Heathkit SG-8 Calibration

2/10/2011 7:12:09 PMMitch

Hello to all,

I have a SG-8 signal generator that is out of calibration. All bands read high on the frequency counter according to the dial setting. Example 1: I hook up my IG-102 sig. gen. (which is truly calibrated) to my IM-2410 frequency counter (which is also truely calibraded) on all bands the readings are within a hair line of the dial positions.

Example 2: After a good warm up period I connect the SG-8 to the frequency counter. Dial in 455 KHz on band A and read 482 KHz. 455 KHz does dial in at dial position 430 KHz. It is the same on all bands. I can get the signals that I want but not according to the dial positions.

The manual reads that there is no calibration required because of factory calibrated coils.

Are my capacitors shot in the Colpitts circuits?

Mitch

2/10/2011 7:56:04 PMPeter G. Balazsy

Here's pure guess work..

Is everything off by the same amount?

Sorry only wild guesses ... and I really don't know since I am not consulting the print but if there is one "base" oscillator and each range is a multiple of it or something.. then maybe the (base oscillator?) is off effecting all ranges.. ?

If so....See if changing the oscillator circuit tube makes any difference... just another guess.

Is it possible that the main knob and pointer are not positioned properly?

2/10/2011 11:58:13 PMEdd

Sir Mitch. . . . .

You need just a wee but more inductance on that A band oscillator coil.

This is just an initial . . . exploratory . . . operation, but find your self a small powdered iron slug . . . hex hole inside of it and have it retained by a hex diddle stick placed inside of the slug.

(So that we will be having 3 degrees of separation and avoiding any hand and capacitive detuning effect.)

Confirm if your dial scale knobs placement is symmetrical on its positions at both ends of its travel.

Note that the RF OSC coil has one winding connected to the plate and the other coil ends up giving feedback to the grid, with B Pluckinbothus feeding the center tap of the coil.

We will be interested in the half that connects to the plate.

Fire up equipmemt and temperature stabilize by run time:

Set your SG-8 dial to 455 and confirm that ~482 killo-giggo-re-cyclers . . as it was before.

Bring your slug near the center of " THE" end of the coil and move toward the coil proper and see the degree of frequency shift acquired.

Is that enough compensation to shift on down to 455 from your initial 482 ?

If so, we have found adequate inductance now.

ZUJ'ing for results . . . . to further show you how to-do-it-to-it . . . . from the mechanical auspices.

73's de Edd

:Here's pure guess work..
:
:Is everything off by the same amount?
:
:Sorry only wild guesses ... and I really don't know since I am not consulting the print but if there is one "base" oscillator and each range is a multiple of it or something.. then maybe the (base oscillator?) is off effecting all ranges.. ?
:
:If so....See if changing the oscillator circuit tube makes any difference... just another guess.
:
:Is it possible that the main knob and pointer are not positioned properly?
:
:

2/11/2011 12:15:04 AMWarren

The Assembly and Operation manuel does say the SG-8 is accurate between 2 and 3% of the dial pointer. There is a way to calibrat with the use of harmonics. Then mark the dial for a more accurate reference. You can do the same thing with your frequency counter. make your own pencil mark on the dial. The 12AU7 could be getting weak. From a cold start check the frequency with your counter. If it takes more than a good 60 seconds to stabilize.