Ampeg SVT-5 Pro distortion

[Mars Amp Repair]

Well,
it turns out the amp we have is an SVT5pro. different config & apparently a different symptom. probably just bad connections on main pcb. We'll keep looking for this, tho...glen

[Grant_More]

Great Work Guys!

Hi Guys,

I'm AMAZED at the amount of experience and expertise you guys have! Thanks for all your suggestions!

Enzo, thank you. I think this is a valid solution. In fact, on the back of your suggestion I’ve put together a SPICE model of the fan circuit and have identified a couple of ways to do just this (I'll share it if anyone wants it). Unfortunately the resistors which need adjusted to make the fan start at a lower heatsink temperature are under the main heatsink and therefore the main PCB assembly would have to come out to get to them. =0/ That said, I think it's a good fix!

Steve, great point made there. Lots of locational variation on the line voltage, plus the transformers are different, adding gears to the variation. I think you’re right – the regulator is marginal so we need a robust solution which works under the harshest corner conditions.

Mark, good suggestions. I checked the 10R resistor you suggested and it’s indeed 10R and 0.5W. If I increase it to 15R the fan will draw less current at startup and the overcurrent/overheat may be avoided. This sounds like a good fix because the fan is in a closed-loop control circuit and hence the loop will compensate for the lower fan current. I think there may be a drawback in that the maximum design fan speed may never be reached – this might be ok, but I’d be worried about cooling at high temperatures. I’m not sure about inserting series resistance in the 7815 supply as the power which would be dissipated in the heatsunk 7815 (lots of it) will be dissipated in the un-heatsunk resistor as you point out. The problem is that I think the resistor would need to be heatsunk as well – probably quite difficult and may also mess with the regulated output voltage of the 7815 if the supply voltage is dropped too much.

I haven’t measured the supply currents because it would mean taking out the PCB and I’m trying to figure out a solution that can be easily applied without taking the PCB out. However, thanks for correcting me on the heater current! I used 300mA in the calcs, but it’s only 150mA (@ 12.6V) as you point out at. The 500mA I’ve used is the tube heater plus the pre-amp current plus the fan current. As the regulator is rated for 500mA, I’m guessing and rating the calculations for a safe 500mA load current (as it can’t be any more!).

Glen, brilliant idea to mount the regulator on the chassis! I think this is the #1 solution if the PCB is coming out. Should be totally rebust – could add a small heatsink to the mounting if ultra-mega-super robustness required!

So, I think the best fix options we have identified are either one or a combination of:

1. Fit bigger heatsink to IC4 regulator and add a smear of heat transfer compound to the heatsink/regulator interface to improve heat transfer.

2. Fit IC4 regulator on to chassis, possibly with a heatsink between the chassis and the regulator.

3. Decrease fan turn-on temperature by adjusting temperature sense/fan control circuit.

4. Decrease fan current by increasing series resistance with fan.

The good news is that both myself and BassBoy49 have checked out solution 1 and, as far as we have tested, it’s fixed the problem. See thread t17571 “SVT 5 Pro Startup”.

Of course, there will be extreme cases where the fix won’t prevent the dropout issues (for example, where the heatsink is very cold and the ambient air is relatively hot), but I think in many cases it will be just the easy fix that is needed.

The Multicomp part number for the alternate heatsink is MC33283 and is available from Farnell (and Newark) under part number 171-0628. It’s got an identical footprint to the original and simply slides on. I have a couple of spares and would be happy to post them out if anyone wants to try the fix – drop me an email!

If the PCB comes out for service reasons, or if a 100% total belt-and-braces fix is required, shift IC4 up on to the side of the aluminium fan bracket on 3 flying wires as Glen suggests.

BTW: I’ve got a couple of unrelated SVT 5 questions that I’m going to post up on a new thread. Comments and experience appreciated!

Cheers guys, good discussion and I'm sure this will act as a great point of reference for others with this issue! Thanks to all who contributed!



Cheers,
Grant

[Grant_More]

Photo of the IC4 heatsink fix. Difficult to get a good perspective, but you can see it's the same footprint, just twice as tall as the original heatsink.

Sorted.

Cheers,
Grant

[anfiore]

Ampeg SVT-5 Pro broken.

I understand you are pretty educated about this specific model. I enjoy reading about how you go about fixing your SVT-5 problems. I bought my amp used. I did not try it out before buying it...STUPID. When I bought it, I did not have a cab for it yet. So a week later I found a ampeg 810. I bought it (still not knowing I had a bum amp) I brought my amp over to the person selling the 810cab. I plugged it in (after trying to find the correct port) and everything sounded okay but still rather muddy- after trying several times to find out exactly where the 1/4inch plug was supposed to plug into the amp. I tried just about every hole because I never had a big amp such as this one. Perhaps I made things worse because it does not even play at all anymore, even after I bought a speakon cable.
I brought the amp over to a local amp repair shop. He said this was out of his league and that I would have to bring it to an ampeg dealer. He suggested it was a cooling problem that something may have gotten too hot. That said I did some research for things I might be able to find on overheating SVT5. I came across your YOUTUBE video, after watching that and reading your thread with the other person on a similar matter, that my IC4 seems to be rather brown where as my IC5 is still for the most part golden/shiney.
Are these my heatsinks?
Will these need to be replaced?
Did this occur because of a cooling malfunction?
Did I set off a cooling malfunction because I was trying out all the 1/4 ports on the amp?
The fan still does work after turning it on for a few seconds, if I hold it like you did in your video it does shut-off.
Anything you could tell me would be much appreciated. I learn fairly quick so I may be able to fix this myself unless I have to go further and inside the sealed areas inside my amp. Please contact me via this website, or my email squeek391mx@aol.com I have it on me 24/7 as I receive email through my phone. Thank you so much.

[Grant_More]

Hey Anfiore,

Sorry to hear you're having problems. Sounds to me more like a simple setup problem than the heatsink problem we've been discussing (although you will probably experience that at some point! ;0) ).

You're right - the SVT-5 does have lots of places to plug in your cabinet and it's not a good idea to plug things around until they work!

Have a look at the users' guide (attached). Check out page 6. Here's a couple of points for you to check:

1. Check that the connections within the cable and connectors you've bought matches the cabinet and the output of the amp. The SVT-5 has two power amplifiers inside - these can run in a number of different configurations and the wiring of the cables is different depending on how you've got it set up. It's all in the manual - take a look.

2. Check the configuration of the amplifier based on the cabinet set up. There are a couple of little configuration switches around the back of the amp - make sure you've got them set correctly. Again, take a look in the manual.

Let us know how you get on!

Good luck!

Cheers,
Grant

[jujumusic]

I need svt5pro schematic,my amp not working now
Any brother can help me?
My email: guizilaile1975@hotmail.com

Cheers
Lee

[pontiacpete]

I have this;