Powerbass asa 700.5

[imraneesa]

Hi all,

the above mentioned amplifier came with a complaint that it is getting hot and is not playing loud.

I opened the amplifier checked the fets and everything is alright. when i checked the gate frequency was 300khz which was too much at idle. i changed the TL494 and the frequency came back to 31khz and the fets were still getting hot. i checked two pnp driver transistors and were good. i removed main rectifiers and was still getting very hot. i took gate resistors which were 100ohm and replaced them with 47ohms and without rectifiers amp came on and no idle current. as soon as i replaced rectifiers amp pulling too much current. all the outputs are in good condition. power fets used are irfz44n.

 

[Perry Babin]

Do you have the outputs for one channel getting hotter than the others at idle?

There are two power supplies. Dis you try installing the rectifiers for only one supply at a time?

 

[imraneesa]

Do you have the outputs for one channel getting hotter than the others at idle?

There are two power supplies. Dis you try installing the rectifiers for only one supply at a time?

Yes the problem is with one power supply. The supply for channel 5 is good. The supply for channel 1,2,3 and 4 is giving trouble. And none outputs are getting hot.

 

[Perry Babin]

How much it 'too much' current?

 

[imraneesa]

How much it 'too much' current?

0.8 Amps when using current limit. And I have only one fet on either side of transformer primary coil. And also no rectifiers on board for that power supply. And one of the two fets gets very hot cant even touch with in few seconds.

 

[Perry Babin]

That doesn't really indicate that there is a problem.

What does the drive signal look like on the two FETs you have installed?

Why only 2? Install all of th ones for that supply, unless you think the amp will be unrepairable. Even it if is, you can pull the FETs.

 

[imraneesa]

That doesn't really indicate that there is a problem.

What does the drive signal look like on the two FETs you have installed?

Why only 2? Install all of th ones for that supply, unless you think the amp will be unrepairable. Even it if is, you can pull the FETs.

Replaced all fets. Attaching the gate waveform of One fet that getting hot.

 

[Perry Babin]

Between the quality of the image (focus is on the wicker basket, not the scope) and the extra lines on the scope, I can't tell much. It looks like the signal is not falling quick enough. This is typically due to a bad PNP driver or pulldown resistor.

 

[imraneesa]

Between the quality of the image (focus is on the wicker basket, not the scope) and the extra lines on the scope, I can't tell much. It looks like the signal is not falling quick enough. This is typically due to a bad PNP driver or pulldown resistor.

I will change the pnp transistors and also will attach good picture. It has A1023 transistor, may I know any good alternative that I can use?

 

[Perry Babin]

The possibilities are virtually endless. Do you have the BD140 on hand?

 

[imraneesa]

The possibilities are virtually endless. Do you have the BD140 on hand?

Attaching new picture of gate waveform. Yes I have BD140.

 

[imraneesa]

this half side fets of this power supply which is getting hot is connected to speakers ground with 4.7kohms resistor and 104 capacitor.

 

[Perry Babin]

You're going to have to take the time to write more clearly to better explain what you mean.

 

[imraneesa]

You're going to have to take the time to write more clearly to better explain what you mean.

Ok the power supply transformer primary is connected to two sets of fets. One fet's emitter out of one set is connected to speaker ground with 4.7kohms and 104 capacitor. Does that cause the fets get hot

 

[Perry Babin]

Are you saying that the primary and secondary grounds are connected with a parallel resistor and capacitor?

 

[imraneesa]

Are you saying that the primary and secondary grounds are connected with a parallel resistor and capacitor?

Yes exactly.

 

[Perry Babin]

That's common.

 

[imraneesa]

So I am just chasing a no problem then. And why the fets getting hot on one power supply where as the same drivers are driving both power supply fets. That means my drivers are good.

 

[Perry Babin]

Did you replace the drivers in the hot supply with the BD140?

 

[imraneesa]

Did you replace the drivers in the hot supply with the BD140?

Yes I replaced them. No change.