Kenwood KA-3500 and my blooper

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#164
If you look at the circuit it say 1.2 volts on the emitter line and -1.2 volts on the collector line.

We need to measure between between those points and so on their example we would read 2.4 volts.

For various reason, reading just to those points isn't accurate enough for faultfinding, that's why we do it this way.
 
#165
Lower Tones said:
ok, thanks. 1.9V up to 2.6V with bias at least resistance
Click to expand...

Brilliant... thats good. That means the bias generator is working basically OK.

Now, with it turned so that you have 2.6 volts is there any increase in bias current.

In other words do you measure any voltage across the 0.47 ohms or is it still zero ?

Also, do you see the 330 ohm resistors Re41 and Re43. With the bias pot set to give you this 2.6 volts what voltage is across each resistor .
 
#166
yes. i can see that the emitter of Qe7 is tied to Re19 and the collector is tied to the emitter of Qe5. not sure about the + and - 1.2v yet though. still magic for me. it will come.
still don't understand why i cannot adjust bias on this thing. voltage all seem to be there, no?
 
#167
Lead with the arrow = emitter (Qe5... so its the collector to collector of Qe5 and Qe7)

Voltages seem reasonable.

Here's how it works... this will help you understand what is or is not happening. Each base/emitter junction needs around 0.7 volts across it before it will turn that transistor on.

So if we have 2.6 volts across Qe7 then is that enough... well we have four base/emitter junctions to bias, the two drivers and the two outputs. That works out at around 2.8 volts. That minor difference is enough to stop them all conducting and so we need to now go back and reduce very slightly Re27 as before. And always start the bias from the lowest setting.
 
#168
Mooly said:
Brilliant... thats good. That means the bias generator is working basically OK.

Now, with it turned so that you have 2.6 volts is there any increase in bias current.

In other words do you measure any voltage across the 0.47 ohms or is it still zero ?
Click to expand...

still 0

Also, do you see the 330 ohm resistors Re41 and Re43. With the bias pot set to give you this 2.6 volts what voltage is across each resistor .
Click to expand...
.72 and .68. now, when i pushed a little bit on Re43, it jumped to .82
loose solder?
 
#172
Lower Tones said:
still 0


.72 and .68. now, when i pushed a little bit on Re43, it jumped to .82
loose solder?
Click to expand...

Hmmm... more like tarnish on the leads and the meter probe made a better connection.

Those readings are right in zone...

Try reducing Re27 the smallest amount and then check again but remember to keep turning the preset back for each test (so that there isn't a sudden increase)
 
#178
okay, so looks like somewhere around Qe5, D9, and junction J7 something was awry. after reseating a few of my own solder points (which looked good, i just wanted to be sure), i then focused on an area of the board which would flex mechanically when i applied pressure with the probe tips. i guess by reseating those three components it set things straight.
both bias now set at .040v
 
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