Rocktron velocity 300 Power supply question

[amelbye]

Hey, I have a problem with a rocktron velocity 300 power amplifier (the original 2 unit version from the 90s). I posted here since I believe the problem is power supply related.

It breaks a lot of fuses on power-up, and it makes loud noise whenever I plug or unplug something in the same room.

I had a local tech look at it (for blowing fuses). He couldn't find anything wrong with it and said the only thing I could do was start using a slightly bigger fuse, which is what I've done now for the past 6 years.

However, I'm sure there has to be something wrong at the power input stage for it to make the noises it does whenever there is a fluctuation in line voltage.

Or maybe it's a design flaw. If so, how can I modify the unit to fix this? (I'm on 230V)

I've attatched a full schematic of the unit.

Best Regards
Audun

[Th3_uN1Qu3]

The schematics are unreadable, please post higher resolution versions. But regardless, it sounds like the transformer of this unit does not have soft start so it draws a large current upon startup till the core magnetizes fully.

You need a soft start module. You can either buy or build one. It's not too sophisticated - a power resistor and a relay bypassing it. The timing aspect of things (turning the relay on to bypass the resistor for normal operation) can be handled by a 555 timer.

[amelbye]

that's strange, I'm sure I uploaded gif files of adequate resolution, not jpegs... ah I see, there's a very small max resolution for gif attachments on this forum. Even the max resolution for jpegs is a bit small for these schematics.

Here I've uploaded them to my own site:




I guess a softstart should get around the problem, but don't you think there's something wrong causing it to draw so much current? It breaks the fuse about 30% of the times I turn it on, I'm sure most of these amps do not...

[gtrmann1]

I have extensive experience with the Rocktron products and have a few tips that may help...

1.) Make sure the volume of both channels regardless of bridged/stereo selection is completely down before power up.

2.) Make sure your input volume from your last unit going into the amp is muted. You want to avoid any kind of "popping" as the circuit receives that "spike" as an overloaded signal and ping goes your fuse(s).

3.) Check your cables! Make sure you are using speaker cables for speakers and a balanced or instrument cable for your amp inputs. This is usually the problem. Most guitar players will just use an instrument cable going into their speaker cabs...huge No No! Everytime I have encountered a fuse blown from my 300 it was a bad speaker cable. They don't last forever!

4.) Power up your amp LAST, shut it down FIRST always.

I am surprised that placing a larger value fuse works and did not overload your coils. I would check this list and attempt to go back to the standard fast blow fuses.

Anyway, sometimes it's the simple things, and Rocktron can be very touchy with input signals. Good Luck!

Tim

[amelbye]

Thanks for your reply!

1) I haven't tried this, however I generally keep them low when I power up

2) This would only make a difference if I powered on the velocity before the preamp. I always turn on the power amp last, hence it won't take much notice of any voltage spike from the pre.

3) Cables are fine. In fact, the amp even blows fuses with nothing connected at all.

4) That's what I do.

I guess using a larger fuse does put the coils at risk. However, as long as there's not anything seriously wrong with the unit, even running the thing unfused should be safe. And illegal of course.

I'm pretty sure this has nothing to do with input signals. It only blows fuses at power-up. Never while in use.

[Th3_uN1Qu3]

Originally Posted by amelbye

I'm pretty sure this has nothing to do with input signals. It only blows fuses at power-up. Never while in use.

Inrush current. Some thermistors in series with the AC input will help, a full-blown softstart circuit may not be needed. Try two NTC thermistors in parallel, recovered from ATX PSUs.

[amelbye]

ah, such a simple solution!

do I place the thermistors before or after the fuse though?

The fuse is in the power plug on this device.

And why do I need 2 thermistors?

I have a few AT power supplies in the garage, I'm sure I'll find some in there. Thanks

[Th3_uN1Qu3]

One may not be enough to keep that big toroid happy, cheap PSUs tend to use underrated parts so put two in parallel to ensure they don't blow up. Heck, some units even omit them altogether... Especially the noname specials labeled at 3x their real power. It's recommended to mount them after the fuse. Also keep them away from sensitive parts as they run hot.