Crate VC6210 popped NTC before fuse, need advice..

[dach]

So...had a few too many beers before leaving my buddies house and left my amp with main on, standby off. Came back a week later and noticed my fu and no lights on. My buddy says they had a power blip during a storm that week so I feel optomistic that it may be a fuse. Pulled the blown 3a slo blo and replaced w/ same. Blows again. I give up and play another amp and bring the Crate home. I pull the chassis, take out all tubes, turn main on and watch as the ntc sparks and quickly turn off. The fuse does not pop as it is downstream schematically (i have the schem. btw) . I find the part number (sl22 1008) and order 3 more from digikey and will try to replace the offender when they get here. The transformer primary is not shorted or grounded (light bulb test) I can, (if necessary) pull all secondary wires and power up the p.side and see what the v readings are. My question is would a spike just kill the ntc and replacing it solve my probs, or should I start looking elsewhere for causes. I have a very good electrical background and a so-so electronics understanding. I have all new power tubes to check with in case one of the EL34's died, but I can't see that being an issue since it popped while they were out. Thanks for your attention to this point and thanks for any replies.

[Enzo]

Can? Sure. If you want a certain story, you will never get one. Maybe the storm triggered some damage that had been done a year before, but hadn't shown up until now. Like an overweight truck crosses a small bridge and cracks the main beams, but the bridge doesn;t collapse for six more months yet.

Until you apply power to the amp and find out if more damage has been done, you won't know.

The NTC is a soft start, the amp doesn't need it to work right. For testing you can short across it to see if the rest of the amp works. The thing only take a couple seconds to come on full, so if you have a variac, bring the thing up on that over 2 seconds and teh amp will never know the difference.

It doesn;t matter if the fuse is before or after the NTC, they are in series, the same current flows through both.

[dach]

Enzo, thanks for the reply. I know you have the schematic because I saw in a previous post (2006) that you sent it to someone so I need to know if I try a variac should I connect the sec. leads all at once or try them one set at a time to narrow down the the fault? I think there are 3 sets and 1 looks like it goes to the OT but I am not positive. BTW I knew the current flow was equal across both fuse and NTC but I was thinking about the way it looked on the schematic. We use PTC devices to soft start compressors in the hvac world so it's a little different visualizing an NTC as a soft start. I read up on how tubes are powered up and have a better understanding now. My goal in this is to see if I want to spend much more $ on this amp. I have about $300 in it now and that is all it's worth, If it seems like it needs resoldering I'll take it to a tech. If it's one of the xformers I am probably done with it. Thanks again.

[Enzo]

We don't yet know there even is a fault to track down.

The two parts may be in order on the schematic, but it is AC, so the electrons will be going one way now, and an instant later they will be going the other way, so they bang on thermistors, fuses and what have you from both directions. The reason your fuse didn;t blow was not because it was after the thremistor, it was because whatever was going on in the thermistor was not drawing more current through the fuse than it could handle. ALl that sparking was going through the fuse one way or the other.

NTC means as it warms up the resistance goes down. SO putting one in series with the mains, it starts out cold at its higher resistance, and over the next few moments it warms up and drops down to very low resistance. SO we start with a resistanhce in series with the mains that drops away after a moment. That series resistor limits current, and does so less and less as the resistance goes down. Hence a soft start.

The variac is pointless without monitoring mains current draw. I'd simply restore the main circuit (replace the thermistor or jumoer it for now) and bring the thing up on the variac and watch the current. If the curent starts to ramp up, back off. If the current just does a normal bob as things rise, then turn it up and see what happens.

You may have nothing more than a failed NTC. If there is further trouble, then first place to look is failing power tubes.

[dach]

Well if you can stand one more reply, I replaced the NTC and all went well with that. I checked with my ohmmeter the value at room temp, then checked from the power switch back to the plug through the NTC and got the same value. I reconnected all sec. leads, hooked up a speaker, left all tubes OUT for the first test. I turned power on heard a hum and not much else. I checked the fuse and it ws open. I put in a second fuse, disconnected all sec. wires still have tubes out and did a quick power bump. same hum, lights in the room dimmed a little and I quickly turned it off. I rechecked the fuse and apparently I caught it just in time because it was still good. I had checked the primary to ground and did the light bulb test the other day and all was well. I still checked it to grnd. and found nothing anywhere with the primary side. Since the sec. was unplugged there must be a leak to ground on one of the 2 caps in circuit on the pri. side. I have a VC6112 schematic and I think the 6210 and it are the same. If you can email or link me the VC6210 I'd appreciate it, but for now the only things I see that could cause that much amp draw on the pri. with no load on the sec. are those caps. I even bypassed my power switch just in case and got the same result. I did this procedure a few times, each time letting the NTC cool back down but no change. If you think I'm on the right track or see something in the schematic I should check please let me know, but for now it's just in limbo.

[Enzo]

What does ground have to do with it? All you need is a few shorted turns within the primary and the thing is shot. DImming room lights is a good sign your tranny is drawing large currents. But do check your small caps, a dead short cap across the primary is a dead short across your mains.

So your amp blows fuses, but does NOT light up the bulb tester bright? That doesn't sound right.

[dach]

I can't find any info on the correct resistance for the primary for this amp in particular. I get 2.8 ohms with my fieldpiece and 3 with my Fluke. I called a local amp tech and he said that sounds about right but he doesn't memorize those things. If this was a step down xformer like I'm used to I would know what normal is. This is a different animal so I'm a little hesitant to say this is too low because I don't know the ratio it steps up to. I took the board completely out an looked at the underside, it looks flawless so it may be the xformer after all but that will be the straw that breaks my back so I'm really trying to make sure before I give up. Thanks again.

[Enzo]

A meter won't help unless the failure is of gross dimension. If you have two shorted turns, that is enough to kill the part, but your meter would never measure the difference.

Transformers work on turns ratios, while the resistance through a winding is a factor of wire size and thus resistance per foot, plus number of turns, and also the size of the core. Wrap 100 turns of wire around a pencil, and it takes a certain loength of wire. Now wind 100 turns around a paper towel tube, and the wire is quite a bit longer, so it would measure higher in resistance. But all the transformer cares about is the ration between number of turns in the primary to the secondary. So ther is no standard resistance to a 120v winding.

The test is simple, now that we have gotten this far. We suspect the transformer itself, so we disconnect the thing completely, clip the mains directly to its primary wires. Use a fuse, a variac, an ammeter, a bulb limiter, whatever to protect yourself, and see if the thing draws current all by itself.

Or go over to RG's Geofex web site and find his very simple transformer tester, and put one together and check for shorted turns that way.

[dach]

I re-read one of your older posts about the dim bulb test and I'm afraid I misunderstood it. I did the test again with the xformer disconnected from everything and wired the primary in series with a 60 watt light bulb and it glows like a 60 light bulb should by itself so I am convinced it's a short in the primary somewhere. That's really the only thing that could've caused my lights to dim on power-up with nothing connected on the secondary. I checked RG's Geofex and I don't have a variac or the other transformer and really don't want to spend alot of money on testing equipment I'll use once. I think the light bulb test says it all. I thank you for your time and knowledge concerning this, and who knows, maybe you'll need help with your AC unit sometime and I can return the favor. Do you even have AC in Michigan? I'm in TX. so it's a rhetorical question.

[Enzo]

You found teh wrong transformer testing. we are not rying to measuer impedance. Look under Tech Tips, in teh Tube AMp Tips for Shorted Transformer Tester. It is a neon bulb, a push button, a resistor, and a battery. That's it!


PUll the tranny primary wires off the circuit, clip mains to them with the bulb in circuit. If that lights the bulb, then indeed the tranny is shot. If the lamp stays dim, then your circuit has the issue. REreading, now it sounds like that is exactly what you did

I am willing to believe you are right about it though.


At least go back to Geofex and find the thing I refer to, it is exremely simple and dirt cheap and useful for transformer tests anywhere.


AC in Michigan? Well we do have the electricity now, at least in town. You know they have radios now with a moving picture? Or did you mean air conditioning? We do have snow 11 months a year, but every July we get a month of bad ski conditions. SOme folks put up these things in their windows then.