GK800RB Pwr Transformer question

[T9c]

I have an 800RB on my bench with no reading on the PT secondaries. 120vac at the primary terminals.

My question is, is there a fuse on one of the primary leads under the black winding wrap? I can feel a little bump about the size of a 50x20mm fuse. It appears that it may be, but I' didn't want to cut the wrap without asking the customer first.

Rick
EDIT: make that a 5x20mm fuse...

[Enzo]

HI. Just a pet peeve: please don't say no reading unless your meter goes blank. Zero volts is still a reading.

120v at the primary wires? Ok, unplug it and measure resistance through the primary. is it open or not?

If there is a fuse inside the primary it is a thermal fuse, not a 20mm current fuse. usually a little silver thing that looks to me more like a diode.

[drewl]

There probably is a thermal fuse.
Replaced one in my bassist's 800 after it popped playing four hours in 100 degree heat with him mismatching the impedance.

[J M Fahey]

In case you wonder what you'll find inside, they usually look like this, and, yes, size is roughly like a standard 5x20 mm glass fuse.

[g1]

If it is open, it should be easy to find a replacement. They used to have them at Radio Shack, they are used in coffee makers etc.
If they are not crimp on connections, make sure you heatsink well before soldering or you could trip it with the solder iron.
As seen in JM's picture, there is a temperature rating and a max.current rating, make sure both numbers match.

[T9c]

Thanks all. I was only able to find one closely rated, so I ordered. The original thermal fuse is a Chinese made UMI J121 - 10A 250v 121C degrees axial thermal cutoff. The ones I ordered are from an eBay company out of Newark, NJ. Their item is SEFUSE SF129E 10A 250v 133C.

[J M Fahey]

Can't you edit your order?

For safety, thermal rating is more important than current (being above regular line fuse rating of course) so if not exact match I'd go slightly below than slightly above.

121C is already hair rising enough, nylon bobbins are quite soft at that temperature, if not already molten, and having 121C on an outside layer means a deep one may be well above that..

[T9c]

As I said, this is as close as I could find anywhere on the net. It'll have to do until it opens and I'm required to purchase the 800rb PT replacement kit, which is over $160 according to posts I've seen. The kit is a toroidal with new bottom cover & hardware. I'm going to have a talk with the customer about their spirit load too, to make sure it's correct.

[mozwell]

These thermal fuses are a last "fail safe" against "gross" overload of the transformer. When this fuse blows, it means the transformer has got waaaay too hot. If the thermal fuse is on the outside of the coils, it means the inside will have been muuuuuuch hotter.
Even if you replace the thermal fuse, it should only be considered a "short term fix" until a new transformer is installed.

the transformer may test ok, but its useful life has most likely been severely reduced. Who knows how long the repaired transformer will last. If it blows again within your "repair warranty period" then its another fuse, or a new transformer, maybe at your cost.

i would explain to your customer, that you can do a temporary repair, but the unit MUST have a new transformer installed, within a few weeks, not a few months, or years.

I would not use a 133C thermal fuse to replace a 121C thermal fuse, choose one that is lower than the original thermal fuse temperature rating

If the original transformer was a toroid, those generally only have 105C rated mylar insulation, so if the 121C thermal fuse blew, the insulation between primary to secondary may be seriously compromised, worst case if the insulation fails your customer can get a mains shock.

i would explain to your customer, that you can do a temporary repair, but the unit MUST have a new transformer installed, within a few weeks, not a few months, or years.

[T9c]

Good ideas, thanks.

Edit: I did another eBay search and found the 121c ordered it.