Targa TG -18D (MA Audio HK2000d)

[chrisg4]

Hi all,

I'm working on below amplifier, it has blown one side of the output fets.
weird thing it has two different sets of fets, one side is IRF9640, the other is FQP19N20. the PSU has IRFP064N, those are ok.

which would be the correct output fet to use?

the driver board also looks suspicious, it has a big solder bridge on one of the ICs.

pics:

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[Perry Babin]

The 'IC' is a dual FET. The drains are all soldered together. That's normal. The amp uses both N-channel and P-channel outputs. The ones in it have been used in other amps. The 19N20 and the 12P20 were used in some of the early amps of this type.

 

[chrisg4]

Thanks for the info. Is that drive IC a ir21844?

 

[Perry Babin]

There's no 'driver' IC like a 21844. The 14 pin IC is an LM219/LM319.

 

[chrisg4]

would that need to be replaced?

 

[Perry Babin]

The LM319 is far back in the circuit ad rarely needs to be replaced.

Generally, only one FET for bank fails. Did you try installing the remaining good FETs to see if there were any other problems? The driver board may have survived.

Search the forum for DLM4500 for a similar design.

 

[chrisg4]

I removed all the fets and the amp remained in protect...

will try new fets tomorrow

ok will do

I also noticed the Ic next to LM219, is a 4558? is that correct?

 

[Perry Babin]

I've seen various op-amps in that location. The TL072 was what I used when I needed to replace a defaced IC. The 4558 should be Ok as well.

 

[chrisg4]

I've installed some new fets for testing, both power and protection lights are now on. I removed the protection ic 2904l, then the amp stays on protection,
output pin 1 - no voltage
output pin 7 - 10v

what do i check now?

 

[Perry Babin]

What is the DC voltage on the drains of the output FETs when you try to power up the amp?

 

[chrisg4]

when the remote is applied it builds up to to +- 40v, then drops to 0.01v

 

[Perry Babin]

The drains are directly connected. They can't be ±anything. They have one voltage.

 

[chrisg4]

yes they all have one voltage - 0.01v

 

[Perry Babin]

Does the circuit for the protection IC in the attached diagram resemble the circuit in your amp?

 

[chrisg4]

i've actually been looking at AQ2200 schematic from another user's post... can't seem to make sense of it.

 

[Perry Babin]

Is it just that diagram or is it that you are not familiar with reading schematic diagrams?

 

[chrisg4]

I am able to read the schematic somewhat, is the protection circuit located at D206?

 

[Perry Babin]

Post the DC voltage on all of the pads for the IC you removed and for the TL494/ka7500. Copy and paste the following into your reply.


Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:


Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:

 

[chrisg4]

Below voltages are with the njm2904l installed


Pin 1: 0.0v
Pin 2: 0.0v
Pin 3: 0.0v
Pin 4: 0.0v
Pin 5: 9.9v
Pin 6: 10.5v
Pin 7: 10.5v
Pin 8: 12.1v


Pin 1: 9.8v
Pin 2: 2.9v
Pin 3: 4.7v
Pin 4: 0.01v
Pin 5: 1.5v
Pin 6: 3.6v
Pin 7: 0.0v
Pin 8: 12.3v
Pin 9: 0.0v
Pin 10: 0.0v
Pin 11: 12.3v
Pin 12: 12.1v
Pin 13: 5v
Pin 14: 5v
Pin 15: 4.8v
Pin 16:0.0v

 

[Perry Babin]

Confirm that there is no solder bridge between pins 6 and 7 of the 2904.

With no power applied, what is the resistance between those pins?