Ampeg SVT IIP Fading problem

[Shockwave]

Hey guys.

My Ampeg SVT IIP Preamp has a slight problem, it has two inputs, Normal and "Bright"

The Normal input works fine with no problems, When using the bright input, the volume fades in and out at random times.

The valves have just been replaced.

Any ideas?

Rob.

[Dood]

I thought I had a schematic for that model - but I can't find it - though I suspect there will be one online somewhere - if you can find one - we can take a look at possibilities.

[Shockwave]

[quote name='dood' post='801212' date='Apr 9 2010, 08:21 PM']I thought I had a schematic for that model - but I can't find it - though I suspect there will be one online somewhere - if you can find one - we can take a look at possibilities.[/quote]

Cheers mate!

[Shockwave]

Ok, when it goes quiet, i hit the amp, and it gets louder again for a brief few moments. Valve Reseat? Transistor come lose?

[jbasss]

[quote name='Shockwave' post='801252' date='Apr 9 2010, 08:56 PM']Ok, when it goes quiet, i hit the amp, and it gets louder again for a brief few moments. Valve Reseat? Transistor come lose?[/quote]
I have the schematic somewhere. What I can recall the only difference between bright and normal inputs is a low pass filter, possibly separated with a buffer. Shouldn't be a big issue, most probably a bad solder. Will check.

[Dood]

[quote name='jbasss' post='803244' date='Apr 11 2010, 11:59 PM']I have the schematic somewhere. What I can recall the only difference between bright and normal inputs is a low pass filter, possibly separated with a buffer. Shouldn't be a big issue, most probably a bad solder. Will check.[/quote]


Yep, I was thinking a dry/cracked joint / loose connection too. Sometimes the PCB track lifts through vibration or a sudden jolt - it's just a matter of time before another vibration starts to crack it through. Usually a simple fix though.

[Shockwave]

I found a schematic, gonna take this to my amp tech, easy fix but i dont want to screw it up!

Rob.

[Walker]

I've just bought this off eBay - must be the same unit as it has exactly the same problem! Did you find out what the issue was?

Cheers
Chris

[Mr. Foxen]

Pretty sure the issue was fixed with that one. Same thing can happen on all of them though, the board mounted stuff pulls out of the board. Think it was the jack pulling out of the pcb. Getting it out is a massive ballpain because its made deliberately difficult, basically undo everything on the front panel, and possibly the back one too, resolder it all underneath, and then try and get it all back together without cracking it again. If it is the exact same unit, you'll see a fairly blatantly later addition resistor on the other side of the board to everything else.

Problem dry joints look like this in the middle:

[Walker]

Thanks Mr Foxen.

Even the allen sockets on the front are an odd size! If got a metric and imperial set and neither will fit, so I've got to go and look for an in between sized key now. The normal socket sounds wonderful.