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Ops... As I was re-reading your original post, I realize that I advised you to do measurements you already reported done from the beginning.
You already tested the 3k9 resistors and measured the voltage at the Q1152 base at 103V... so this already determines that the bias point is OK.
If there's no excessive current coming out / in to / from the load (through the 120 ohm resistor)... then I think there's not much to worry.
Maybe only replace the resistor for one of bigger wattage, which will make it run cooler (although the dissipated power will remain the same and therefore, the air temperature in the area won't probably change).
On Fri, Apr 27, 2018 at 08:24 am, Fabio Trevisan wrote:
Looking at your picture, the degree of "browning" that you're referring to is
lighter than I've seen on some hot spots in scopes of this era (and age) so,
there's nothing really worrying in there.
On the actual dissipated power... I made some calculations on quiescent point,
and each pair of transistors; Q1132 with Q1134, and Q1152 with Q1154
dissipates about 0.65W (per pair), and that's just the quiescent power, not
mentioning the added power due to loading, from the circuitry downstream.
T0-92 transistors are usually rated as 0.6W, and TO-39 transistors at 1W
MAXIMUM (i.e. if you would ensure you're keeping the case at 25 degrees, which
is not the case).
If only quiescently they're (each pair) already dissipating 0.65W, (and again,
not accounting for additional power due to current coming from or going into
the load)... I think they really run quite hot!
Not scorching hot, but hot enough to develop some browning of the PCB
throughout 40 years of use.
Besides, the 20K resistor have almost 100V across it, and it alone dissipates
0.5W, which is enough to brown the PCB and heat the surrounding transistors as
For your peace of mind's sake, first of all, substitute that 20K resistor you
replaced for one bigger than the one it's there (use at least a 1W, better 2W
resistor if you want it to run cool). and measure the voltages on the circuit
to make sure they meet the calculations and there's no component drifted.
The 3K9 resistors at the emitter of the top transistors should have about 25V
Lift one of their leads and measure their resistance, to be sure they're still
If the resistors are within tolerance, and the voltage is about 25V, then you
know the collector current of the output transistor pair is correct.
Measure the voltage at the collector of Q1136 and Q1156... they both should
measure about 4V (relative to ground)... this will ensure the bottom part of
the bias resistor string is not drifted and that the bottom transistor is
operating at the intended bias point.
Finally, measure the voltage across the 120ohm resistors at the output... It
will give you an idea of how much current is being drawn by the load circuitry
By no means this current can be more than the quiescent bias current going
through the output transistors, which is about 6mA.
My wild guess is that it shouldn't be any greater than half of it, or about
3mA, because otherwise this would seriously upset the quiescent point of a
Class A amplifier.
Please do those checks and, if it measures right, than just let it be like the
Tek engineers designed it... It lasted 40 years... how bad can such a project