Quantum Audio QCA4000D

[S Merlington]

Amp is using IRF1010's for power supply fets with 47 ohm gate resistors..

The drivers are C2383 and A1013 ..

Im gonna replace the fets with IRF3205's since the 1010 are defective,,


The drivers are also defective..


Would the BD139's and BD140's work as good subs ??

 

[Leolabs]

Supply voltage???

 

[Perry Babin]

Yes.

 

[Leolabs]

Switching frequency how much??

 

[S Merlington]

Replaced all power supply fets and drivers..

If i power the amp up it will idle fine for 1 to 2 mins then draw excessive current and the transformer will start crackling and popping ..

If i twist it i can watch the current draw increase and decrease..

I cant find a short in it..

Also not sure if the transformer would cause this but the power supply driverboard get very hot when the amp draws current..


Should i unsolder 1 side from the board to see it i can find the short or whats the best was of doing this??

 

[Perry Babin]

Post a photo of the power supply section of this amp.

 

[S Merlington]

Heres a couple of pics of the power supply

 

[Perry Babin]

Follow the traces on the bottom of the board. Do you see a direct connection between the center taps of the two transformers?

 

[S Merlington]

I got the amp to idle fine for now..

Problem im having with it now is i have 2.03 volts on pin 8 of all op-amps and have 53.6 volts on pin 4 of all op-amps..

The regualtors are D669A and B649A ..


Pins 4 and 8 of the op-amps connect to leg 1 of theese Not sure if this is normal or someone has them in backwards..


If this is normal is there a good replacement for them??

 

[Perry Babin]

With the probes directly on pins 4 and 8, confirm that you see 50v+ of DC.

 

[S Merlington]

With probes directly on pins 4 and 8 the voltage starts at 30 volts and climbs to 50 volts

 

[Perry Babin]

Remove the two regulator transistors and carefully check them.

If they are OK, make sure that there are no shorts between the pads. Power up the amp and post the DC voltage on the pads for both transistors.

 

[S Merlington]

The D669A doesnt read at all the meter stays on OL at all times i trippled checked this but cannot get a reading ..


The B649A tests fine..


Heres what i get on the pads..

B649A

Pad 1:-72.6
Pad 2:-72.6
Pad 3:-0.6


D669A

Pad 1:0.00
Pad 2:64.5
Pad 3:0.1

 

[Perry Babin]

Confirm that leg 3 of the 669 goes to pin 8 of the op-amps and leg 3 of the 649 goes to pin 4 of the op-amps.

 

[S Merlington]

The way they were installed when i got the amp

Pin 1 of the B649A is going to pin 4 of the op-amps

And Pin 1 of the D669 was going to Pin 8 of the op-amps..


Does it appear theese were installed bacwards ?


Someone has worked on this before

 

[Perry Babin]

Post a photo of the area where these are installed.

 

[S Merlington]

Heres a pic of the location around them

 

[Perry Babin]

DZ1 appears to be the diode for the positive regulator. The striped end connects to pin 3 of the D669.

The other zener connects to pin 3 of the B649.

Check the diodes. It appears that both are shorted.

It also appears that you have a solder bridge between pins 1 and 2 of the B649 transistor's pads.

 

[S Merlington]

I will have to go to the store in the morning and grab to new diodes..

They are 1N3600 .. Unless theres another number that will work

 

[Perry Babin]

Do you have any 15 or 16v Zeners? If so, use them. The striped end of the zener goes to the 3rd leg of the D transistor. The non-striped end of the zener goes to the 3rd leg of the B transistor.

Confirm that you have ±15v on pad 3 before installing the transistors?