Hughes and Kettner Swtichblade 100 FU2 keeps blowing

[Mr Johnny Birchwood]

Hi guys, I have a Hughes and Kettner Switchblade 100 head on the workbench. FU2(schematics/service manual attached) was blown and new 630ma fuse in FU2 blew instantly. 800ma fuse in FU2 blows too. 1a fuse in FU2 does not blow but U2 which is a 5v voltage regulator gets very hot very quickly, much hotter than U1 which is warm to the touch after 1 minute. U2 output is exactly where it should be, read 4.9vdc on the DMM. None of the chips on the preamp board gets hot to the touch. All the LED indicators on the front panel light up and the amp makes clicking noises when powering up and powering down, not sure if this is normal. I do not have the midi effects outboard controller on hand to determine functionality of the midi pre-amp circuit. Of course the problem HAD to be in the op-amp rats nest that is the preamp pcb. My hope is that a diode is blown or a small capacitor is shorted or leaking DC which is pulling the extra current. I'm not qualified to replace a mini surface-mount chip if that turns out to be the problem. Can anybody point me in the right direction as far as what I should be looking for here?

[soundguruman]

As you see, the fuse 2 is protecting 22V, 15V, and 5V power supplies.
Soooooooooo...the thing I would do is disconnect, unplug, whatever, the 3 voltage rails. disconnect 5, 15, and 22 volts from the rest of the amp...
I would turn on the power supply, without any outputs from the power supply connected. Now measure 5,15,22 volts from the supply. Do you have all three voltages?
If the fuse still blows, then the power supply has a bad component...if the fuse does not blow, then the power supply LOAD(s) may be shorted.
But which load? 5Volt, 15 Volt, or 22 Volt? connect the outputs one at a time to the rest of the amp...as usual...which one is loading down, and dropping the voltage? Is 5 loaded down? is 15? is 22?

[Mr Johnny Birchwood]

I don't really have a good way to connect the supplies independently, they are all on the same 14 pole ribbon connector. The clicking on power up draws my attention to the preamp relays. Stuck relay would pop the fuse? Gonna smack em around a bit and see if I can un-stick 'em. Gotta go replenish my supply of 630ma fuses too, so I will know if the problem is resolved. H+K tech support sent me the appropriate manual but they are pretty aloof when it comes to specifics of what the problem may be.

[g1]

Just to verify, you are using slow-blow fuses?

[Mr Johnny Birchwood]

yes using slo-blo's. I had to get the schematics printed as it is near impossible for me to follow these solid-state schems on the small laptop screen. On my way to pick up some more fuses too. Switching between the crunch and lead channels produces a single click in the relay network. There is a click-click-click when powering up the board and a single click when powering down. Standby until I get back with the prints and fuses after lunch.

[Mr Johnny Birchwood]

I read 300ma with the DMM hooked up across the FU2 terminals. Powering up spikes the AC current to max of 370ma as measured on my DMM. Channel switching appears to be functioning normally.

[Jazz P Bass]

I read 300ma with the DMM hooked up across the FU2 terminals. Powering up spikes the AC current to max of 370ma as measured on my DMM. Channel switching appears to be functioning normally.

So how does that blow a 630ma fuse?
Is the regulator still getting hot?

[Mr Johnny Birchwood]

yeah, the 5v regulator still gets pretty hot within 20-30 seconds. Why the fuse blows when I only measure the current at a max of 370ma is a mystery to me. The clicking of the relays on power up has me a bit curious, but all the switches on the channel selector do what they are supposed to do as far as I can tell. I'm thoroughly stumped. R65 the 22ohm measures a 4v drop across it, varying by .1-.2 v when various channel switches are engaged. That drop indicates a 180ma draw through the regulator. Seems normal to me. The spec sheet for the regulators says they are high-temp high-current units. Makes sense it would get hot dropping 18v input to 5v output. Maybe it was designed that way.

[Mr Johnny Birchwood]

So...I left the pre-amp board powered up with the 1A fuse for 90 minutes, in hopes that a failing or weakened part would get noticeably hot or let the magic smoke out and indicate the location of the problem. No such luck. 5v regulator gets much hotter than the 15v regulator, almost too hot to touch with my calloused, battle scarred fingers, but after 20-30 seconds it has reached a stable temp and does not get hotter. The customer is bringing over the FX board later today, and if all the FX work I'm gonna replace the worn out power tubes, bias, and put it back together. Guy wants to take it on tour and I will have to advise him not to. Will advise customer to get a simpler amplifier and use outboard effects. If there is a malfunction in a particular effect then that may help narrow down the problem. As far as I can tell, the DC op points on the preamp board are right where they should be. Got a buddy that is more of a solid-state guru than myself coming over tonight to poke around the preamp board. If everything works I have to assume that a short blew out with the 1A fuse. Unless, that is, someone can come up with a better explanation...