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Old 11th June 2003, 05:30 PM   #1
jesimon is offline jesimon  United States
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Default car amplifier repair question....

I am just getting into amplifier repair. I have just recently built the leach amp with great success.

I have a car audio amplifier that will not turn on. The chassis as well as all the collector and the emitter on the output transistor are sitting at + 12 Volts. A working amplifier of the same type has + 30 Volts on the output transistors' collector and the chassis (heatsink) is at ground.

Does anyone have an idea to what the problem is? I can go into more detail.

Jerad
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Old 11th June 2003, 06:57 PM   #2
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What brand and model is it? What PSU controller chip does it use? Probably the TL494.......... You have to check a few voltages around controller chip to see why it's not working.

Post a few more details, and maybe we can figure it out,

Lukas
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Old 11th June 2003, 08:22 PM   #3
jesimon is offline jesimon  United States
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It is a hifonics centurion. I will look at it and post more details when I get home from work.

Thanks.
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Old 11th June 2003, 08:40 PM   #4
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its still here? not moved to car audio? its been here 3 hours. the mods are slacking (no offense meant, dont ban me ).
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Old 11th June 2003, 08:55 PM   #5
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Quote:
Originally posted by Matttcattt
its still here? not moved to car audio? its been here 3 hours. the mods are slacking (no offense meant, dont ban me ).

Not slacking, just having a very nice meal

But, now you have my attention, I suppose my "god like" powers could use some exercise
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Old 12th June 2003, 12:40 AM   #6
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99% of car amplifier problems
are the power transistors mounted
to the heatsinks whether output
stage or power supply. Remove all
those power transistors and use
a DMM measure each transistor "out-of-circuit", find the bad ones.
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Old 12th June 2003, 04:14 AM   #7
Diode is offline Diode  United States
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Unless you have a schematic, you'd be best be advised to take out the suspect transistors and then check them on diode check AND ohms check. Check the switching fets, probably IRFZ series, TO-220 packs. If there is a short, the PWM chip will not turn on into a fault situation. It goes into, what we call, burp mode. It will burp out a few pulses and see if all is OK but if not, it shuts down and won't start. It is a protect mode that switchers use.

Good luck,

Chris
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Old 12th June 2003, 05:07 AM   #8
jesimon is offline jesimon  United States
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Some more information...

The power supply controller is a korean product KIA494P. I found the datasheet after looking for about 2 hours. A replacement part after comparing onsemi's products would be TL494-D if needed in the future.

I didn't get time to hook up the amplifier to a power supply. But I did completely remove the board from the sink. I can see that the board actually connects ground to the sink, but I know I measured 12 Volts on the sink. How would that be possible without blowing the fuse?

Quick question: I must remove each transistor from the board to insure an operating device? I see that I should do this, but am nervous about doing this (although I am very good at desoldering).

Thanks guys for your help. I appreciate it. We go over circuits in class, but never how to find problems in them. Again, I have always wanted to fix an amplifier and hope that I can repair this amp. Thanks again.

Jerad
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Old 12th June 2003, 06:06 AM   #9
jesimon is offline jesimon  United States
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Removed the Power Mosfets from the board. They are IRFZ44 power mosfets made by Fairchild. Here are the DMM measures for the 6 devices.
1. Vsd = 0.505V
2. Vsd = 0.501V
3. Vsd = 0.502V
4. Vsd = 0.507V
5. Vsd = 0.512V
6. Vsd = 0.510V

1. Rsd = 2.7 M ohms (IR806H)
2. Rsd = 2.69 M ohms (IR806H)
3. Rsd = 2.69 M ohms (IR806H)
4. Rsd = 2.7 M ohms (IR806H)
5. Rsd = 2.7 M ohms (IR806H)
6. Rsd = 1.4 M ohms (IR803C)

Gate to Source & Gate to Drain were all infinite R for all devices.

Does these seem like good values?
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Old 12th June 2003, 07:42 PM   #10
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Which amplifier is this?

If those values were "short" then = bad

It's a good chance they are good, but it's also
possible to have an "open" circuit. But I never
found open circuits during car amp repair, they
always shorted when bad.

Go to step two.

Remove output stage power transistors
and test them.

Later........
When you re-install the transistors and mount them
on the heatsink, typically, you need to isolate the
transistor tab from the metal heatsink. After
mounting them, do a last minute ohms check
between tab and heatsink to make sure there
is no contact. There are a few exceptions to this,
some companies many not have insulators due
to some esoteric heatsink design, I think RF
might do this on some amplifiers.
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