Vox vt50 & vt100

[tbonuss]

Hello all--

I'm new to the forum. I've searched and have not been able to find any good links to the VT series amps. Perhaps I'm just missing this, but it seems like just about everything EXCEPT these are available.

I've got a dead VT100, and would like to either fix it or put the effects loop onto the 50 (the 50 doesn't have one). Either way, it might be easier with the schematics.

Thank you in advance for your assistance.

[DrGonz78]

Here is a link to download the VT50
http://bmamps.com/Schematics/vox/Vox_vt50_sm.pdf

What specific symptoms/problems are you experiencing with the VT100?

Edit: please refer to this post for VT100 schematics. (Note: there is a claim that perhaps the AD100vt is similar to the 100vt output)
http://music-electronics-forum.com/t31986/

AD100vt schematic. I have not worked on 100vt or AD100vt so I am really not sure if the output section is even close to each other...
http://bmamps.com/Schematics/vox/Vox...e%20Manual.pdf

[tbonuss]

First--thank you so very much. That was fast!

I haven't opened it up yet, so I can't comment yet on how closely it does/does not match the AD100VT. Plus, its been an eternity (the 80's) since I worked as an electronics technician, so I'm not sure how well this is going to go...

Anyhow,

Here's what I've got so far:

1. On power up in runs through the same sequence of LEDs as my fully operational VT50.
2. The power tube is glowing.
3. It had a bad headphone and input jack which threw me off at first. Its more functional than I originally thought when I posted.
4. I'm now getting good output out of the headphone jack
5. All of the preamp controls and the modeler seem to work just fine (listening through headphone jack).
6. Effects loop return works just fine (line level input to return; listening through headphone jack)
7. Nothing from power amp / speaker output.
8. Also, none of the controls seem to do anything at all to the effect loop return -> headphones. Maybe thats normal, but there are essentially 3 "volume" controls (volume, master and the attenuator) and none of them do anything to the loop return to headphones?

[DrGonz78]

Look on the PHONE OUT (Jack PCB) board and you will see all sorts of references to VT100. This gives us hope that we can use the VT50 preamp schematic interchangeably for the VT100 preamp section. On the Jack PCB board area I would check continuity on the board especially from wire to wire (connections from board to board) and even R295/R296 on the board itself. Do you know for a fact that this speaker is working? The VT30 I worked on a few years ago had a bum speaker, so always report that you checked that one off the list.


Edit: I guess I missed this key point below and that would mean checking R295/296 would not matter, sorry. I guess my only good advice there was to make sure that speaker is good working condition.

6. Effects loop return works just fine (line level input to return; listening through headphone jack)

[tbonuss]

Thanks! (Should I ask mod to move thread?--I don't know what is customary here and don't want to be unintentionally rude).

I haven't had the chance to crack it open yet, but I did take some voltages / resistances and looked at the schematics. It looks like there are commonalities across the different circuits. They all seem to use the VR (Valve Reactor) circuit, and the ADV and VTs have a lot in common beyond that, and the VT50 and VT100 are very close as well, but there might be some minor differences (e.g., the 100s have an effects loop) I'll be happy to document anything anyone is interested in, once I actually crack it open.

Here's what I got with the limited time I had last night:

1. Both speakers = infinite resistance--visual inspection shows contacts / braided wire intact => Open Coil on both
2. Main speaker out ~ 64VDC
3. Aux speaker out ~ 64VDC
4. Looking at the schematic for the VT50 (assuming P/S voltages for the 100 are roughly the same), +VCC = 34VDC, -VCC = -34VDC, so...if IC1 full ON (through a direct fault of IC1 or its biasing network), that could explain it as the delta would be ~ 68 volts, and then drop a little for the resistors on the output could get it around 64 (???). Its been 30 years since I worked on electronics every day, but that's what a quick look tells me. Does that make sense?

Again--thank you in advance for any help at all.

[DrGonz78]

So both speakers are burnt open and probably due to the 64v on the output. So looking on the AD100vt schematic they have a STK404-130S for the power output stage, is this the same type used on the 100vt?

[tbonuss]

STK404-130. No "S".

[g1]

The S will replace the original. Be extremely wary of any of these coming from China as they are probably counterfeit.
What is your country/location? Someone here may be able to recommend a reputable source.

[tbonuss]

Virginia, USA.

[Jazz P Bass]

Audio lab of Ga

[tbonuss]

Are we sure then that this is the fault. I was going to check i/p and bias voltages to see if something is locking it ON.

[Jazz P Bass]

That chip is bad if you have volts dc on the output.

[tbonuss]

Okay--Thank you. I have ordered from the company above. You are all very kind to be so helpful!

[tbonuss]

So all the big caps seem to have been hit with dollops of adhesive...is that normal?

[g1]

Not sure about "normal", but it's not uncommon.