Ampeg SVT 4 Pro schematic needed

[Englfireball82]

I just tried everything u mentioned and still no sound from power amp A side, thanks for the reply....any other ideas?

[Enzo]

You didn;t answer whether both relays are clicking on or only one.

[Englfireball82]

oh ok, well heres how i noticed what was going on....im using it through 2 2x15 mesa road ready cabs, and of course on the back of the svt there are 3 speakon outputs while looking at it from the back from left to right power amp B then bi amp then power amp A.....so i connected one speakon to one mesa cab from power amp b then the other speakon to the other cab from power amp a......and only the power amp b side was working...for each power amp there is a speakon connection and 2 1/4 connection outputs basically allowing you to run 3 different cabs from each power amp if you wanted to.....so hense theres no sound coming out of power amp A side cause i tried all 3 outputs on that side and no sound at all.....im still quite new to this amp so i dont know much about it.....if it was a relay that wasnt kicking on how would i be able to tell or find out? Thanks again.

[pecorporation]

If the unit is set to "Bi-amp" and "Mono bridge", theamp will not operate correctly. The bridge output is not a third amp, it's a colmination of A and B.

[Englfireball82]

Thanks for the reply.....its not set to bi amp, im using it to run 2 cabs, as in one cable from power amp B to a cab, and another cable from power amp A to another cab....the bi amp output isnt being used. It is set to Stereo and not mono/bridge. Im not getting any output to the cab from power amp A side.....B side works just fine. Both cabs work great and the cables are great, ive tried swapping all cables and cabs around and just running on power amp A side alone, and still no sound from power amp A side at all.

[Englfireball82]

Im still new to the head....how could i tell if it was set to "bi-amp"? I see only the switch that says stereo or mono/bridge.....it is currently switched to stereo.

[pecorporation]

I've seen a lot of those units with failing solder connections on the output amplifier board, mostly components like output FETS and large components like filter caps that are suseptable to heat and vibration stress. I usually reflow all these suseptable components, including all jacks, pots, free standing drive transistors, even the relays and the multitude of pc plug pins to ensure the amp will be reliable on the road. Visually inspect every solder connection closely on that board looking for a cracked solder ring around the component lead.

[Englfireball82]

Ok cool, so your next step would be to go into the head itself and browse the soldering connections...i usually install my own pickups so im familiar with soldering. Should it be quite easy to detect a broken solder when i get in there or would it be a pain to see? As i dont have a diagram of what it should look like. Thanks

[Enzo]

SO lets label your stuff. We'll just call it all A and B. SO your setup is Output A through Cord A to speaker A, and then Output B through cord B to speaker B.

Sound comes from speaker B.

Unplug cord A from the amp and set it aside. Now plug cord B (still going to speaker B) into amp output A. Still no sound, or does sound now come out through speaker B?

Just making sure there is no troubloe in the speaker and cord.


I believe you when you say no sound comes out one side, but I still would really like to know if BOTH output relays click on or just the one on B channel. Usually you can hear it. A few seconds after turning on the power switch you hear a click inside. Actually there should be two clicks or at least a sort of "cli-click." Leave the speakers off or at least don't put any signal to the amp, so you can hear this. What I usually do is put my fingers on top of each relay, and I can feel it clicking into place inside it.

This piece of information is a valuable clue as to what is happening inside. If it DOES click on, there is a very limited list of probably faults. If it does NOT click on, then the relay circuit has detected a problem in the power amp.

[Englfireball82]

exactly how you just said it....i have done that already....both cabs and speaker cables work great as ive ran both of them through power amp B cause its the only one working (and side B does run both cabs together)....and ive tried swapping them out and using them in power amp A and yes.....there is no sound at all....ive tried switching every button on and off thats pushable or switchable on the amp and have had someone else constantly playing a guitar through it while i was working on it just to see if there was any shorts or anything and still nothing from side A. I have also (just now) taken the amps top off to inspect for broken solders or loose wires and everything looks perfect inside it...no broken solders or loose wiring, and nothing touching metal. If there is a relay that isnt working i would have no idea what it looks like or what i should replace or look for in this particular amp. If i only had a circuit board diagram for the amp then i could see what the relays at least look like on the inside.

[Enzo]

Did you read this whole thread? In post #3 of this thread, I posted the schematics AND board layouts for this amp.

428XXp9.pdf is the file with the power amp board layout. On the furst page of it is the board layout. Upper right area there are four large round things - your main filter caps. Immediately to the left of those are two relatively large sorta rectangular things. The left end of each is taller than the right end with a step in the middle. They are numbered K101 and K201. THOSE are your speaker relays. They are plastic boxes on the circuit board.

[Englfireball82]

ahhh yes i remember seeing those and thought they may be the relays when i was in there. So in this case relay K101 would be for power amp A correct? How do you go about replacing it....does it just pop out of position by pulling it out (like replacing a power tube)? I wouldnt want to break anything. Thanks again.

[Enzo]

One would desolder it from the circuit board and solder in a new one.

BUT!!!

I am not suggesting the relay is bad. It might be, but what I am more worried about is that it is not clicking on because something is wrong in the amp circuit. The relay action is merely a clue. Like a detective looking at a bed to see if it was slept in. The detective is not concerned that the bed is untidy, he is looking for what the bed tells him. There is a circuit for each relay. The circuit detects trouble in the amp, and when it finds trouble, it leaves the relay not energized.

If the relay clicks on but there is still no sound, then we might suspect the relay after we measured the resistance of its contacts. But we could just as easily suspect the input stages of the power amp, possible bad IC. But if the relay does not click on, then we want to find out if the amp has failed to a DC voltage condition.

[Englfireball82]

ok yea....so it could be numerous things then. I will mostlikely have it taken to a shop and worked on. Thanks

[LoGruvz]

Amepg SVT-4pro Side A no or very little sound output

I have the same issue as some of the others. Taking it in to be checked. However, there is another thing you can do to test. Just plug your bass straight in to the amp A "in" on the back I did this and the volume was good and strong. If I plug in to the front input the sound is so low you can barley hear it but there is sound. I play out every weekend so it has been banged around and dropped. Maybe just a loose preamp tube. Side B works fine.