Re: 7603 Z-Axis board transistor Q1152 running a little too hot?

Fabio Trevisan

Hello Al,
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 well.

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 accross them.
Lift one of their leads and measure their resistance, to be sure they're still at 3k9).
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 downstream.
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 be?



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