Help with Xtant x603

[shawnk]

Hey Perry.

Unfortunately my Astron doesn't have a meter on board. I actually wanted to ask you about the possiblility of adding a Veleman amp meter as apposed to getting a new power supply. I can get a 30amp panel meter from Parts Express for cheap $. This particualar meter uses a shunt instead of running in series with the power lead. Will this type of meter be accurate?

The only capability I have right now is to do what I said in post 3. I do not have any RCA or speakers connected to the amp, and the 3ohm 40w resistor is in series with the power lead. With the resistor inline, the amp did not blow a 10 amp fuse so I felt confident that I could run my multimeter in series with the power lead and get a current reading. I get a fluctuating reading from 1.2-1.8 amps that coincides with the audible pulses. I know this probably isn't as accurate as an analog meter but it's all I have right now.
Thanks a bunch!

[Perry Babin]

The 3 ohm resistor would limit the current to ~4 amps so there's no way to blow the fuse.

With the transistors tightly clamped to the heatsink and the 10 amp fuse in the B+ line (no resistor), does the amp power up without blowing the fuse?

The AIM6030A should work well. Most meters on power supplies work off of a shunt so this is really no different.

[shawnk]

Ok. I removed the resistor. Powered up the amp for about two seconds and the overcurrent indicator (yellow led) tripped. Also R183 &R184 started to smoke. These are the same resistors that I had previously replaced. Also R381 & R361 (in the low current power supply) started to smoke as well. I had noticed before that these two resistors were discolored.

[shawnk]

Got a chance to mess with the amp a little tonight. I found that if I remove R183 & R184 I can power the amp up and R361 & R381 no longer overheat. There is no longer an audible pulse from the transformer either.

Perry- So I can comprehend what's happening here can you enlighten me as to what function R183 & R184 serve? I read through the section in the tutorial about this particular amp, but the schematics I saw don't have this part of the circuitry.

I also did a basic diagnosis again and sure enough I found a couple of faulty transisors I must have missed before. (ahh the glory late night troubleshooting!) I found shorted Q47 & Q48 in the mono channel. I believe these are driver transistors for the output fets correct? I'm going to replace these in the morning and hope for the best.

Thanks a bunch

[Perry Babin]

183 and 184 are there to provide better filtering of the power supply feeding that channel. They also often act as fuses to help protect the low voltage supply.

Yes, Q47 and 48 are drivers. Are they BF422 and BF423 in this amp?

There are often other damaged SMD resistors, examine the top surface of each one with a lighted magnifying glass to see if any have any surface defects. If they do, confirm that they're within tolerance.

[shawnk]

Hey Perry.
Ok, so I think I'm making a little headway with this amp. Yes. Those transistors are BF422 & BF423. Since we talked last, I have replaced those drivers and checked all surrrounding smd resistors. I have also replaced R183 & R184 yet again. I powered up the amp, and those resistors did not heat/burn up. Unfortunately the overcurrent led lit up again and the transformer would make a constant winding sound. So after a little extremely technical diagnosing (aka the feeler test) I noticed that one of the rectifiers, D53 (MUR1620C), was heating up pretty quick after turning on the amp. I removed the rectifier and measured it. With the common lead on the center leg and the meter set to diode check I'ld get a reading of .46v on each of the outer legs so I'm under the assumptiong that the rectifier is good. Without reinstalling the rectifier I powered up the amp and it will no longer throw the overcurrent led, nor is there an audible transformer anymore.
None of the other rectifiers seem to heat up. So like I said some progress, but I'm not quite sure what to check next.
Thanks for your time.

[Perry Babin]

The diode was likely overheating because the sub channel was pulling excessive current. Removing it broke the connection to the outputs so they could no longer pull excessive current.

Pull all of the outputs in the sub channel and check each one to be absolutely sure that none are leaking. The gate should read infinite resistance (OL) to either of the other legs.

[Perry Babin]

shawnk, email me.

babin_perry@yahoo.com

[shawnk]

Still having some issues with the amp. I may be creating more problems than I am fixing them. Oh well, have to learn somehow.

I pulled all of the output transistors in the mono channel. None are leaking. Even with the fets removed the overcurrent led lights up. I went back in and started testing individual components again. To my dismay I discovered a couple of the driver fets had shorted again. I know I had checked these before and they were fine so something had caused them to short again. I also noted a couple of the small diodes directly in front of the output fets had shorted as well ie. D33 D35 D37 What is the purpose of these diodes? I also noticed transistor Q57 (I'm assuming this is a Darlington circuit?) was shorted which I had not previously tested. I believe at this point I have removed all of the faulty components in this channel and I still get the overcurrent light. I'm not even sure if this is even a legitimate way to test, but I gave it a shot anyway.

Now I'm jumping over to the other channels to check for problems there. Perry, at one point you asked me to check the voltage across all of the emitter resistors apon powering up the amp. If there is voltage there what could be the cause? I ask you this since I now have ~.1 to ~.2 volts, depending on which resistor is being measured, across the emitter resistors on the left channel. I tested this channel before and there wasn't any voltage. The right channel however, seems to be fine.
Thanks!

[Perry Babin]

Those diodes help to protect the FET gates from over-voltage. If they short, they can cause the drivers to fail.

I could be wrong but I believe that 'darlington' only applies to bipolar transistors. I'd appreciate it if someone would correct me if this is incorrect.

If you're reading 0.1v across the 0.15 ohm source resistors, that could be causing the amp to go into protect. Are you sure you have the bias pot (20k pot) set to the fully counter-clockwise position for this channel?

If so, pull the outputs out of that channel and check them for leakage also. Leave them out for now.

Does the amp power up with the outputs removed from those two channels?