# Diagnose Problem B&K Reference 50 Preamp



## barnsbee (Apr 3, 2015)

I am hoping that someone here is willing to help me diagnose and potentially repair an issue that I am having with my home theater preamplifier. I have a B & K reference 50 preamplifier. There are line level outputs for 7.1 channels with RCA connectors. The one that goes to my right rear speaker is not working properly. When I go to adjust speaker levels this speakers volume is greatly reduced and static filled. Sometimes when I fiddled around with the connection it would work properly but soon revert to its low volume and static. Directly above the output is a come positive video output. While fooling with the wires I noticed that if I touch the center post of this RCA speaker connection to the composite video and then plug it in it will work correctly for a brief time and then fade back to reduced volume and static. This seems to happen somewhat reliably.

I would appreciate any feedback.

Thank you
Michael


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## Lumen (May 17, 2014)

barnsbee said:


> I am hoping that someone here is willing to help me diagnose and potentially repair an issue that I am having with my home theater preamplifier. I have a B & K reference 50 preamplifier. There are line level outputs for 7.1 channels with RCA connectors. The one that goes to my right rear speaker is not working properly. When I go to adjust speaker levels this speakers volume is greatly reduced and static filled. Sometimes when I fiddled around with the connection it would work properly but soon revert to its low volume and static. Directly above the output is a come positive video output. While fooling with the wires I noticed that if I touch the center post of this RCA speaker connection to the composite video and then plug it in it will work correctly for a brief time and then fade back to reduced volume and static. This seems to happen somewhat reliably. I would appreciate any feedback. Thank you Michael


Would you like to start out with some basics? I assume your preamp is feeding a power amp. First try swapping the cable from a working channel to the suspected one. If the problem goes away, then you need a new cable, or might be able to get away with tightening up the outer sleeve. If the problem persists, then it might be the connector on the preamp. Many of these are directly connected to a circuit board and can loosen over time. You mentioned the static didn't return for a while after wiggling the cable/connector, so either one of these could be the cause. If it's the preamp connector, then re flowing the solder may fix it. Try swapping cables first and we'll go from there, okay?

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## willis7469 (Jan 31, 2014)

+Lou.


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## lcaillo (May 2, 2006)

It sounds like you might have either a bad jack or a bad solder connection somewhere if you could mess with the connection and get it to work. Bad solder joints are, by far, the most common problem with most electronics.


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## willis7469 (Jan 31, 2014)

Yep. Fixed a few of those!


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## barnsbee (Apr 3, 2015)

Hello and thanks for the replies.

When I first was checking this out I reversed the cables and tried other cables. The problem is tracked to the output of the right rear channel. I have looked inside the unit and it appears that there is a bank of goldplated RCA connectors that go somewhere that is currently not visible. I will have to remove some circuit boards to visually assess that. I have fiddled with the connector it does not appear that tightening the sleeve is the problem.

A second thought them having is that this unit has balanced interconnect outputs as well as the RCA outputs. My amp is not balanced. I was thinking about trying the balanced output for this channel to see if it is putting out the correct signal. I don't know how to connect a balanced output to an unbalanced amplifier connection to test it. Is there a way? Can I check the output with a Digital Multimeter?


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## Lumen (May 17, 2014)

barnsbee said:


> Hello and thanks for the replies. When I first was checking this out I reversed the cables and tried other cables. The problem is tracked to the output of the right rear channel. I have looked inside the unit and it appears that there is a bank of goldplated RCA connectors that go somewhere that is currently not visible. I will have to remove some circuit boards to visually assess that. I have fiddled with the connector it does not appear that tightening the sleeve is the problem. A second thought them having is that this unit has balanced interconnect outputs as well as the RCA outputs. My amp is not balanced. I was thinking about trying the balanced output for this channel to see if it is putting out the correct signal. I don't know how to connect a balanced output to an unbalanced amplifier connection to test it. Is there a way? Can I check the output with a Digital Multimeter?


Yes, there are adapters to convert an XLR (balanced) connection into a single-ended one. XLR pin out can be found here http://www.clarkwire.com/pinoutxlrbalanced.htm. Measure between pins 2 and 3. A procedure for measuring maximum vs useful output voltage is detailed here http://www.hometheatershack.com/for...-your-system-measuring-avr-output-voltage.pdf. The results will only be conclusive if your AVR derives its balanced signal from the single-ended one. With a truly balanced topology from input to output, you'd be able to measure a signal on the XLR connector, but the single-ended RCA outputs could still be dead.

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## Lumen (May 17, 2014)

Some more troubleshooting ideas... It looks like the rear panel connectors are not directly soldered to a circuit board, is that correct? If so, then tracing the wires will lead to one eventually. However, they will not have been subjected to the stress seen by the direct-connect type. In fact, it's unlikely they've been subjected to any stress at all unless the unit was dropped. Check instead for damaged insulation that may cause shorting. Also check for conductive material like small screws or metal shavings that may cause shorts. If the unit has been operated in a humid environment, dendritic growth is a distinct possibility, but may not be visible to the naked eye. But if that were the case, more than one circuit would more than likely be affected. Another possibility: board to board connectors may have worked their way loose enough to make intermittent connection resulting in the interrupted sound you're experiencing. Check for these as you disassemble the unit, and carefully reseat them.

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## Glen B (Jun 11, 2013)

barnsbee said:


> A second thought them having is that this unit has balanced interconnect outputs as well as the RCA outputs. My amp is not balanced. I was thinking about trying the balanced output for this channel to see if it is putting out the correct signal. I don't know how to connect a balanced output to an unbalanced amplifier connection to test it. Is there a way? Can I check the output with a Digital Multimeter?


I always keep several sets of these XLR/RCA adapters on hand for interconnecting pro and consumer gear. They are plug and play, and I have never had issues with them.

http://www.parts-express.com/xlr-male-to-rca-female-adapter--240-438

http://www.parts-express.com/xlr-female-to-rca-female-adapter--240-428


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