Ampeg SVT-VR&CL ground loop hum solution

[Mars Amp Repair]

hello folks,
I don't know if anyone has encountered this issue, but I have a few times & have finally found a solution to the overly noisey, hummy, front end on some of these Ampeg SvT-VR's & probably the Svt-CL's, too.

Even with the volumes fully down the amp has hum & noise. If you insert an input in the 'Amp in' jack, the hum/noise all goes away.

The first indication you can try to determine if this design snafu is your problem is to remove the green grounding lead that goes to the preamp chassis. If the nasty ground hum/noise is remedied, then continue on.

You might have also noticed on these amps that in attached pic that the jumpers that are supposed to chassis ground the Star ground spot on the board have been left out. This was to remedy an initial ground loop issue. It worked until another ground was created via the heat sink of IC-1 12V regulator. It punctures its way through the paint in the chassis & then becomes the new grounding point. See pic showing heatsink under main pcb. If you loosen the 3 pcb mounting screws & gently lift the pcb up so the heatsink doesn't contact the chassis, the hum should go away.

The solution is of course to electrically isolate the regulator from the heat sink & then isolate the heatsink from the ground trace.

This can be done easiest by replacing the 7812 with the plastic version 7812CP. see pic.

Then you can dremmel out the connection from the center lead of the regulator to the heatsink. see pic.

Since I received this amp second hand from another service center, I don't know if Ampeg had any kind of insulator in there as a counter measure to keep this heatsink from contacting the chassis or not...I just know what I was up against.

Amp was nearly completely quiet after this countermeasure.

Hope this helps someone down the line...glen

[douglas parks hodges]

Thank You

Thanks...I experienced this problem with my new SVT VR and I will try this solution...I will take these directions for fixing to the nearest authorized repair place

[Mars Amp Repair]

Always great to help...glen

[malu]

Hi Glen,
I have an SVT VR with a similar problem (50Hz hum). If I apply a signal to the power amp input, the hum goes away, but if I disconnect the grounding lead that goes to the preamp chassis, the noise remain (and maybe increase).
However, should I try your voltage regoulator mod or it's better to look for something else?
Thank you very much!
I'm sorry for my poor english.

[Mars Amp Repair]

you should first verify that this is the problem. You should loosen the bottom circuit board & move it around to see if the heatsink getting grounded is the problem.
then you could perform the mod if it is indeed the problem. thanx, glen

[malu]

Thank you Glen!
It's a bad idea to put a piece of rubber (like a bicycle tube) between the heatsink and the chassis?

[Mars Amp Repair]

Definitely a bad idea to use anything but something designed to transfer heat. rubber would not only act as a heat insulator between the sink & regulator, but it would probably melt, too.

It's best to use the plastic variety of the regulator if you can get one. If not, then you have to deal with a mica insulator (or teflon sheet) as well as getting the plastic collar to keep the mounting screw insulated from the regulator.

You could also find a way to keep the entire heatsink from touching the chassis. On some models, they have a piece of material wedged between the chassis & heatsink. I think it's a pretty poor way to do it, but would also serve the purpose if you cannot get the insulated regulator IC.

glen

[Vukefal]

I cant see the photos..is it possible to reupload them since I think that I'm having this problem

Thanks

[drewl]

I can see them.

[gilthethrill]

Hello Glen. Just bought a used 2006 USA SVT-VR and have this hum noise which is driving me nuts. Can you tell me what year your VR is?
I will try the simple remove the ground wire from the pre-amp as you said to do first to see if this hum goes away.

[Mars Amp Repair]

To GillTheThrill:
the SVT that i posted about belonged to a customer of my business 'Mars Amp Repair'. At this stage of the game i have no way of determining what year that amp was.
i would suppose given the date of the post that around 2006 would be good to assume.
If your amp had all the same symptoms eg: when you lift the ground the hum improved, etc and it had the heat sink mounted regulator, your chances are good to perform the mood as described.
you may email me directly if you wish.
MarsAmpRepair@gmail.com.
good luck...Glen

[SimTech]

I added two very thin lock washers under each of the 3 standoffs at the chassis. This elevated/lowered the pcb just enough that the heatsink can't touch the chassis. Then to make sure this didn't add any stress to the pots and jacks I reflowed the solder for all the pots and jacks with them fully installed. Infinitely easier fix than drilling out the RIVET they used to mount the 7812 to the heatsink (what idiot rivets a regulator to a heatsink). The heatsink only has to clear by the slightest amount since there's no real voltage difference between the heatsink and the chassis. Arcing isn't gonna happen unless it gets struck by lightning and that's a whole different ball of wax,