Ampeg SVT 3 Pro

[bsco]

Thought I'd post this...Had this amp for repair and the complaint was that when the tube gain knob was adjusted, the unit would make lots of crackling and popping noises...(tubes had already been replaced).....Removed the front PC board and checked for broken solder connections and the shape of the pot...Found a bunch of solder connections on some of the controls that were kind of bad....fixed them up....same problem.....Downloaded a schematic and noticed that the tube gain pot actually varies the plate voltage to two tubes....a 12AX7a and a 12AU7.....but there was a 12AX7 in the 12AU7's location.,,,Changed that out....problem was really weird as it sounded like a bad connection because when you tapped the boards while you were varying this control the amp would make all kinds of strange noises for a few seconds and then settle down....once it settled down you couldn't get the amp to act up until you varied this tube gain control......I removed the power supply/output board and did a good visual inspection..I noticed a dis-colored section on the circuit board which could not be seen from the top due to the natural color of the circuit board itself and the large amounts of silicone present....There were two 16 volt zener diodes there for the low voltage supplies along with two 220 ohm 10W resistors and two 47uf 25V caps....The diodes tested fine and I even put a heat gun on them to see if they went haywire when heated up...the only thing that happened was the forward voltage drop would decrease with applied heat.....and I checked out a few other diodes that I had just to be sure....I did the same with the resistors and they didn't budge from their rated value......however, I did notice that there were poor solder connections to the leads of these resistors...I cleaned up the leads and the pads and then checked the two caps...They are installed between these two power resistors and looked as if they had the outside insulation shrinking on the cap..Tested these and they were out of spec but not by very much...Giving them a quick shot of cold spray would cause the capacitive rating to drop....So I replaced those with 100uf 35v caps for better filtering......Also soldered some bad connections to some jacks and connectors.....Re-installed the board and connected everything back up and tried it out...Worked fantastic....So I am thinking the two power resistors with the bad connections were probably the main cause and it is possible that some of the other bad connections might have also played a part in some way...I replaced the caps based on the fact that they were not up to par, the unit would probably be back, and I would have to tear it apart again...but I don't think the caps were actually the root cause...I think the bad solder connections on the two resistors were.....but you had to look real close....at first glance they were not noticeable....Hopefully this helps somebody out in the future......
Cheers,
Bernie

[Zouto]

Thanks, Bernie!

[bsco]

You are quite welcome,Zouto....

[Brian Swatton]

The one on my bench today has a rather more obvious problem :/

[bsco]

Wow!!!! One thing puzzles me.....You would think that the main fuse would blow before this kind of damage occurs...I have had equipment that was burnt up and the fuses were fine.....or...the fuses would finally blow but only after all the damage was done.....makes me wonder sometimes about the quality of the design and or the quality of components used....Please let me know what you find....I would be very interested......

[Brian Swatton]

This looks to be a shorted turn within the transformer. No other faults are evident, rectifiers, output transistors etc look fine.

The thing with a shorted turn is that current within it will be huge compared to the primary current, the turns ratio will have gone off the scale.

[J M Fahey]

And mains fuses blow .... if they are the proper mains fuses.
Now: nails, bolts, .22LR cartridges, wads of aluminum paper, bale wire, and similar stuff do not.

[bsco]

I know what you mean JM.....I have had equipment with tin foil wrapped around fuses.....or fuses three times the rating installed......but i have seen some equipment with the rated fuses and still the unit burns up before the fuse pops...These are the ones that puzzle me..........

[Enzo]

That is because you have the popular idea that the fuse protects the amp. But the fuse is really there to prevent the whole thing from catching fire and burning down your house. Individual parts are free to croak. When a resistor burns up, ask yourself how much power it took to do that of it were a 1/2w resistor. 2w? 5w? 10w? Now ask yourself how many watts must flow through that main fuse to blow it? SVT3 Pro has a 10A fuse for 120v operation, so we need to draw 1200 watts before it even thinks about it. Now ask yourselves, how many things in that amps could burn themselves out at less than 1200 watts? I'd say most of them.

[g1]

Which begs the question, how does a fuse prevent a fire? Most outlets are on a 15A breaker anyway, that's 1800W. How many watts does it take to start a fire and how does a fuse prevent that?

[bsco]

A couple of the Marshall's that I have seen....the DSL and TSL series, had had boards burnt up to the point they were written off......so the fuse did blow....only after ALL this damage.....With circuitry like this who says it wouldn't burn your house down???? Maybe the fuses are overrated???? I don't know...that is why I asked the question.....to blow out a halh watt resistor is one thing....to blowout a 10 watt or burn up a transformer, or burn up a circuit board???? If a power supply filter shorts or a bridge rect shorts the fuse blows....If an output tube shorts the fuse may or may not blow.....or if your 10 W resistor burns up, it should be able to take out the fuse due to the current draw from the mains.....and a lot of the amps fave fuses in the secondary side of the supply...these should definately blow out.....anyway, I'll just have to make sure I do not leave anything turned on while I go the the bathroom.....I wouldn,t want my work area to catch fire....

[J M Fahey]

Which begs the question, how does a fuse prevent a fire? Most outlets are on a 15A breaker anyway, that's 1800W. How many watts does it take to start a fire and how does a fuse prevent that?

It isn't a watts problem but a temperature one.
An overfused (or bypassed) shorted SVT Classic amp will very probably trip your 15A wall breaker/fuse in a couple seconds, before its internal temperature does not rise that much.
A similar non fused 15W practice amp, whith a PT primary resistance of , say, 20 ohms will not (120V/20 ohms= 6A), will melt surrounding plastic in a minute or two, will be red hot in another 3 to 5 minutes , happily start burning the table/chair/carpet it's on, nearby furniture and curtains, etc.
But a proper 250 or 500mA fuse will avoid that.

Hope this answers your doubt .

FWIW *my* shop and half my house burnt because of a 100W filament lamp too close to some wood, go figure.

I have also seen some amps catch fire onstage or in rehearsal, always with a way too large or bypassed fuse.

[bsco]

Well JM...I would assume you had insurance coverage...I will most definately keep your advise in mind.....I don't leave anything turned on while un-attended....I for one can't afford to take the chance...and I sure as hell don't want to learn a lesson the hard way....
Cheers,
Bernie

[J M Fahey]

Unfortunately no insurance

As usual, bought some *after* the fact.

But I was fully working again in a Month or so, thanks to good friends and commercial relations.
*Everybody* stopped cashing my checks (including one supplier for 6 months and another for a full year), all offered me to ask for whatever I needed (parts/copper/aluminum/wood/tools/ads/hardware/paint/Tolex/etc.), just signing the bills with no specific due payment date, ..... which OF COURSE I started paying within my possibilities after a couple Months.

I bet that's what good Karma stands for.
In a way, "better than Insurance "

[bsco]

*******I bet that's what good Karma stands for*********
You are so right there.....One thing I don't want to do in this business is make bad relations with anybody...I don't know about your area but mine is not all that big and bad news travels much faster that good news...there are only a couple of tv repair shops here in town and one guy doing antique radio work...it looks like I am pretty much the only one here on a full time basis attempting repairs on music equipment....there may be a couple more doing it as a side line after their regular work but I am the only one trying to make a go of it......so I go out of my way to accommodate people the best that I can....seems that they appreciate that....