Vox AC15 Reverb Issues

aienco

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Hello to all., I am new to this forum. (Couldn't find a Vox Forum). It is suggested many here own vox amps.

I am hoping someone maybe able to assist me with a reverb problem I have. The problem is it doen't work. I have the full schematics and I have done some initial testing. I was just about to remove the board and replace a few components. HOWEVER I plugged the in and out of the reverb to each other and the reverb knob does increase the volume. This indicates it is working???? I have replaced the RCA cables and installed a brand new tank, but still get no difference when I try the reverb. Tremlo and all other things on the amp appear to be working fine.

So pulling the board out seems to be pointless. BUT I have no idea where else to look if the tank and leads and connectors to this are all good and functional. It is just the reverb does nothing????

Anyone have this experience or know where else I could look please?

Thanks Tony.
 

Jon Snell

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Have you got reverb drive to the tank?
I think from your description, you have reverb recovery. Check the recovery by thumping the reverb tank and listen for the springy noises from the amplifier. Adjust the level of reverb to check it is producing springy noisies at adjustable volume.
Which one is missing?
 

Pete Farrington

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I plugged the in and out of the reverb to each other and the reverb knob does increase the volume
There should be a massive degree of gain when doing that. If not, signal is being lost somewhere.
The above ‘thump’ test should indicate whether it’s going astray on the drive or recovery circuit areas.
Model id, schematic link?
 

aienco

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Have you got reverb drive to the tank?
I think from your description, you have reverb recovery. Check the recovery by thumping the reverb tank and listen for the springy noises from the amplifier. Adjust the level of reverb to check it is producing springy noisies at adjustable volume.
Which one is missing?
Thanks for the answer. Sorry I will clarify. If I hit the tank or even move it, Yes I get a heap of noise from it through the amp. This is why I am confused. It seems that that is all the reverb does. Turning the reverb dial does make a noise when you hit the tank, but there is no NOTICABLE difference in the guitar tone. Definiatly not what I know as reverb. There does not seem to be any difference unless you hit the tank.
 

aienco

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There should be a massive degree of gain when doing that. If not, signal is being lost somewhere.
The above ‘thump’ test should indicate whether it’s going astray on the drive or recovery circuit areas.
Model id, schematic link?
There is. When I even move the tank or springs, it comes through very loud. So to me that would mean it should be working. However, I don't notice any difference in the guitar tones. I mean I have other amps, and you definatly know when reverb is on.
 

Jon Snell

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Depending on the actual model number and chassis fitted, there will be variations in the circuit.
Please state exact model number as it would be very helpful and maybe post a schematic.
 

aienco

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I do have an electronics background, however I haven't been doing a lot for the last 15 years, so very rusty, mainly on the troubleshooting side.
Here are schematics. Really grateful for any help. Like I said before, I am getting plenty of noise from the box when i knock it, but no reverb sound through the guitar, just normal sound as if reverb is off.
 

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Jon Snell

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There are five ICs withing this model.
U1 (IC1) is used to pass signal to U2 U3 and U4 and to U5. U4B is the recovery amplifier and that is working. U1 is the path for the normal and treble chanel; that works.
U2, 3 and 4A are the power drive for the tank. You should have signal on pin1 of Con3.
If the wrong tank is fitted, the reverb will be very weak.
 

Spanngitter

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Pls check FS3 which is a 2R2 fusible Resistor. If this one degrades or blow you lose the +/-27Vdc supplying the OpAmps.
Since there is apparently still some operation of the OpAmps it is most likely degraded and Voltages on the OpAmps is insufficient => check on Pin4 resp 8 to GND
 

aienco

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There are five ICs withing this model.
U1 (IC1) is used to pass signal to U2 U3 and U4 and to U5. U4B is the recovery amplifier and that is working. U1 is the path for the normal and treble chanel; that works.
U2, 3 and 4A are the power drive for the tank. You should have signal on pin1 of Con3.
If the wrong tank is fitted, the reverb will be very weak.
Thank you Jon. I had suspected it may be an ic but not sure what I sould be seeing on each one. I will give this a go today and post back. And yes the tank is the original PLUS I purchased a brand new MOD one which is a compatible tank, but both yeild the same results. Again grateful for your assistance. I will post back.

Hi Jon, I have had a chance to check. Pin 1 of Conector 1 is giving me 68k ohm. The out put of ic3,4&6 ie: Pin 1, all measure 16Volt.
 
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aienco

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Pls check FS3 which is a 2R2 fusible Resistor. If this one degrades or blow you lose the +/-27Vdc supplying the OpAmps.
Since there is apparently still some operation of the OpAmps it is most likely degraded and Voltages on the OpAmps is insufficient => check on Pin4 resp 8 to GND
I will check this today and post back. Very grateful for all the assistance.

I have just checked this. Fus/Resistor checks ok. Pin 4 of U1,2,3,4,5 all measure 16Volt.... Pin8 of U1 = 0, Pin8 of U2=22mV, Pin8 of U3=22mV, Pin8 of U4=0V, Pin8 of U5=0.! When I was meauring pin 8 on one of the chips i got a loud noise sounded like it was echoish or revervb. I would say reverb. It was on full. I think it was U3 or U4.

I hope this helps you guys, help me? :)
Thanks again.
 
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Jon Snell

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Hi Jon, I have had a chance to check. Pin 1 of Conector 1 is giving me 68k ohm. The out put of ic3,4&6 ie: Pin 1, all measure 16Volt.
We are not looking at resistance, we are looking for signal. Place your oscilloscope on pin 1 (of connector 3!) and you should see audio. If you do it proves either the tank or its wiring is faulty, if you don't get audio signal, follow the signal path to pin 3 of U2A, if none, go back to pin 5 of U1, then if you need to pin 3 of U1.
Pin 1 and 7 of all ICs should be around 0 volts DC.
Pin 4 -25v and pin 8 +25v or thereabouts.
 
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Spanngitter

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16Vdc is insufficient on Pin 4/8, it will work but have not much headroom...did you count pins correctly?
Pin1 is denoted by a dot on the IC and usually a square mark around the contact hole in the PCB.
25Vdc to 27Vdc is what should be measured there against GND, see also the Schematics you posted.
Also check at R84/78 to verify the loss of voltage is not somewhere in between.
OpAmp PinOut.jpg

I mentioned these Voltages not because of seeing it on the schematic, its pure experience as I had already a few CC here where either F3 or one of the Zener crapped out due to the Capacitors just shorting out...
Only if Voltages are ok then continue with tracing Signal with an O-Scope
 

aienco

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We are not looking at resistance, we are looking for signal. Place your oscilloscope on pin 1 (of connector 3!) and you should see audio. If you do it proves either the tank or its wiring is faulty, if you don't get audio signal, follow the signal path to pin 3 of U2A, if none, go back to pin 5 of U1, then if you need to pin 3 of U1.
Pin 1 and 7 of all ICs should be around 0 volts DC.
Pin 4 -25v and pin 8 +25v or thereabouts.
Unfortunatly sold my crow last year. I had it for 40 years. I was looking at getting one of the new digital ones, but been reading they are not as good. I realize the voltage should be around 27, but 16 is all I am getting. Anyway. It is solved now. I had ordered 6 NJM2147D chips. I just got sick of it and replaced the chips U1 to 5. and it is working now. So you are correct. Definatly one of those chips faulty. Thank you kindly for all your assistance. Very helpful.
 

aienco

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16Vdc is insufficient on Pin 4/8, it will work but have not much headroom...did you count pins correctly?
Pin1 is denoted by a dot on the IC and usually a square mark around the contact hole in the PCB.
25Vdc to 27Vdc is what should be measured there against GND, see also the Schematics you posted.
Also check at R84/78 to verify the loss of voltage is not somewhere in between.
View attachment 108224

I mentioned these Voltages not because of seeing it on the schematic, its pure experience as I had already a few CC here where either F3 or one of the Zener crapped out due to the Capacitors just shorting out...
Only if Voltages are ok then continue with tracing Signal with an O-Scope
Ye I realized I should be getting around 27 volt. But 16 was all I am getting. I went off the schematic you put in. Anyway, turns out it was one of the chips. All working for now. Thank you kindly for all your patience and assistance. Much appreciated.
 


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