Xtant 403A not powering on

[DiyFan]

I'm not positive the pinout on Q83 but if you look at my diagram top pin is 5vdc, middle 12vdc, and bottom 5.65vdc of the working amp. I think it would be top E, middle C, bottom B, correct?

Non working top .47vdc, middle 12.3vdc, and bottom .26vdc.

 

[Perry Babin]

Where is remote voltage coming into the diagram?

 

[DiyFan]

It's coming in at the junction of R219 and the zener D78. There is a jumper pin labeled 'remote enable' bringing it there, and I did verify that the remote 12v is at the junction.

 

[Perry Babin]

Voltage across D78?

 

[DiyFan]

I get about 7V across working amp diode and 12v across non working amp diode.

 

[DiyFan]

I checked both working non working amp zeners with multimeter and they measure the same.

 

[Perry Babin]

For a 6v-ish zener, you should not have 12v across it. Is it open? You may have to pull it to know for certain.

Is it the original diode or has it been replaced?

Was this amp working normally and just quit?

 

[DiyFan]

Yes, it is the original diode and the amp was working and then stopped a while ago.
I’ll pull it and test it.

 

[DiyFan]

Progress! I replaced the zener diode and now it is powering up normally without having to jump A56.

Now I am getting normal level sound out of the left front channel but a much lower level out of the right channel. I checked the gain settings and they are set equal. I checked all the op-amps again and they are all getting +/- 15vdc now.

The rear mono channel I am getting no sound at all out of.

 

[Perry Babin]

Have you tried removing and replacing the gain jumpers to see if there are any bad connections?

Have you tried operating all of the pots and switches through their entire range with a signal source connected and a speaker connected to the dead channel to see if the channel ever produced any sort of sound (even if it was just static/noise)?

 

[DiyFan]

I played around with all the jumpers and still output is a lot quieter in right front channel. I can't get any sound or noise out of rear mono channel.

I did determine that the rear mono channel is not working. There is +/- ~33vdc on the drains of the n-channel and p-channel output mosfets but only +.6v and -.3v on the gates. On the working amp there is +1.9vdc and -4vdc on the gates of the n and p channel. Also the non working is only idling at about 1.5amp and the working is at 2.6amp.

 

[Perry Babin]

Does the idle current change when you adjust the bias for the mono channel?

The FETs must be clamped tightly to the heatsink when adjusting the bias.

 

[DiyFan]

No, fiddled with the bias no change at all.

 

[Perry Babin]

Is there a TL431 regulator in the sub channel?

 

[DiyFan]

Yes, it is labeled D59.

 

[Perry Babin]

DCV measured from anode to reference (pin 1-2 for TO-92)?

 

[DiyFan]

Anode to reference pin measuring -.18vdc and there is .5vdc at the reference pin.

on working amp A to R -1.6vdc and -2v on reference pin.

 

[Perry Babin]

Check all of the low-value resistors in the channel. It's common for the 10 ohm resistors to open or go well out of tolerance.

What are the exact markings on the faces of the 431 regulators?

 

[DiyFan]

All the resistors seem to check out.
It is a thru hole TL431C and hard to read maybe TGADFUM above and mexico below TL431 marking.

 

[Perry Babin]

It could be defective.

When working normally, they should read 2.5v from anode to reference. There is a low-voltage version but those should read 1.25v.

Do you have a replacement?

If not, do you have an assortment of spare resistors?