Ampeg Jet J-12 Tremolo Problem

[rf7]

I've tried the Fender "bug" and the $35 part that fliptops sells and I still can't get the circuit to work. The oscillator is working fine. Any ideas?

schematic

[Twist]

How did you determine the oscillator was working? Try replacing all the 0.047uf caps in that circuit. Check the switch and the footswitch jack.

[Enzo]

I would ask the same question.

But assuming it does oscillate, what appears at the top end of the neon bulb? LFO? DC?

The Fneder part lets yopu look down the end and see the bulb flashing. Do you see it?

I wonder if the bulb is being driven, or if it is, is the photocell doing its job. We need to isolate the problem to one side or the other of the process.

If the bulb flashes, then disconnect the photocell leads and monitor what the resistance does. An analog meter would be great for this, but the trend scale on a digital meter should suffice.

[rf7]

How did you determine the oscillator was working? Try replacing all the 0.047uf caps in that circuit. Check the switch and the footswitch jack.

The oscillator output looks fine on my scope. I already replaced all of those caps. The switch seems to work correctly, it stops the oscillator when off and lets it run when on.

I would ask the same question.

But assuming it does oscillate, what appears at the top end of the neon bulb? LFO? DC?

The Fneder part lets yopu look down the end and see the bulb flashing. Do you see it?

I wonder if the bulb is being driven, or if it is, is the photocell doing its job. We need to isolate the problem to one side or the other of the process.

If the bulb flashes, then disconnect the photocell leads and monitor what the resistance does. An analog meter would be great for this, but the trend scale on a digital meter should suffice.

The top end of the bulb, on the scope, looks like the bottom portion of a sine wave climbing up to a point (see picture) and then it's kind of flat for a longer period, and then it repeats. My digital meter shows the photocell resistance going from 140K to 180K ohms when the tremelo is on. This is with the Ampeg LDR.

With the Fender bug, I get the same wave shape and the resistance goes from 20K to 50K ohms. And I hear a clicking sound on the output, but no tremelo effect on the guitar .

[IMG][/IMG]

[Enzo]

OK, so the bulb is lighting.

The waveform is great. A neon lamp takes a certain voltage before the gas will ionize and glow. Once it fires, then the voltage across the lamp drops. See the sine wave ramps up to the firing threshold, then the bulb fires and WHAM the voltage drops to whatever the arc represents. Then when the sine wave falls below the minimum voltage that sustains the arc, it drops out of conduction.

The flat area is the bulb under conduction. Note that it is somewhat higher than the minimum of the sine.

I think it is hard to measure the photocell with the LFO running. Stop the LFO with the bulb ON. Measure the resistance of the cell, now either short across the bulb - those large series resistances will prevent this from hurting anything - or just turn off the power, and measure again. That is what the part is capable of range-wise.

The 5.6M resistor and the 1M volume pot form a voltage divider for the signal passing on to the next stage. The photocell is in parallel with the 5.6M. so the variation in resistance causes a variable division in volume. So the range of change has to be appropriate. The 5.6 insures a continuous path, but with the trem bulb on, it largely disappears I think. SO if the cell varies 20k-40k or something over top of a 1M resistance, that isn't going to make much of an effect , I wouldn;t think. We need a large variation, don't we?

[rf7]

When the amp is off, my DVM set on it's 20Mohm scale measures both the fender bug and the Ampeg LDR as open circuit. In circuit, with the bulb on and tremelo off, the Fender measures 14K ohms, the Ampeg measures 70K ohms.

I remeasured each one, with the Tremelo on and with my meter set on different ranges, and I got a variation of 17k through 84K for the Fender bug and a variation of 115K through 250K for the Ampeg LDR. The higher ranges on my meter showed higher values for the variation's upper limit.

I forgot about the 1M pot in series there. I guess I need a LDR with a much higher resistance range for this to work. Anyone need a $35 Ampeg LDR?

What about taking the oscillator out thru a bypass cap to the junction of R25 and R26, after lifting that junction away from ground? Leaving the cathode resistor and cap tied to ground, of course.

[rf7]

So, is there some other LDR that I can use for this?