Disassembling an old rack mount Ampeg SVT 2 Pro...

handofseven · #1 10-16-2011, 12:08 AM

I picked up cheap old beat to crap rackmount Ampeg SVT 2 Pro head off Craigslist... Other than dents and dings (it looks as though the previous owner dropped it every time he moved it

), there are two issues I would like to address in getting this head up and running:

1. Shorn gain pot: I have some soldering experience and I figured this wouldn't be too hard to repair myself. Unfortunately, I didn't realize how much trouble it was going to be to get access to the back of the control panel! Has anyone ever taken apart one of these heads? It seems like the only way to get access to the back of the front panel, is to remove the four screws from the front handles. The problem being that the two gigantic transformers are in the way of access with the screwdriver. Is there a special kind of sideways ratcheting driver that will do the trick? It seems like by the time I can get access to the broken pot, I might just have the whole amp disassembled on the bench!

I know, I could just take it to a tech, but where's the fun in that? Also, I will need to remove the control plate in order to flatten out all the severe dents. So somehow it's got to come off...

2. Scratchy/static ridden graphic EQ: Is there an easy way to clean this or am I just better off replacing it? I have a mind to just leave it be, considering I will probably never use it anyway, but again, where's the fun?

Anyway, I'm just wondering if anyone out there has ever had to get inside one of these old rack mount heads. Any help would be appreciated... I have a feeling I might just have to take it to a tech and ask to watch him take it apart!

Sinomis666 · #2 10-16-2011, 12:57 AM

Take it to a tech there are very high voltages in ampegs even after they are turned off you could end up getting a very bad shock &trust me it ain't nice!

ampegfuzz · #3 10-16-2011, 01:08 AM

If the gain knob still turns set it and for get it and they make a cleaner/lube you can just spray into the fader then just like new

toomanyslurpees · #4 10-16-2011, 05:53 AM

I would second the advise to stay out of that amp if you're not too familiar. I work in aircraft electronics and dable in tube amps, even the guys with tons of knowhow on the tube amp building forums are cautious of these things because the voltage in these amps (that can be stored in the capacitors and will still be there after you turn the amp off and unplug it) is high enough to kill. I've read the recommendation when working on these amps is to keep your left hand in your pocket so your less likely to make a circuit through both hands that will send current through your heart.... When I opened mine to replace the op amp that feeds the direct out I had it turned off for a long time and was checking for voltage with a meter everywhere before I actually touched anything.

*just noticed you're in Vancouver, it's been ten years since I lived there, but assuming Ho's electronics is still up and running, he's the guy to go to.

handofseven · #5 10-16-2011, 11:42 AM

All good advise guys... Thanks. The gain knob does have just enough post left to turn, so it is functional at least. I have read a lot about the danger of capacitors (I work with robots and industrial welders) and consequently have been very very careful, having only removed the top and bottom cover. All the outer panels need work banging out dents of various severity. Oh well back together it goes and off to the tech!

@ toomanyslurpees : Yeah, I remember Ho's.My guitar playin' bandmates always took their Marshall's in to Ho's for maintenance... That was back in the early '90's though... I think they are still in business. I see they are still listed. I'll give them a call tomorrow... There is no such thing as too many slurpees!!!

BassmanPaul · #6 10-16-2011, 01:33 PM

Quote:

Originally Posted by handofseven

Oh well back together it goes and off to the tech!

Thank you. You have made a wise decision. The HV rail in that amp is around 600 odd volts. Definitely lethal if touched. Trust me the tech's bill will be much less than your life is worth!

**B-string** · #7 10-16-2011, 01:40 PM

Quote:

Originally Posted by BassmanPaul

Thank you. You have made a wise decision. The HV rail in that amp is around 600 odd volts. Definitely lethal if touched. Trust me the tech's bill will be much less than your life is worth!

I was wondering how close you are to the OP? Close enough to be of any help?

handofseven · #8 10-16-2011, 03:02 PM

Now, with all this talk of the lethality of being shocked by the stored charge of the caps, I am wondering what steps the technicians take, other than extreme care, to remain safe while working on these amps... Don't worry, still taking it to the tech.

cjmatt · #9 10-16-2011, 03:19 PM

Quote:

Originally Posted by handofseven

Now, with all this talk of the lethality of being shocked by the stored charge of the caps, I am wondering what steps the technicians take, other than extreme care, to remain safe while working on these amps... Don't worry, still taking it to the tech.

I believe they use a high voltage shorting probe and carefully remove any remaining voltage from the capacitors before they proceed. The tech would probably wear electricians rubber gloves and goggles while doing this. Maybe a full face shield.

handofseven · #10 10-16-2011, 03:28 PM

Aaaah drain the voltage! That makes sense. The average dood wouldn't have the equipment or know how to get that done at home. I was briefly imagining a gigantic game of "Operation" ... As fun as that sounds, it doesn't seem very practical.

**B-string** · #11 10-16-2011, 03:56 PM

Quote:

Originally Posted by handofseven

Aaaah drain the voltage! That makes sense. The average dood wouldn't have the equipment or know how to get that done at home. I was briefly imagining a gigantic game of "Operation" ... As fun as that sounds, it doesn't seem very practical.

Actually many times we MUST work on live circuits!

BassmanPaul · #12 10-16-2011, 03:59 PM

Quote:

Originally Posted by cjmatt

I believe they use a high voltage shorting probe and carefully remove any remaining voltage from the capacitors before they proceed. The tech would probably wear electricians rubber gloves and goggles while doing this. Maybe a full face shield.

No they don't. Shorting a capacitor can damage it's plates and ruin it. They should be discharged using a 1kΩ 10W resistor wired to a pair of insulated leads and insulated croc clips. I use red and black to remind me of which to connect first: Ground! Start at the first filter cap wait a few minutes and then repeat for all the other filters.

BassmanPaul · #13 10-16-2011, 04:05 PM

Quote:

Originally Posted by B-string

I was wondering how close you are to the OP? Close enough to be of any help?

You could well be closer to me!

I thinks it's around three thousand miles.

Reminds me of my Dad's first visit to Toronto from the UK. He asked if we could take a side trip to see the Rockies. I had to explain how big this country is.

Gawd I miss him.

cjmatt · #14 10-16-2011, 04:26 PM

Quote:

Originally Posted by BassmanPaul

No they don't. Shorting a capacitor can damage its plates and ruin it. They should be discharged using a 1kΩ 10W resistor wired to a pair of insulated leads and insulated croc clips. I use red and black to remind me of which to connect first: Ground! Start at the first filter cap wait a few minutes and then repeat for all the other filters.

interesting....never would have thought of doing that....then again, I don't work audio circuits very often.

**B-string** · #15 10-16-2011, 04:43 PM

Quote:

Originally Posted by cjmatt

interesting....never would have thought of doing that....then again, I don't work audio circuits very often.

Yes never trust that bleeder resistors even if present in the design, are functional.

**B-string** · #16 10-16-2011, 04:45 PM

Quote:

Originally Posted by BassmanPaul

You could well be closer to me!

I thinks it's around three thousand miles.

Reminds me of my Dad's first visit to Toronto from the UK. He asked if we could take a side trip to see the Rockies. I had to explain how big this country is.

Gawd I miss him.

Showing my poor knowledge of geography

handofseven · #17 10-16-2011, 10:53 PM

So once the caps have been bled, is it 'completely safe' to work on the amp? I imagine the caps don't 'charge' again until you plug it in and turn it on?

**B-string** · #18 10-16-2011, 11:52 PM

Quote:

Originally Posted by handofseven

So once the caps have been bled, is it 'completely safe' to work on the amp? I imagine the caps don't 'charge' again until you plug it in and turn it on?

They can recover small amounts of charge, "completely safe" ? You can not count on that ever. Always respect that voltage can be present. Care must be used around high voltage paths during repair so you don't create a undesirable path to ground or lower voltage circuits. Wire dress must be payed attention to so you don't create new noises in the unit.