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SOLVED: Sound Technology 1000A DUAL SWEEP not working

thornev

thornev

Super Member
I've used my 1000A to tune my 500-C and other receivers/amps. In following the 500-C FM alignment instructions and also the 1000A instructions, they have steps that involve the DUAL SWEEP function. I've tried using it several times but can never get the suggested image on any of my 3 scopes. What I always get is an image with moving waves emanating outward larger and larger from a center dot on the screen. No matter how I try on the scope to tune in the suggested wave, I never can get it.

Can someone recommend a good forum where I can get help? Or someone here maybe can help? I've studied the schematic (I have the manual) and measured some of the parts, but nothing sticks out as a cause of DUAL SWEEP's not working. Frankly I can't figure out where the DUAL SWEEP circuit is located. The 2 main filter caps are 1500uF and both of mine measure 1800. ESR meter claims they're good.

I did calibrate the DC BALANCE which was a bit high (I got it as low as it would go) and the +5V circuit as described in the manual measures 16V, but I figure the manual is wrong in stating that the Q13 emitter is where I measure. I believe Q15 is the correct part and it does measure in the high 5's.

I'd like to get this thing working because the 1000A is the only test equipment I have that transmits a modulated FM frequency (EDIT: not true as I also have a Heathkit SG-8). Already been to Stan Curtis' web site.

EDIT: I've been scouring the internet for 1000A info and came upon this claim: "IIRC the "CW" position outputs 10.7MHz through the RF allowing you to directly inject it into IF sections." I didn't think the 1000A can generate a 10.7MHz signal.

Also the manual states that if the internal oscillator's Total Harmonic Distortion is set too low, it may affect the operation of DUALSWEEP and SCA functions. Maybe a previous owner set it too low for their purposes?
 
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If you're trying to make what you see on the scope with a 1000A look like what is in the alignment manuals for the Fisher, its not going to happen. They work in a different way and the screen looks different. You have to use the 1000A alignment instructions and match it to what those look like.

this also might get more useful traction in the Tuners area
 
No, Gadg, I'm not doing that. I'm following the 1000A instructions when I'm trying to get the passband image. The Fisher alignment instructions don't have any images to expect from a scope.
 
I believe I fixed DUAL SWEEP. I put a frequency counter on the 1000A and nothing was coming out the INT OSC (internal oscillator) jack. I adjusted R105 until I got a signal and now 1kHz is coming out (actually 9900Hz). The manual does state that R105 adjust THD and if set too low, it can affect DUAL SWEEP mode. Someone must have set it too low.

EDIT: I misquoted myself in this post. I meant 10kHz is coming out, not 1kHz. 10kHz is the spec for DUAL SWEEP at the INT OSC port.
 
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990.0 hz is fine, 9.9 khz is pretty far off the mark. I don't believe that frequency is overly critical but it should be somewhere in the ballpark of spec.
 
Gadg... I misquoted myself in post #4. I said 1kHz was indicated when I should have said 10kHz which is the spec for DUAL SWEEP at the INT OSC port. I'll correct the post. Thanks for pointing it out.

I made the oscillator "improvements" per http://ham-radio.com/k6sti/st-1000a.htm. Unfortunately that mod ruined the frequency indicated at the SCA port. Indication should be between 60k and 74kHz, but now I get 44kHz and the tops and bottoms of the sine waves are flat (distortion). A previous owner had put a 1.5k resistor between R100 and R105 pot which is what R104 is supposed to do. I don't know if that has anything to do with the change, but I'll have to decide if I've really improved my 1000A or remove the mod. I doubt I'll ever test the SCA (Subsidiary Communications Authority) subchannel so no loss. SCA is for signals like Muzak as I understand it.
 
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yeah the SCA output should be 67khz, and ideally it ought to be quite accurate otherwise adjusting the trap won't work right. On the other hand I don't know that much is even broadcast with SCA these days so functionally speaking it might not matter too much. I guess it will serve to deal with any harmonics from the 38khz oscillator though.
 
In addition to SCA at 67 kHz, there are other subcarriers at several frequencies. There is also digital information. For todays environment, the 67 kHz filter should be replaced with a low pass filter at 53 kHz (top of the stereo subcarrier band) and cut out everything above that.
 
Wow, Fred. subcarriers go below 60kHz which was previously believed (from what I've read) to be the low end for SCA. Interesting. Are these subcarriers actively used in today's AM and FM radio market?
 
Yes. Subcarriers at 57 kHz, 67 kHz, and 92 kHz. The 57 kHz is for text output such as a car radio showing the name of the song playing or other similar info. Subcarriers are allowed at lower frequencies if the station is not broadcasting stereo. There must still be a guard band around 19 kHz to prevent false triggering.
 
thornev said:
Gadg... I misquoted myself in post #4. I said 1kHz was indicated when I should have said 10kHz which is the spec for DUAL SWEEP at the INT OSC port. I'll correct the post. Thanks for pointing it out.

I made the oscillator "improvements" per http://ham-radio.com/k6sti/st-1000a.htm. Unfortunately that mod ruined the frequency indicated at the SCA port. Indication should be between 60k and 74kHz, but now I get 44kHz and the tops and bottoms of the sine waves are flat (distortion). A previous owner had put a 1.5k resistor between R100 and R105 pot which is what R104 is supposed to do. I don't know if that has anything to do with the change, but I'll have to decide if I've really improved my 1000A or remove the mod. I doubt I'll ever test the SCA (Subsidiary Communications Authority) subchannel so no loss. SCA is for signals like Muzak as I understand it.
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Did you ever get your 1000A worked out? I did the mod and had the same results: improvement on dual sweep, clipped signal on the other settings. I pulled the mod back out
 
I just finished an ST 1000A. Replaced every resistor and capacitor along with several transistors. End result is 1020Hz (1000Hz) and 396Hz (400Hz) on the internal oscillator. THD is about 0.025% on either 1000Hz or 400Hz @ 3.00VAC. Pilot is 19.000kHz and stable at about 2.4VAC. Dual sweep is 10.225kHz and SCA is 67.000kHz with a little drift. Both around 3.2VAC. I was able to adjust SCA with C44A after removing a resistor that was on the foil side of the main board. It was 120Ω on my unit.
 
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