Hifonics BX1500D no TL494 voltage

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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?
 
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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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.
 
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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
 
With the same voltage drop and value, the operating temperature should be the same. It could be that your meter isn't reading higher frequency voltage. If you have a scope, look at the waveforms on the primary windings on both transformers to see if the ringing is excessive on the transformer with the resistor that's running hotter.
 
The signal is very messy and noisy. Cant make anything out of it. Cant even really capture the entire thing whats happening there. Whats interesting, I traced it back to TL494 pin 9. Both 494 output pins have the same wierd signal. Its not a normal squarewave. Theres like 2 different traces that are mixed with eachother. The amp works normal tho. 56VDC rail voltage and a clean output. Also measured the frequency on the hot resistor. The hot one has 76kHz over it and the cold one has 38kHz. A harmonic? (38x2=76 as the 2nd harmonic). I have changed the 494 when i was looking for that missing Vcc. I also changed the 494 output driver transistors(one was open) previously.

Also when increasing the capacitor (C31P1) value (adding one parallel to it) makes the resistor to smoke. But at the moment it seems like the 494 output is not OK either? Cant take a decent picture of the scope, because i cant get settings to have a readable image on the whole screen of the scope (too wierd signal).
 
Tried measuring some simple signals from my signalgenerator.
THE OSILLOSCOPE IS BROKEN! Very blurry images.

Took an oldschool one. Now with some pictures. The sqarewave is ok. Not like i stated before.

20V/div 2microsec/div (AC only)
GND - cold resistor (probe tip in between the resistor and the capacitor)
http://www.upload.ee/image/4257355/20140912_110501.jpg
GND - hot resistor (probe tip in between the resistor and the capacitor)
http://www.upload.ee/image/4257364/20140912_110630.jpg

They are identical BUT
20V/div 2microsec/div (AC only)
Ground probe on one resistor end and the other probe on other end (measuring the cold one)
http://www.upload.ee/image/4257363/20140912_110553.jpg
nothing.

but the same thing on the hot one
http://www.upload.ee/image/4257365/20140912_110608.jpg

Almost 60V spikes
 
When capacitors swapped, both resistors get hot. The one that was about 60C before is now about 40C. The cold one is just very warm. Now both have spikes.

Swapped back and tried some other capacitors and seems that a lower value reduces the spikes. Original was 22nF, replaced with a 10nF one. Spike is about 16V peak to peak. Resistor is cold.

Can i leave it like that?
 
The spikes you see represent energy/power being dissipated. This would otherwise be seen as ringing on the rising edge of the drain waveforms. Lower spikes mean that less energy is being removed by the snubber (not necessarily a good thing). The resistors are large because they're expected to be dissipating a significant amount of power. 60C is not hot for a resistor. I'd replace the capacitors with the same value as the originals. Mylar/polyester are generally preferred but not required.

If this is your amp, you can leave it as it is. If it's for a paying customer, restore it to its original state.
 
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