Ampeg SVT 5 Pro Startup

[Bassboy49]

I have a used SVT 5 Pro with the following symptom. When cold and turning on, the amp stays on for about two minutes until the fan starts. It then goes into the protect mode, shuts itself off for 15 seconds, turns itself on again, until the fan starts. It may do this cycle anywhere from two to four times. With the help of this forum, I have already doen several things (see previous post). I have replaced the +15 voltage regulator, checked and cleaned all the boards, and installed a new fan (the other was real noisy). This has greatly improved the overall performance of the amp, especially the tone controls. Now, today I turned the cold amp on, after first removing the sole tube from the small PCB tube board. The amp stayed on, even after the fan turned on, and did not go into the protect mode. I have checked all the connectors to the tube PCB. OK there. Then the voltage on the tube socket, to see if there is any voltage drop when the fan turns on. Negative on that. I do not have a tube testor. Thanks.

[Steve Conner]

Sounds like the power supply is struggling to power the tube heater and the fan at the same time.

Check for bad, bulging, or dried up electrolytics in the +15/heater/fan supply. (I assume the tube heater is DC, otherwise my diagnosis doesn't make sense.)

[Grant_More]

Hi Guys,

Have a look here for a debug of this issue:

YouTube - Ampeg SVT-5 Pro Problem

YouTube - Ampeg SVT-5 Pro Problem

I recon two fixes are possible:

1. (Easy) increase the size of the heatsink on IC4.

2. (Hard) adjust the turn-on temperature of the fan by adjusting the control circuit.

I'm going to try 1, I'll let you know how I get on.

Cheers,
Grant

[pecorporation]

This may be a silly question but here goes...

Is the value of R25 still 10ohms? How does the solder look on that resistor?

Ok, that was 2 questions.

Steve

[Grant_More]

Hi Steve,

Yep, on the unit I have it's 10 Ohms. I guess it's current limiting for the 250mA/24V rated fan.

I've checked the continuity to the +15V supply and down to the diode in series with the fan and it checks out sub-1 Ohm so I guess the soldering is ok. I can't see the solder because it's a leaded resistor and the solder joints are on the back side of the PCB (I'm trying to avoid taking it out of the case!).

Have you seen a similar issue?

Cheers,
Grant

[pecorporation]

I have on other SVT models and it's usually been cold or failing solder on that resistor or the +15vdc regulator (that you already replaced). How does the solder look on that tube socket? See any cracks in them?

On these units, I usually go through the entire power supply & output section & reflow all large components and all components suseptable to heat...output transistors, load resistors, all filter caps, all voltage regs., rectifier bridge, tube sockets and even found failing solder on the pc board plug pins that attach to the multi-wire cable connectors. It's a lot of digging but it makes the amp road ready and reliable. May want to take it to a tech if your not comfortable with the work.

Good luck!

Steve

[Grant_More]

Hi Steve,

Thanks for that, great advice! I'm planning to gig a load with this amp, I need it to be reliable!

BTW, I've debugged the original problem a bit more and think I have a solution - see the other threads I'm discussing on for what I'm thinking if you're interested.

Thanks again!

Cheers,
Grant

[Bassboy49]

Really cool technical stuff in your last post, and, sounds like a great plan. I'll be waiting for the results of your bigger heatsink experiment. If it works, hope I can get one of those here in the states. My wife says you are a great techie wizard and should have your own TV show..."Sparky and Shorty Fix Amps". Thanks again, patiently awaiting the outcome.

[Grant_More]

Hey Jeremy,

Great idea! I wonder if the BBC will let me have my own show!

I've got five heatsinks on order from Farnell, but the fix only needs one. If it works, I'll post one of the spare heatsinks over to you along with some heat transfer compound. I'll put together a quick crib sheet on how to apply the fix and you can help me out by independently verifying the fix and checking out my instructions - in return for the heatsink you can give me some feedback! We can then give everyone else a simple and verified fix which players can apply without being technical experts. Winner!

How does that sound to you?

Cheers,
Grant

[Bassboy49]

Thanks Grant. I checked the web, and, there's a outfit in New Jersey that I can order the same Multicomp heatsink from...costs about a dollar (US). That might be faster than the "post" from across the pond. I'm happy to test your theory out on this side. It's only a few bucks, even with the shipping. I don't know if the sink will come with any directions (I doubt it), but, you can post instructions on this thread, if you want. I noticed that when I put a new regulator in that the existing heatsink just lifts right off. Is adhesive necessary? Thanks again, I think you're on to a real simple fix for the SVT-5 Pro.

[Grant_More]

Hi Jeremy,

Sounds good!

Heatsink should slide right on, admittedly after a gentle push - the heat transfer compound is a grease rather than adhesive.

The original regulator is a 78M15 in a TO-220 package. What have you replaced it with? The TO-220 is a standard package with standard dimensions... =(

I've seen some low-cost TO-220 devices in all-plastic packages (i.e. without the metal tab on the back), but as far as I'm aware, 78M15 devices in To-220 packages should all have the metal tab.

Can you point me at a datasheet for what you replaced the original regulator with?

Cheers,
Grant

[Bassboy49]

Hey Grant, I purchased the regulator from West Florida Components, check their website at westfloridacomponents.com. The regulator is a TO220 7815 1A 1amp +15V, stovk #T170. It has a metal top with a circular hole in it. I'll order the heatsink (I think I'll order three, there are two other heat sinks, one next to IC4, IC5, and another on the main board), after I log out of the Forum. Keep you posted. You're in the home of my favorite author's (Ian Rankin) protagonist, Edinborough. Thanks again.

[Grant_More]

Hey Jeremy,

Ok, I think you've maybe got a TO-257 package there. Supplier is MicroSemi and their 7815 type regulators don't seem to come in a TO-220 package (could be wrong, I did have a quick look). Although very similar (and easily mistaken for a TO-220 I guess), it appears the physical package specs are slightly different. That's probably why you've got a problem with the heatsink sliding off there.

TO-257: http://www.st.com/stonline/products/...kage/to257.pdf

TO-220: http://www.fairchildsemi.com/dwg/TO/TO220Y03.pdf

BTW: I wouldn't bother replacing the other two heatsinks - the current is waaay less through the -15V regulator (IC5) and the fan IC (the other IC with the heatsink) is lower current too.

When you replace the heatsink, apply the heat transfer goo and also, if you've got a glue gun, put a dod of glue gun glue on the PCB and heatsink to make sure it can't slide off or move around.

Heatsink should be a tight slide-on fit. If it's not, you'll have to pop out the regulator and put in a TO-220 device or you'll get a poor thermal contact...

Let me know how you get on! Off for a pint of Rankin's favourite now! ;0)

Cheers,
Grant

[Grant_More]

Jeremy,

Should also say, don't let the heatsinks touch each other - 7815 and 7915 regulators can have different voltages/ground connection to the metal tab - be careful! =0)

Cheers,
Grant

[Bassboy49]

Hey Grant,
Received my heatsinks and compound yesterday afternoon. I pulled the top off the SVT this morning and installed the heatsink, with a little grease as per instructions. It was a very tight fit, and, I think in giggling it back and forth, may have loosened up one of the solder points on the regulator. That's the bad news.
The good news...after I turned it on, the amp did not go into the protect mode, even after the fan turned on. Everything seems to be working as it should. I started it up twice now, after letting it cool off completely, and it worked fine both times. I haven't put a load on it yet. First, I want to take the main board out (again), and have the regulator soldered by my friend at the music store. I can't seem to get the nice "nipple" type solder point with my home equipment that is present on all the other joints. Hope your experiment has the same results.
Can't thank you enough. It seems like such a simple fix. I suppose Ampeg would be hesitant to send this out to the field because the regulators may be stressed from overheating and also need replacing. So, have a pint for me, and take claim to the Superstar of SVT 5 Pro troubleshooting. Stay in touch, if you like. I'll see if I can figure out how to send you my email without having available to everyone. Thanks again, Jeremy

[Grant_More]

Hi Jeremy,

Excellent news! My heatsinks arrived yesterday and I’ve tried them out with the same result as you – the problem has vanished, at least in the ambient temperatures in my lounge! 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 soldering issues you’ve got might be due to the solder composition. I’m guessing here, but I think that between the time they made the amp and now they’ve switched to lead-free solder which from experience I know doesn’t flow quite as nicely as old lead solder. You’ve probably got lead-free rework solder there.

You might want to see if you can get some lead solder (they do still sell it exactly for the purpose of repairs – it’s difficult to come by though) – what you want is 60/40 tin and lead solder.

Also, try a “flux pen” – it’s a pen with stuff for making the solder flow when you apply heat. All you do it use the felt end of the pen to really wet the solder joint before you solder it, then when you do it helps to form the solder joint correctly. Just make sure you remove the reside with a suitably solvent (sometimes they sell them in a 2-pen pack, flux and remover). ;0)

No worries at all for helping you out – post the fix and tell other bass players about it! Barry was really kind to share his schematics with me to help me find the fix and fixing stuff is all about sharing information!

Happy playing!

Cheers,
Grant

[Bassboy49]

Well, I think this should be my final post on this problem. As mentioned previously, the new heatsinks have solved the startup problem. I did resolder the regulator, and got much improved solder joints this time around. I set up my usual rig with the 2 BagEnds (biamped for the woofer) plugged in the old Zon and...worked perfect. Huge sound, as good or better as my old SVT-4 Pro, this time with factory octave and dirt. Many thanks to Grant, who sniffed out the problem and provided the fix.