Ampeg SVT II (SLM) Gnarly Hum

[priesty]

I have recently taken delivery of a 1992 St Louis Music (SLM) Ampeg SVT II. When the amp is turned on (standby on) there is only a very faint sound of my bass and a very dominant gnarly 50hz "triangle wave" type hum. The hum is also present with nothing connected. I'm pretty sure it relates to the pre-amp as the hum disappears when I plug direct into the "power amp in" jack socket on the rear panel. The amp played ok (albeit with no volume or EQ controls) when the bass is plugged straight into "power amp in" I have re-soldered some dry looking joints on the pre-amp board, and have changed out all four 12AX7's. Have not changed power tubes or 12AU7's as the "power amp in" bass test proved the power amp tubes, PI/Driver are working fine.

Any thoughts where I go with this next??

[nevetslab]

Preamp Mechancal Grounding high resistance?

While I haven't worked on the SVT II, I had similar issues with an SVT 2 Pro. Not suspecting mechanical grounding issues, I went thru repairing any and all solder joint fractures, had one power tube's heater coil open, but after not finding the problem to be component related, I looked at the ground continuity, starting with the front panel pots. I had high resistance there (painted chassis....with the Paint serving as an insulator). Then, there was a single Grounding standoff that was supposed to take the preamp's ground circuit to chassis. It wasn't. High resistance.....once again, paint insulating the ground path.

I scrapped away paint where the chassis panel should have grounded the pots' metal housing, restoring ground to the pot cases. Then, I had to remove the standoffs, grind away the paint on both the chassis and the standoffs, so I finally got 0 ohm resistance when I put it back together. Did the same on the power amp, as there is also one standoff used there for the chassis ground reference. That got rid of the loud hum and the sensitivity to high noise when you banged on the unit.

You may have similar issues with the SVT II. At least, worth looking at.

[priesty]

Success! I took out the pre-amp board and examined it. Nothing untoward found. Took the chance to clean up input jack sockets, and pulled out and re-made all the spade terminals and push connectors. That appears to have sorted it, played for 30 minutes with no issues. Maybe in removing and replacing the pre-amp board I have sorted a bad ground on a pot, as you have described here. Many thanks for your reply.

[Luthin You]

Guys Im new and I cant see how to post a new thread , My question is about a SVT2 , I need to know values of 3 components , can anyone help me please ?

[The Dude]

The schematic is here:

AMPEG SVT II Service Manual free download, schematics, eeprom, repair info for electronics

Otherwise, if you can provide more info (photos, part descriptions, designation numbers, etc.), you might get more help.

[Enzo]

You cannot start a new thread while you are in an existing one. Find a section you want for it, like Maintenance and Repair. Then on the index page for that section, just above the top left corner of the index is a button that says "Post New Thread"