Cadence Ultrashock A7+HC

[Dr Zeus]

Got this amp in for repair; its been repaired before by another tech back ~4 years ago. I see what happened; either the insulator strip behind the fets caused them to short, or the wrong fets were used to previously repair this amplifier. I also found an IRF9640 in the output section with 1 of three legs detached.

Question: The previous rebuild used IRF3205 in the power supply and 22 ohm gate resistors. What should I use to replace? I have more IRF3205, some IRFZ44, and very few FQP50n06 available, with several resistor values to choose.

I saw a Cadence A5+HC on the DVD using IRFZ44 and 47 ohm gate resistors.

[Perry Babin]

I don't know what the original power supply transistors were in the A7 but I think it would be safe to use the 3205s.

[Dr Zeus]

22 ohm resistors OK? I have to replace all of them. If 47 ohm or something else would be a better option...

[Perry Babin]

The 22 ohm resistors are OK.

[Dr Zeus]

OK. I put one IRF3205 in the amp and it powered up, but after 5 seconds the new IRF3205 got very hot. Not sure why at this point so I'll keep on troubleshooting.

[Perry Babin]

At the very least, you'd need one per bank.

[Dr Zeus]

I took out the 4 rectifiers and the now (two - one per bank) IRF3205 power supply fets still get extremely hot after just a few seconds. I removed the power supply fets and checked the waveform of the fet's first leg; a clean +10v square wave. With a few fets installed, the waveform is not square, but slanted jagged starting from +10v and jagged down to about +3v on the scope.

Im thinking there might be a bad driver in the ps section of this amp unless removing the Rects did not disconnect the PS from the output section.

[Dr Zeus]

I should probably already mention; but this amp probably was not in the greatest of previous repair condition for re-build this time around. None-the-less, I love working on these things no matter what as I dont <yet> do this for a living.

From the looks of it, this amp had these parts rebuilt from the past:

All PS fets,
All Output fets for both channels
Output pre/driver section from Q202 through Q207, and several resistors in between
Output pre/driver section from Q402 through Q407, and several resistors in between

Currently, R211, R206, R406, and R411 look burnt/overheated but are still measuring 3.3k.

In the PS section, previously Q15 and Q21 were replaced with A1266 transistors

R25 was replaced
Voltage regs U51 and U52 were removed/replaced.

The two chokes/coils in the center of the amp; underneith the PCB it looks like one of the coils overheated and darkened the board. Both coils are now covered in black glue (Epoxy?) In between these coils theres a 1/4 inch hole in the PCB where I can only guess a mounting screw used to live.

Alas, I recived this amp which only appeared to have a failed PS. The output section seems to be relatively in tact.

If it werent for all the privious repairs, I might not be so temped to bin this amplifier...

[Dr Zeus]

I appologize to myself for my last comment. I really actually would like to learn/fix and work with this amplifier even though it is in pretty rough shape.

I removed all four main rects from the amp. Should this have divided the PS from the audio sections, and enabled me to power on the amp with isolation from the audio side?

With the rects removed the power supply fets still get instantly hot.

What transistors are for driving the power fets? Q15 and Q21?

[Perry Babin]

I really don't have any information on this amp so I don't know which transistors are the drivers. Follow the circuit back from the gate resistors.

With the power supply FETs out of the circuit, do you have a clean gate drive signal at the gate pads?