vox viscount

hello ive got a vox viscount guitar amp and i resently lost 90% of my output on the guitar chanel in clean mode, please help.

here i was going to post the schematic of the guitar chanel pre(how do i do that?)

Hi,

You have to have the image located on the web- if you haven't got your own webspace (some ISP's provide you with a Meg or so free- check what they can offer!) you can get it hosted at a third-party place. There are sites that do this specifically for images- a sort of on-line photo album.

Just take the URL of the image and pop it between the brackets (click "Img" box in post)

If it's a big image, just link to it via the tags to save forum-bending stuff h...d I'll host it while discussions go on! Mark

thanks

Bingo...



bed now...

Mark

the above schem is the guitar/normal stage of the vox viscount, this is the only chanel that is giving me any problems at all please help.

Go to geofex.com look around for the vox section. R.G. has a few articles about older Vox SS amps IIRC. you could also ask here

http://www.diystompboxes.com/forums/stompboxforum/

Does the relay work and do the contacts have low resistance? Thats kind of cool it looks like a fuzz face set to max gain in the switched distortion section.

Dirty contacts can cause a problem like you posted.

yea the relay works. and what do you mean by do the contacts have low resistance?

Hi frob,

Just a few questions first-

1. Does the amp sound/behave exactly as it used to when in Distortion Boost mode?

2. When you say "90% less output" in clean mode, what type of "less" is it- less guitar sound with more background noise, or just less guitar level with no noise- as if the volume has been turned down?

3. When in clean mode, does the "Top Boost" make any difference to the sound, and can you hear the bacground "hiss" increase/decrease whilst switching it?

4. When you switch from clean to distortion, can you hear the relay click?

If you look at the above schematic, you can see that the input signal from the guitar is split 2 ways at the relay. The normally-closed contact (the contact connected to the common terminal when no power is applied to the relay) connects directly into the preamp. This is the clean channel. The other relay contact, the Normally-Open contact, goes to the Distortion Boost circuit. So when a voltage is applied to the relay, the guitar signal is connected to the distortion booster circuit, and the output of the ditortion circuit goes to the clean preamp circuit.

What Gus means is that when a contact in a relay becomes oxidised (due to age, humidity, lack of use etc) it has a higher resistance. If this resistance becomes too large, it stops the audio signal passing or becomes intermittent. Often just "working" the relay (switching it on-off quickly for a few minutes- make sure volume of amp is down!) will budge the oxide.

Small signal relays such as those used in audio can become intermittent due to the fact that the current passed by the contacts is so small. Relays carrying higher current are "self cleaning" and tend to develop less problems.

The best way to ascertain the problems in a circuit such as this is by signal tracing on the board. But check the notes above first.

Mark

[quote author="Mark Burnley"]
1. Does the amp sound/behave exactly as it used to when in Distortion Boost mode?[/quote]

yes, i dont have a pedal to operate the relays so i have to use a jumper wire to activate it(i just could never find a plug that fit the jack its a six pin like thouse used on old astatic mics)

[quote author="Mark Burnley"]
2. When you say "90% less output" in clean mode, what type of "less" is it- less guitar sound with more background noise, or just less guitar level with no noise- as if the volume has been turned down?[/quote]

no the hiss dose not go up as mutch as it should when i turn up the normal input volume, however i know the power amp block of the amp is working properly as bolth the briliant and bass chanel work fine. also if i use the jumper to switch it to distortion then i can play with the volume turned way down, wile if its in clean then i have to turn it half if not all the way up.

[quote author="Mark Burnley"]
3. When in clean mode, does the "Top Boost" make any difference to the sound, and can you hear the bacground "hiss" increase/decrease whilst switching it?[/quote]

let me see...
...yes thier is a noticable difrance in the ambiant hiss

[quote author="Mark Burnley"]
4. When you switch from clean to distortion, can you hear the relay click?
[/quote]

agien let me see...
...yes i can hear the relay click


thank you for helping me ive been trying to fix this off and on for a while

Okay,

How confident are you with electronics? I was just wondering what level of knowledge you've got working with "live" gear- and have you got access to a DMM etc. Have you peeked inside the amp yet?

Let me know!

Mark

long distance bet.
1. relay contacts
2. bad transistor.

Find a voltmeter and let Mark talk you through it.

im preitie good with electronics i can follow schematics well enuph diy and mods, i know what will hurt me and i know how to discharge caps.
yes this is not the first time ive had to fix /mod it only mods are a new power cord, and some nice alinco 16 ohm speakers (yes it suposed to have 16 ohm and their rated for this amp) however repairs made replacment of the cracked speaker wire and car stareo speakers, the rotary power switch had to be replaced with two switches(the rotery was custom to theise tvamps and are no longer available). and right now the amp is laid out in bench test mode

Cool,

Just checking where we can go with this...

Okay, I'm not familiar with this particular Vox, so could you tell me how the relay is connected- I presume it's mounted on the PCB? One way to tell if the relay is a duffer is to place a jumper wire across the Normally Closed relay contacts when it is in Clean mode. If the sound re-appears healthy and normal, then you know it's a relay contact gone dodgy.

The jumper would have to go from after the input resistors (the resistors directly after the input jacks) and connect to the cap marked "408"?? (it's a bit unclear on the schem- but it's the cap directly after the relay- orientated North-South on the schem) Connect the wire on the relay-side of the cap.

If you can solder a wire between these two points, power up, and see what happens. With the jumper, make sure you don't switch the amp into "Boost" mode because the Boost amp will get caught up in a feedback loop because you've effectively disabled the relay with the jumper!

Hope this helps,

Mark

thanks ill do this tonight. unfortunaly this is the worst point to pot wiring job ive ever seen and ive though long and hard about rebuilding it. although when it worked it sounded great.the chasis is a metal box that is opened at one endthen thier are guild rails goin acrosed it that hold the pcbs in place their is a tangle of barly orginised wires goin from point to point stufed deep inside this metal box is a series of relays only one has a .22 cap on it so i guess thats it if all else fails ill just have to trace it.

i jumped past the relay and it did jump in volume ill have to test it before mid night to see if i can get the true volumes out of it though.

Cool,

Looks like it could be a go-er!

Fingers crossed...

:grin:

Mark