Hughes & Kettner Switchblade 100 head FU2 Fuse issue

[Smitty02]

Hey all I got a H&K switchblade 100 head with the FU2 630ma internal fuse blowing issue which from what Ive read all around the web and this site too... Seems to be a common problem... Ive read many postings about this but none have resolved, or posted the findings a to weather it is a common fix... I was wondering if anyone has fixed one of these and can share their solution.... Here's what I have found and am about to dig into... Huges and Ketner Switchblade 100 head, blown FU2 fuse. Tubes power up front panel does not/ no output.... Found bad power tube, replaced with a temp pair, staggered them replaced fuse with on hand 600ma sb. It Did power up, I played it for a bit seemed ok. then shifting positions of chassis, seems to blow fuse...Sorry I dont have copy of service manual but seen one posted on here, but couldn't download. Went to schemo unlim, can view it, but not download. It asks for signature... I am going to dig into it this weekend starting with buying some fuses and chopsticking at first... if that dont show a bad connection or the culprit. I will start testing the 3 voltage rails this fuse supplies.... I just thought I would ask if anyone has run into this and found the issue and fixed it and would care to share their solution.... Sorry for limited info and no schematic, just thought Id throw it out there see if i can catch some knowledge .... Thanks boys and girls.... Peace

[Jazz P Bass]

I have attached the schematic.

The 630 ma fuse is on the 460 Vdc B+ rail.
If anything on that rail draws more than 630 ma of current, it will open.
I don't know about 'a common problem' other than bad output tubes that can blow that fuse.

Moving the chassis & causing a fuse to blow does not make any sense but it may be a clue.

You do not need all four tubes installed for troubleshooting, if that will help narrow down a bad tube.

[Smitty02]

Thanks Jazz for the schematic, Are you sure we are talking abut the same fuse ? The one the back of the amp is a 630ma also I think your talking about that one it is the b+ fuse, FU1 Jazz. I talking about FU2 that supplies the 22v, 15v & 5v rails for the switching and preamp fx or what not, I do know the amp front panel will not light up and no output if it blows. It s fed by the 17vac section of the trans. Jazz.. I think you are thinking of wrong fuse buddy. On page 37 or page 5 of 11 you shared with me, right above the heaters you will find it. It is a issue with these amps Ive read many postings bout this H&K know it too Im sure...Thanks for the schematic. I am going to stick to my plan so far to see if its a bad joint or a relay or preamp issue... I hope I can find it. Oh how I love these double sided board DSP amps, they were such great ideas... Hey !! lets put tubes in a dsp amp it it will sound like a vintage classic or better, with all the fx you could ever want, and such a bargain at $1,500 plus and SO dependable you could drop it on a feather bed covered in bubble wrap and may just may, if your lucky still work ... Heh Heh..

[Enzo]

Don't worry about it being a "common problem" and don't worry about what someone else found with theirs. Just find the problem in yours. If that fuse blows, then the first thing to check is bridge D6. Check all four sides of it for proper diode drop. SHorted filter caps are possible, but not very likely. Then ohm meter on the 22v rail, is it shorted to ground or shunted by a low resistance? And check the output of the +15 and +5 regulators, either shorted?

[Smitty02]

+1 Enzo... Just looking for some insight. Actually these amps are built pretty good, just not my cup o tea. Good components. I dont like the double sided boards they use these days. Any tips on de-soldering ? Ive worked on a few of them that it seems like they use a different type of solder that dont flow like regular solder it seems. Just my opinion, the stuff is like chunky lead. Tough to unsolder parts Ive found, some are not to bad, others are like pulling wisdom teeth lol... Yes, my plan is to chop stick and meter it and see what I find. i will share my findings if I nail it down. I know others may like to know as i did how to fix .. or what to look for... There is another post on here about the same type of issues..

[Enzo]

That is lead free solder. it is mostly tin. It takes more heat to melt, doesn;t wet as well, generally sucks.

Ever notice that a damp sponge picks up water better than a bone dry sponge? If you8 need to melt the solder in a hole and it is the lead free solder, just melt some fresh real honest-to-god old lead solder into the joint. NOW it ought to melt a lot easier

[Smitty02]

Ok I thought I would post results now that Ive got time. Seems the issue with this amp was a shorted power tube and plate resistor for one. Fixed that. The issue with the FU2 blowing all the time came down to only blowing when the pedal board was plugged in after repairing plate resistor ?? Anyhow pedal board was the issue. I took the end off and the midi cable was the worst solder job Ive ever seen. I opened the board and that was terrible also. I repaired both ends and that took care of the problems. Now new tubes are on the way and biasing per the service manual, there is also a hum adjustment tucked up in there also... Hopes this may help anyone who has issues of the same sort, they seem to be common with the pedal board specially..