leak current??? From where? Why?

I repeat my words...

"if someone has experience of problems or symptons like this, (in any
circuit), I'd be thankful of some hints and suggestions of what type
of fault to look for."

PS. The poweramp is fine and really sounds good. So if anyone is
interested in building a simple, good sounding amplifier take a hint.

 

[Robert Baer]

Yes I was sure it would be. Note my line "without describing the
circuit".


No. The circuit doesn't have negative and positive supply voltages.

It's impossible to post the schematic to everyone or to describe it in
detail (or draw it with ANSI graphics), howerer...

You, (or anyone interested), can check the circuit from

http://www.redcircuits.com

http://www.redcircuits.com/Page65.htm

to be specicic.

Its the 60W guitar amp's preamplifier's 1st and 2nd stage. First is, I
believe it is (I'm a beginner), a darlington. Second one is almost
identical without the complementary transistor. When simulated with a
software, or calculated, it works fine. In real life... no.

After the R21 the voltage should be around 59 V because the main
supply voltage in circuit I built is about 63 V, not the 60V. This
shouldn't do any harm howerer. Without transistors all the preamps
voltages seems to be ok but when transistors are put on their places
the Q1 (BC546) shorts through its collector to base. The supply
voltage decreses to about 32 V and resistors R21 and R8 (both 330 ohm)
have to handle at leat 3 W of power. Even 5W resistors heat too much.
No sound. No signal present after R1 or R2.

I really don't know what I should look for to get it working.
Everything seems fine. The transistors should be ok and I tested them.
I'm not sure whether the author of the page really built the circuit
him/herself but the poweramp operates and the preamp seems logical so
I gather it should work, and I'm mostly sure that ..I.. have made an
error. Just don't have any clue, that what it could be. No there is no
short from collector to base,.. except when the transistors are put on
their place. All the teething problems in the circuit should be fixed
by now..

BTW. I send him email. No answer. Otherwise I wouldn't be bothering
you.

Now I'm looking for the clues of what should I be looking for to get
this circuit operating. Leak current??? From where? Why?

I repeat my words...

"if someone has experience of problems or symptons like this, (in any
circuit), I'd be thankful of some hints and suggestions of what type
of fault to look for."

PS. The poweramp is fine and really sounds good. So if anyone is
interested in building a simple, good sounding amplifier take a hint.

***Amplifier:
First, i see no darlington configuration in this section (Page65.htm).
Q1 is a simple amplifier, Q2 is to bias Q3/Q4 to class AB1 or AB2; R3
is adjusted for zero signal standby current thru Q3/Q4.
Q3/Q4 is a "standard" complimentary push-pull output stage.
I have no idea as to the setting of R3; before powering up, set it to
zero (not 2K which very likely will "short" out a wimpy power supply).
Slowly (!) increase the post setting to adjust the standby current to
specified value

***Pre-amplifier:
I would not call the Q1/Q2 configuration a (complimentary) darlington
due to the resistive coupling.
The circuit is not unreasonable.
....and the current is not going from collector to base in the
transistor.
If the Vcb of Q2 in the preamp is near or at zero, check C3 (shorted
as well as for polarity).

Seems to me you may have a problem from the over-biasing in the
amplifier stage that i first mentioned.

 

[Ken Taylor]

Yes I was sure it would be. Note my line "without describing the
circuit".


No. The circuit doesn't have negative and positive supply voltages.

It's impossible to post the schematic to everyone or to describe it in
detail (or draw it with ANSI graphics), howerer...

You, (or anyone interested), can check the circuit from

http://www.redcircuits.com

http://www.redcircuits.com/Page65.htm

to be specicic.

Its the 60W guitar amp's preamplifier's 1st and 2nd stage. First is, I
believe it is (I'm a beginner), a darlington. Second one is almost
identical without the complementary transistor. When simulated with a
software, or calculated, it works fine. In real life... no.

After the R21 the voltage should be around 59 V because the main
supply voltage in circuit I built is about 63 V, not the 60V. This
shouldn't do any harm howerer. Without transistors all the preamps
voltages seems to be ok but when transistors are put on their places
the Q1 (BC546) shorts through its collector to base. The supply
voltage decreses to about 32 V and resistors R21 and R8 (both 330 ohm)
have to handle at leat 3 W of power. Even 5W resistors heat too much.
No sound. No signal present after R1 or R2.

I really don't know what I should look for to get it working.
Everything seems fine. The transistors should be ok and I tested them.
I'm not sure whether the author of the page really built the circuit
him/herself but the poweramp operates and the preamp seems logical so
I gather it should work, and I'm mostly sure that ..I.. have made an
error. Just don't have any clue, that what it could be. No there is no
short from collector to base,.. except when the transistors are put on
their place. All the teething problems in the circuit should be fixed
by now..

BTW. I send him email. No answer. Otherwise I wouldn't be bothering
you.

Now I'm looking for the clues of what should I be looking for to get
this circuit operating. Leak current??? From where? Why?

I repeat my words...

"if someone has experience of problems or symptons like this, (in any
circuit), I'd be thankful of some hints and suggestions of what type
of fault to look for."

PS. The poweramp is fine and really sounds good. So if anyone is
interested in building a simple, good sounding amplifier take a hint.

Did you use a printed circuit board for the construction? Are you sure you
have the right transistor in Q1's position? What manufacturer of BC546 are
they?

For information - since you're a newbie - the 'B+' is your 60V and the
'B-' is the 0V. You have to have + and - supplies, but whether they are +
and - in respect to earth is dependant on the design.

Cheers.

Ken

 

[Rich Grise]

Teemu_K wrote: ....

***Amplifier:
First, i see no darlington configuration in this section (Page65.htm).
Q1 is a simple amplifier, Q2 is to bias Q3/Q4 to class AB1 or AB2; R3
is adjusted for zero signal standby current thru Q3/Q4.
Q3/Q4 is a "standard" complimentary push-pull output stage.

Actually, I think they're darlingtons, as per the parts list; they're
just drawn like that for simplification.

I have no idea as to the setting of R3; before powering up, set it to
zero (not 2K which very likely will "short" out a wimpy power supply).
Slowly (!) increase the post setting to adjust the standby current to
specified value

And, of course, I'm sure you'll agree, that'll be twice as much for
darlingtons as for singles.
Cheers!
Rich

 

[Teemu_K]

***Amplifier:
First, i see no darlington configuration in this section (Page65.htm).
Q1 is a simple amplifier, Q2 is to bias Q3/Q4 to class AB1 or AB2; R3
is adjusted for zero signal standby current thru Q3/Q4.
Q3/Q4 is a "standard" complimentary push-pull output stage.
I have no idea as to the setting of R3; before powering up, set it to
zero (not 2K which very likely will "short" out a wimpy power supply).
Slowly (!) increase the post setting to adjust the standby current to
specified value

***Pre-amplifier:
I would not call the Q1/Q2 configuration a (complimentary) darlington
due to the resistive coupling.
The circuit is not unreasonable.
...and the current is not going from collector to base in the
transistor.
If the Vcb of Q2 in the preamp is near or at zero, check C3 (shorted
as well as for polarity).

Need to check it out. The capasitor is alright, not broken, ceramic
doesn't mind of polarity. It was my first suspect but turned out too
be ok. If the Vcb turns out to be what you said, (zero or near it),
what could cause it? How can it be fixed? There should be around 27 V
difference.

Seems to me you may have a problem from the over-biasing in the
amplifier stage that i first mentioned.

Could this over-biasing cause transistors in the pre-amp to act as if
they were shorted?

 

[Teemu_K]

I will test the pre-amp circuit with a lower supply voltage and see
what it does...

There seems to be no faults in

Need to check it out. The capasitor is alright, not broken, ceramic
doesn't mind of polarity. It was my first suspect but turned out too
be ok. If the Vcb turns out to be what you said, (zero or near it),
what could cause it? How can it be fixed? There should be around 27 V
difference.

The Vcb at Q2 is abot 0.88 V. That's the voltage drop over transistor.
It ain't right and was just what I already suspected. The cap is ok.
Why does the transistor let all the voltage pass through?

 

[scada]

Yes I was sure it would be. Note my line "without describing the
circuit".


No. The circuit doesn't have negative and positive supply voltages.

It's impossible to post the schematic to everyone or to describe it in
detail (or draw it with ANSI graphics), howerer...

You, (or anyone interested), can check the circuit from

http://www.redcircuits.com

http://www.redcircuits.com/Page65.htm

to be specicic.

Its the 60W guitar amp's preamplifier's 1st and 2nd stage. First is, I
believe it is (I'm a beginner), a darlington. Second one is almost
identical without the complementary transistor. When simulated with a
software, or calculated, it works fine. In real life... no.

After the R21 the voltage should be around 59 V because the main
supply voltage in circuit I built is about 63 V, not the 60V. This
shouldn't do any harm howerer. Without transistors all the preamps
voltages seems to be ok but when transistors are put on their places
the Q1 (BC546) shorts through its collector to base. The supply
voltage decreses to about 32 V and resistors R21 and R8 (both 330 ohm)
have to handle at leat 3 W of power. Even 5W resistors heat too much.
No sound. No signal present after R1 or R2.

I really don't know what I should look for to get it working.
Everything seems fine. The transistors should be ok and I tested them.
I'm not sure whether the author of the page really built the circuit
him/herself but the poweramp operates and the preamp seems logical so
I gather it should work, and I'm mostly sure that ..I.. have made an
error. Just don't have any clue, that what it could be. No there is no
short from collector to base,.. except when the transistors are put on
their place. All the teething problems in the circuit should be fixed
by now..

BTW. I send him email. No answer. Otherwise I wouldn't be bothering
you.

Now I'm looking for the clues of what should I be looking for to get
this circuit operating. Leak current??? From where? Why?

I repeat my words...

"if someone has experience of problems or symptons like this, (in any
circuit), I'd be thankful of some hints and suggestions of what type
of fault to look for."

PS. The poweramp is fine and really sounds good. So if anyone is
interested in building a simple, good sounding amplifier take a hint.

I believe the setup description should read adjust "R3" and not "R9" to give
1/2 the supply voltage at the +C7. This is the zero crossing point of the
Push-Pull output stage, the waveform needs to "Ride" on top of a zero volt
reference point, that way the output sits at zero with no signal, this
prevents distortion in the output wave.

 

[scada]

I believe the setup description should read adjust "R3" and not "R9" to give
1/2 the supply voltage at the +C7. This is the zero crossing point of the
Push-Pull output stage, the waveform needs to "Ride" on top of a zero volt
reference point, that way the output sits at zero with no signal, this
prevents distortion in the output wave.

To explain further:
By setting the level at +C7 to half the supply voltage, the output can swing
+ & - from the mid-way supply voltage (30V level: + to 60V, - to 0V). If for
example the +C7 point were set at 50V, then a positive signal would clip at
just above a10V peak creating distortion! By setting the +C7 level at
mid-supply voltage the waveform can swing equaly + & -. This is the output
waveform "0V" reference level. Also Q1 and Q2 are not configured as a
darlington!Q1 is an amplifier, Q2 is used to bias Q3 & Q4 for the zero
crossing set point.

 

[Teemu_K]

To explain further:
By setting the level at +C7 to half the supply voltage, the output can swing
+ & - from the mid-way supply voltage (30V level: + to 60V, - to 0V). If for
example the +C7 point were set at 50V, then a positive signal would clip at
just above a10V peak creating distortion! By setting the +C7 level at
mid-supply voltage the waveform can swing equaly + & -. This is the output
waveform "0V" reference level. Also Q1 and Q2 are not configured as a
darlington!Q1 is an amplifier, Q2 is used to bias Q3 & Q4 for the zero
crossing set point.

I gather that you're talking about poweramp now. It works perfectly
fine when set on the mid. It could clip but to my ears it's not
audible. I don't own an oscilloscope.

My problem is however in the preamp. Mistakenly the parts in the both
of the circuits, pre- and poweramp, have been named the same way. I'm
a beginner and don't exactly know whether the preamp's first stage
Q1&Q2 is a darlington or whether it is not. It looks like one to me
except that there's an negative feedback capasitor C3.

The author and designer of the circuit sent post to me yesterday and
suspected transistor problem. Somehow it just seems hard to believe
since the transistors I used were new. Just bought and even tested
when not found working alright.

I made some new tests and found out that they only seem to operate
normally in quite low voltages. At 15 V the signal was passed through
transistors but it wasn't amplified. Rather lowered. The transistors
should take 65 V so it is obvious that they're not working properly.

Now the question is: is this caused by something in the circuit or are
the transistors just simply broken. What are the odds that six new
transistors are broken when bought? (See I had substitutes to switch
with if the ones first fitted wouldn't operate.)

I'll buy some new transistors soon and try with some new models too.
If they don't work then it is sure the problem is in the circuit,
however I already think it is in it. It's obvious that I have made a
fault since the author got it work and it's clear to see it should
work. Just don't know what to look for...

 

[Robert Baer]

I gather that you're talking about poweramp now. It works perfectly
fine when set on the mid. It could clip but to my ears it's not
audible. I don't own an oscilloscope.

My problem is however in the preamp. Mistakenly the parts in the both
of the circuits, pre- and poweramp, have been named the same way. I'm
a beginner and don't exactly know whether the preamp's first stage
Q1&Q2 is a darlington or whether it is not. It looks like one to me
except that there's an negative feedback capasitor C3.

The author and designer of the circuit sent post to me yesterday and
suspected transistor problem. Somehow it just seems hard to believe
since the transistors I used were new. Just bought and even tested
when not found working alright.

I made some new tests and found out that they only seem to operate
normally in quite low voltages. At 15 V the signal was passed through
transistors but it wasn't amplified. Rather lowered. The transistors
should take 65 V so it is obvious that they're not working properly.

Now the question is: is this caused by something in the circuit or are
the transistors just simply broken. What are the odds that six new
transistors are broken when bought? (See I had substitutes to switch
with if the ones first fitted wouldn't operate.)

I'll buy some new transistors soon and try with some new models too.
If they don't work then it is sure the problem is in the circuit,
however I already think it is in it. It's obvious that I have made a
fault since the author got it work and it's clear to see it should
work. Just don't know what to look for...

Transistors do not "break".
I strongly suspect that there is absolutely wrong with the transistors
that you bought.
Double check that the specified part types are placed in the circuit
as indicated by the schematic.
Then also double check all other component values by marking and/or
color code; also measure resistors (power *off*) and calculate what one
should see if alternate paths exist.