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XTi1002 Dead CH2


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#1 johnnycrown

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Posted 29 July 2014 - 07:04 AM

Hi Folks,
I have a XTi1002 with ch2 not working, actually the amp dosn't complete the power up cycle.
It starts up puts CROWN on the display for 5 ish seconds then fires the crowbar and shuts down.

Things I've done so far:-
Removed both OP device heatsinks with devices attached, they all meter as good.
Removed both driver transistors Q132 & Q133 which have exploded.
Removed pre drivers Q129 & Q131 these meter OK, although I do have replacements.
Removed R225 & R226 340ohmRs as they are both open circuit.
Removed D189,D188,D187 & D190 as they are all short. Metered between collector connections of Q132 & Q133 and the 100ohm resistor R258 looks good.
Checked Zeners D160,D151,D158 & D150 with a 20vPSU, a 1K resistor as a load and a multimeter and they all correctly reverse bias at 16v, checked D155 & D153 while I was at it and there ok.
Metered the paralleled emitter resistors of the OP devices, all appear ok at 0.47 Ohms.
Checked all the other transistors of ch2 on the board and they all appear at least with two good junctions.

The amp now correctly powers up, all be it no ch2, but my question are:-

1. Should there be anything else I should check before putting new drivers  Q132 & Q133, new pre drivers Q129 & Q131, new R225 & R226 340ohmRs, new D189,D188,D187 & D190 bias diodes?

2. Why did the drivers explode in the first place :o

thanks
j

#2 Fspatola

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Posted 29 July 2014 - 07:15 AM

Hi Johnny, sorry to hear you are having problems with your amp. I just repaired an xti1000 with the same issues. Q133 & q132 shorted, D187, D188, D189, D190 also shorted. But also found one output transistor shorted from collector to emitter. I would double check the output transistors. I replaced all parts and I am back up and running. Hope this helps.
Good Luck. Frank.

#3 joust

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Posted 31 July 2014 - 08:37 AM

Hello Johnny,

Perhaps you should review the output transistors as Frank advises. Also, if not already done, try to swap the Ch2 output devices (and heatsink) with Ch1.

From my perspective, you did all right with your visual inspection; All the usual suspects have been inspected. Verify the outputs as per Frank's suggestion.
Do you have your rails voltages for Ch2?
If you do have the rail voltages, do you have voltage on the output rail?
Verify the base junction voltages : the drivers should be conducting and the outputs very close to conducting; If voltages don't concur or devices not conducting, you'll need to go upstream.
Inject some signal : Do you have signal appearing at the input of the amp section? At the driver section? At the output section?
If you measure directly on the heatsinks with reference to ground, once the power up cycle terminates, you should have the same measurement as Ch1.

Hope this helps!

A.

#4 johnnycrown

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Posted 15 August 2014 - 06:54 AM

Hello again,
Well I took on board your suggestions and dusted off my trusty Levell TM12 transistor tester and measured all OP devices, they all measure good with Hfe of about 60-80ish and no leakage. I could not measure Vcbo as the Levell only puts out max 172v and the Vcbo junctions of all trannies would not avalanche at this voltage (These  transistors are speced at Vcbo 400v) so I have to assume that all transistors are good.
The rail voltages in the XTi1002 are + - 75V so, the amp, under any fault condition could not exceed max Vcbo of the output transistors.

I put the OP devices back in after replacing the other duff drivers, bias diodes and resistors and powered the amp up, hey presto, amp powers up now with both channels and no DC on the outputs.

I updated the firmware and set up some nice crossover preset settings (Ch1 LP 140Hz 48db/oct Ch2 HP 140Hz 48db/oct) for my set up stuck the amp in my rack and gigged the amp last night, all performed flawlessly including the other 2 musicians!

So again the question why did the drivers explode, maybe it was just a tired transistor :(
j

#5 Patlover1983

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Posted 28 August 2014 - 04:25 PM

Today i have recieved also an XTI1002 on my workbench with also 2 blown driver circuits. Also the power transistors where OK (measured with my Peak DCA55 component analyser). After a couple hours of searching i found out the BIAS potentiometer whas faulty. When measuring it was ok, but after using the amplifier the amp runs very hot in a couple minutes. When the screwdriver hits the potentiometer the amplifier imidiatly goes into protection. After replace the potentiometer the amp is running fine for a couple of hours now.

Just my two cents, hope you can so something with my info!

#6 joust

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Posted 02 September 2014 - 08:24 AM

Thanks for the input! Yes, the info may become handy! I Haven't discovered this type of fault yet!

Alain