Topics

FG-504 not-so odd behavior

John Ferguson
 

I now have two FG 504's.  The first doesn't oscillate despite all manner of exercising the buttons and switches and has a defective VAR pot for the frequency vernier - knob turns through 360 degrees.

The second one seems to be functional except for square wave.  I haven't tied down the frequency but it puts out a very high frequency triangular signal.  It seems to be within the range of my 2445b scope, so tomorrow when my spouse lets me return to this I should be able to measure it.

The manual for this instrument is really good. It runs you through some exercises for first time power on (or first time use for me) and except for square wave and the functions associated with it all seems ok.

Could this be a defective Schmidt trigger (circuit)  and if so, is it possible for only the square wave functions to be affected by it?

john ferguson

keantoken
 

Start by checking the PSU voltages.
See where the oscillation stops, starting at the oscillator board on the left side of the plugin. J300 is the output connector for the oscillator. If the oscillator is dead, check the open case trimmers for open wipers as this will stop oscillation or peg the output frequency (check any voltages marked on the schematic for hints). Replace with sealed Bourns trimmers as necessary, working them back and forth may sort of bring them back to life but they will drift or die again. The frequency vernier var pot may be open if it is broken, or you may just have a broken coupler allowing the knob to spin freely.

This is on the triangle wave generator schematic in the service manual (page 138 in the PDF I have). Check any voltages marked on the schematic. The start, stop and var pots may stop oscillation if they go open.

On my FG504, the open case trimmers are all losing wiper continuity due to age.

I also had the regulator trimmers go open so check your voltages. On either side of the power output transistors on the right side of the FG504 I had the large MLCC capacitors short out, they crumbled when I replaced them. Oddly enough I have had no problems with the tantalums so far, but check them and see if any are getting hot.

keantoken
 

For the unit with frequency pegged high, I would look for an open x40 cal trimmer or dial min cal trimmer. Anything pulling the U30A/B inputs low will peg the frequency high.

John Ferguson
 

the pegged frequency is only on the square wave. I looked at it briefly and it isn't square but triangular.  fwiw, I'll check frequency at the 10to the second setting and compare it with what I'm getting on the sine and triangular.

john

On 7/10/20 6:55 PM, keantoken via groups.io wrote:
For the unit with frequency pegged high, I would look for an open x40 cal trimmer or dial min cal trimmer. Anything pulling the U30A/B inputs low will peg the frequency high.

keantoken
 

The oscillator has two outputs, a triangle output for the triangle and sine functions and a square output for the square function. J300 is the triangle output. The oscillator always outputs both when it is working as they are both parts of the feedback loop.
Since both waves come from the same oscillator it is very strange if the frequency changes when changing function, as that only changes which output is used from the oscillator. On Friday, July 10, 2020, 09:45:57 PM CDT, John Ferguson via groups.io <jferg977=aol.com@groups.io> wrote:

the pegged frequency is only on the square wave. I looked at it briefly
and it isn't square but triangular.  fwiw, I'll check frequency at the
10to the second setting and compare it with what I'm getting on the sine
and triangular.

john

On 7/10/20 6:55 PM, keantoken via groups.io wrote:
For the unit with frequency pegged high, I would look for an open x40 cal trimmer or dial min cal trimmer. Anything pulling the U30A/B inputs low will peg the frequency high.


John Ferguson
 

with the FG 504 set at start freq 10, stop freq 40, multiplier at 10^2, sweep off,

measured on my 2445b, Sine freq 982.3 Hz, Sawtooth 975.9 Hz and Square wave 24.5 mHz.

The square wave is an asymmetrical  sine wave, not the Sawtooth I was expecting.

Since I have two of these things and this one at least does this, and also the sweeps and other functions, all except a square wave, and the other one does nothing, this might be the one to fix.

What do you think?


john

keantoken
 

10*10^2=1KHz, so it seems to be partly working. All functions should be 1KHz so something is off.
Check that the oscillation is coming from the oscillator. The oscillator may be behaving normally and the output amp is oscillating on it's own. If they are both oscillating at 25MHz then I would look at the decoupling caps or a broken ground causing feedback between the output amp and the oscillator.
I would try a little harder to identify the fault in the non functional one. It could be that when you find the problem it will suddenly work fine with no issues. On Saturday, July 11, 2020, 09:51:04 AM CDT, John Ferguson via groups.io <jferg977=aol.com@groups.io> wrote:

with the FG 504 set at start freq 10, stop freq 40, multiplier at 10^2,
sweep off,

measured on my 2445b, Sine freq 982.3 Hz, Sawtooth 975.9 Hz and Square
wave 24.5 mHz.

The square wave is an asymmetrical  sine wave, not the Sawtooth I was
expecting.

Since I have two of these things and this one at least does this, and
also the sweeps and other functions, all except a square wave, and the
other one does nothing, this might be the one to fix.

What do you think?


john