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Fuse's or MOV's? 11A72 and 11A52 fuse question.

wkibler
 

Today I connected my P6204 FET probe to my 11A72 rather carelessly
and must have accidentally shorted the TekProbe connector killing
the +5v to the probe.

The mainframe threw a fuse error for that plugin so I checked the
fuses and found typical small soldered fuses. Not wanting to pay
Mouser's shipping cost for a single fuse I opened up a toner 11A52.

In the 11A52 instead of fuses I found what looked like MOV's in
the same position with the same "+5 fuse" silkscreen. Part F1003
on the schematic.

The MOV's are labeled "FV4B 50V X110". Can someone kindly
explain how these MOV's can replace fuses? I swapped one
into my 11A72 and it restored the +5v rail.

I tried looking up more info on these mov looking things but only
found eBay listings selling them for rather cheap. What are these
things? I am so confused.

In the 11A52 extended service manual the component is:
A1F1003 FUSE.WIRE LEAD:0.75A,125V,FAST.

Any info would be greatly appreciated.
-Will

Ed Breya
 

Those are not MOVs (voltage clamps), but PTCs (kind of like a resettable fuse). From the marking I'd guess they are rated 50V, and either 1.1 amp or 110 mA. If the original regular fuse type was 3/4 amp, then the PTC is likely a 1.1 amp part.

Ed

wkibler
 

Thank you. that makes a lot of sense. I wonder if this was a factory thing or a field mod.

Ed
 

The fuses are not PTCs, they are a fast blow SMT fuse similar to the big ones but in an SMT package. The others that look like through hole style disc capacitors may be PTCs they were very common. If there was originally an SMT PTC in there it should have recovered when the short was removed. Also, PTCs are slower to open than a fast blow fuse. That being said, replacing the fuses with PTCs should be OK.

ed

 

On Sat, Apr 18, 2020 at 03:48 AM, Ed Breya wrote:


Those are not MOVs (voltage clamps), but PTCs (kind of like a resettable
fuse).
They are called Polyfuses see https://en.wikipedia.org/wiki/Resettable_fuse

Here's part of an article from Wizards Workshop of Oct 92:

//
Polyfuse P/N 307-1592-00.
The polyfuse is a direct replacement for the 159-0235-00 and the 159-0153-00 fuses.
In addidtion, it is strongly recommended that the polyfuses be installed in all plug-ins that come in for servicing.
Changing these parts will greatly reduce the return activity due to blown fuses. This in turn will reduce the failure rate of the 11k plug-ins.
//

/Håkan

Dale Chayes
 

I recently (last year) used PTCs in a new design - another opportunity for life long learning ;-)

There is lots of info on them available on via the ‘net.

As a user, one thing to keep in mind is that (at least some if not all) take time to reset - they do not reset instantly.

-Dale

On Apr 18, 2020, at 10:52 , zenith5106 <hahi@...> wrote:

On Sat, Apr 18, 2020 at 03:48 AM, Ed Breya wrote:


Those are not MOVs (voltage clamps), but PTCs (kind of like a resettable
fuse).
They are called Polyfuses see https://en.wikipedia.org/wiki/Resettable_fuse

Here's part of an article from Wizards Workshop of Oct 92:

//
Polyfuse P/N 307-1592-00.
The polyfuse is a direct replacement for the 159-0235-00 and the 159-0153-00 fuses.
In addidtion, it is strongly recommended that the polyfuses be installed in all plug-ins that come in for servicing.
Changing these parts will greatly reduce the return activity due to blown fuses. This in turn will reduce the failure rate of the 11k plug-ins.
//

/Håkan


robert Johnson
 

the other ting you need to realize is they do not open they still conduct  some

On Saturday, April 18, 2020, 07:57:23 AM PDT, Dale Chayes <dale@...> wrote:

I recently (last year) used PTCs in a new design - another opportunity for life long learning ;-)

There is lots of info on them available on via the ‘net.

As a user, one thing to keep in mind is that (at least some if not all) take time to reset - they do not reset instantly.

-Dale


On Apr 18, 2020, at 10:52 , zenith5106 <hahi@...> wrote:

On Sat, Apr 18, 2020 at 03:48 AM, Ed Breya wrote:


Those are not MOVs (voltage clamps), but PTCs (kind of like a resettable
fuse).
They are called Polyfuses see  https://en.wikipedia.org/wiki/Resettable_fuse

Here's part of an article from Wizards Workshop of Oct 92:

//
Polyfuse P/N 307-1592-00.
The polyfuse is a direct replacement for the 159-0235-00 and the 159-0153-00 fuses.
In addidtion, it is strongly recommended that the polyfuses be installed in all plug-ins that come in for servicing.
Changing these parts will greatly reduce the return activity due to blown fuses. This in turn will reduce the failure rate of the 11k plug-ins.
//

/Håkan


wkibler
 

This is the exact reason I enjoy repairing vintage Tea gear. There is much to learn that I am able to use in my career.


I recently (last year) used PTCs in a new design - another opportunity for
life long learning ;-)

wkibler
 

Apologies, I meant TEK, not Tea. Damned auto correct. I swear the Apple autocorrect and predictive type they use on the Mac OS has got more aggressive.