DIY Fix: Reverse engineer power board from pictures?

Thread Starter

VonBarth

Joined Mar 29, 2019
5
Hello, I got a Vox Valvetronix VT20X "Modeling Combo Amp" from my neighbor, but it's missing a small board which was broken, removed and never replaced. In fact I don't even have the broken board. However I do have pictures which I found online here.

Can you help me reverse engineer the circuit pictured below, or suggest a DIY replacement of some kind?

The Board takes a 12V input from an external (laptop-like) power supply and feeds power to the main board via three wires. There are no markings on the board regarding voltage. As this is an audio application maybe it's purpose is to "clean" the power?

VT20X.JPG
VT20X Amp.jpg
KLM3507.JPG

A few other details:
  • I have contacted vox customer support directly, and they don't provide anything beyond operational support for their products.
  • I also contacted several authorized VOX service centers (I literally called like four of them in my area) and understandably they all want me to physically bring the amp in so they can look at it. I explained to them it's a DIY fix project, but none could provide schematics or spare parts.
  • As far as obtaining a new PCB, they seem to be only sold as a set of 5 boards costing more then the value of the amp.
  • Also, you may say "forget fixing this, not worth it!" which I agree, however this is a fun problem solving project and i'm having my son help out.
Thanks for reading!
VonB
 

KeithWalker

Joined Jul 10, 2017
1,933
Hello, I got a Vox Valvetronix VT20X "Modeling Combo Amp" from my neighbor, but it's missing a small board which was broken, removed and never replaced. In fact I don't even have the broken board. However I do have pictures which I found online here.

Can you help me reverse engineer the circuit pictured below, or suggest a DIY replacement of some kind?

The Board takes a 12V input from an external (laptop-like) power supply and feeds power to the main board via three wires. There are no markings on the board regarding voltage. As this is an audio application maybe it's purpose is to "clean" the power?

View attachment 186298
View attachment 186294
View attachment 186293

A few other details:
  • I have contacted vox customer support directly, and they don't provide anything beyond operational support for their products.
  • I also contacted several authorized VOX service centers (I literally called like four of them in my area) and understandably they all want me to physically bring the amp in so they can look at it. I explained to them it's a DIY fix project, but none could provide schematics or spare parts.
  • As far as obtaining a new PCB, they seem to be only sold as a set of 5 boards costing more then the value of the amp.
  • Also, you may say "forget fixing this, not worth it!" which I agree, however this is a fun problem solving project and i'm having my son help out.
Thanks for reading!
VonB
What is the model number of the amplifier. There was a large number of different models sold over the years.
You may find the information that you need here:
https://www.google.com/search?q=Vox...kAhWTlp4KHUdZAKwQsAR6BAgGEAE&biw=1024&bih=639
 

dendad

Joined Feb 20, 2016
3,921
It may be that is a zener, so it will feed 0V, +12V and the zener volts too.
Where did you get the picture?
Is there a bit clearer one, like with the electro lifted away from the board so the traces under it can be seen?
It will not be hard to draw the circuit, but all the board needs to be visible.
PowerIn.jpg
This is my guess.
The other parts are just bug stoppers and you can get away without adding them if you so desire.
The 3 pin connector wiring may not be correct, but I think is is ok.

D24 may be a 9V 1W zener.
 
Last edited:

Thread Starter

VonBarth

Joined Mar 29, 2019
5
Hi Guys, thanks for the replies. Keith I agree there are schematics available for many VOX Amps, but most are of the "vintage" category (you know back when people & companies use to assume things would be fixed instead of put in landfills.)

deddad thanks for the sketch, that's very helpful!! There's a link above in the thread with where the pictures came from, I don't believe there were any other detail shots that would reveal the tracing.

I'll breadboard that circuit and give it a try. Any idea how I might be able to tell by looking at the other end of the 3 pin connector which is the 12v and which is the Zener? (see pictures below)

PowerConnectTop.JPG PowerConnectBottom.JPG
 

Thread Starter

VonBarth

Joined Mar 29, 2019
5
Just to wrap this thread up, after a few minutes with a multi-meter I discovered that the two white wires were ground, so the other hot. I hooked up 12 volts and voila, amp turned on!

I assume (as you guys pointed out) that the "missing" circuit board is just a bug-stopper for people attempting to use noisy power supplies. I'm using a PC type power supply atm, so I think it'll be good.

thanks for all your help!
 

rizast

Joined Jan 20, 2020
1

Dodgydave

Joined Jun 22, 2012
10,060
The connector has the same pinout ok, the side pins do the DC switching just make sure you get the polarity correct, looks like centre pin Positive?
 
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