[Radio Wrangler]

The spectrum analyser needs a wide range of sweep speeds. They don't need to be especially accurate and only a 1-2-5-10 sequence of steps is really needed, so to get the range without switching the timing capacitor, someone has had the idea of using a logarithmic converter, so that a linear series of increments produces a logarithmic series of sweep rates. The logarithmic converter is current in/current out and is formed by U16B and Q11B.

U16B runs with its inputs sitting several volts positive of ground. Circuit block B makes +10v (with a deliberate tempco) to act as a general reference for the whole ramp-current source.

Q11A is used to control an offset to the reference voltage in order to temperature compensate the log converter transistor Q11B which is the other half of a dual part. this offset voltage affects the speeds of all sweeps, so it has the trimmer for adjusting the slowest speed (blurred R66?)

Schematic block 'C' is a bank of switched resistors to ground, modified by a temperature dependent voltage set by the Vbe of Q12, which is used as just a diode.

The resistance to ground of block 'C' modifies the gain of U16B and hence the output voltage of U16B. Note that this is all with respect to the reference voltage. Note also that the 'gain' of U16B is rather low if you think of the grounded ends of block 'C' resistors as being an input to U16B.

The difference between the output of U16B and the reference voltage sets the base-emitter voltage of Q11B. The result is that the collector current of U11B is the anti-log of the current through the switched resistors in 'C' and so it covers an expanded range.

Why on earth did they use such a complicated arrangement, bring ing in all sorts of touchy temperature compensation etc? OK, it saves switching some capacitors but it doesn't use fewer components. There is only one benefit from using a single unstitched capacitor, and that is that if you push a button to change sweep speed part way through a sweep, the speed changes and continues from the place you pushed the button. With switched capacitors for different speeds, the sweep would jump to whatever charge there was left on the new capacitor every time there was a change.

So this explains what all the stuff before C16 is doing, but C16 should have a nice linear ramp on it. No curviness, no spikes.

There are switches to allow the sweep to be halted to make a pause for the frequency controller to operate, of for the signal frequency counter to run. This will put a flat spot on the ramp - just a bit of horizontal time, not a spike.

There is a switch to reset the ramp to the start. This uses U1B as a feedback system to pull C16 voltage down until the output of the buffer amp (Q17, Q18) is held at a voltage set by R109, R110. This seems to be working.

TP17 looks at the ramp after the buffer so your scope doesn't load the ramp capacitor C16 directly. On TP17 you see a curved ramp and spikes. The spikes are fairly fast and would take a lot of current to make C16 move that fast. It's unlikely that this is happening. Besides, as Jeremy says, C16 would show a lasting effect from such a jump.

What about something downstream putting spikes onto everything? Q17 is a JFET and its gate should be a reverse biased diode. If something big enough happened downstream the FET junction could become forward biased, but that doesn't explain the curvature.

I think Q17 isn't happy and it's taking gate current which increases with input voltage, thus slowing the ramp on C16, and curving it. Maybe its glamorous assistant, Q18, isn't good and Q17 is trying to drive the output alone?

This points to replacing Q17 and Q18.

But first, I'd check the voltage on C17 for spikes, and the ESR of C17. I'd check that U18 wasn't trying to do anything during the ramps by probing R110.

HP's usual resistors are long-lived. Their capacitors are showing age, and there are some transistors which definitely have shown long-term issues. Q18 is probably an 1853-0020 (good old 2N3906) and marked 3-020 usually. I'm not going to guess Q17


David