SVP-PRO blowing fuse

[toomanyslurpees]

Ok, so I have an SVP-PRO (a one space tube bass preamp) that twice now has blown it's fuse randomly, both times it was between tunes with no signal running though it. The first time I thought possibly just the fuse failed, it's a .625A fuse with the spring and the resistor in it, I looked for problems and put the tubes back in one at a time to see if that was the problem, everything back together and it worked fine for a long practice and half a gig before the fuse went again. I've read about solder joint problems so I'm going to take the board out and check that this time (the main board is 90 degrees from the tube socket board, read about problems there)

Don't know if it's of any consequense, but I'm running the signal from the effects send into my amp because the preamp out is to hot and causing noise problems, so I use the gain and drive knob to set the level and leave the master at 0. Was wondering if having no load on the preamp out could cause a problem? Figured not since it's only a preamp but thought it could be worth mentioning.

Anyway, thanks as always. On a side note I'm just about finished my first real amp build, a point to point job of a pro jr, soon time to fire it up and see how much smoke comes out huh?

[John]

soon time to fire it up and see how much smoke comes out huh?

don't forget your light bulb limiter! :)

[toomanyslurpees]

Well, couldn't find any suspect solder joints, swapped out all four 12AX7s (don't have another 12AU7 handy) and it seems fine, though that's what is does, stupid intermittant faults....

[toomanyslurpees]

Ok, I should probably do some more ground work before I come looking for ideas here (I know, it's stirring up tons of interest...) Well, I think my problem is the single 12AU7, after switching the 4 12AX7s I left it on and after two hours I went to bed, when I got up the fuse had blown, I discovered I did have a 12AU7 stashed away and took it to work with me, swapped the tube and left it on all day and the fuse never blew.

I'm just learning and I know there's lots I have yet to see as far as problems but is that common for a tube to have a problem like that? run for hours before blowing a fuse in a repetitive fashion, even in a cool spot with no vibations and no signal running through it?

(assuming this tube is the problem and it's not just the amp having some fun with me)

[HotBluePlates]

I highly doubt that a preamp tube is causing the fuse to blow. At least, not without some other fault.

Preamp tubes have a load resistor of 10-220k, and a supply voltage between 100-400v. Considering a worst case of 400v supply and 10k resistor, even if the 12AU7 is a dead short, only 400v/10k = 40mA could be pulled through the stage and load resistor. 400v across a 10k load resistor also implies 400v * 40mA = 16w, and that would burn open any common load resistor and kill the short.

Fuses pop because of excessive current. With the cause you have in mind, excessive current would have resulted in smoke. That didn't happen, at least from your description.

Do you have a schematic for this thing? We'll have to look for one and see what makes sense to pop a fuse. You also should do yourself a favor and use a lightbulb limiter, especially for troubleshooting. Fuses pop (often) in the blink of an eye. What caused the fuse to blow? If it's not very obvious, it is hard to find, unless you can observe the amp in its fault condition. With the limiter, you can see what is drawing more current/dropping more voltage than it should. Your fault is likely near it.

[toomanyslurpees]

Yep, you called it, not the tube.... I had it on for 8 hours yesterday it the fuse never blew but decided to leave it running over night to make sure and sure enough, it blew by the time I got up this morning. Somebody mentioned I could try reflowing all the solder joints? I do have a schematic for it but I'm a little hard up for ideas on how to track down a fault that only shows it's face once in however many hours.

[HotBluePlates]

It's probably not a cold solder joint. That would be a good call for a new build, but faults in previously-working manufactured gear are typically different.

Can you post or link your schematic? I found a schematic for the SVT-PRO 2, but I don't know that it will actually correspond to your amp.

Since the fuse pops after it's been on for a while, I'd be more inclined to suspect a solid-state part in the power amp running away and drawing the extra current when the whole amp gets hot. But let's figure out what you got, then how to attack this.

[toomanyslurpees]

I've been trying to post the schematic but the file is too big and I'm somewhat dumb when it comes to computers, I'll keep trying, thanks for the interest. One thing I was thinking about is whether some thing is intermittantly conusming a bunch of current or whether the the preamp is constantly running on the verge of blowing that fuse. (actually come to think of it maybe I should just run it with my meter on there to actually answer that... but it's Saturday night and I'm feeling pretty lazy) How bad of an idea is it to bump that fuse up a notch? Probably a bad one considering the next size is 1/4A I'm guessing so double... at least then I might have something more obvious to fix...

Edit: Interesting, it's a .125A fuse and I put my meter in series with the fuse and I'm getting 145mA right off the bat, going to let it run for awhile and see if it goes up or down but obviously that's more current that I would figure should be there... (that's with a decent quality true RMS meter)

Looking at the schematic the fuse that's blowing goes the transformer that then goes to a bridge rectifier and three filter caps providing 275Vdc for everything down stream, which is a bunch of stuff

[Ritchie200]

Ok, fuses are rated at no more than 60 minutes at 135%.  You are at about 115% so we are talking a few hours, nothing wrong with the fuse.  May be a long shot, but I have found faulty components with a laser thermometer.  Probably not much heat here, but if all else fails...  I would not bump up to .25.  It may not seem like much, but 100% is a huge jump.  What does the voltage look like after the filter caps?  Was the 275v listed on the schemo or was it what you measured?

Good luck!
Jim

[sluckey]

You originally said...

The first time I thought possibly just the fuse failed, it's a .625A fuse with the spring and the resistor in it

And now you say...

Interesting, it's a .125A fuse and I put my meter in series with the fuse and I'm getting 145mA right off the bat

So, which size is it?

[toomanyslurpees]

Oops, you found my typo, .125A is it, and I'll resist the temptation to try a .25.

Lets see if this says anything to you guys, when I fire it up the voltage that's 275VDC on the schematic climbs to 280V then comes back down and stablizes at 246VDC. (over the span of less than 10 seconds)

[Ritchie200]

Check the bridge.

Jim

[toomanyslurpees]

So on the advise of the bench tech at work I popped all the filter caps out of the board to check them out of circuit, they're 3 100uF caps that all read 94ish uF on my meter, I checked some brand new 40uF caps I have for something else and they all read 45ish uF. So based on that would filter caps sound like a reasonable suspect for this thing drawing too much current or my I be barking up the wrong tree?

[PRR]

> I be barking up the wrong tree?

Wrong tree.

50u, 100u, 200u caps will all work fine and NOT affect current. 94 instead of 100 is within spec (often +/-20%).

What size fuse does the CHASSIS say to use?

I have never seen a 0.125A line-fuse in transformer-power gear. The parasitic current of small transformers on 120VAC will often blow 0.25A fuses, I'd never use less than 0.5A. If you are in 240V land, these values would be half.

Is the box thermally hot?

[PRR]

http://www.ampeg.com/pdf/SVP-PRO.pdf

http://www.bgavinsound.com/reference/music/ampeg/SVP-PRO%20Schematic%202000.10.05.pdf

[toomanyslurpees]

Hey PRR, thanks for posting those, I was trying to figure out how to post the schematic but the PDF file was too big, I tried photographing the hard copy but that failed miserably, never had found them online...

So one less tree to bark up, diodes are good and I've been trying to follow that +275 around to where all it's going but haven't found anything suspect... hmmm

it does say .125A 250V right on the chassis, I'm assuming since nothing makes reference to changing the fuse size to switch to the 230V setting it's not required.

[toomanyslurpees]

Ok, now that I've spent some more time looking at this schematic I realized I wasn't really getting how this power transformer works, so I see that on 230V the two coils on the primary are in series and on 120V the two coils are parallel and F2 is used where it wasn't on 230V. So that means the fact that it's F2 that's blowing might be of interest. With F2 blown I loose the power light, if I have a good fuse in F2 and no fuse in F1 I do get a power light..... Oh crap, as I think this out it makes me realize I may have been barking up the wrong side of the forest.....

Ok, seeing that I checked the other voltages out of the PT (yeh, probably should have done that in the first place) I have the 8V and -8V but each side of the tube heater lines are about 2.8VAC, and I get that AC at the tubes too, it looks like there's a recifier in there for the tube heaters is there not?

[PRR]

Are you in 230V land and wired for 230V?

Are you sure it is F2 blowing?

In 230V connection F2 does NOT feed the preamp. It apparently does feed C77, the radio-hash bypass cap.

So how would you ever know that F2 had blown? The preamp would still work fine. 

That's a very odd connection and I wonder if it may be mis-drawn.

If correct, then:

1) C77 is breaking-down at odd moments

2) you have frightening voltage spikes

3) both 1 and 2

It's just not making sense. And I hesitate to suggest more mucking-around with lethal voltages.

I suggest you ask your Ampeg repair depot.

[toomanyslurpees]

It is F2 blowing, I am in the world of 120V, sorry, forgot to mention that earlier.... From what I hear there isn't much Ampeg tech support since Loud took over SLM, and hey, I'm a noob, I gotta learn somehow, if I was afraid of sounding like a moron once in awhile I wouldn't be posting on here, don't worry, I know enough not to fry myself at least.

Anyway, may not be the wisest thing to do but I found there's .2A slow blow fuses which I'm going to use temporarly because I'm playing the next 3 nights  (I run this signal in parallel with a clean signal so if this thing does blow again I don't loose all my sound, just the tube warmth and growl) Just getting sick of playing without it.

[PRR]

> With F2 blown I loose the power light
> in the world of 120V

Both PT primaries should be getting 120V.

Yet when F2 is out, the box is dead?

UN-plug. Remove F1. Read the resistance across PT pins 1 and 2 at the PCB solder-blobs. A few ohms is normal. Infinite ohms is wrong. If wrong, re-check probing the stub of the actual pins (possible or probable bad solder blob).

If that's OK:

Be very sure the 115/230 switch is ALL the way in 115 position.

If the switch us funky, if you are never going to use it in 230V world, I'd be real tempted to remove the switch and hard-jumper the 115/230 function to 115 connection.

If the PT winding 1-2 is bad:

If the PT does NOT run hot after some hours, you could just double-size F2 and run only on the 3-4 primary. Using only half the primary means more heat and sag, but it may not be loaded to full rating, and your DC voltages seem acceptable.

> the voltage that's 275VDC on the schematic climbs to 280V then comes back down and stablizes at 246VDC

The climb and fall is normal: tubes suck no current until the cathodes heat-up, around 10 seconds.

275V versus 246V.... well it's all +/-10% depend on tolerances and your particular wall-voltage. You are 11% off. You may only have one working secondary. However the 11% off won't bother the tube. Just adds "tube color" insignificantly sooner (lower level).

> there's a recifier in there for the tube heaters is there not?

Not. The heaters are heated with AC, raw 6.3VAC. You seem to have 5.6VAC, 12% low. That's at the low end of acceptable. In a preamp it is not a problem.

They use a different AC winding to make a +/-DC supply for the chips. The voltage on pin 1 of 7808 should be at least 11V DC. It can be anything up to 35V; the '08 will regulate. I would guess 12V-18V DC.

> It's probably not a cold solder joint. That would be a good call for a new build, but faults in previously-working manufactured gear are typically different.

Yes, but bad joints happen. Even in factories. And in dip-soldered PCB work, the "large" leads (transformers, switches) are prone to a condition where solder "touches" the large cold lead, maybe enough to conduct on final-test, yet fails as tarnish sets in. The Samsung IBM MDA monitor was prone to a bad joint on one of the heavy leads to the yoke. Smacking made it work for a little while. I salvaged a bunch by knowing where to suck all the solder out of the joint, scrape the now arc-corroded lead, heat and tin the lead, then flow to PCB.

[toomanyslurpees]

I guess this one doesn't get much closure but I thought I would mention it anyway, but I've had the .2A in place of the .125A and the amp has been working fine since, a whole swack load of shows later. Thanks for all the help though.