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B&K Components 125.2 Repair Assistance - DC voltage speakers terminal
B&K Components 125.2 Repair Assistance - DC voltage speakers terminal
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Old 13th May 2019, 01:18 AM   #1
vicsd is offline vicsd  United States
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Join Date: Mar 2013
Location: Palisades Park, NJ
Default B&K Components 125.2 Repair Assistance - DC voltage speakers terminal

Hello,

This is is my first post on the forum. I am new to power amp repair and not sure where to start the checks. Looking to repair the B&K components 125.2 power amplifier.

The amplifier speaker terminals reflecting DC voltage (rail) of 70 V on both channels. At first checked all capacitors and resistors. Also, I have replaced the all output MOSFETS but the issue still persist.

Thank you in advance,
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File Type: pdf REF_125.12357_Amps_SM.pdf (122.0 KB, 48 views)
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Old 13th May 2019, 05:49 PM   #2
Mark Tillotson is offline Mark Tillotson
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Join Date: May 2018
Location: Cambridge UK
Both power rails are up?

I would take care with +/-70V, thats 140V in total. But you'll need to measure the voltages on the LM4702 inputs and outputs - I'd start by checking the bias voltages between J1 and J2, and J3 and J4. (the outputs).

If the outputs are high one a channel the non-inverting(+) input should be at a higher voltage than the inverting(-) input. If not that LM4702 is toasted. If so then the feedback circuit is misbehaviing and the loop isn't closed.
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Old 14th May 2019, 12:48 AM   #3
vicsd is offline vicsd  United States
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Join Date: Mar 2013
Location: Palisades Park, NJ
Hello Mark,

Both rail are up. Power Supply PCB has +70V/-70V going to iZone board.

This is two channel amp, so only J1 and J2 are present. Both channels shows positive voltage:

J1 +67.46 V
J2 +68.0 V

I will measure the voltage on U1 and post the results.
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Old 14th May 2019, 06:03 PM   #4
Mark Tillotson is offline Mark Tillotson
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The bias is backwards by the look of it.
J1 and J2 are part of one channel, so you must have J3 and J4 on the other channel?
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Old 15th May 2019, 04:28 AM   #5
vicsd is offline vicsd  United States
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Join Date: Mar 2013
Location: Palisades Park, NJ
I've measured the voltage values on the U1.

Pin13 + 68.2
Pin14 + 67.8
Pin15 - 68.3

Pin2 +67.5
Pin3 +67.6

Pin5 +67.7
Pin6 +67.8
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Old 15th May 2019, 06:11 PM   #6
Mark Tillotson is offline Mark Tillotson
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R2 open circuit perhaps? The inputs are matching the outputs so clearly very little is pulling them down to ground. Could be internal fault or the feedback network.
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Old 16th May 2019, 04:42 PM   #7
vicsd is offline vicsd  United States
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Join Date: Mar 2013
Location: Palisades Park, NJ
Ordered replacement for U1. Will post an update once installed. Thank you Mark.
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