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
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
).
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
Good luck,
Chris
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
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
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?
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?
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.
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.
Some more probing information.
When the amp is hooked to a power supply with remote at ground.
All pins on output and power transistors read 6.97V. The chassis also reads 6.97V.
When the remote is connected to +12 volts. All pins on output and power transistors slowly charge to the battery voltage.
Also, my working amp draws initial current (electric spark) when reconnected. But the bad amp does not spark like the good amp. The good amp's chassis sits at ground. And its output transistors when the amp is on with no signal have around +30 and -30 volts for the power rails.
I will be removing the output transistors tonight to begin testing them.
Jerad
When the amp is hooked to a power supply with remote at ground.
All pins on output and power transistors read 6.97V. The chassis also reads 6.97V.
When the remote is connected to +12 volts. All pins on output and power transistors slowly charge to the battery voltage.
Also, my working amp draws initial current (electric spark) when reconnected. But the bad amp does not spark like the good amp. The good amp's chassis sits at ground. And its output transistors when the amp is on with no signal have around +30 and -30 volts for the power rails.
I will be removing the output transistors tonight to begin testing them.
Jerad
Check for shorts to the chassis, things like melted mica on the input fets and bipolars would be a good place to start.
Also if the psu is ok and the bipolars still read the battery voltage check the transformer, The secondary should be isolated from the primary winding. The enamel could have been melted of or scratched at some point and it conducting.
THe first think I'd do is to measure teh resistance from the source to drain of the mosfets in circuit but detached from the heatsink (not powered up). you shouldn't get a reading (or an extremly high one >40Mohms), If you're getting a short a FET's been punched through. From then on just isolate the problem down to the culpurits.
Also if the psu is ok and the bipolars still read the battery voltage check the transformer, The secondary should be isolated from the primary winding. The enamel could have been melted of or scratched at some point and it conducting.
THe first think I'd do is to measure teh resistance from the source to drain of the mosfets in circuit but detached from the heatsink (not powered up). you shouldn't get a reading (or an extremly high one >40Mohms), If you're getting a short a FET's been punched through. From then on just isolate the problem down to the culpurits.
You can power an amplifier with as little as a single MOSFET in each bank for test purposes. However, don't try to run it at anything near full power or these transistors will probably fail. The lowest Rds-on models like IRFZ48N, HUF75344 and better may survive alone for some time if they are well heatsinked, though.
Some amplifiers also allow to be tested with the hearsink completely removed as long as no speaker load is connected, but do this with care because some models *do* require a heatsink even without load because some devices always get hot. PSU MOSFETS and diodes should *never* get hot until the amplifier is putting a reasonable amount of power to the load, though, otherwise there is something wrong with the PSU.
Some amplifiers also allow to be tested with the hearsink completely removed as long as no speaker load is connected, but do this with care because some models *do* require a heatsink even without load because some devices always get hot. PSU MOSFETS and diodes should *never* get hot until the amplifier is putting a reasonable amount of power to the load, though, otherwise there is something wrong with the PSU.
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