Help with Xtant x603

[shawnk]

Hello all.

I snagged this amp the other day and I just started to go through it and start the repair process.

I have found a shorted transistor (Q86). It is marked:
108
C936AG

I can't seem to find a replacement. I've tried Digikey, and Mouser to no avail. Are there any other sources I could try? Any suitable replacements?

Perry- If your are reading this, I would just like to say "thank you, thank you, thank you!" I purchased your tutorial a while back on ebay. What a fantastic piece! I was an installer/bay manager for nearly 12 years up untill recently when the economy really went to hell. I've always wanted to repair amplifiers and when I finally discovered your tutorial it was like hitting the lottery. Then, when I saw that you were on this forum I felt compelled to join. Your knowledge, and willingness to help others is superb. Something that's not all too common these days so cudos to you and everyone else here who shares the knowledge!

Thanks in advance. Shawn

[Perry Babin]

It's a J108 jfet and it will appear to be shorted. If you look at the 'Checking Semiconductors' page, you can see a J108 reading 6.7 ohms out of the circuit.

I'm glad to hear that you're happy with the tutorial.

What's the amp doing (or not doing)?

[shawnk]

Hey Perry. Boy I feel silly. I referred back to the tutorial and saw the page you were talking about. Sure enough this jfet is fine. Reads just over 6 ohms. I've read through the tutorial twice so far, but man there's just so much information. Hard to remember all of it. I feel I'm at the point where I just need to start getting my hands dirty. I've always been a firm believer in "dive in and learn from your mistakes", as long as it's not at a cost to others.

So far I have only replaced obvious faulty components. Two output transistors: IRF9530, IRF530 along with their respective emitter resistors. I will replace all other parallel components once I know the amps is up and running well; unless I should do so now? I have also replaced burnt R183 & R184 10ohm smd. I have checked all other transistors, including drivers, for shorts, and they all seem to be fine. The only other obvious problem, from a visual inspection, is discolored R361 &R381 in the low current power supply section. This has me concerned as I have another X603 here in really bad shape where it's low current power supply is severely burnt.

Here's what's going on with the amp. I can now at least fire it up with a 3 ohm 40w resistor in line. Amp turns on, but you can here the right-side large transformer pulse approximately once every second and the 3ohm resistor definitely heats up. I can run my meter (digital) inline with B+, resistor still inline, and current seems fluctuate with the pulses from 1.2 to 1.8 amps.

Before we even get there, I must say that I don't have an O-scope just yet. Times are tough as I've been out of work for a while now. Very soon I will though as tax returns should be here in a week or two. Can't wait as I know this is really a necessity.

Thanks Perry

[Perry Babin]

With the transistors clamped to the heatsink and the bottom cover removed (so you can access the bottom of the emitter resistors), measure the DC voltage across each emitter resistor as the amp is trying to power up. Do any read more than ~0.001v DC?

[shawnk]

In the mono channel I do see that there is a pulse across all of the resistors of ~.04v when I power up the amp. If I leave the amp on, voltage will drop down to .001v then pulse up to .008v every ~1sec. As I had stated in the previous post, the pulse is very audible. The transformer makes a screeching sound upon each pulse. The other two channels have different results. One channel stays at .001v upon power up and doesn't seem to fluctuate, but the other with pulse ~.008v across the resistors.

[Perry Babin]

There is a small 20k potentiometer for each channel. Turn them fully counter-clockwise. Does that reduce the voltage across the emitter resistors?

[shawnk]

There are two pots for the mono channel. I'm assuming these are for biasing? I turned them both fully counter clockwise. Upon powering up the amp I still get a voltage spike between .02-.03v. Slightly less than before. Interestingly though, the pulses are much more drawn out now. I'ld say maybe 3 sec or more in between pulses. I know these symptoms may be a little odd, but I'm just trying to provide you with as much information as possible.

Thanks

[Perry Babin]

The 500 ohm pots are for DC offset. The 20k pot is the bias pot.

Are you measuring the voltage directly across the resistor?

[shawnk]

Ok I got you on the pots.

And yes, I am measuring directly across the resistors.

[Perry Babin]

Do you have the amplifier connected directly to the power supply or do you have a current limiting resistor inline?

What's the current rating on the power supply?

How much current is the amp drawing?

Do you have any RCAs or speakers connected to the amp? If so, disconnect them.