Sound Technology ST 1000A SCA Calibration

[jblmar]

Hello,
My ST 1000A works fine except that the SCA output reads 63.3 KHz not the specified 67 KHz. The manual doesn't cover alignment of the SCA frequency nor the tolerance of the frequency.
Anyone familar with calibration of this or if it is within spec?

 

[audiohead]

Check out this link:Updating the Sound Technology 1000A.

Art

 

[jblmar]

Thanks audiohead.
I have that link on my computer.
My concern is if the SCR should be 67K exactly or is there a tolerance (%). It's off by around 6%.

 

[audiohead]

The SCA channel occupies the space from 60 to 74 khz, and its center is 67 khz.

You may have to look up the "FM stereo transmission standards" for allowable SCA center frq. tolerances.

Do you have a trim cap C44A as shown in the schematic? If yes, then adjust it for exactly 67 khz with the function sw. in SCA mode.

Art

The FM generator of the choice is Leader 3216.

 

[jblmar]

That's a very good suggestion. It may be within FCC limits where it is.
I'll check for C44A. I would rather not mess with it right away and risk misaligning something that may well be aligned.

 

[audiohead]

It is real easy Just connect your frq. counter or your scope to pin6 of MC9, with function switch in SCA mode, and adjust C44A for exactly 67 KHZ.

Art

 

[jblmar]

Thanks, Art.
I have the Fluke 1900A frequency counter. Very accurate.
Couldn't I connect the 1900A to the In. Osc. output and measure/adjust there?

 

[audiohead]

It is even easier, Yes you can use J4 (INT OSC) BNC/RCA jack to measure and adjust if needed.

Art

 

[jblmar]

Art,
Thanks for your time. Before touching anything, I'll get some info on the 'net regarding allowable limits.
There is very little info regarding ST gear on the 'net.

 

[audiohead]

This is really not a big deal, all you need is an isolated screw driver/alignment tool.

You monitor the Osc. output, and adjust C44A trim cap.

Internet is wealth of info, but you don't know who is providing it.

Just do it!

Art

 

[jblmar]

Agreed.

 

[Murray Allen]

Adjustment of the trim cap may not be enough to bring the the oscillator back on frequency.

Check the values of the carbon comp resistors around the modulation oscillator. You will likely find that they will all be at the top edge or slightly out of of tolerance. It is well known that carbon comps do not age well and will have drifted high in value.

I recently re-calibrated 4 ST1000's that all exhibited low modulation frequencies. Replacing these resistors with 1% metal films put the oscillator bang on frequency without further adjustment.

 

[Murray Allen]

Adjustment of the trim cap may not be enough to bring the the oscillator back on frequency.

Check the values of the carbon comp resistors around the modulation oscillator. You will likely find that they will all be at the top edge or slightly out of of tolerance. It is well known that carbon comps do not age well and will have drifted high in value.

I recently re-calibrated 4 ST1000's that all exhibited low modulation frequencies. Replacing these resistors with 1% metal films put the oscillator bang on frequency without further adjustment.

 

[jblmar]

Thanks Murray.
Eariler I was going to adjust the cap, but my training got the better of me. I need to look into this futher before doing something I'll regret.