Boston Acoustics GTA 800m

[outoftheblue]

Hi,

I own a gta 800m that recently died. It's out of warantee and I have experience as an audio tech, so I opened it up. I replaced the output and power transistors, along with the driver transistors for the TL494. I also replaced a 22uF cap near the output relay that appeared to be bulging. The amp is still in protection mode and I only get 6v at Vcc of the TL494.

 

[Perry Babin]

Can you post a photo of the inside of the amp?

 

[outoftheblue]

I've also included a closeup of the TL494. The traces around the IRF3205's were really burnt up. The hot glue was me insulating the traces I had to fix, along with the 47 ohm resistors I had to replace.

 

[Perry Babin]

Does Q1 supply 12v to pin 12 of the 494 via the 10 ohm resistor?

If so, post the DC voltage on all 3 pins of Q1.

 

[outoftheblue]

No, the 10 ohm resistor feeds the IRF3205 drains. I did some tracing on the bottom of the board and pin 12 of the 494 comes up between Q14 and R17, and again between R30 and Q9.

 

[Perry Babin]

Are you referring to the 10 ohm resistor next to Q1?

Did you find that connection with your meter or did you follow a trace?

Did you find any low resistance connection between any pin on Q1 and pin 12 of the 494?

 

[outoftheblue]

The center pin of Q1 has continuity with pin 12 of the 494.

 

[Perry Babin]

Please answer all of the questions.

 

[outoftheblue]

Sorry.

I am referring to the 10 ohm resistor next to Q1.

I found that connection with my meter.

There is a low resistance connection between the center pin of Q1 and pin 12 of the 494.

 

[Perry Babin]

If you look at the drains of the power supply FETs (with a scope) after you get the amp working, you'll see that the resistor is not connected to the drains. The connection that you found between the resistor and the drain is through the transformer's primary winding.

Does the other end of the 10 ohm resistor connect to the firs leg of Q1?

What do you mean by 'low resistance'?

 

[outoftheblue]

Yes, the other end of the 10 ohm resistor connects to the first leg of Q1.

By "low resistance" I meant there is continuity between pin 2 of Q1 and pin 12 of the 494.

 

[Perry Babin]

Post the DC voltage on all 3 legs of Q1 with the amp powered up.

 

[outoftheblue]

Q1:
pin 1: 11.31v
pin 2: 6.02v
pin 3: 10.51v

 

[Perry Babin]

If it's not extremely hot, it's defective.

 

[outoftheblue]

It's not! Should I just replace it and fire the amp up with a 10 amp fuse? That's what I was doing in the beginning.

 

[Perry Babin]

Yes if you have an amp meter on your power supply to confirm that the idle current isn't excessive. If it powers up and idles at less than 2 amps, reinstall it in the sink and test it more thoroughly.

 

[outoftheblue]

Q1 is a B631K PNP transistor.

The voltages on the board without the transistor are:

Q1:
pin 1: 6v, jumps to 12v with remote off
pin 2: 0v
pin 3: 12v

It looks to me like it acts as a switch. I don't have an exact replacement on hand. So I replaced it with a RS-2026 PNP transistor. I just turned it around because the pinouts are reversed.

Now as soon as power is applied the red LED for protection mode lights. When the remote is switched on the LED lights up brighter.

 

[Perry Babin]

I can't find any information on the RS transistor.

Do you have something like a 2n6491?

If so, install it (reversed from the original transistor). Does the amp power up?

 

[outoftheblue]

I cross-referenced the b631k to a nte374.

I installed the nte374 and it dies immediately, leaving everything as it was with the dead b631k.

I foolishly tried this twice to no avail.

 

[Perry Babin]

Do you have something like a 2n6491?