SVT II Repair

Discussion in 'Amps and Cabs [BG]' started by cmassa, Nov 29, 2003.

  1. cmassa

    cmassa Hello. My name is Chris, I'm a bass-aholic.

    Nov 27, 2001
    I have a rack SVT II and after a visit
    to the shop have an unsolved question.
    >>Also, whilst troubleshooting the SVT-II yesterday, to find why it isn't as punchy as it should be, I noticed that most all the general power supply voltages seemed significantly lower than they should be (about 40-60 Volts). Further research found that the A.C. voltages coming out of the power transformer are also low, even though I don't have an EXACT schematic diagram of this model with the specs as to what the voltages are supposed to be, the "dead give-away" was the fact that the "heater" winding, which is ALWAYS going to be 6.3 V.A.C., was only putting out about 5.5 V.A.C., which is enough of a difference to know that SOMETHING isn't right. This is a strange circumstance, whereas, it's as though the transformer was meant to be used with about a 140 V.A.C. voltage source, but there is only one voltage "tap" in the primary winding!?!?! (ACTUALLY, there are two seperate power transformers, one just for the "heater" line and preamp, and the other for the higher supply voltages, but both are reading low).

    Just thought I'd get some opinions.

  2. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Are the filaments in the SVT II run on DC? If so, the heater transformer could be less than 6.3 Vac...

    Perhaps you could take some resistance measurements on the various windings, send them to Ampeg Insider and see what he has to say about them.
  3. cmassa

    cmassa Hello. My name is Chris, I'm a bass-aholic.

    Nov 27, 2001
    I don't really know. this was the email I
    recieved from the repair guy. He's got a
    name here in Chicago and 20+ years experience.
    And rebuilt many an amp, but for some reason
    this stumped him. I took it back and have
    been using it at practice with the Billy Sheen

    As there are a few very tech types here,
    and Ampeg reads the group, I thought I would
    post the problem here.

    All input is appreciated.

  4. What's your wall voltage? Is the PT getting at least 110 volts AC? You might have a bad power cord which is shorting intermittently. If it's one of those detachable ones, you might need to change the jack on the amp.
  5. Obviously, he needs a $175 power cable.
  6. Wanted to clarify that I was being facetious about the power cable....

    But isn't BillyB correct--aren't the heaters DC?

    And if the amp has a problem at two places--both the tech repair shop and at the owner's house--then that rules out the voltage at the wall outlet. If there's enough resistance in the power cable jack to cause that much drop in voltage, you would think the tech would notice. Something would be getting hot, even if he didn't feel the hot area, he'd probably smell it.

    I guess I would start by getting the schematic, finding out what the voltages are designed to be, then work from there.....
  7. I've seen insulation wear on power cables lead to intermittent grounding of the hot leg to the chassis or the ground wire which would be consistent with these symptoms without necessarily heating up the cable.
  8. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Have you had this amp for a while, or is it new to you?

    If you have had it for a long time, I would be wondering about the shop that checked it.

    If it's new to you, the sky is the limit for kludges inside by the previous owner etc.

    From what you are saying, you are getting about 10% low on all or most of the AC voltages. Measuring AC should let out any DC issues.

    It is possible to miswire so that the voltages are HIGH, by connecting for a lower voltage. The transformers are intended to be connected for 100, 120, 220 or 240, and run on 120 wired for 100 would be high.
    But that isn't your problem.

    The voltages come from two different transformers, which points back to the wall supply again....unless both transformers are screwy.....or something else is.

    The only suggestion I can make is to have it checked to absolutely verify the AC wiring.

    There is a way for SOME (not all) of the voltages to be low if somewhere somehow a person re-wired it wrongly and ended up running one transformer off of the 100V tap on the other. Its possible to do it, but I don't know how likely it'd think it would be noticed.

    One other goof-up would be getting a portion of one in series with the other, but I think probably there would be a pretty serious problem in short order if that were the case.....dead parts/blown fuse/etc.

    Most other mistakes would probably just blow a fuse pretty soon.

    I have to suggest you get the AC wiring checked out by another good shop, preferably where they know about SVTs. Tell them what was found, and have them verify it and check AC connections if it still shows up.

    This bugs me....If it is really true that the voltages are low, there is just enough chance of a safety problem due to connections etc that I think it ought to be looked at.
  9. cmassa

    cmassa Hello. My name is Chris, I'm a bass-aholic.

    Nov 27, 2001
    Thanks for the answers and suggestions.

    I have had the amp for a year and it was an
    eBay purchase. I like to buy the ugly ones.
    This one came from a pawn shop. I have him go top to bottom on all my purchases. New tubes, caps, re-solder, what every it needs. The sticker serial number was trashed way before I got it. Same with my recent purchase of an Selmer SVT.
    My only request to him is make it 110% as I
    am prone to play on 7. :eek:

    There is an Official Ampeg repair station next town over and he does Mesa 400+.. Woohoo got one of them too.

    So I'll print out the emails and drop it off there next week. The problem I have is my hand size so instead of hunting for basses I can play, I just buy heads and play my MM Stingray 5 through them.

    The original repair person has re-built many an
    early toggle SVT, one for me and is an "official" Fender and Marshall service center. I just dropped off a "Selmer Made SVT" white rockers to him. As I left his shop a tour bus pulled up dropping off some other amps.

    So who knows? But if I find an answer I will post it here.

    Thanks again.
  10. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Somewhat local inquiring mind needs to know...what shop are you talking about?
  11. cmassa

    cmassa Hello. My name is Chris, I'm a bass-aholic.

    Nov 27, 2001
    I go by Mac Amps. Mac just moved from the city
    by wrigleyville to Berwyn. (708) 484-8338.
    I live in Crete so the "official" repair station
    is in Chicago Hts. Which is close.
  12. cmassa

    cmassa Hello. My name is Chris, I'm a bass-aholic.

    Nov 27, 2001
    Well it's back and seems the tubes were not all that they
    should have been. The repair station claims the rack head
    now is push 300-400 watts

    So tomorrow I pick it up and see if it has improved. When I
    asked about the voltage thing they said it was the tubes.