Help with Xtant x603

[Perry Babin]

An open gate resistor can most definitely cause excessive current draw.

Only one defective resistor would only account for one gate-gate measurement that was not correct (unless you used that gate as the reference). For all of the FETs with 22 ohm gate resistors, you should read 44 ohms gate-gate. For the FETs with 47 ohm gate resistors, you should read 94 ohms gate to gate. Measure them again to confirm that's what you have.

[shawnk]

Also found R6 open. I didn't see this gate resistor before as it's in a different location from all of the others. So R6 and R75 are the two open gate resistors that coincide with the two fets that had improper readings.
I'm going to swap these out in the morning. Is there anything else I should check before I call it a night?
Thanks a bunch!

[Perry Babin]

Check R87.

[shawnk]

Damn you are good!

R87=45k

[shawnk]

Well I'm a happy camper!! I replaced the open resistors and the amp fires up seemingly with no issues.
Now I'm looking foward to using my new scope (should be here soon) to set the bias for each channel. However, I do have a question about the DC offset pots. How do I go about correctly setting the bias AND offset? Bias first then offset, or visa versa? I know that many amps do not have offset pots. Are they (the pots) available for more of a fine adjustment tool compared to other amps that do not have them?
Thanks a million for all of your help Perry! I certainly have learned a lot with this.

[Perry Babin]

You don't need a scope to set the bias. Does your power supply have an amp meter on it?

Set the DC offset by measuring the DC voltage across the speaker terminals for the channel being tested. After setting the bias and the DC offset, you'll want to let the amp 'settle' then recheck them.

Amplifiers without bias pots are typically class B amps and have no bias current.

Amps without DC offset pots typically have capacitors in the ground path for the gain setting resistors. This prevents DC amplification. If there is no capacitor, there can be significant DC offset. The pot allows the offset to be set low enough to be acceptable.

[shawnk]

Unfortunately my power supply does not have a meter on it. I still need to order the Velleman meter for this purpose.

I got my new scope however. I was eager to try it out right away so I fed a 1khz sinewave into the amp using the cd supplied with the tutorial. I now have a couple of new issues to work out

The left channel seems to be fine. The bias is still at its minimum setting, yet the sinewave appears to be clean on the scope (no notch distortion). Is this common? Shouldn't I see at least slight distortion in the signal?

The mono channel has a clean signal up untill clipping. I noticed the bottom half of the signal clips before the top so I know there's some more investigating needed there.

The right channel does not have any output at the speaker terminals. I did verify that there's a good input signal, but I havn't been any further than that.

[Perry Babin]

For the dead channel, confirm that you have signal on leg 1 of the j108 muting transistor. Compare the signal to that on leg 1 of a working channel.

For the sub channel, does the amount of clipping on the bottom half of the waveform change (relative to the amount of clipping on the top half) when loaded/unloaded?

Sometimes, the bias range is very limited so it doesn't go far enough to make much of a difference. With a speaker (or dummy) load, a 15k test tone and about 1v of audio on the speaker outputs, if you don't see any distortion on the outputs, the bias is likely OK. Set the scope to 0.5v/div so that the waveform covers ~3 vertical divisions. Set the timebase so that ~4 full cycles are displayed.

[shawnk]

I confirmed the signal on the muting transistor. Signal is identical to the other two channels. Q68 Q77 Q86 are all identical.

[Perry Babin]

Is the gain control set to the full clockwise position?

Is the channel absolutely silent?

Do you see the audio signal on any leg of any output transistor in this channel?