Presto 8dg monster fuse removal

[sameal]

Has anyone successfully replaced the mega fuse block on their presto 8dg?

From what i understand of electricity and everything ive been told and seen in operation you should not fuse both legs of your 120v. Which the presto does, with fuses probably rated for industrial machinery, when the presto isn't much different then an open reel tape machine that runs off a regular household fuse.

I did finally pop a fuse fixing something and it did exactly what i thought it would, it blew only the hot leg. I can't find this stupid fuse anywhere. I think presto just used this because it's what they could get at the time, not because it was the best engineered solution.

Has anyone just put in a single 3a fast acting 250v fuse? I can't see why i couldn't.

[boogievan]

Is it double-fused as a protection against polarity reversals / phase-to-ground?

I don't have the schematic for that lathe but there's a cool YouTube from 2 yrs. ago of an 8DG in semi-refurbishment. Maybe you could help Junkyard Studio along. Or maybe he's got an answer for you.

https://www.youtube.com/watch?v=5kB_sUvjTCE

Are those "current limiting fuses"? Filled with sand?

The internets say, 'When there is a fault, the current through [such]... fuse[s] will heat up the surrounding sand and melt it into glass, turning it into an insulator.' Also, it says that the blown element may ionize and allow an arc to continue, but the sand cools the broken metal down. 'Sand is usually used in higher amperage fuses (30+ amps) and in fuses for higher voltages (480V and up).'

This type of fuse is in my 3-phase one-arm-bandit switch boxes and also the single-phase ones with only 20 Amps wiring, but the fuses are briefly 60 Amp-tolerant, as there are motors on the rooftop for heat-assist and airco So, perhaps, due to the motors in the lathe, the Presto was given these bulky fuses to allow for brief surges of high current without blowing them, but, if it does blow, to allow safe snubbing of current (and heat). Don't want any fires.

My lathe came with slow-blow fuses because of the initial powering current surges - but I put in crow-bar protection and had to move to fast-blow, of course, since the working theory of crow-bars is that immediate 'protection' - albeit, via failure of a small section of conductor - is better than not having to keep changing fuses.

Show us your Presto! (:

[sameal]

"Is it double-fused as a protection against polarity reversals / phase-to-ground?"

No, it actually had no ground connected from the 120vac coming in. When i replaced the cracking two prong for a 3 i ran the ground, but haven't connected it to the table. I think judging by the schematic i could connect it to the purple on the motors. I could probably keep it off chassis too.

IMG_20230214_161820.jpg

"I don't have the schematic for that lathe but there's a cool YouTube from 2 yrs. ago of an 8DG in semi-refurbishment. Maybe you could help Junkyard Studio along. Or maybe he's got an answer for you."

He is me and I am him. I took some time off audio projects for awhile. The schematic in the manual was not the greatest. No mention of fuse type or ground from 120, because I'm certain it didn't have it oem.

"Are those "current limiting fuses"? Filled with sand?"
I'm not 100% sure. It's small, like 3 amp. I believe them to be R5K types. The motors in the presto are single phase, with a cap, run at about 1800rpm at 1/15hp one at a time. So it seems like they set up the fuses for something much bigger than that.

A ampex or scully tape machine runs 3 motors at once. Each with a cap and rated very similar to these motors, but running off a regular ol' 3? 5? Amp?

But now, my mind has drifted towards a motor circuit breaker. I could wire it for single phase, dial in 3 amps up to 4 or 5 and i think it would protect the motors probably way more then the fuses. For now i just wrapped the old fuse in copper, slapped it on the neutral and put the good fuse on the hot.

[boogievan]

Cool channel (Junkyard Studio)! Looking forward to Part 2 of the Presto video.

I have only single, slow-blow, 4-A fuses in my lathe's motors, but they're for relatively low, DC voltages (+/- 15 V). However, the 5-V(DC) logic supply is going through a 4-A, fast-blow fuse, and, in use, that one's current is usually over 3 Amps, due to it powering 70+ TTL ICs. That's the crow-bar-protected line, forcing the 4-A fuse to blow when the DC voltage exceeds 5.6 V (when the civilian and military logic chips would be broken, even if the current didn't appreciably increase).

I could upgrade to breakers, but, for now, I re-fuse. );

[sameal]

Cool channel (Junkyard Studio)! Looking forward to Part 2 of the Presto video.

Thanks!

I have it half filmed right now. I'm trying to get faster at videos but i tend to work on 4 things at a time. I need to focus more on one or two, but sometimes it's refreshing and helps clear the mind to jump between two projects.

I have only single, slow-blow, 4-A fuses in my lathe's motors, but they're for relatively low, DC voltages (+/- 15 V). However, the 5-V(DC) logic supply is going through a 4-A, fast-blow fuse, and, in use, that one's current is usually over 3 Amps, due to it powering 70+ TTL ICs. That's the crow-bar-protected line, forcing the 4-A fuse to blow when the DC voltage exceeds 5.6 V (when the civilian and military logic chips would be broken, even if the current didn't appreciably increase).

I could upgrade to breakers, but, for now, I re-fuse. );

Doing a little more reading on inrush current i think these fuses might have been rated for that as you suggest, but i can't find a figure on inrush current for those motors, so I'll have to get with the math. That still doesn't explain fusing both legs without a ground reference.

In industrial applications ive seen they have a contactor, mpcb and overload relay. My switch panel would do the job of the contactor, so now I'm looking at overload relays, but i think the mpcb covers that. But i still think this is way over the top for what this single motor pulls.

[sameal]

Welp. I blew my last fuse, so I've moved to a circuit breaker. I guess there's a stray 6vac on my chassis someplace because the ground lug i had disconnected tapped the frame and shot the fuse. When i measured it it had 6vac on it somehow.

Off to the multimeter i suppose