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HT fuse blows in SVT 3 Pro

#21
agedhorse said:
Never.



There are no output tubes.



No, it's not "within the design spec". The fuse is selected by the designer based upon the needs of a circuit, if the needs are 20% higher then the designer would have spec's a 20% higher fuse.
Table 1: MIL-STD 975M derating specs for resistor, fixed carbon

Standard MIL-STD 975M
Component Resistor, fixed carbon
Parameter Power
Derating Specs 60% up to 70C and further decreasing to 0 at 110C

As it is, as long as the caps haven't been "upgraded", the sizing is adequate.
Click to expand...
well I saw SVT and my brain went to TUBES Duhh , as far as over rating a component in a circuit they are always over rated , the % of over rating varies. ( not trying to argue with you here )

Table 1: MIL-STD 975M derating specs for resistor, fixed carbon

Standard MIL-STD 975M
Component Resistor, fixed carbon
Parameter Power
Derating Specs 60% up to 70C and further decreasing to 0 at 110C


I am not a designer I was trained as a troubleshooting tech and worked with some of the best engineers in this country so I do have background, however you are right in not telling an inexperienced person to try stuff they know nothing about.

send to a qualified tech like @agedhorse to be fixed
 
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#22
mike buzzetti said:
well I saw SVT and my brain went to TUBES Duhh , as far as over rating a component in a circuit they are always over rated , the % of over rating varies. ( not trying to argue with you here )

Table 1: MIL-STD 975M derating specs for resistor, fixed carbon

Standard MIL-STD 975M
Component Resistor, fixed carbon
Parameter Power
Derating Specs 60% up to 70C and further decreasing to 0 at 110C


I am not a designer I was trained as a troubleshooting tech and worked with some of the best engineers in this country so I do have background, however you are right in not telling an inexperienced person to try stuff they know nothing about.

send to a qualified tech like @agedhorse to be fixed
Click to expand...
How do derating specs for a resistor apply to a discussion about fuses?
 
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#25
mike buzzetti said:
well I saw SVT and my brain went to TUBES Duhh , as far as over rating a component in a circuit they are always over rated , the % of over rating varies. ( not trying to argue with you here )

Table 1: MIL-STD 975M derating specs for resistor, fixed carbon

Standard MIL-STD 975M
Component Resistor, fixed carbon
Parameter Power
Derating Specs 60% up to 70C and further decreasing to 0 at 110C


I am not a designer I was trained as a troubleshooting tech and worked with some of the best engineers in this country so I do have background, however you are right in not telling an inexperienced person to try stuff they know nothing about.

send to a qualified tech like @agedhorse to be fixed
Click to expand...
You are suggesting the OPPOSITE of what you posted above.

A designer will DE-RATE a component when used outside of the baseline specified temperature. For example, a resistor specified at 1 watt dissipation from 25 deg C to 50 deg C may require a 30% derating if used at 70 deg C in order to have the same reliability.

A fuse has certain derating characteristics as well, based on both operating temperature and inrush current versus time curves. This has (presumably) already been done by the designer, and is why somebody modifying a circuit's components can affect other aspects (such as fuses) that they do not realize (like the inrush current versus time curves). This is why I asked the question about modifications.

The fuse appears to be protecting the high voltage winding which can be wound using very fine wire (a lot of turns for the step-up), over-fusing this winding could ruin the (expensive and IIRC no longer available) power transformer.
 
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#26
mike buzzetti said:
Invalid Link Removed
From Digikey , Invalid Link Removed
Click to expand...
If you wanted to make a case for fuses, why didn't you link this in the first place instead of information about some component not being discussed? Lot of people here aren't techs and don't know a fuse from a resistor, from a bagle.

Got me thinking about tolerances. Is every 1 amp fuse guaranteed to blow at exactly one amp? Or is there a range?
If that's the case, and I slightly over-fuse beyond the engineer's specs can I be sure that it will blow at the precise current shown on the fuse, or if it's on the high side of a tolerance, am I now more than slightly over fusing the circuit? It just seems that tolerances should be a factor along with derating values.

For my simple technical mind it just doesn't make sense to over-rule the knowledge and experience of the guy who designed the device. And if I were to do that, I would at least know that there could potentially be a risk. A lot of folks here don't have your background and just do what they heard someone else say, without even knowing if it's appropriate to question it.
 
#30
agedhorse said:
You are suggesting the OPPOSITE of what you posted above.

A designer will DE-RATE a component when used outside of the baseline specified temperature. For example, a resistor specified at 1 watt dissipation from 25 deg C to 50 deg C may require a 30% derating if used at 70 deg C in order to have the same reliability.

A fuse has certain derating characteristics as well, based on both operating temperature and inrush current versus time curves. This has (presumably) already been done by the designer, and is why somebody modifying a circuit's components can affect other aspects (such as fuses) that they do not realize (like the inrush current versus time curves). This is why I asked the question about modifications.

The fuse appears to be protecting the high voltage winding which can be wound using very fine wire (a lot of turns for the step-up), over-fusing this winding could ruin the (expensive and IIRC no longer available) power transformer.
Click to expand...
over rating, derating 2 sides of the same coin IMO , I should have used derate Busted ,

and Yes resistors and many other electrical components have temp DERATING for them ( or from a design standpoint , lets OVER RATE that component for safety etc. )

I had no idea the transformer was no longer available, however a 20% increase like I said before is not enough IMO to be an issue, this is a intermittent problem , these as you know are the hardest problems to diagnose.

call Tech support, my only response anymore.
 
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#31
Old Garage-Bander said:
Got me thinking about tolerances. Is every 1 amp fuse guaranteed to blow at exactly one amp? Or is there a range?
If that's the case, and I slightly over-fuse beyond the engineer's specs can I be sure that it will blow at the precise current shown on the fuse, or if it's on the high side of a tolerance, am I now more than slightly over fusing the circuit? It just seems that tolerances should be a factor along with derating values.

For my simple technical mind it just doesn't make sense to over-rule the knowledge and experience of the guy who designed the device. And if I were to do that, I would at least know that there could potentially be a risk. A lot of folks here don't have your background and just do what they heard someone else say, without even knowing if it's appropriate to question it.
Click to expand...

Fuses have a time versus current range for opening, from a design perspective this is a delicate dance between providing adequate protection and premature opening. A real engineer will use these curves to be sure that their ideal protection target falls within the appropriate range given both the inrush and steady state currents. Both of these must be accounted for.

Since I haven't heard of any issues with the amp blowing this fuse randomly, I would assume that the designer chose an appropriate value.

mike buzzetti said:
over rating, derating 2 sides of the same coin IMO , I should have used derate Busted ,

and Yes resistors and many other electrical components have temp DERATING for them ( or from a design standpoint , lets OVER RATE that component for safety etc. )

I had no idea the transformer was no longer available, however a 20% increase like I said before is not enough IMO to be an issue, this is a intermittent problem , these as you know are the hardest problems to diagnose.

call Tech support, my only response anymore.
Click to expand...

You don't replace a factory fuse with a larger fuse to diagnose a problem, that's not how diagnosis is properly or safely done.

IF there is an intermittent failure of a component, you want the fuse to open to protect the rest of the circuit, that's the whole point of the proper fuse value in the first place. In this case, the transformer is no longer available (AFAIK), therefore, the entire amp could be ruined from damage to this single winding (which is a particularly vulnerable winding)

Respectfully, I encourage you to think about the safety aspects of your suggestion, it's just plain bad advice. You just don't (or can't) understand why, in spite of several of us telling you why.
 
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#32
agedhorse said:
Fuses have a time versus current range for opening, from a design perspective this is a delicate dance between providing adequate protection and premature opening. A real engineer will use these curves to be sure that their ideal protection target falls within the appropriate range given both the inrush and steady state currents. Both of these must be accounted for.

Since I haven't heard of any issues with the amp blowing this fuse randomly, I would assume that the designer chose an appropriate value.



You don't replace a factory fuse with a larger fuse to diagnose a problem, that's not how diagnosis is properly or safely done.

IF there is an intermittent failure of a component, you want the fuse to open to protect the rest of the circuit, that's the whole point of the proper fuse value in the first place. In this case, the transformer is no longer available (AFAIK), therefore, the entire amp could be ruined from damage to this single winding (which is a particularly vulnerable winding)

Respectfully, I encourage you to think about the safety aspects of your suggestion, it's just plain bad advice. You just don't (or can't) understand why, in spite of several of us telling you why.
Click to expand...
I agree with what you are saying except for one point Purely from a troubleshooting an electronics circuit that is intermittent , from My experience over 40+ yrs there are times ( not in all circuits ) that the fastest way to find an intermittent component is to over stress the circuit , Mind you this is from a production standpoint ! ( assuming you have replacement parts ) , as far as this case goes . We have different views on how to deal with intermittent failures and thats fine.l

SEE A TECH. if all else fails , or IF it fails !!! ??
 
#33
You DON'T want to over-stress this circuit and especially the power transformer.

I don't know of any qualified techs that will intentionally allow an intermittent unit to intentionally go into an overcurrent condition. In fact, best practice is to power the circuit from a current limited supply so that if and when this fault occurs, you are both monitoring the current AND foldback limiting the current to a known safe value for the transformer. This is pretty standard procedure in this industry. Brute force troubleshooting methods are Mickey Mouse at best, sometimes dangerous in the event of a failure such as an electrolytic cap explosion, and usually result in more damage (sometimes catastrophic to the amp).

I say this as a design engineer (EE) as well as somebody who taught college level circuit fault analysis and structured troubleshooting as part of root cause documentation.

Sorry, we will have to disagree.
 
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#34
I cobbled this together from various pages in the schematic binder.

upload_2022-8-22_3-7-43.png


Blue and Green is AC, Red is DC, and Orange is the fuse.

As far as I can see, this circuit only feeds the tubes.

The preamp schematic includes this table of test points:
upload_2022-8-22_3-9-37.png
 
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#35
Again, and I cannot stress this enough, most people do not have the technical background to understand the risks of over fusing, or using whatever "contact cleaner" is handy, or a myriad of other potentially damaging actions we see discussed here almost every day.
All they are going on is, "I read about a guy on the internet who fixes amps by putting in larger fuses." And unfortunately, fuses are inexpensive and easy to find at just about any hardware store. This makes the matter all that much worse.
 
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#37
I had a svt pro 4 for a while back in 2000s. It had similar problems. Had it repaired by qualified Ampeg service several times, never was really fixed. Had another electronic amp guy look at it and he declined getting in there because of the supposed safety risks. I sold it for parts and never looked back. It may not be worth fixing at all, those Ampegs are prone with failures.
 
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#38
landrybass said:
I had a svt pro 4 for a while back in 2000s. It had similar problems. Had it repaired by qualified Ampeg service several times, never was really fixed. Had another electronic amp guy look at it and he declined getting in there because of the supposed safety risks. I sold it for parts and never looked back. It may not be worth fixing at all, those Ampegs are prone with failures.
Click to expand...

I have a few other amps that are in working order so this one is just a project for me and I don't mind taking my time. I'm more interested in finding the problem just to learn a few things about the design of the amp. I definitley found a good handful of cracked solder joints on front/back panel pots, as well as a few cold looking solder joints here and there. Footswitch jack contacts were also starting to lift w/o cable inserted. The board itself looks fine to me just looking it over with a x5 eye loupe. At this point it's a roughly 20 year old amp.
 
#40

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