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Old 20th March 2008, 07:17 PM   #1
shagone is offline shagone  United States
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Question xtant x604 dc offset

i'm working on an xtant x604 (later version of the 604x, same board i believe) the 2 rear channels have high DC offset. 1.4 volts on the left and .5 volts on the right. the right still sounds fine to me but the left sounds a bit distorted and lower volume than the right.

here is some history on this amp: this guy brings it to me from his dessert rat truck and says he just wants it cleaned and that it was working fine last time he used it. he said it had beer spilled on it and that it worked after that but he disconnected it and took it out of the truck and it sat for a while. the board was filthy! it has encrusted dessert sand stuck in every crack and crevice and dried up beer spots. i brushed of what i could with a paint brush and then i used acetone on the bad spots. this got most of the mess off but some ares looked bad still so i sprayed contact cleaner in all the pots and cleaned some more. i noticed a broken lead wire from the remote circuit and a small electrolytic cap with a lead broken from the board. i repaired these items and hooked the amp up. big suprise, it actually worked. well it sorta worked. the rear left channel was distorted. i noticed a fried 100 ohm resitor that led to a small smt pnp transitor. i replaced these items and noticed 2 bad 100 ohm resitors in the right channel along with another smt pnp. so i replaced these too. the other bigger transsistors (BF422's and so on) in both channels appear to check ok as well as the other resistors. after this i still got the same sound or therabouts and the DC offset is at 1.5 volts left and .5 volts right. the front channels have nearly no DC offset.

what else should i look for? i see 3 electrolytics in each channel that i think i will replace. the one's in the right rear look to have been changed before because they are different brands and color but same value. i'll change them anyways.

it's amiracle this thing works at all but i think i can get it back to full strength with a little help from you guys.
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Old 20th March 2008, 09:49 PM   #2
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If you get another amp that's that filthy, run it through the dishwasher or spray it with 409/fantastic and wash it with a pressure washer (at the car wash). Keep the wand a safe distance from the board so you don't blast any components off of the board. Remove excess water with an air compressor and set it in the sun to dry for several hours. Before applying power confirm that it's completely dry (including under that capacitors).

Doesn't the amp have DC offset pots? They look like the gain pots but don't have the knob.
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Old 21st March 2008, 03:33 AM   #3
shagone is offline shagone  United States
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good cleaning tips! i'll try that next time.

there are 4 small pots near the input voltage selectors for front and rear?? and there are 2 more small pots near the driver stages of each channel?? one of the small pots is near the idividual gain knobs in each channel. i have no clue what is what inside this monster of an amp. for DC offset pots do you turn them up or down to lower the DC? i guess i could try them and see what they do
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Old 21st March 2008, 04:20 AM   #4
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Don't turn the ones near the jumpers. I think they're to set the balanced input null point (I could be wrong).

The DC pots will be near the audio output transistors. The gains will be marked as gains. The DC offset pots probably will not be marked other than their circuit board designation.
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Old 21st March 2008, 04:35 AM   #5
shagone is offline shagone  United States
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the gain is marked clearly but there are 2 smaller pots for each channel. could one be for bias and one for DC offset? do i turn them up or down to lower the DC offset?
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Old 21st March 2008, 04:58 AM   #6
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When you turn them, watch the current draw from the power supply. If the pot is the bias adjustment, the current draw from the power supply will change. If it's the DC offset, there will be no change in current. Do this with no speakers connected to the amp. Turn them both ways slowly and observe the changes in DC offset or idle current.

If the 604 is like the 603, the bias is connected to a TL431.
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Old 21st March 2008, 05:32 AM   #7
shagone is offline shagone  United States
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Thanks perry! i'll go try that now.

I own a couple of 603x's that are still new in the box along with a couple of 302A's. i was thinking of running a system using them. What are your guys opinions out there on them as far as sound quality? How will they compare with ADS? i have some smaller ADS amps available too but the Xtants just look great but i know i won't be showing them off much if at all.
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Old 24th March 2008, 06:27 PM   #8
shagone is offline shagone  United States
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It's Alive

Thanks again Perry. the DC offset pots were maked 500 and the bias pots were marked 22 or 22k. the DC offset pots worked great although i found another open resistor in my left rear channel so i was still getting only half of the waveform on that channel after reducing the DC to null. I used the small lighted magnifier that you recommended to find the cracked/fried resistor hiding behind a cap that i had previousely missed. after changing the resistor and re-setting the bias all was well and the amp was playing loud and clear again.

Next up is an X1001 from the same guy. it was not from the sand this time but out of his boat (no major corrosion issues that i can see). it looks to have bulging/leaking large bulk filter caps and some issues in the power supply fet driver section.
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Old 24th March 2008, 06:34 PM   #9
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Nice to know you got it going. Those lighted magnifiers are great time savers when troubleshooting amps that have blown SMD resistors.

Start a new thread for the 1001. Leave the transistors clamped to the sink while troubleshooting.
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