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Powerbass asa  700.5
Powerbass asa  700.5
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Old 6th June 2019, 08:03 PM   #1
imraneesa is offline imraneesa
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Default Powerbass asa 700.5

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.
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Old 6th June 2019, 08:09 PM   #2
Perry Babin is offline Perry Babin  United States
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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?
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Old 6th June 2019, 08:17 PM   #3
imraneesa is offline imraneesa
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Join Date: Apr 2016
Quote:
Originally Posted by Perry Babin View Post
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.
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Old 6th June 2019, 09:13 PM   #4
Perry Babin is offline Perry Babin  United States
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How much it 'too much' current?
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Old 6th June 2019, 09:18 PM   #5
imraneesa is offline imraneesa
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Quote:
Originally Posted by Perry Babin View Post
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.
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Old 6th June 2019, 09:22 PM   #6
Perry Babin is offline Perry Babin  United States
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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.
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Old 6th June 2019, 10:00 PM   #7
imraneesa is offline imraneesa
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Join Date: Apr 2016
Quote:
Originally Posted by Perry Babin View Post
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.
Attached Images
File Type: jpg 20190606_175827.jpg (967.6 KB, 37 views)
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Old 6th June 2019, 10:25 PM   #8
Perry Babin is offline Perry Babin  United States
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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.
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Old 7th June 2019, 01:22 AM   #9
imraneesa is offline imraneesa
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Join Date: Apr 2016
Quote:
Originally Posted by Perry Babin View Post
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?
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Old 7th June 2019, 03:07 AM   #10
Perry Babin is offline Perry Babin  United States
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Join Date: Nov 2003
Location: Louisiana
The possibilities are virtually endless. Do you have the BD140 on hand?
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