ta dave

Dave , that resistor I burned out , you said it looks like 100 Ohm .. would that be 0.5 w

 

1/4W will do .

 

1/4W will do .

thanks dude

 

Take well focused photos of both sides of the board so that we can reverse engineer the circuit.
Take the photos as one board and also as two sections (six photos in total) so that we can get good close ups. We want to be able to read the colours on the resistor.

Also, write down the part numbers of the black transistors and their locations.

 

Take well focused photos of both sides of the board so that we can reverse engineer the circuit.
Take the photos as one board and also as two sections (six photos in total) so that we can get good close ups. We want to be able to read the colours on the resistor.

Also, write down the part numbers of the black transistors and their locations.

 

Ok thanks. I'll get on it.

 

Have those two output transistors been checked yet" At this point it could be either an open transistor or a shorted transistor.
Do we know if the amplifier is direct coupled or is it capacitor coupled?

 

Have those two output transistors been checked yet" At this point it could be either an open transistor or a shorted transistor.
Do we know if the amplifier is direct coupled or is it capacitor coupled?

Yes , I got these readings earlier using common ' zero point' as seen on page 1 .
T1 - . 103 .002
T2- .301 .201

measuring across the transistors once was 0.72 the other 0.02 .

Im sorry but I dont know if its direct coupled or not ? I dont know how to tell . learning basics here

 

Have those two output transistors been checked yet" At this point it could be either an open transistor or a shorted transistor.
Do we know if the amplifier is direct coupled or is it capacitor coupled?

He's already checked the 2n3055 and one is shorted out.
I would say that it's DC coupled looking at the circuit board .

 

He's already checked the 2n3055 and one is shorted out.
I would say that it's DC coupled looking at the circuit board .

Not much reason to use a bipolar supply if you are not direct coupling the output.

 

Not much reason to use a bipolar supply if you are not direct coupling the output.

According to some folks, using a bipolar power supply for the output reduces the distortion from the coupling capacitor. That is not my assertion, it comes from other folks.
My reason is that a split supply is just a better choice. The detailed explanation is rather tedious.
And the big benefit of a direct coupled amplifier is that it makes servicing more difficult and it allows any transistor problem to take out the power transistors and burn out the speaker.

 

This is designed and sold as a bass guitar amplifier. The output to the loudspeaker is DC coupled.
The preamp and tone controls are all dual supply LM741CP (don't tell this to Audioguru again).
The power output stage is a standard quasi-complementary push-pull output design. I have been too busy to come up with a circuit. We want to know why that resistor blew. Could be one of the driver transistors is gone.

The circuit is similar to this. I should have the circuit done today.

 

This is designed and sold as a bass guitar amplifier. The output to the loudspeaker is DC coupled.
The preamp and tone controls are all dual supply LM741CP (don't tell this to Audioguru again).
The power output stage is a standard quasi-complementary push-pull output design. I have been too busy to come up with a circuit. We want to know why that resistor blew. Could be one of the driver transistors is gone.

The circuit is similar to this. I should have the circuit done today.
View attachment 304939

geez .. you guys are going deep .... way over my head .. all sounds great though .

 

This is circuit I have drawn. No guarantee on accuracy. Some component values are my guesses. The blown resistor is R19.

 

geez .. you guys are going deep .... way over my head .. all sounds great though .

This is circuit I have drawn. No guarantee on accuracy. Some component values are my guesses. The blown resistor is R19.

View attachment 304980

wow Mr chips . that 's fantastic . I wish i knew how you did that . another world to me that Im just getting a peak at . so ... what do we do with it ? whats next . Im just waiting for goodies to arrive in the post.

 

wow Mr chips . that 's fantastic . I wish i knew how you did that . another world to me that Im just getting a peak at . so ... what do we do with it ? whats next . Im just waiting for goodies to arrive in the post.

so R19 is the blown one and R12 is the other one . I'm looking for my blown resistor ..where would that be ?

 

The blown resistor is the blown resistor that you showed us that was blown. (See post #46.)





We need to know under what circumstances the resistor blew.
Was the resistor already blown before you got started?
Was the resistor already blown before you removed the two large power transistors, 2N3055?
Did the resistor blow after you removed the two transistors?
Did the resistor blow after you reinstalled the two transistor?

The correct answer will help us diagnose what caused the resistor to blow.

 

Here are DC operating voltages from simulation, assuming that the supply voltages are +40V and -40V.
I meant to choose 2N3906 for Q5. It will be changed in next update.

 

The blown resistor is the blown resistor that you showed us that was blown. (See post #46.)

View attachment 304991

View attachment 304990

We need to know under what circumstances the resistor blew.
Was the resistor already blown before you got started?
Was the resistor already blown before you removed the two large power transistors, 2N3055?
Did the resistor blow after you removed the two transistors?
Did the resistor blow after you reinstalled the two transistor?

The correct answer will help us diagnose what caused the resistor to blow.

nope. that resistor blew after I removed both 2N23055 transistors.

I was just wondering where that blown resistor is on the schematic you made . just learning

 

nope. that resistor blew after I removed both 2N23055 transistors.

I was just wondering where that blown resistor is on the schematic you made . just learning

I did say R19 in post #74.

So your are just learning. Here are some trouble shooting tips.

Have measured the DC on speaker , just for a second ..saw 19 volts .

You did not state the sign, +19V or -19V.
Where was the RED meter probe connected?
Where was the BLACK meter probe connected?

Details such as these can make or break the diagnostics.

From what I surmise, the original fault was a bad transisor Q5, BFR79.
Whatever replacement you use, make sure that you check the transistor lead identification and that the transistor is installed correctly. The PCB hole orientation might not match the transistor.