troubleshooting a dbx 163 (long involved post)

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arkestra
alignin' 24-trk
Posts: 50
Joined: Fri May 14, 2004 12:47 pm
Location: Northampton, MA

troubleshooting a dbx 163 (long involved post)

Post by arkestra » Thu Nov 13, 2008 1:30 pm

Hi all, so im trying to get into the whole repair thing. I have had limited success with some amps and am working on a stereo dbx 163 now as a learning project. One channel works fine and the other barely works at all. here are the schematics :

http://www.dbxpro.com/support_Classic.php

click on manuals then choose 163-164

The unit has no output unless you slide the compression thing all the way to the right then it is loud and very distorted. there is effectively no "clean" signal. The meter shows no gain reduction.

At this point I have determined that the power supply is good and the input stage up until R17 and R53 respectively is showing signal on a scope.

I am assuming that QA 2 and 3 are the vca transistors. I also disconnected R72 to see if disconnecting the RMS detector changed anything. it has not. So, I am left with assuming there is a problem with both the vca AND the rms detector (lack or change or of g/r meter readiing, and no output when disconnected)

Questions:
Does this sound logical? how would one go about checking these circuits? How do I determine if these transistors are shot? with chips I assume if the voltage and input/ output look ok its fine. I guess im really just asking for a good way to test the transistors in this circut. I want to tackle the vca first then the rms detector since it seems more involved. Thanks. . . :shock:

???????
resurrected
Posts: 2383
Joined: Mon May 22, 2006 6:15 pm

Re: troubleshooting a dbx 163 (long involved post)

Post by ??????? » Thu Nov 13, 2008 8:45 pm

arkestra wrote:Hi all, so im trying to get into the whole repair thing. I have had limited success with some amps and am working on a stereo dbx 163 now as a learning project. One channel works fine and the other barely works at all. here are the schematics :

http://www.dbxpro.com/support_Classic.php

click on manuals then choose 163-164

The unit has no output unless you slide the compression thing all the way to the right then it is loud and very distorted. there is effectively no "clean" signal. The meter shows no gain reduction.

At this point I have determined that the power supply is good and the input stage up until R17 and R53 respectively is showing signal on a scope.

I am assuming that QA 2 and 3 are the vca transistors. I also disconnected R72 to see if disconnecting the RMS detector changed anything. it has not. So, I am left with assuming there is a problem with both the vca AND the rms detector (lack or change or of g/r meter readiing, and no output when disconnected)

Questions:
Does this sound logical? how would one go about checking these circuits? How do I determine if these transistors are shot? with chips I assume if the voltage and input/ output look ok its fine. I guess im really just asking for a good way to test the transistors in this circut. I want to tackle the vca first then the rms detector since it seems more involved. Thanks. . . :shock:
This might sound obvious, but I gotta ask--

Have you tried spraying contact cleaner like DeOxit in the slider?

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