Hifonics BX1500D no TL494 voltage - diyAudio
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Old 31st August 2014, 09:12 AM   #1
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Default Hifonics BX1500D no TL494 voltage

Hi!

Got this amp with power fets blown and output fets blown. Also a small capacitor near the coil (C31P1) was burnt. Removed all the fets and replaced the capacitor.

All the fets removed, the VCC on TL494 is 0VDC.
Removed the 494. On the pad (pin12) still 0VDC. The remote terminal has 13,4VDC and right after the first resistor (R54P) its about 6,2VDC.

The PCB tracks are somewhat covered with glue, so im not sure if the voltage should go straight to 494 or are there some transistors (C3198/A1266) envolved.

Should there even be a normal VCC on the 494 pad when all the fets are removed and the 494 chip itself too?
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Old 31st August 2014, 02:59 PM   #2
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With the black probe on the emitter of each transistor (Q51P and Q52P), post the DC voltage with the red probe on the collector and base.
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Old 31st August 2014, 03:58 PM   #3
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Q51P :
black probe on emitter - red on collector = 3mV
black probe on emitter - red on base = 0,7V
Q52P :
black probe on emitter - red on collector = 50mV
black probe on emitter - red on base = 0V
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Old 31st August 2014, 04:07 PM   #4
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It appears that the main fuse is missing or blown or there is some other break in the B+ line. If not, post the voltage on all 3 terminals of Q52P with the black probe on the amp's ground terminal.
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Old 31st August 2014, 04:23 PM   #5
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I have 13,2 VDC on B+ and on remote. The main fuse (2x60A) is OK. The 13,2V reaches also the center pad of the power fets (No fets are in).
The voltages are accordingly ECB 3,6mV/-0,51mV/3,6mV
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Old 31st August 2014, 05:12 PM   #6
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The emitter of Q52P should have 12v. Find the break between the B+ input and the emitter of that transistor.
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Old 31st August 2014, 06:06 PM   #7
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Cant find any breaks. Dont have the schematic either.
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Old 31st August 2014, 06:28 PM   #8
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A schematic won't help. If your amp is the same as the one in the photo below, you should read 0 ohms between the emitter and the B+ terminal of the amp. Follow the traces. If the trace goes under another component, confirm that you have continuity under the component if not, pull the component to see if the trace is supposed to be connected or if it's two different traces.
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File Type: jpg DSCN4270x.jpg (226.1 KB, 64 views)
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Old 31st August 2014, 06:38 PM   #9
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Yes, its the same. But there is no obvious trace from the emitter to the B+. Already pulled those big caps to see if the trace goes somewhere there. Will look again.

FOUND IT! It went to the large resistor R55P and from there it went to the 494 IC pads where it went trough a 494 pad on the upper side of the PCB again and near the C61C to B+. The trace was burnt under the glue that holds the C61C cap.

Last edited by mekaiver; 31st August 2014 at 06:51 PM.
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Old 10th September 2014, 10:01 AM   #10
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Got the parts and replaced the power fets and the output fets. Also the HIP. All seems ok, draws about 1,7 amps without any load. But what i noticed is that one large resistor (R37P1) gets really hot. At the same time on the other coil the same resistor is cold. I mentioned before that when i got it, the capacitor (C31P1) was blown. Its connected to the resistor. I replaced it with the same value (223) but before there was this blue plastic square. I repleced the cap with a small ceramic one. Can that be an issue or the cause why the resistor gets hot? The voltage drop on the resistors is same - about 0,02VDC
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