Rocktron velocity

gtr-fxr · Post by **gtr-fxr** » 23 Nov 2011, 00:28

does anyone here have any experience with the older Rocktron velocity 300 ?
I bought a used one with hopes of repairing it and need a little assistance,
first off when i got it it needed a transistor, i replaced that then when i powered it up to see what was going on with it it started smoking resistors,
i have a schem. that I'll post as soon as i find it.

gtr-fxr · Post by **gtr-fxr** » 02 Dec 2011, 01:36

here it is now if only someone could give me a hand figuring out whats wrong

collingtech · Post by **collingtech** » 16 Dec 2011, 15:50

i had one of these, old velocity 300 dual space rack , i cant open your schem , and i cant find mine, so repost and i could give you a hand tracing the prob, also especify the section where you are burning the resistor too

gtr-fxr · Post by **gtr-fxr** » 16 Dec 2011, 17:46

try this one

gtr-fxr · Post by **gtr-fxr** » 16 Dec 2011, 17:59

the last ones i burned up are R28-29. and U5 and a jumper that goes to ground . it seems that one side of the burnt jumper is ground and the other has -50vdc also pin 6 of U5 has -50vdc doesn't seem right to me. any help would be greatly appreciated.

teemuk · Post by **teemuk** » 17 Dec 2011, 10:18

First, built yourself a current limiter so that further testing will no longer burn up components in case the amp still remains faulty. Something like a common lightbulb in series with the mains circuit will do. If the amp draws too much current the lightbulb will absorp most of the energy and while it does this it will also indicate a faulty circuit by burning brightly.

With that done, you start from the basics. Transistor tests for everything, resistor tests for everything, capacitor tests for everything, IC tests for everything, etc. Judged by your description it seems the amp has several faults already and you - testing the amp without appropriate safety measures - have furtherly escalated them to the point where even parts of minor circuit functions (e.g. clipping indicators) start failing. It may have started from blown output transistors, the thermal effects of running a failed amp without current limting have spread the problem across the board.

So, patiently start from basic part testings and replacements until everything you have tested faulty has been replaced. At that point you can power on and test the amp - with current limited - and if the current limiter still shows some faults then we'll discuss it and pinpoint the remaining problems based on your test measurements and description of symptoms.

gtr-fxr · Post by **gtr-fxr** » 17 Dec 2011, 15:24

sounds good, I'll throw together a limiter and get back to testing . i will post my findings.
Thanks

gtr-fxr · Post by **gtr-fxr** » 18 Dec 2011, 19:00

so i built a light bulb current limiter with a 60w bulb (all i have on hand at the moment)...and when i turn on the velocity to test it im still burning up R28-29 does this mean i need a larger or smaller bulb?

mozwell · Post by **mozwell** » 19 Dec 2011, 00:24

Just to check, the light bulb is in SERIES with your load, not in parallel with it.
The bulb shoudl be connected between the active of the wall plug & the active of the socket your amp will plug into for the testing. Neutral is connected to neutral & earth to earth.....

There are four internal fuses in the power amps, remove them temporarily. This will disable the power transistors from drawing current.
Before powering up, meaure the following with an ohm meter.
Power transistors Q13, Q17 etc.
Meaure across op amp power supplies, pin 8 to pin 4.

Turn on the amp, measure the +18V & -18V supplies, are they correct. If they are high, then the voltage regs U9 & U10 may be blown, and will need to be replaced. All op amps will need to be replaced too. If you are replacing op amps, install ic sockets, and then power up the amp with th op amps not installed to make sure we have the correct +/-18V supplies.
Measure the +/-50v supplies, are they ok ?
Once we have made sure the power supplies are ok, we can move forward.

gtr-fxr · Post by **gtr-fxr** » 19 Dec 2011, 01:16

just to make sure on the power transistors you test base to collector and base to emitter and if thats the case they all look bad "reading about 5 ohms either way ,also with leads reversed"

gtr-fxr · Post by **gtr-fxr** » 20 Dec 2011, 02:10

oh boy--- its all kinda screwed up . power transistors are looking bad 18v is bad, time to start replacing parts i guess. ill re post when i get everything replaced and can start testing again.

mozwell · Post by **mozwell** » 20 Dec 2011, 23:27

I just normally check collector to emitter on ohms & measure both ways. If they read low, they are most likely shorted.
If i replace power transistors, i do all of them and the driver transistor too.

There are LF353 & 5532 opamps in this amp, and the variable regs will give out +/- 17V.
LF353 are rated for +/- 18V absolute maximum. Personally i think the +/-17V is a bit too close to the 18V max rating of the ic's.
i woudl change R84 & R85 to 2k7, for +/-15.5V output.

First step is to make sure the power supplies are ok, then we can go onto the rest of the amp.

mozwell · Post by **mozwell** » 20 Dec 2011, 23:42

i think the design of the power supply is marginal in this amp. The LM317 etc are rated for 40V max across them. With a main DC level of 50V, and and output of 17V for each reg, this is 33V across teh reg. No wonder they have a heatsink on them. If mains supply is high, especially with 115V systems in the US, we coudl very easily get to more than 40V across the reg.
Fortunately this is easy to fix.
We just need to insert a resistor in series with the input to each regulator. We can try 220 ohm 5W, then measure the input voltage to each regulator. We shoudl aim to get about 25 to 30V on the input to each reg.
All the opamps will need to be installed for this, as we need to size the resistors for the actual current draw.
If op amps are blown, they will need to be replaced.
We can get the op amp part of the circuit & power supply working first, then we can size the resistors to suit.

If U5 pin 6 has -50V on it, it is fairly likely that all the op amps have gone.
With power off, measure ohms between the +18V line & 0V, and between -18V line & 0V, does it read very low ?

So, remove the four fuses in the power stage.
With the light bulb limiter in circuit, measure the +/-50V rails & the +/-18V rails.
If the 18V rails are more than 20V, the ic's will all need to be replaced. Install sockets if you do this.
Get teh +/-18V rails working correctly & change R84 & R85.
Install new ic's, and install a resistor in series with the input of each regulator. It is ok to use a different resistor value for each regulator, as it will be determined by current draw. Aim for 25V to 30V on the input to each regulator.

Nothing so serious here that we cannot fix it.

mozwell · Post by **mozwell** » 21 Dec 2011, 00:07

may i also suggest you start a thread at Ampage http://music-electronics-forum.com/ on this topic.
There are plenty of people more knowledgable that me there, (Enzo, Jazz P Bass etc), and they may be able to get your amp fixed quicker than with any help from me. i will also be away over the holidays for 2 weeks....
If you start a thread, go through what you have already done, step by step & post the schematic.

the guys on the Ampage forum are exceedingly generous with their time & are really helpful.

gtr-fxr · Post by **gtr-fxr** » 21 Dec 2011, 08:28

thanks for all the help so far, it is looking like the ic's are fried so im gonna take a break until the parts come in.

gtr-fxr · Post by **gtr-fxr** » 22 Dec 2011, 16:34

I'll try to post on ampage later today, but for now i went through and replaced all blown/burned parts except the power transistors since i dont have them yet.and so far so good "no smoke" butttt Q22 is getting really hot within about 5-10seconds of flipping the switch

gtr-fxr · Post by **gtr-fxr** » 29 Dec 2011, 09:06

i posted on ampage...no luck yet. so far i got the 18v and 50v straightened out,but when i pit in fuse 2 on channel 1 the limiter lights up like.....well..a light bulb very bright! so somethings going on after the fuse but not on channel 2 light is barely lit but still no signal passes through.

gtr-fxr · Post by **gtr-fxr** » 30 Dec 2011, 19:43

ok...i got the reg in 18v is good "actually 15.5v" 50v is good . no op amps in and reading 46v across pins 4+8 and Q10 is burning up now WTF!

gtr-fxr · Post by **gtr-fxr** » 30 Dec 2011, 20:19

ok never mind ,i cant seem to get the -18 working right it keeps coming out -46v. this is getting frustrating.

gtr-fxr · Post by **gtr-fxr** » 31 Dec 2011, 07:14

in other words U10 keeps going out. oh and i changed R84-85 as recommended to drop to 15v.