Help for a VOX solidstate

[AnnMar Studios]

Hey guys,

I am going crazy trying to fiqure out the problem with this VOX solid state amp which has the 12AX7 preamp tube in it.

The sound is unstable. its fine for a minute, then silence, then back for a bit, a little crackly , etc. its the kind of deal where when you smack the chassis it cuts in and out. I thought it would be a simple and obvious problem when i opened it up, but as it turns out its not.

I changed the preamp tube, and wire another speaker for testing. same thing. I sprayed the pots and clean the boards which were filthy. No change.

Here are some things i have noticed during my testing. Seems like there is an open ground on the thing. Also on the preamp tube, seems like if you tap it you can almost hear it in the speaker.

I had the scope on it, and the voltmeter and I also noticed something bizarre. The transformer wires out to 4 or 5 leads on the main board. When testing the leads the power seems to decrease. Like it i stick the meter on and it reads 10v it starts decreasing from there.

Any thoughts on common problems on these solid state junk boxes? Or any ideas on what I need to be looking for?

I am learning how to use a scope and I am handy with the DM.

thanks,

chris

[J M Fahey]

Hi AnMar.
At least post what model it is and as much info as you can.
What do you mean by

i stick the meter on and it reads 10v it starts decreasing from there.

Where do you "stick" it? What voltage do you actually measure? Where? What value does it eventually reach? How long does it take? 10V DC or AC? Does it happen after you turn the amp off?
As a side note, comments like

these solid state junk boxes

won't get you very far around here.
Do you want to repair it or just babble against SS amps?

[AnnMar Studios]

The model is Vox 100AD, and yes its a piece of junk I am looking at for someone. I didnt mean to insult whatever SS amp you have, I was only speaking to this one.

I was speaking about 10vdc declining on the transformer leads.

I guess my main question would be, if there is crackling and intermittent audio cutting in and out , what would be the likely culprit? I have checked alot out and I cannot seem to find anything. Any help would be great.

[Enzo]

First suspect is any jack on it with a cutout contact. For example FX return jacks or power amp in jacks. And if it has a headphones jack that cuts off the speaker, check it too.

COuld this be an AD100 rather than a 100AD? The AD100 has a series/parallel speaker switch and some cross connecting through the speaker jack cutouts, so maybe clean those.

[AnnMar Studios]

Hi Enzo,

yes I am sorry. It is the AD100.

I will check what you have said and report back later today. its driving me nuts.

thank youi

[chuckedup]

wassup AnnMar.. i've got nearly exactly the same problem with'a
'Vox AD30VT-XL'... thought it was the "extra" built in gain on the amp model's. A tube cange only helped a few minutes.. I've read two possible causes/cures for similar problems on Vox Valvtronics amp's.. here it is for what its worth..

first guy claimed it was'a cold solder joint said he'd given up and decided to re-solder everything inside the amp.. used a sharpie and the one inch square method.. marked the board off an inch at a time and resoldered everything inside each square.. i think this was at Strat Talk..

the seccond guy claimed it's a common problem with the 'attenuator' and related circitry.. also solder related.. me thinks..

hope someone with enough knowledge can step in and say... "you know what??" that's just dumb!! why would you even say that.. cuz.. thats just dumb!! or you know what?? that sounds like a big ole definite maybe right there.. them power sinks'll do that some times.. sum' like that anyway..

mine acted like it really liked the tube change untill i turned the power up at the attenuator.. was nice and clean with the gain turned down till then..
you could turn the gain up and it would distort nicely then clean up when you turned the gain back down.. after i turned the power up full at the attenuator, not so much.. harsh distortion at low gain settings, notes seem clipped/wont sustain any note on any amp setting.. hell, it wont even come to full volume now, though it coulda been the amp setting cuz', well, its goofy like that....laters,chuck

[JHow]

Kurcze blade! Nice z tego nie rozumiem! Czy facet pisze po angielsku?

[allante666]

Hi chuckedup. I AM that first guy! and resoldering it all cured it completely.....for two whole days! then it came back. so I went over a few bits again and it lasted a day. Its relegated to a corner for now, i too was hoping that guy thats going to come on and say 'you know what? thats just dumb' would be of help!
Mine is just a bad crackle that comes on when it feels like it. it also goes away when it feels like it. sounds for all the world like a bad solder joint but flexing the board doesnt seem to bring it on or stop it.
Whats this attenuator fault you speak of?

[R.G.]

Mine is just a bad crackle that comes on when it feels like it. it also goes away when it feels like it. sounds for all the world like a bad solder joint but flexing the board doesnt seem to bring it on or stop it.

If it sounds like a bad contact, it probably is a bad contact. The issue, obviously is - where?

Solder flaws were very, very common in stuff manufactured starting when the factory shifted over to RoHS-compliant solders and lasting for a couple of years. The cheaper lead-free solders were very, very touchy about soldering conditions, and often led to tented joints which looked good to visual inspection, but were really just a joint-shaped cover over the lead which went intermittent long after the unit passed inspection and was shipped. Many of these were thermally sensitive, and went open when the joint was later heated by operation.

However, if the joints have all been hand-resoldered and a good job been done of it, and there is still an intermittent, then the problem is somewhere else. Where?

It could be anyplace in the box. It could be
- a dirty contact on a jack
- a jack or pot bushing that's not making good contact with the chassis
- a cracked resistor or cap body or lead
- a jack, pot, or switch that's cracked
- a connector, especially the small square-pin style connectors adopted from computer usage
- a tube socket, for amps with tubes

How do you find it? You use the First Law of Intermittents:

Find something that makes it happen when you want it to. Then you can do some real testing and find it. You can't find an intermittent when it's not happening, except by dumb luck.

[allante666]

Thats just the thing, I cant make it do it. flexing the board doesnt do it, ive cleaned and checked and even bypassed the jack sockets, ive been round individual components with freezer spray, poked them with a stick and tweeked them with pliers. its sulking in the corner until i can muster up a whole load of enthusiasm again. Fortunately it belongs to me so i have nobody waiting for it.
It doesnt happen through headphones and the only control which affects the volume of it is the 'power output' pot on the back

[R.G.]

Thats just the thing, I cant make it do it. flexing the board doesnt do it, ive cleaned and checked and even bypassed the jack sockets, ive been round individual components with freezer spray, poked them with a stick and tweeked them with pliers. its sulking in the corner until i can muster up a whole load of enthusiasm again. Fortunately it belongs to me so i have nobody waiting for it.
It doesnt happen through headphones and the only control which affects the volume of it is the 'power output' pot on the back

If it won't do it with cold, how about heat? Use a hairdryer.

[J M Fahey]

Allante: if

It doesnt happen through headphones

then you practically have it solved.
It either is a problem at the headphone jack, which usually cuts the speaker ground through a built-in leaf switch, or, more probably, cracked flexible leads on some of your speakers or a speck of iron in the speaker's gap.

[allante666]

Sorry i forgot to mention, I tried heat with a hairdryer also.
I must admit the speaker isnt something i have looked at but now im beginning to doubt myself when i said it didnt do it through headphones, was it this one or was that some other one? they all run into one after a while!

I will try again tonight with the headphones, Im wrongly thinking if its ok through headphones then it must be in the power amp and not the preamp but of course thats the line output im thinking of, which this doesnt have one.
Must be my age, the rot is starting to set in!

[allante666]

Sorry Anmar, Ive just realised, I seem to have hijacked your thread!

[AnnMar Studios]

no your fine.

Its the exact same problem.

I have not sorted mine out yet, I am still trying to find time. I am hoping tommorow I will be able to get on the bench and solve this.

I am going to see about the headphones as well, and all other jacks.

I will report back....please do the same on your situation.