Quantum Audio QCA4000D

[S Merlington]

I put in some 1n4744a's in and had 15 volts on pad 3 for 30 secs then the amp blows the diodes..

Any ideas?

 

[Perry Babin]

What is the value of the resistor connecting the collector and base of the regulator transistors?

 

[S Merlington]

10K ohms

 

[Perry Babin]

I don't think that that diode could be damaged through a 10k resistor (the only thing, except for possibly a capacitor, connected to that diode with the transistor out of the circuit).

Confirm that there are no solder bridges between the base and collector pads of the regulator transistor.

 

[S Merlington]

Ok for the D669A on the pads i get

Pad 1:0.00
Pad 2:21.6
Pad 3: 14.6


For the B649A on the pads i get

Pad 1:0.00
Pad 2:-27.4
Pad 3:27.3


For the D669 transistor i installed a 1N4744A and get the correct voltage on pad 3

For the B649A transistor i havent installed the diode yet and get 27 volts on pad 3 is this noramal or do i have a problem somewhere??

 

[Perry Babin]

The voltage on the 3rd leg of the B649 should be negative, not positive. Do you have the polarity right?

 

[S Merlington]

Checked again

With no diode installed and the B649A removed i get the following voltages on the pads..


Pad 1:-0.10
Pad 2:-27.66
Pad 3:-27.79

 

[Perry Babin]

That's normal as long as there is no short between pads 2 and 3. You should read ~10k ohms.

 

[S Merlington]

Ok now i get -14.56 volts on Pad 3 for the B649A transistor..

And i get 14.79 volts on the 3rd pad for the D669A

Heres what i get in diode check for the B649A

Legs1-2 Legs1-3 Legs 2-3 OL in all posistions..

Probes reversed i get
Leg1-2:OL
Leg1-3:0.606
Legs2-3:0.606


Heres what i get for the D669A
OL in all posistions with probes reversed i get OL also i cant get it to read anything..

As soon as i stick it in the amp the diodes blow..

So im guessing this is defective..

Is there any good Subs for theese 2 transistors??

 

[Perry Babin]

There are many possible subs for these. I don't know what you have on hand.

 

[S Merlington]

I have the following on hand..

A1381 and C3503

Tip41C and Tip42C

A1535 and C3944

2N6488 and 2N6491

A1700 and C2800

A1837 and C4793

D600K and B631K

 

[Perry Babin]

6488 - D transistor

6491 - B transistor.

Install facing the opposite way. The ±15v will connect to leg 1 of the 6488/91.

 

[S Merlington]

After putting the transistors in facing the other way I get the +15 volts But only get -2.82 volts for the negative voltage..

As soon as i powerd up the amp with the 2n6491 installed it blew the diode again

 

[Perry Babin]

Those two things contradict each other. How could you have -2.82v after installing the transistors but as soon as you powered up the amp the diode failed?

 

[S Merlington]

Sorry..

I meant to say that as soon as i power up the amp it blows the diode for the -15 volts..


When i went to check the -15 volts on the op-amps i was only getting -2.82 volts but i discoverd that it blew the diode again ..


So im not sure why this amp keeps blowing the diodes for the -15 volts

The diode connects to legs 1-3 of the rectifiers also..

Wondering could one of theese could cause the diode to blow if it was leaking??

 

[Perry Babin]

I don't see how you could have ±15v on the base pads for the regulators and then the diode fails. It would be more likely to destroy the regulator transistor if there was a load fault causing the failure.

Did you use continuity only to determine that the zener connects to legs 1 and 3 of the rectifiers?

 

[S Merlington]

I measured the ohms from the didoe to the rectifiers and get 0.3 ohms from the diode to legs 1-3

 

[Perry Babin]

You're seeing the secondary ground via the secondary winding of the transformer.

 

[S Merlington]

Ok I finally have + - regualted 15 volts on all op-amps..

I have + - 64 volts of rail..

Amp has no output ..

Where do i go next?

 

[Perry Babin]

What was the cause of the blown zeners?