Sundown SAZ-1500 excessive current draw

[brandes.cm]

Originally the amp had failed supply and output fets. Supply is operational, output fets replaced, gate resistors verified. On power up amp with draw excessive current and it climbs with B+ voltage increase.

I can get the amp to stay out of protect by lifting D13 and using a current limit resistor. The outputs are IRF640N/IRF9640 pairs, essentially the AQ2200 schematic, same driver board with 4 transistors to drive the fets. The gate signal is roughly 40Khz on either rail. I only see a 5V square wave though, that seems to be my issue.

Thoughts,

An externally hosted image should be here but it was not working when we last tested it.

IRF640N gate signal, gnd is two divisions above center.

An externally hosted image should be here but it was not working when we last tested it.

 

[timwebb]

ive have those output drive transistors to fail very easy. ive have plenty that tested good but were defective.

 

[brandes.cm]

Note: 1)These amps do not like bringing the supply voltage up slowly, gets caught in some unknown region and causes excessive current.
2) The nomenclature on the board is wrong for the rca inputs, what says "out" is in.
3) Output section resembles AQ2200 and a host of Hifonics etc boards that use DWM1216 card.

 

[timwebb]

are the output fets in, in the second picture?

 

[brandes.cm]

Lifting D13 allows the amp to function, producing clean audio output. I need to hunt down the source of this protection issue.

 

[brandes.cm]

Anyone recognize this board, maybe have a schematic for another amp that is close.

An externally hosted image should be here but it was not working when we last tested it.

 

[Perry Babin]

How much difference is there between the AQ2200 that you mentioned and this amp?

 

[blazer40]

This reminds me of a hifonics hercules, but I think that the hercules has a diff driverboard and more outputs, but the rest looks the same. Are those irfp064's in the ps section? Does it run a ka7500 on the ps driverboard?

 

[blazer40]

I had a sundown 3000d in here last week with protect issues, turned out to be the npn transistor hooked to pin 8 on the driverboard (but it's a bigger driverboard than your 1500).

 

[brandes.cm]

The AQ2200 uses a power supply driver board. I can't find why the protect trips right on power up. I lifted the inductors inline with the current sense for the outputs, still tripped.

Not sure where to start.

 

[Perry Babin]

The circuit could be the same laid out onto the main board.

 

[brandes.cm]

It is, the lm293 is behind the output driver module.

 

[brandes.cm]

I lifted pin 1 of U6, fault condition occurred. Replaced pin 1 and removed R19, fault condition gone, does that point to Q5?

 

[blazer40]

Q4 is what i had go out on the 3000d I just had in.

 

[brandes.cm]

unfortunately removing Q5 eliminates the error condition, so Q4 is still in the circuit. The output of the comparator is reading 1.191V and the base of Q5 = .667, this is referenced to -rail.

Don't know, thoroughly confused at this point, haha.

 

[brandes.cm]

Dropped in my spare DWM1216 board and now the voltages on the op-amps look good. The output is 0.032V, non-inv = ~6V and inv = 12V. I still see .5V on the base of Q5. (reference is -rail)

 

[Perry Babin]

Did you measure the DC voltage on the base with the meter directly on the base and emitter of Q5?

What's the DC voltage on the junction of C25 and R15 with the black probe on the emitter pad of Q5?

 

[brandes.cm]

The reference point was the -rail bus bar.
Using the emitter I have .032V at that junction R15.

 

[brandes.cm]

Q11, Q7, Q19 and Q22 get hot. Hot being; you can place your finger on them for 15 seconds before you feel them. Is this normal?

 

[Perry Babin]

They get very hot under normal operating conditions. That's why they have such a high failure rate. After the amp is on for about 5 minutes, they are generally so hot that you can't hold your finger on them for more than a few seconds.