Updating the Sound Technology 1000A

The Sound Technology 1000A FM Alignment Generator is an excellent FM stereo signal source whose design dates from the late 1960s. While its performance was adequate at the time, FM tuner distortion continued to improve over the years, eventually falling below that of the ST-1000A. Signal generator distortion sets a lower limit on aligned tuner distortion. The maximum-THD spec for the ST-1000A is 0.1% in mono and 0.2% in stereo for 1 kHz modulation deviated 75 kHz. Simple circuit modification can improve these figures by an order of magnitude. Other modifications improve accuracy, versatility, and ease of use.

Internal Oscillator

Modulation linearity is mainly limited by the internal audio oscillator, a Wien bridge sinewave generator. A P-channel JFET acts as a voltage-controlled resistor to automatically set the oscillator loop gain to one. Variation in JFET channel resistance with drain-source signal voltage distorts the sine wave.

Trimmer R105 in series with the drain sets the oscillator operating point. This adjustment trades distortion for reliable oscillation. The ST-1000A alignment instructions say to adjust R105 for about −64 dB (0.06%) THD. Go much lower and the oscillator may stop. The internal oscillator also generates tones for dual sweep (10 kHz) and SCA (67 kHz) modes. Oscillation may fail at one of these frequencies first.

A trick to linearize JFET channel resistance may not have been known when the ST-1000A was designed. Applying one-half of the drain signal to the gate makes any variation in channel resistance symmetrical, cancelling even-order distortion. This kills the dominant second-order JFET distortion product, greatly reducing THD.

Another oscillator limitation is residual signal on the control gate. Any variation at the oscillation rate will distort the waveform. Filtering the control signal more heavily solves this problem.

You can modify the internal oscillator to incorporate these improvements as follows:

Cut the trace to the gate of Q8. Bridge the cut trace with a 100kΩ resistor. Solder another 100kΩ resistor between Q8's gate and drain (the ungrounded terminal). Finally, add a 10 F 15V electrolytic capacitor across C49 (observe polarity).

Adjust R105 for minimum distortion consistent with reliable oscillation in all modes. If R105 must be considerably advanced to obtain oscillation in one mode, distortion may be unnecessarily high in the other modes. By altering the capacitor ratio for the sluggish mode (C43/C47 for 10 kHz dual sweep, for example), you can make oscillation occur more readily. If R105 is set too close to the onset of oscillation, oscillation may fail at some temperatures. After modification, it is normal for the oscillator amplitude to take a few seconds to stabilize after starting or changing frequency.

This shows the spectrum of the 1 kHz internal oscillator after modification. Horizontal is 0 to 5 kHz, vertical is 10 dB/div, and the analysis-filter bandwidth is 30 Hz on an HP 3580A spectrum analyzer. Depending on the individual sweep and temperature, a tiny second-harmonic pip may occasionally appear somewhere near the −89 dB noise floor. The 400 Hz spectrum looks similar.

The signals in the spectral images are not deemphasized. A tuner's deemphasis network will attenuate the 1 kHz fundamental about 1 dB, the second harmonic about 3 dB, and the third harmonic about 5 dB from the levels shown.

Stereo Modulator

The ST-1000A stereo modulator uses a pair of 2N3819 JFETs as switches. Capacitors C111 and C112 (68 or 100 pF) slow the gate signals, which are 38 kHz square waves. This may cause the FETs to linger at a resistance higher than when fully on, increasing modulator distortion. Disconnecting C111 and C112 can lower it. I unsolder the nongrounded lead of each capacitor, leaving each part on the PCB supported by a single lead.

This is the composite stereo spectrum from 0 to 5 kHz after modification. I had adjusted the spectrum analyzer sensitivity to place the 1 kHz pip at the top of the screen in mono. Here it is about −6 dB for single-channel stereo. The second harmonic in stereo is at −82 dB. Its level varies somewhat with temperature but is always less than −80 dB.

This is the composite subcarrier spectrum from 33 to 43 kHz. The 37 and 39 kHz fundamental sidebands are about 12 dB down. Third-harmonic sidebands at 35 and 41 kHz are less than −81 dB. These harmonic levels also vary somewhat with temperature.

Disconnecting C111 and C112 normally has no adverse effect, but in one ST-1000A low-level switching glitches appeared in the composite output. The resulting spectral spurs were low enough, but after modifying the stereo modulator it's a good idea to check the composite waveform for glitches and the spectrum for spurs.

One ST-1000A with 68 pF capacitors had no visible baseband distortion products, but the subcarrier products were greater than −80 dB. Removing the capacitors reversed the distortion levels. 30 pF capacitors yielded the best balance between baseband and subchannel distortion, with everything below −80 dB.

In another ST-1000A the baseband second harmonic was well above −80 dB. Removing C111 and C112 mainly improves the gate signal fall time. To quicken the rise time, I added 22kΩ across R9 and R15. That brought the second harmonic down to −82 dB. It also reduced the gate drive, which is 8 V worst case for 2N3819 cutoff. If you modify R9 and R15, check the distortion at 150% modulation to verify that the FETs turn completely off for large signals.

Check the pilot phase alignment after any change to the stereo modulator. Occasionally I've been able to adjust R93 to bring the phase within spec, but the trimpot is all the way to one end. To permit full adjustment, I add 3.9kΩ across R92.

Be sure to check distortion for both left and right inputs when modifying the stereo generator.

RF Distortion

I used a Marconi 2305 modulation meter to evaluate the overall ST-1000A distortion, which includes the distortion of the internal oscillator and stereo modulator plus that of the varactor frequency modulator. The Marconi contains a wideband, low-distortion, charge-pump FM detector. To check its own residual distortion, I built a pulse-count FM detector and attached it to the Marconi's 1.5 MHz IF output. I looked at an FM signal with multiple harmonics below −60 dB. The harmonic spectrum was identical for the pulse counter and the Marconi detector, except for some difference in second-harmonic levels below −70 dB.

To characterize the Marconi second-harmonic distortion, I used Bob & Ken's spectrum inversion trick. I tuned the Marconi local oscillator above the ST-1000A signal frequency and measured the distortion level. Then I tuned it to the low side and repeated the measurement. These LO settings invert the phase of the ST-1000A second harmonic. The ST-1000A and Marconi distortions sum in one case and subtract in the other. I adjusted R52, RF distortion compensation, until the residual second-harmonic distortion was the same at both LO frequencies. It was −68 dB, and this is the residual Marconi second-harmonic distortion. This procedure nulls the ST-1000A second harmonic and is the best way to adjust R52.

This shows the demodulated spectrum of a 97.5-MHz monophonic signal deviated 75 kHz. In some units the second harmonic is anywhere between the noise floor and −75 dB, depending on temperature. I obtained these results before I knew about the spectrum inversion trick. The second harmonic here probably is a combination of ST-1000A and Marconi distortion. The third harmonic is stable, here at −75 dB (−70 to −80 dB in other units). Based on tests of the Marconi detector, I believe this distortion product is due to the ST-1000A.

This is the demodulated baseband stereo spectrum (9% pilot, 75 kHz deviation). The spectrum analyzer sensitivity is the same as for mono. It is hard to identify distortion products in the residual noise. At some temperatures they may just appear at the noise floor. Harmonics are much lower in stereo than mono because the baseband signal level is about 6 dB lower and harmonic reduction with level is much faster than linear.

This shows the demodulated distortion products around 38 kHz. All are less than −82 dB.

Intermodulation distortion products may appear on the demodulated 19 kHz pilot signal. Here they are less than −79 dB. In some units these products are below the noise floor.

Equalizing Deviation Levels

When switching from mono to stereo, the pilot increases total deviation. To keep the deviation constant, you must reduce the internal oscillator level. The ST-1000A will do this automatically if you install an optional resistor in the pads provided. One end of the resistor connects to the junction of R120 and C54. Function switch section S1-6 grounds the other end in stereo mode, forming a voltage divider that lowers the internal oscillator level.

The yellow rectangles outline the pads for the optional resistor. In my ST-1000A, 160kΩ equalized mono and stereo deviation for a 9% pilot. This resistor is not mentioned in the ST-1000A manual nor shown in the regular schematic, but it is indicated as R206 on the option M3 schematic with a value of 47kΩ.

You may also see a deviation change when switching to L+R or L−R. In my ST-1000A the deviation rose in these modes. Adding 62kΩ across R113 equalized the levels. The pads for this 10kΩ resistor coincide with the lower line of the yellow rectangles. Its value is 8.2kΩ on the M3 schematic.

Pilot Injection

To better measure pilot injection, the TEST pushbutton kills the modulation and increases meter sensitivity by a factor of ten. To check the times-ten accuracy, apply an external 19 kHz in mono, adjust the input level until the meter reads exactly 100%, and then measure the AC voltage at the COMP jack. Remove the 19 kHz, switch to stereo, press TEST, and adjust the pilot level until the composite voltage is exactly 10% of the previous reading. The meter should read 100%. Adjust the value of R142, a factory selected part, to correct any error. The green rectangle in the righthand image above outlines the pads for R142.

SCA Frequency

If your ST-1000A has option M2, which adds a switch to select an internal oscillator frequency of 400 or 1000 Hz, you'll find that the SCA frequency varies with the switch setting. This stray-capacitance effect is not mentioned in the ST-1000A manual. Pick one switch position as standard and adjust C44A for an SCA frequency of 67 kHz. The switch has a negligible effect on the 10 kHz dual sweep frequency.

Auxiliary Input

Option M1 provides a wideband auxiliary input on the rear panel that bypasses the stereo modulator and lowpass filter. It's mainly intended for injecting SCA signals at frequencies other than 67 kHz, but it has many other uses. I use it to precisely check a tuner's deemphasis accuracy, avoiding any frequency response error due to the lowpass filter. I apply signals near 114 and 190 kHz to test susceptibility to HD Radio self-noise. You can connect a composite stereo generator to test stereo separation at high frequencies. (Separation for the internal stereo generator is specified above 1 kHz only with option M3 installed.)

To add option M1, install a BNC socket in the prepunched hole on the rear panel, install R34 and C150 on the PCB, and run a shielded cable from the BNC to the PCB. 75 kHz deviation requires about 2.4 V RMS. Although normal SCA injection levels require a fraction of this level, it may be difficult to generate enough signal for full-deviation tests. I added 9.1kΩ across R34 and 100 pF across C150 to increase sensitivity. Adjust C150 for equal deviation at 1 kHz and 100 kHz to obtain a flat, wideband response (+0.03 dB at 50 kHz and −1 dB at 200 kHz). To precisely set C150, measure the RF deviation. After setting C150 with a modulation meter, COMP output was 0.44 dB greater at 100 kHz than at 1 kHz; a meter reading of 100% at 1 kHz became 105% at 100 kHz. If you can't measure the RF deviation, adjusting C150 for either of these alignments should be adequate for all but the most finicky. The composite output slew-rate limits (1% THD) at 150 kHz for 100% modulation and 240 kHz for 50%.

Variation with RF Frequency

Although I've never found it necessary, you can calibrate the RF LEVEL dial by loosening the setscrews in the internal shaft coupler. The output level spec is 1 dB at 98 MHz, but variation with frequency isn't specified. I measured +0.2/−1.4 dB from 88 to 108 MHz. Deviation also varies somewhat with frequency. For 75 kHz at 98 MHz, I measured +0.0/−2.1 kHz from 88 to 108 MHz. When modulated, the second-harmonic level also varies. It's best to align the unit at a clear midband frequency and make measurements there whenever possible.

dBf Scale

Later ST-1000As have an RF LEVEL dial with a second scale calibrated in dBf. This scale was originally intended for use with the Model 100 50:300Ω matching transformer, which has a 1:1 voltage transfer ratio (7.8 dB loss). If you loosen the two set screws on the knob, you can rotate it and the dBf scale with respect to the V scale, which is fixed on the shaft. This lets you recalibrate the dBf scale for any amount of external loss. For no loss, 73 dBf should line up with 1k V. If you alter this alignment to account for the loss of your 50:75Ω matching network and test cable, the dBf scale will indicate the power delivered at the end of the cable.

Model 100

Peter Livengood unpotted a Model 100 50:300Ω matching transformer. Inside he found a small ferrite transformer and resistors as shown above. Assuming a 1:2 turns ratio and no additional losses, the voltage transfer ratio is 1:1.11 (spec is 1:1 10%). A 50Ω source impedance becomes 364Ω at the output. A 300Ω load transforms to 51Ω at the input. It yielded an input return loss of 2426 dB across the FM band.


A schematic is here and part locations are here. A schematic for option M2 is here and part locations are here.


Stan Curtis has more about the ST-1000A here.

October 6, 201388108 MHz