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Discussion Starter #1
Howdy folks.

I’ve got a ‘66 tremolux that was in pretty rough shape when I got it. I had a local tech replace the filter caps and install a 3 prong power cable straight away.

Right after that, I replaced power tubes, biased it, and swapped preamp tubes with brand new ones known to work perfectly.

So when I got it, the front panel was dented at the intensity knob for the trem. The pot had been pushed in really far, and I assumed thats what was causing the trem tocnot work.

So I replaced the pot, punched out the dent, and still no joy.

Where should I start looking? I’m comfortable with a soldering iron and poking around inside amps.

Thanks in advance,
Scott
 

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I've forgotten which years, but many Fender trems will not work without the footswitch or a shorting plug at the trem footswitch jack.
If you are using the footswitch, it would be my first suspect.
 

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While earlier models used bias tremolo, the Blackface AA763 model uses a photoresistor and lamp. Possible the lamp is burnt out.
 

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I have three different schematics for the Tremolux. Only one (labelled 63-66 AA763) uses the optoisolator (P/N VTL5C1). If that is in there it could be the problem.
If not look to bad connection, the footswitch jack, or a bad capacitor.
It would help to know the chassis number.
 

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Only one (labelled 63-66 AA763) uses the optoisolator (P/N VTL5C1).
Dan, that part number is for the Vactrol opto (LED type) that they used in late 80's stuff. I think you meant the neon type, 0037836000.
 

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Discussion Starter #8
Update: I’ve plugged an RCA into the footswitch jack with no change. Should I try something with the RCA? Like maybe grounding it?

The photocell in the amp flashes when I switch the standby switch off. I haven’t seen if it lights up while playing yet.

The chassis number is A06282
 

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Discussion Starter #9
Update:

I’ve plugged a shorted RCA into the footswitch jack. I can hear the tremolo pulsing (almost clicking) through my speakers, but my guitar signal is unaffected.
 

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There's not much left in the circuit after that, the photoresistor, the intensity pot, or else it's not connected after the pot.

Is the intensity pot scratchy?
 

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Looking at the schematics for the AA and AB763, that was exactly my thought as well. I was going to suggest one or both of the electrolytic caps off the 12AX7 in the LFO circuit being dry and bad, since they set the gain of that tube. But if the light source is flashing, then that indicates the LFO subcircuit is fine.

The photoresistor bleeds off the Vibrato-signal to ground to produce the effect. At less than maximum depth, that bleedoff function MUST rely on the connection between the Intensity pot and the audio signal path, since the photocell is connected to the wiper of that pot. But at maximum Intensity, the photocell is tied directly to the audio signal path.

So that provokes the diagnostic question: does the audible pulsing change as a function of either or both Rate and Intensity settings, or some "special" combination of them?

Addendum: Re-reading your post, I misunderstood. You note that the light source flashes when powering up, but did not specify that it continues to flash once powered up. So....
the 7025 tubes used for the preamp are essentially interchangeable with the 12AX7 tube used for the LFO, albeit lower noise for audio portions of the circuit. Swap out the 7025 from the normal channel with the 12AX7. If the source of the problem is a bum 12AX7, then the tube swapout should result in a) the Vibrato resuming function, and b) the Normal channel ceasing to work very well, if at all. If the tube swapout does NOT fix the situation as described, then that would start to point to one or both of the electro caps in the LFO subcircuit.
 

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Discussion Starter #12
There's not much left in the circuit after that, the photoresistor, the intensity pot, or else it's not connected after the pot.

Is the intensity pot scratchy?
Nope. The intensity knob is brand new.

Looking at the schematics for the AA and AB763, that was exactly my thought as well. I was going to suggest one or both of the electrolytic caps off the 12AX7 in the LFO circuit being dry and bad, since they set the gain of that tube.
Which 12AX7 governs the trem?
So that provokes the diagnostic question: does the audible pulsing change as a function of either or both Rate and Intensity settings, or some "special" combination of them?
Only changes with the speed knob and only when the footswitch jack is shorted.
 

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Which 12AX7 governs the trem?
Only changes with the speed knob and only when the footswitch jack is shorted.
The schematic shows only one 12AX7. The two preamp tubes are shown as 7025, and the phase-splitter for the power section is a 12AT7. Quite possible that over the years, one or both of the 7025s have been changed out for 12AX7s, since, as noted, they are essentially the same, just lower noise. The drawing below shows that there are 4 smallish tubes in a row. The LFO tube is the one next to the 12AT7.
http://www.thevintagesound.com/ffg/schem/tremolux_aa763_layout.gif
 

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I have a new pot that was put in a bass where one pin is not connected internally to the wafer.

You know the light is going, so there's the photoresistor or some connection not being made.
 

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Maybe, but the OP notes the presence of audible "pulsing". One test of whether the issue is the Intensity pot connection is to simply lift the connection between the photocell and the wiper of the Intensity pot, and then link (temporarily) the wiper to either outside lug of the pot. If the problem is the pot itself, then varying the intensity would NOT also vary the amount of bleedoff of the audio signal to ground. If twiddling the Intensity pot does vary the volume - in effect a manual vibrato - then the problem is with the LFO subcircuit, rather than the Intensity pot.

I'm trying to identify "tests" one can conduct that are not especially invasive or destructive, and don't require spending any money. If we find that something has to be purchased and replaced, perhaps by a pro, then that will make itself known eventually.
 

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If he can hear pulsing, and it changes with the speed pot, then the LFO must be good, as far as I know.
Or is it very weak pulsing you hear?
If the intensity pot is new, perhaps it was miswired?
 

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Discussion Starter #17
Could be, but I made sure to connect it the exact same way it was hooked up originally. I’ll check my connections to the pot and trace the leads to make sure there isn’t a disconnect anywhere.
 

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Discussion Starter #18
Update: IT WORKS (sorta)

The LDR lights up and pulses with the effect on. The speed of the pulsing changes with the speed knob though when I go from slow to fast quickly, the photocell goes dark and then a half second later, starts pulsing at the new speed.

I traced the leads and they’re all attached to where they need to be.

The effect works for about 2 minutes and then stops on its own and then starts on its own about 2 minutes later, and repeat.

I’m thinking maybe a capacitor?
 

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Yes, if it is stalling out like that, it is the LFO as Mark suspected.
There are only 5 caps in that circuit, including the 2 cathode caps. Just change all 5, they're not expensive.
(this is assuming you have already tried a known good tube in the trem. position)
 

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Discussion Starter #20
Yes, if it is stalling out like that, it is the LFO as Mark suspected.
There are only 5 caps in that circuit, including the 2 cathode caps. Just change all 5, they're not expensive.
(this is assuming you have already tried a known good tube in the trem. position)
Yep there’s a new tube I know works well in there.

I’ll check the caps to see if they’ve drifted out of spec
 
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