Laney VC15-110 overheating?

[Stompfrog]

I have a Laney VC-15 110 guitar amp which works perfectly... but only for a short amount of time.

After 5-10 minutes it goes silent. The red power LED remains lit but the tubes go out and it is silent. If i switch it off and come back a while later it will work again.

I did some googling and found an old thread mentioning a thermal fuse, also referred to as a polyswitch being responsible for this kind of issue with this amp.

I found a low res schematic and proceeded to take the amp apart.

I think I have found the polyswitch (two in fact) and they are labelled as JK60 030.

It would be great to understand from someone more knowledgable than myself if my self-diagnosis matches my amps symptoms and if so, what i should be replacing these polyswitches with. If you need any further info or photos of the amp circuitry please let me know.

Many thanks in advance for your help.

[nickb]

I think your conclusions are reasonable. As a test just short out the polyswitch and see if the problem is fixed. Then you have to decide if the one that is in there is faulty or if it's a design shortcoming. The purpose is to protect the transformer. You can try the next lower resistance one JK060-040 but the fault current will be higher before it trips. That said, the tolerance on these things is huge so consider an anti surge fuse instead.

[Stompfrog]

Thanks for the quick reply Nick.

I'm pretty inexperienced with this kind of thing. What would be the best way to short one? Should I do both or just one? If just one, which one?

[nickb]

As far as I can see the schematic doesn't show them. My guess they are in parallel and replace the 5A fuse in the heater circuit (bottom right of sch). If you have a meter you can use the ohms range to verify that they connect to the heaters and also whether they are in parallel.

The quick answer - short one out and see what happens.

[g1]

The VC30 uses both for the HT winding. (edit: actually not HT but LV)
If it's the same you will have to short both of them.

[nickb]

The VC30 uses both for the HT winding.
If it's the same you will have to short both of them.

g1: I think you meant LV - they are for the +/-15V supplies.

I bet the VC15 is the same especially since looking at the photos. If that is true and if you are seeing the tube stop heating up then you have a different problem. I'd be looking for a dry joint of bad connection in the heater circuit.

Get your meter out and determine which part of the circuit those polys are connected to.

[g1]

Right Nick, LV not HT, thanks!
That VC30 also shows a polyfuse for the heaters, F4 in VC30 (shown beside hum adjust pot).
I've had to replace a few in other Laney models. Again, it can be shorted with a wire to test.