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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Voltage at emiter Q7
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Etaoin



Joined: Jun 30, 2005
Posts: 760
Location: Utrecht, NL

PostPosted: Fri Nov 04, 2005 12:28 am    Post subject: Voltage at emiter Q7 Reply with quote  Mark this post and the followings unread

I have a reasonably well working Soundlab. My only problem is signal bleed through from the square waves of the LFO and OSC2 into the noise generator. There's another thread about that somewhere.

However, buried in that thread is an observation that might be related (or the cause). In the docs for the Soundlab, it says:
Quote:
You should see a positive voltage at the emitter of Q7 of between 4.5 to 6 volts (depending on the impedance of your meter or scope it could be lower but it should at least be a couple of volts above ground).


However, on both a digital voltmeter (unknown impedance) and on my scope (around 1M impendance, I think) I see a negative voltage there, slightly over -1V. I've already tried several different transistors for Q7 and have replaced C15. Resistor values are fine (and I even measured them in the event they were a different value than their rings indicated), I have tried several different IC7s as well.

Should that voltage really be several volts above ground?

I did notice that if I connect the scope to the emiter, the signal bleed through lessens...
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Uncle Krunkus
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PostPosted: Fri Nov 04, 2005 2:38 am    Post subject: Reply with quote  Mark this post and the followings unread

The way I see it.
Based on my fairly limited circuit analysis skills.
You need to ask "what is the resistance of the reverse biased EB junction" if it's exactly 1M (R96) then you'd read 0V. If it's >1M then you'll read a positive voltage, if it's less then you'll read a negative. This is assuming you actually have + & - 9V across the junction/R96 divider (ie: - the supply, check this too)
Now if that is all okay then a reading of 1V will mean that the junction has a resistance of 1M25. Lowering R96 to about 360K would force the reading to around 5V. BUT,........ This will increase the current through the junction, which could change it's resistance. (I doubt it though)
I'm going to check out what the reading is on mine.
I'll let you know what I find out.
Andrew

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