Ampeg SVT3 PRO transformer stray field?

[lesuedois]

Hello everybody,
this is my first post on this forum so i present myself shortly. i am amp technician in rouen/france , i allso build (my own, no clones) effect pedals, amp's, studio stuff... with good local success.
here is my problem:
actually i have 3 Ampeg SVT3 pro in my workshop, two of them have no real problem, but the third one has a realy strange problem. the transformer has a enormous stray field! when I approach the amp with my bass guitar I get one of these enormous humm, if i'm about 2meters from the amp it's OK. The others don't have this problem...
anyone has an idea? These SVT's really die like Fly's over here...

[J M Fahey]

Bonjour le suédois, ça va?
Strange problem indeed ... and we should check that you *really* have a problem.
1) you say that 2 meters or more away, you have no hum problems, with none of them ?
2) you say, that, say, at 1 meter, you have "enormous hum" with the 3rd one, but not with the other two ?
I´d recheck this, with all three, one by one, with the amp in the exact same place on the bench, the settings in the exact same position, the bass in the exact same place too.
You may stick some pieces of tape on the bench to mark the positions of head and bass, so you repeat the experiment with the three of them, under the exact same conditions.
Otherwise you are comparing apples to oranges.
I´ve chased gremlins/ghosts many times before trying to solve "problems" which really weren´t so, simply different sttings (which went unnoticed) or which later couldn´t be reproduced.
Oh well.

Why do I insist on experimenting in such a "rigid" way?
Because in fact, it is *normal* for a Bass (or Guitar) pickup to pick up transformer hum at a short distance, and for transformers to have stray field .
What I don´t get is one having *lots* more than two exact "brothers".
The only condition I imagine causing that would be the culprit having a shorted turn, or trying to power a shorted PSU or power board , real gross failures which would blow fuses or cause heavy overheating in a short time; you report none of that.
Good luck.

PS: *unless* two of them have, say, toroidal transformers and the suspect one, being from another year, has a conventional EI transformer, but I doubt it, just mention it as *something* that could explain two similar VA rated transformers having a very different stray field.

PS2: *maybe* the culprit is unstable or oscillating, (wouldn´t surprise me at all) placing a heavy load on the PSU (and the transformer) and transmitting all sorts of interference.
Just in case, scope the output when that happens.
Maybe *you* hear it as "hum" with human ears, but strange RF things are happening ... which you don´t directly hear, only its artifacts.
A bientôt.

[Steve Conner]

A man goes to see the doctor.

"Doctor, I am suffering from terrible flatulence. Luckily the farts don't smell or make much noise, but it is annoying nonetheless."

The doctor gives his patient some pills to take, and tells him to come back after a week.

He comes back even more worried. "This is terrible, I am farting just as much as before, and now they stink to high heaven!"

"That's good, it shows your sense of smell is coming back. Now we'll get your ears checked."

The link to the SVT should be obvious. For instance, maybe the loudly humming one is the only one of the three that works.

[MarkusBass]

here is my problem:
actually i have 3 Ampeg SVT3 pro in my workshop, two of them have no real problem, but the third one has a realy strange problem. the transformer has a enormous stray field! when I approach the amp with my bass guitar I get one of these enormous humm, if i'm about 2meters from the amp it's OK. The others don't have this problem...
anyone has an idea?

The simplest idea is to try transformer from another amp. And since you have 3 of them (and at least 2 are OK) this shouldn't be a problem. Transformer in this amp can be easily and quickly replaced.
BTW, the amp has steel enclosure. Does the problem exist with closed enclosure?

These SVT's really die like Fly's over here...

I fixed many SVT3Pro but not because they are so poor quality but beacase there are very popular here. If SVT3Pro dies, it was usually treated very bad, or was poorly fixed the last time it was fixed. I'm not related to AMPEG but I have to defend their amps - they are very good amps (if they are treated correctly and serviced correctly) .

Mark

[lesuedois]

bonjour J M et Steve,
comment ça va?
sorry that i didn't mentioned that all three amp's where tested under the exactly same conditions and settings. I know how important this "rigid" testing is...
but trust me steve, it stink heaven high till the beginning. no need to check the ears ;D
All three have toroidial transformers, the two normal working amps are my control group if you want to. with the control group i must approach the right side of the amps to about 50cm to capture transformer noise (which is normal i agree). with the suspected amp, the humm is at the same level at about 1,5 meters away. When i cut the volume on my bass the amp is dead quiet (like the others). the PSU seems OK (diodes, condensators, tensions...), I suspect a shorted winding or some voodoo action. Maybe I got some time today try to swap transformers and see what's happen...

bon courage,
simon

[MarkusBass]

I don't think that this is voodoo. Just swap the transformers or check whether the enclosure is connected correctly to the ground of the amp.

Mark

[Enzo]

Because you have three of them, Markus's swapping transformers is a good idea. SImple and fast.

But before you do that, check very close that the mains wiring is exactly as the schematic suggests. And check to see if the noisy unit is wired any different from the other two. The transformer has a lot of wires and there are numerous posts for them on the board. The difference between the 230v and 240 arrangements wouldn;t make much difference to amp performance, but could make the transformer field less even.

ANother test would be to disconnect the transformer secondaries from the circuit, then power up the mains winding with an ammeter in series. Does the bare transformer draw any more current from the mains than the "good" ones?

ANything is possible, of course, but I tend to doubt the shorted turn theory. If there were shorted turns, I would expect the secondary voltages to be low and the transformer to get hottter than the others.

And you can also go to the Geofex site and find the simple transformer tester. Make one and use it.

[J M Fahey]

I mentioned the possible shorted turn specifically to discard it, because you didn´t mention any of the symptoms that would justify this, yet I still don´t discard the possible instability, because it has not been tested.
Please set the amp controls "flat", scope the output, and putting the bass closer (at 1.5m and closer) watch what you get in the speaker output.
If pure hum, you should see basically a 50Hz sinewave with some small but visible spikes, because line voltage is never "clean", lots of junk-making stuff is plugged in the mains, but any oscillation or unstability, maybe a strange kinking of the waveform at certain points or "fuzz" or "grass" should be visible, if present.
The test is valid even if you really have a *much higher* induction field, you´ll clearly *see* it using your own bass as a detector.
Why do I call it much higher?
To reach 3 times as far (with a given intensity) it has to be 9 times as strong at the source.
A big difference which would need a strong justification .... which so far we don´t see.

Now, if the only symptom you have is "somewhat more induced hum" , these tests show nothing strange, and the amp behaves normally otherwise ... I wouldn´t worry very much and leave it as-is.
As in "don´t fix what´s not broken".
Good luck.

PS: congratulations on already having tested it in a "scientific" way on your own, we often get not-too-precise musician´s descriptions of problems which throw us off the path, which sometimes are not real problems at all but the result of certain settings which later are not repeated.