Crossfire "Distinct Audio" TD-1002 problems

[Dr Zeus]

This is a pretty big amp for not having a lot of stuff inside. I get power at the FETs, +/-35vDC power at the rails, and all the major transistors check out. The amp produces no sound. Theres .25vDC on the speaker terminal outputs. Amp turns on, green LED

R93 in the center (Right blue resistor) gets hotter than you can touch, even after I replaced D103 just above it (1N4744). R93/R94 are connected the bottom FOUR large black diodes to the right of center, and all four of these diodes are connected in a pattern to the amp's two rectifior transistors.

R93/R94 still measure 470k.

Faults fixed:
1. The old diode in D103 was shorted, and the new diode is still measuring OK after replacement and testing the amp. I replaced it with a 1N4743 because I dont have any 1N4744.
2. I also found one trace on the back of the PCB broken like a popped fuse so I fixed that - it connects to RCA shields and R126 just left of center.
3. I also found at the back of the RCAs, center was shorted to shield probably from someone playing to hard on the RCA inputs. Easy fix with a little pressing.

Ive not fixed an amp in the rectifier section. What else can I look at?

[Dr Zeus]

Sorry thins one got upside down from the picture above.



Edit - I was measuring C51/C61 wrong. They are fine (at least while measured on the board)

[Perry Babin]

What's the DC voltage across each of the large resistors?

What's the DC voltage across D102? (can't really see it) and D103?

[Dr Zeus]

R93 has 30.66vDC across it's wires
R94 has 20.50vDC across it's wires

D102 has 15vDC across it's wires
D103 has 5vDC across it's wires

R93 is directly connected to D103's anode.
R94 is directly connected to D102's cathode.

The two caps in between the Diodes check out 'ok' while on the board.

This picture shows underneith. The screw driver is between R93 and D103's anode.

[Dr Zeus]

To take out some of the equation, I released J73, J75, and J76 from interfering with this problem section. R93 still gets hot.

[Perry Babin]

Are you sure the 1N4743 isn't defective?

If it's not, the capacitor connected between it and ground may be defective.

With no power applied, what's the resistance across the pads for the Zener (preferably with D103 out of the circuit)?

When working with this type of regulator, it's important that you NOT power up the amp with the Zeners out of the circuit.

[ian33]

i had a problem almost like that but it was a leaky diode one out of the four that is close together

[Dr Zeus]

I was wondering about those 4 diodes myself. Seems I have plenty of spares to try replacing them. They -do- check out OK on the DVM however.

Is this amp any good? Seems to be 200x2 RMS Class AB made in Korea. Nothing on ampguts. By comparison, I have an MTX amp thats 40x4 which uses 4 of the same power fets as this one. This amp uses 6 power fets. I dont think its truly 200x2 RMS, and on the front cover is says 1000w :/

[Dr Zeus]

With both D102 and D103 off the board I measured resistance across pads:

D102 pads: 9m Ohms in one direction, and -3.2m ohms (Rises slowly like a charging cap) measured in the other
D103 pads: 2.7m Ohms in one direction and -1.7m Ohms in the other dirction. Resistance barely changes

I pulled the hot R93's corosponding electrolitic cap and it measures OK on the bench. In circuit it seems to measure at 3-10m ohms.

[Perry Babin]

Something's limiting the voltage across D103 to 5v. This is generally due to a leaking zener or a leaking (electrically) capacitor.

Are you sure the 1N4743 isn't defective?