Problems with my Vox AC15C1

[grip]

Hi All,

Firstly,the schematic:
dealers.korgusa.com/svcfiles/AC15C1_Smanual.pdf

I have an issue with my Vox AC15C1. The Top Boost (TB) channel is very quiet, even with master and TB channel volume cranked up. Also the normal channel, although loud, was not breaking up when cranked, it was very clean with no character.

Starting with the obvious solutions, as it suggested to me the power tubes, I replaced them and the preamp tubes and found that there was a little more volume to the TB channel and I could get the Normal channel to break up OK when cranked. Strangely, at this point, increasing the volume on the normal channel increased the volume on the TB channel with nothing plugged into the normal channel.

The reverb isn't working, the tremolo works fine, I'm not sure whether the bridge rectifier has anything to do with this, it's something I'd rather not change due to it's sturdy 4 legs not going to be easy to remove from the PCB, although I did buy a replacement component.

So far I have reflowed the solder from both sides of the tube PCB's, replaced all of the Potentiometers, looked for any signs of burned out resistors.

The Amp is usable on the normal channel, but Ireally want both channels working. I guess I'm faced with various capacitor and resistor replacement, and my question is which ones are most likely to need replacing?

Any help or suggestions will be greatly appreciated.

Kind regards,


Chris

p.s. will be seeking advice on another forum also and will update this with any forthcoming suggestions.

[Charlie Hall]

Hi Chris,
The bridge rectifier is for the 3 preamp valve heaters. If they light up as normal the rectifier is probably OK.
I am thinking there could be a bad connector to earth somewhere. With the amp switched off and drained and disconnected from the mains power, try disconnecting and re-connecting all suspect connectors on the preamp board and see what happens.
Regards,
Charlie

[grip]

Hi Charlie,

Thanks for your response, information regarding the Bridge rectifier and the heaters appear to be working fine.

I did try during my initial trouble shooting to parting those blessed connectors and was only able to confidently part 2 of them without having to exert a force likely to break the thin cheap plastic holding the connectors. I may have to finish up hand wiring those if they break during removal. I tried to pull apart the spade connectors from power transformer and switches just to give me a bit more manoeuvrability with the "mainboard" and the first one I tried - a blue crimped wire from the standby switch became un-crimped and is now soldered and leaves me doubting the security of all the other crimped connections in that little "nest" of spade terminals. They have a pressed hole on the male terminal and the female has a raised part that locates in that hole and I'm really not sure of the methodology to part them, I don't want to risk snapping the fairly (or should that be "unfairly"?)thin PCB. I have to leave it a couple of days before looking again as lifting it, unscrewing it etc. on the floor plays havoc with my leg and causes it to swell up. I'll update this when I've got inside it again.

It's just occurred to me that I do have something else to check...my mod of replacing R80 with a 1024Ohm 15Henry choke, before I do anything else, I'll try it with the resistor in place of the choke.


Kind regards,


Chris

[Charlie Hall]

Hi Chris,
I have to say I don't envy you having to work on that amp. It sounds a nightmare design. Hope you can sort it out OK.
Regards,
Charlie

[grip]

Hi Charlie,

Yes you are right in what you say, the design was very much profit motivated. It certainly wasn't designed or built for ease of working on.

Kind regards,


Chris

[grip]

Mar 18, 2019 13:52:31 GMT grip said:

It's just occurred to me that I do have something else to check...my mod of replacing R80 with a 1024Ohm 15Henry choke, before I do anything else, I'll try it with the resistor in place of the choke.

Chris

Hi All,

I finally got around to giving this 1 more look whilst hands legs and feet felt OK, and I swapped out the 15H Choke and all was fine, if only I'd started there
I'm not sure what happened with the choke, but the resistance of the coil had dropped from 1024 Ohm to 872 Ohm and 10.3 Henry and caused a lot of non required work from the idiot working this keyboard....surely anyone would start with looking at the the changes they made, can feel my face reddening as I type.

Kind regards,


Chris

[Charlie Hall]

Hi Chris,
Interesting, especially as chokes don't normally fail. What did you replace the faulty choke with, the original resistor or another choke?
Regards,
Charlie

[grip]

Hi Charlie,

There doesn't seem to be any"scorch" marks or anything else to indicate why it failed or the fact that it HAD failed. I replaced the resistor with like for like 5W 1kOhm.

Kind regards,


Chris