Hello So it's time to start fixing the next amp - this time it's a VoX AC30 CC2 that has two issues (which I kind of think is connected). First - it does not let any sound through until you bash the strings, this happens typically a few minutes after turning it on. The second problem has to do with the tremolo (I think). When the tremolo is on it usually puts out this extreme noise when pulsing (see/hear attached clip). The file is recorded with my iPhone so it doesn't sound like it is in the room, there is a lot of low end going on as well, but you'll get the picture. If I turn the treble and bass (top boost channel) fully clockwise it stops. It does not matter if I play in the top boost or normal channel. I started thinking this could be DC leaking through to the tone controls but I have changed the caps in the tone stack without any change. Has anyone experienced this kind of behavior with this or a similar amp before?
A quick measurement of the treble and bass potentiometers show that there is DC pulsating on the lugs when the tremolo is on, its fluctuating from about negative 250 V DC to about the same positive V DC. Is this supposed to happen in this curcuit? The noise stops then the bass is fully clockwise.
I would first rig up a way to pump some music into the return jack of the effect loop section .
This will confirm that the last preamp tube ( phase inverter section) is working and also the complete output section is working.
The output of your iPhone cranked up should have enough output level ( voltage ) to hear some good sound out of the speakers , although it will not be loud.
A easy thing to do would be to take the signal out of the send jack of another amp and pump that into the return jack of the Vox.
All this being said your issue with the amp sounds like a blown / shorted output transformer which the test I posted about above will confirm.
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Stevem wrote: ↑Fri Nov 19, 2021 11:22 am
I would first rig up a way to pump some music into the return jack of the effect loop section .
This will confirm that the last preamp tube ( phase inverter section) is working and also the complete output section is working.
The output of your iPhone cranked up should have enough output level ( voltage ) to hear some good sound out of the speakers , although it will not be loud.
A easy thing to do would be to take the signal out of the send jack of another amp and pump that into the return jack of the Vox.
All this being said your issue with the amp sounds like a blown / shorted output transformer which the test I posted about above will confirm.
I tested with some music through the FX-return and there is lots of sound, actually its very loud when I turn up the master. The problem with no sound getting through is not present at this time (using the amp input). But the noise issue is still present. The amp works fine if the tremolo is turned off (depth to 0) or if the bass is turned fully up...
Edit: maybe its just as silly as a bad Bass-pot... I messed a bit more with the pot and the problem is now almost non existing. I guess I'll have to pull the board (if I can somehow loosen those nuts on the mini switches) and try to fix or replace the pot.
So it's time to start fixing the next amp - this time it's a VoX AC30 CC2 that has two issues (which I kind of think is connected). First - it does not let any sound through until you bash the strings, this happens typically a few minutes after turning it on. The second problem has to do with the tremolo (I think). When the tremolo is on it usually puts out this extreme noise when pulsing (see/hear attached clip). The file is recorded with my iPhone so it doesn't sound like it is in the room, there is a lot of low end going on as well, but you'll get the picture. If I turn the treble and bass (top boost channel) fully clockwise it stops. It does not matter if I play in the top boost or normal channel. I started thinking this could be DC leaking through to the tone controls but I have changed the caps in the tone stack without any change. Has anyone experienced this kind of behavior with this or a similar amp before?
