Re: TM506


On Sat, Jul 25, 2020 at 02:42 PM, Harvey White wrote:


I think someone gave you a direct link.  You may have to more or less
match with more modern components.
Yes. I posted before seeing Michael’s post with the link to the list.
I found what I needed. All these are 2N2905A and 2N2219A pairs.

I’m gonna change the pair that was directly connected to the 33R resistor that was charred.
It’s very difficult to probe anything with a scope in that area without an extender.

One thing I can tell you tight of the bat, is that when my scope is DC
And the trace is on the center horizontal line, when I plug in the FG502,
The trace goes directly off screen to the bottom. Even if the FG has the
Offset off (pushed).
This is not the case when the scope is AC coupled.
That means I have Neg DC bleeding through when I shouldn’t have any. I
either have to move the trace back
up on the scope, OR use the DC offset switch to inject Positive DC to bring
it back up. But that crushes the signal to the point where it almost a
straight line.

Can be one of two things, or perhaps a combination.

1) negative driver turning on too much

2) positive driver not turning on enough

what I *think*, because the + part of the trace is distorted, is that
the + driver chain is not driving the output properly.

what the circuit does is

1) take a single ended input (from the amplitude pot)

2) convert it to a differential (+ going and - going) signal

3) amplify it, running it through a bipolar amplifier, which is capable
of both + and - excursions.  This is an alternate approach to a push
pull output.  the "upper" transistors take care of the + excursions, the
"lower" transistors take care of the - excursions.  In a push pull tube
amplifier, there's generally a driver transformer, or a circuit that
splits the input.  That is applied to two output tubes fed out of phase,
and the audio output transformer puts those phases back together, since
the amplifier is likely a class B or there abouts.

In transistors, there exist transistors that can run off - voltages, so
you typically have an output stage where one transistor (the + one)
pulls the output +, then is turned off where the - transistor pulls the
output -      tubes can't do this well, but it's possible.  (Tektronix
did it in their vertical and horizontal scope amplifiers).

So you're dealing with a similar design, but no transformers means DC
response possibilities, so really, no LF limit unlike transformer
coupled designs.

Since transistors can drive low impedance loads directly, no output
transformer is needed.
All of this makes perfect sense. Thank you Harvey.

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