Fuse Finding Help

[mrarroyo]

Recently I obtained a circa 1977 Technics SU-7300 integrated amp. The unit has several fuses (4), of which I have not been able to find the following:

3 Amp 125 Volt (Size 1.25" by 0.25") - need one

3.3 Amp 125 Volt (Size 1.25" by 0.25") - need two

The closest I have found are: 3A 3AG FUSE | AllElectronics.com

Would it work two replace the 3.3 amp fuse? Thanks.

[n_maher]

In my head there's little risk in replacing a fuse with a lower current rated model, the real question is are the fuses that you're taking out slo or fast blow types. I wouldn't want to replace a fast-blo with a slo-blo and there's no good way to tell what type of fuse it is from looking at the package as far as I can tell. Last question, were the fuses blown? If so, simply replacing them is probably a bad idea.

[kevin gilmore]

found the exact schematic.

The 3 amp fuse is in fact the input power fuse and is a fast blow.

The 3.3 amp fuses are slow blow and are the amplifier output fuses.

So anything <= works fine.

xba1f30nu100 is the 3 amp powr source fuse

xbas1a3301 is the 3.3 amp output fuses

Edited November 20, 2010 by kevin gilmore

[kevin gilmore]

http://gilmore.chem.northwestern.edu/su7300.pdf

[mrarroyo]

Thanks for all the information. I received the amp with a blown fuse, the unit was covered in grime and the yellow residue of an ex-smoker. So far all I have done is clean it up, thanks again.

[kevin gilmore]

tobacco does evil things to audio gear.

especially the high voltage stuff.

[The Monkey]

I feel like I'm always asking this question, but fuses confuse me:

I have an 8A 32V fuse. I will replace it with like kind if possible, but if I can't find one around (likely) what kinds of ratings can I use to at least see if this thing works?

For example, would a 4A 250V fuse suffice or is that what makes things go boom?

[The Monkey]

Ok, so I did some research and replaced the 8A 32V slow blow fuses with 4A 250V fast blow. So far so good. I will let the amp run a bit to make sure all is ok (OR UNTIL ONE OF YOU TELLS ME I WILL BURN THE HOUSE DOWN), then shut it all down and find the correct spec fuses.

[mypasswordis]

I'm surprised the turn on current surge didn't blow the 4A fast blow if the original was a 8A slow blow. Using a lower rated fuse just means the fuse will blow faster/more often, using a too high rated fuse means the fuse won't do its job breaking the circuit before the electronics start blowing up and shit. So I think it should be alright for now but you should find a similarly spec'd fuse asap in case the circuit really doesn't like seeing over 32V, and is seeing over 32V.

[The Monkey]

Good advice, thanks. Printed on the fuseholder is "300V maximum" so I think it's ok, but I will get the same fuses asap. In other news, a little arcy-sparky tonight when I stupidly tried to put the amp cover back on while the thing was running and caused a nasty spark on top of one the big caps. Huge "SNAP" and spark. No shock to me, but it was scary. Seared part of the PCB, too. But the thing is still running fine. That surprises me. I'm not so good with the electronics.

[The Monkey]

Some pics:

Inside Amp, see scorched PCB:

Inside top cover with sear marks, yum:

[Emooze]

From what I know, the voltage rating on a fuse is at what point a busted fuse will arc. Having a fuse with a voltage rating over what it's being used with is a good thing. But I agree with MPI in finding an 8A slow blow fuse.

Edited February 21, 2011 by Emooze

[mypasswordis]

Brain fart , the voltage rating being higher is fine, it is just the current that causes it to blow. If 4A fast blow is working now, 8A fast blow should be fine as well and possibly a bit safer but you can't go wrong with using the same as what was in there before (unless it's a Singlepower).

Assuming the places where it's burnt are the leads to the power caps, it looks like they short circuited when you put the top back on and thus created arcy sparky. It ought to be fine especially if you say the thing still works but check for any other burn marks throughout the amp and make sure the capacitors aren't bulging or anything. And as a rule of thumb, try to minimize messing around with a circuit while it's on, it can be quite dangerous especially for power applications like an amp. I do it somewhat often but that doesn't mean it's safe, it just means I'm an idiot.

Edit: Also remember that caps store charge, especially ones that don't have some sort of bleeder resistor, so even when the amp is off you do not want to short the leads of a capacitor or more arcy sparky may happen.

Edited February 21, 2011 by mypasswordis

[The Monkey]

Thanks mpi. Lesson learned here. And I gave those caps a good long time to discharge before I put the cover back on.

[gortari]

Hello everybody.

I have problems with my technics su7300. I perfectly understand your explanations about the three fuses mentioned. But what about the fourth one (F2, 500mA)? is it slow or fast blow? I don´t understand the schematic!

I hope some of you could help me.

Thanks in andvance.

[wink]

Looks like F2 only protects the meter lamp PL1 ( 6.3Vac @ 250mA) . Since lamps have a low resistance when cold the current rating is higher than the current drain when they are at operating temperature.

Any 500mA fuse would work.

The fuse part number XBA2F05NU100 is a Panasonic number. $4.51 each.

http://www.ued.net/ued/viewItemAction.do?itemCode=MSCXBA1F05NU100