Hello from central TX!
I have a DC503 counter where it's difficult to get the input A level
adjusted correctly. Connecting a scope at any point between the input JFET
pair and the TTL logic, you can see that a minuscule adjustment of the
level knob causes the signal to swing from one extreme to the other.
If it's just been turned on, with effort I can get it adjusted so that it
will work. As it warms up it will need additional adjustment, and it
becomes more difficult to find the sweet spot.
Q35 and Q40 are JFETs at input A, both 151-1022-00 (selected 2N4392). Q35
gets the input signal, while Q40 is connected to the input level pot. Q40
gets extremely hot in operation, so much so that I'm surprised I don't have
a circle burned into my fingerprint where I touched it. I thought this fet
may be damaged due to a voltage excursion on the rail [1], but my (very
cheap) component tester shows similar parameters for Q35 and Q40.
Transposing the two transistors gives the same results: the one at Q40
becomes extremely hot, the signal coming out looks the same, and it's
difficult to adjust the level.
Does it seem reasonable to assume the two transistors are supposed to be
around the same temperature?
If I feed a 1kHz square wave into inputs A and B and check the signal
between the JFETs, on input B I get a nice square wave while on input A I
get a flat line, with spikes alternating up and down where each edge should
be. Input B uses a different JFET but the circuitry before the JFETs is
identical to A, so I'd expect the waveform to be similar. If anything,
input A should be better since it's good to 100MHz while B is only good to
10MHz.
The component tester also indicates Q50 (the next transistor in circuit) is
ok. Probing the pads with power off and the transistors (Q35, Q40, Q50)
removed, my ohm meter shows values that seem reasonable for the connected
circuitry. There's drift, but it doesn't seem sufficient for the level of
heating. For example, R35 (to +5) is spec'd at 100 ohms and measures 108,
while R40 (to -22V) is spec'd at 510 and measures 508. Resistance in the
level pot seems ok and changes smoothly as the knob is turned.
Power rails are in spec.
What would you recommend doing next? Look elsewhere because it's normal for
the transistor to get so hot? Desolder adjacent components to measure them
individually? Find a replacement jfet? Or something else?
[1] Q40 is connected to the -22v rail through a 510 ohm resistor. The
rail's regulation failed due to a bad pass transistor in my TM503, putting
the rail at 30-something volts below ground.
Regards
Mark
