Gibson GA-8T 120hz.Hum & No Tremolo

[Helmholtz]

I made a stupid mistake above (please see Edit). The trem depth wiper must not be disconnected, rather it should be at ground potential.
The idea behind is to prevent HF generated by a possible malfunction of the trem generator entering the power stage.

[misterc57]

All tests and readings have been with both trem pots at zero. Tone at 5, Volume at 5.

Enzo, I tried your approach and I had to change the time base because it was way beyond double the waves.

My earlier scope pic had the wrong setting on the scope probe X factor. It was 10X and should have been 1X. I am not using an actual probe, I have a dummy load box and the connection to the scope is coming out of the dummy load box, see pic.

I have taken more pictures. One with time set to 5 ms, the rest the time is set to 5 us which is the only time base showing a legible wave form. The different pics show different settings on the scope.

I do not have enough experience using the scope for this type of analysis and I appreciate everyone's help and patience.

Thank you

[Helmholtz]

My earlier scope pic had the wrong setting on the scope probe X factor. It was 10X and should have been 1X.

So oscillation amplitude is actually 10 times lower.

I have taken more pictures. One with time set to 5 ms,

The first pic (with 5ms/div) shows trigger instability.
Find the trigger section on the front panel (probably on the far right).
Press the trigger source button and select "line".
Then carefully adjust the trigger level knob to get a stable display with a single curve.
It might help to select "HF reject" in the trigger mode menu.

In case of doubt consult the scope's manual.

[g1]

The first pic (that is not triggering right), seems to be 60Hz (the curve of the waveform looks like it will be 3+ divisions at 5mS per div.)
If you put a short across the input jack, is the hum still there with volume up?

[misterc57]

The first pic (that is not triggering right), seems to be 60Hz (the curve of the waveform looks like it will be 3+ divisions at 5mS per div.)
If you put a short across the input jack, is the hum still there with volume up?

Both inputs have shorted jacks and there is no plug inserted into them.

[g1]

Both inputs have shorted jacks and there is no plug inserted into them.

Yes. And if the shorting part malfunctions, you could get oscillation like this. Short the tip to ground manually to be sure.

[misterc57]

So oscillation amplitude is actually 10 times lower.



The first pic (with 5ms/div) shows trigger instability.
Find the trigger section on the front panel (probably on the far right).
Press the trigger source button and select "line".
Then carefully adjust the trigger level knob to get a stable display with a single curve.
It might help to select "HF reject" in the trigger mode menu.

In case of doubt consult the scope's manual.

I am reading the scope manual.

trigger source option includes "ac line", when I select that I cannot get to a single curve.

If I choose the autoset function which is suppose to set all the params and display the results, it changes the trigger source to CH 1 and DC coupling and displays the following pic.

[Enzo]

Don't confuse the high frequency oscillation with hum.

[misterc57]

Don't confuse the high frequency oscillation with hum.

But HF would not be audible, right?

I hear hum at the output/speaker and also at V1A plate (pin 6). Both increase with the VOL control. My frequency analyzer phone app tells me the speaker output is 120 Hz. This app has been very good at differentiating 60 Hz and 120 Hz.

Should I try to move all ground connections to one place?

When I first got the amp it had a rats nest of ground wires, filter caps, resistors, and no OT. I think all grounds were connected to pin 3 of the rectifier and I believe that is grounded via the white wire connecting all the pot and input jack grounds (so the grounding was probably the input jack contact to the chassis).

Below is a pic when I first got it.


Thank you

[Helmholtz]

The first pic (that is not triggering right), seems to be 60Hz (the curve of the waveform looks like it will be 3+ divisions at 5mS per div.)

Looking at the lower envelope line (and with a little fantasy) I might read something like 1.7 divisions between peaks, resulting in 118Hz.

Obviously the 120Hz component is much smaller than the 41kHz oscillation, making it hard to trigger and display.
What I was hoping to see was a dark band of HF with the upper and lower envelopes showing the 120Hz modulation.

[Helmholtz]

If I choose the autoset function which is suppose to set all the params and display the results, it changes the trigger source to CH 1 and DC coupling and displays the following pic.

Your output signal consists of a strong HF oscillation plus some 120Hz ripple.
The autoset function only works for the larger (HF) component.

Anyway you must get rid of the oscillation and changes are that will also cure the 120Hz hum.

[misterc57]

Good news. Hum is gone.

I removed all wires going to rectifier pin 3. The two PT center taps, one green striped and one red striped moved to my new ground point with all the filter caps.

The floating black wire removed from the circuit. The white wire coming from the pots/jacks was already at ground so I did not connect that one. Was this white wire causing a ground loop?

Thank you everyone!

Next, Figure out why I have no tremolo. I have not started to look at that issue yet. Suggestions?

MarkO

[Helmholtz]

Good news. Hum is gone.

HF oscillation also gone?

[misterc57]

HF oscillation also gone?

I am not sure, I now see this on the scope. No input connections, TREM pots at zero, VOT and TONE pots at 5. This signal varies with the VOL control.

I also noticed that while playing a guitar it sounds horrible with the VOL past 7 and gets worse at 10.

Thank you

[Helmholtz]

Looks like the same HF oscillation frequency as before but way lower amplitude.

Watching the scope, does chopsticking wires affect the oscillation?