Vox Tremolo Problem, Troubleshooting HELP needed

[toobnoob]

Hi everyone, new guy looking for help here, I hope someone can help with troubleshooting this oscillator circuit.
I have a Vox Cambridge Reverb here that has a non-functioning tremolo.
I'm familiar with the Marshall 18w circuit and this is similar.
I've lurked and read as much on the subject as I could find through searches and tried everything i could think of.
I'm stumped and could really use some help.

Here's what I've done so far
Tried multiple tubes in the position, have a high gain JJ in there now.
Checked to see if circuit was oscillating by strapping a red LED from the cathode to ground.
The LED did not light.
Disconnected cathode resistor and bypass cap and left LED only.
The LED never flashed or lit up.
I measured 1.6v at the cathode prior to altering it (pin 3 v7b)

Replaced all timing caps (twice)
Traced circuit and verified all connections.
Measured all components in the circuit; pots, resistors, caps.
Cleaned pots.
Replaced all electrolytics; power supply and bypass caps.
Verified grounds.
Voltages are all within 10-20v of the schematic.

What puzzles me is I have no oscillation with all new parts in the circuit, I measure 1.6v at the cathode but the LED doesn't change with the speed or depth control and doesn't light.

It seems like I've checked everything multiple times, the 220k plate resistor seems fine, the voltage calls for 270v, I measure 280v.
The caps are new, the tube is strong, good grounds in the network.
What am I missing?
I'm really stuck and don't know what else to try.

[shooter]

If you don't have a scope put your led back in, verify the 1.6v then try disconnecting the wire going to the trem 1/4" jack (the one to the footswitch).  also try unsoldering the wire coming off the middle of your depth pot.
good luck

[Willabe]

Here's a link to a thread on a Marshall trem that was resolved, it might help you.

There's a lot of good info in there, I linked to the final fix, but if you have time go back and read the whole thing.

http://el34world.com/Forum/index.php?topic=16282.msg160098#msg160098


                    Brad   

[sluckey]

Is this an old amp that once had a functioning tremolo? Or is this a homebrew that the trem has never worked? I ask because the troubleshooting path will be a little different for the two cases.

I would start by removing the LED and put a good resistor/cap on the cathode. Get the circuit back to stock. (You can always do the LED mod later.) Monitor the plate voltage to see if the oscillator is working. If plate voltage is steady, it ain't working. If plate voltage is fluctuating, it is working. A scope or analog voltmeter would be very nice, but even a DMM will tell you if the voltage is fluctuating, but the display will just appear erratic.

[toobnoob]

Ok  awesome some replies!!
Thanks, I did read that entire thread and the related offshoots before I posted, lot of good info there but nothing there helped.
The circuit is stock, I presume it was working at one time but not since I've had it.
The cathode resistor is good and there's a new bypass cap re-connected.
So the plate voltage should fluctuate?
I will take a measurement right now.
I tried grounding the foot switch, nothing helped there.
I didn't think to unhook the wire from the middle of the depth, what will that do?
Be back in a few minutes.

[sluckey]

I tried grounding the foot switch, nothing helped there.

That circuit requires the footswitch to be OPEN to enable the oscillator.

[eleventeen]

Does an LED in a trem circuit require any sort of current limiting resistor? Even a small one?

[sluckey]

Does an LED in a trem circuit require any sort of current limiting resistor? Even a small one?

It has the tube and plate resistor for that.

[toobnoob]

Hey guys it's working, thanks for your replies.
Upon examination I discovered that there was an error in the power supply wiring. Someone had replaced the main filter caps and mis-wired part of it and I missed that until some of your suggestions prompted me to take a closer look.
I did learn a lot about how the circuit works from all the information in the other threads referenced.Sounds I replaced all the filter caps, made sure it was all correct and the amp sounds great, thanks for your help!

[toobnoob]

Now I have another problem surfacing.
The reverb is noisy.
With the reverb control up there's a hum, maybe this is normal, it doesn't change much or go away if I move the cables around, but there's a distortion with the note when I hit it hard, and there's a runaway oscillation with certain settings; different combinations of reverb and tremolo.
So if the tremolo is on in any setting, there's a place on the reverb control, a little past halfway, that causes trouble, runaway oscillation sounds like a hip hop DJ scratch kinda thing.
What should I be looking for here?
I cycled through a number of 12AX7's (both reverb recovery and tremolo are on one tube) and not much difference.