SVT 3 Pro Dead?

grumpyjfc · #1 10-30-2011, 02:50 PM

My American made SVT 3 Pro (one of two, I love these things), won't make any noise. It powers up, goes through the warm up cycle, I hear the relay kick, but zero output. I've tested all the tubes & checked both fuses. I've tried the gain, EQ, Master and all the switches in different positions and same result. I've verified that there is signal coming out of the bass. There is no activity on the red clip light no matter where any/all the settings are.

Anybody have a clue?

JimmyM · #2 10-30-2011, 03:11 PM

Have you cleaned the effects loop jacks recently? If you don't use them, they can build up corrosion which can cause shorting out of the system. If that's not the problem, then it's tech time. The good news is they can almost always be fixed if you throw enough money at them.

grumpyjfc · #3 10-30-2011, 03:14 PM

I've never used the effects loop so I couldn't tell ya if there're clean or not. I'll take a look. Thanks!!

Jerrold Tiers · #4 10-30-2011, 06:29 PM

If the input clip does not light, then there is probably no signal getting through even the first part of the preamp.

it is possible that tube #1 is bad, or that the preamp filament supply is 'out". The tubes are all in series for filaments, so one open will shut them all off in the preamp.

grumpyjfc · #5 10-31-2011, 09:31 AM

Hey Jerrold, with the help of another amp, I've isolated the problem to the power amp section. Any suggestions on where to go from here?

BassmanPaul · #6 10-31-2011, 11:24 AM

I would seriously suggest a tech.

randysmojo · #7 10-31-2011, 11:30 AM

My first thought with these is cold solder joint. If you are not qualified to solder or work around high voltage caps, please take it to a tech. It could hurt you or worse.

Jerrold Tiers · #8 10-31-2011, 09:22 PM

If it is in the power amp, then the "clip light" should have lit. The tubes in the power amp are not on the same filament supply as the preamp, and the preamp clip light should DEFINITELY have lit.

Anyhow, there are a lot of reasons the power amp might not work.

Does the fan run at full speed, or at a slow speed when the unit is "on"?

grumpyjfc · #9 11-01-2011, 04:13 AM

After troubleshooting with a seperate amp I discovered that the battery in my test bass was dead - hence no clip light. Replaced the battery and the clip light came on when I hit a string like it should. so then I took turns bypassing the pre and power amp sections using the preamp out and power amp inputs on the back. This is how I isolated the power amp section. Just need to know where to look next.

The fan runs at full speed just like my good SVT3 pro.

Randy, thanks for the concern about soldering. I install and service marine electronics for a living, I'm around high voltage, large power supplies and capacitors all the time. And I use a soldering iron almost every day!!!

WingKL · #10 11-01-2011, 05:23 AM

If you really want to troubleshoot this yourself, you will need a signal generator (your PC soundcard will work), an oscilloscope and schematics for your amp. Send in a sine wave at the power amp input and trace the circuit till you find where the signal is not going any futher. It would also be good to have an voltmeter so you can confirm that your power supply rail voltages are correct.

randysmojo · #11 11-01-2011, 05:53 AM

What was wrongs with mine when having the same issue was cold solder joints on the jack for the mute footswitch on the back. If you notice the clip light lights up when it is muted. Cold solder joints on that jack puts it in a muted state so clip light lights up and no sound. I thought it was the power amp too at first. Of course I had a lot more cold solder joints then just that, so while you are in there, inspect all of the boards, etc.

grumpyjfc · #12 11-01-2011, 06:30 AM

Yeah I have already had my other 3 Pro apart resoldering the input jack that came loose during a sound check one night. I think at this point I'll pull the board and inspect the solder joints, couldn't hurt.

Does anybody know where one can score a schematic for this amp?

WingKL · #13 11-01-2011, 06:53 AM

If anyone does, it would be Jerrold.

Jerrold Tiers · #14 11-01-2011, 07:04 AM

Yeah I have the schematics.... PDF, send me an email address to get the info.

But your "good" 3PRO is apparently "bad"......

The fan should not run at full speed, unless the unit is hot. The fans are temperature-controlled. Either the fan circuit or the temperature sensor circuit is messed up if the fan runs full speed.

If the temp circuit is messed up, the amp *may* decide it is too hot, and open the relay, or decide it is too hot to ever close it. That would produce a "no sound" condition for sure.

Conditions that open the relay are over temp, and under-voltage (really low, like turn-off). And there is a delay at start-up.

I know you mentioned hearing the relay, but did you for sure hear it CLOSE, or was it opening? That is one thing to check.

Since your fan control seems to be messed up (fan runs full speed when cold) you would need two problems to get a no-sound also, unless the temp circuit is stuck in a 'too hot" output condition.

I don't like co-incidences, so check that first. Especially check it because "it can't be the problem, I heard it close"...........

grumpyjfc · #15 11-01-2011, 07:09 AM

Thanks Jerrold, good advice. Email is pm'd

grumpyjfc · #16 11-01-2011, 07:55 AM

Okay so, sheepishly, I must confess that I have never really paid THAT much attention to the speed of the fan motor; they are both running the same speed which, this morning because it's 42 degrees in the garage, is very VERY slow after the initial spin up at power on. Having said that, maybe I can breathe a sigh of relief that both of the amps arent faulty!!!

Jerrold, I'm not sure if the relay is opening or closing when it clicks. Looks to me like I can only access it from underneath to test, is that correct? I,ve had it in the back of my head that the relay may be at least part of the issue since there is ZERO noise from the cabinet after it clicks; no hiss or anything. Is the relay a "usual suspect" in this scenario?? Thanks!!

WingKL · #17 11-01-2011, 06:00 PM

grumpyjfc, you should really PM your email address to Jerrold instead of posting it out in the open, unless you don't mind getting spammed when the internet-crawling email address harvesters finds it. You might still be able to edit your post to remove it.

grumpyjfc · #18 11-01-2011, 06:05 PM

WingKL,
Point taken, good advice, PM sent, post edited!!!!

randysmojo · #19 11-01-2011, 09:14 PM

I hope you get it figured out quickly enough. Is this a SLM model with the blue and red logo? That's what mine is. I basically had to resolder everything, but it works great now.

grumpyjfc · #20 03-24-2012, 10:06 AM

It was the f****ng tube in the power amp section. So much for the accuracy of my Sencore tube tester.