Express just blew a fuse?!

[Gargoyle]

Guys,
Need some feedback from the Trainwreck collective brain.

Just plugged the amp in (Chassis only) had it sitting on the cab, made some RF type noise (from sitting on the cab) went to turn it off made a small tick noise and then nothing.
No power light , noise or anything. Checked the fuse and its blown. Had a pretty good look around inside and can't see any burn marks or any nasty smells either.
I still had the chassis screws in the chassis after trying out a cab for it on the weekend. One of the screws can be moved enough to touch one of the lugs on the input jack, couldn't see any marks on the lug or screw though.
I am pretty uneducated when it comes to looking inside these things so I am hoping there maybe some simple solutions I can try before a trip to the tech.
Photos attached.
Cheers
Mick

[passfan]

Have you tried to re start it using a current limiter (light bulb rig) just to see if it is showing an actual short ? Perhaps it blew due to a surge and your fuse is too small. I have no experience with the VVR rig you have there so I'm not sure if it has an increased current draw when you start turning down.

Also noticed you don't have any grommets in that transformer lead hole by your power switch. Did you try to pull those leads up to make sure they didn't get cut on a burr, did you de burr those holes good and round the edges ? Sometimes a burr will work through just enough to touch the inner conductor.

[sliberty]

Replace the fuse, and try running the amp without a head cab and without those mounting bolts. If it blows the fuse again, bring the amp to a tech. If it is stable, then maybe it was related to the bolts touching something. Only do this one more time though - don't tempt fate.

[Structo]

I don't think if the bolt touched the input jack it would blow a fuse.
There is no juice on that.

More likely it is a result of the VVR or your power supply is not wired correctly.

Do you have a light bulb current limiter?

If not, easy to build and can be a real parts saver.

On a new build I will use a 25 watt bulb. That limits the amp to less than 250ma so it most likely won't fry something if there is a short circuit.

Just so you know, when you turn on the amp if there are filter caps being powered with the switch, the bulb flashes briefly then settles down to a dull glow.
You do this the first time with NO tubes in the amp.
If it stays bright, you have a short or other wiring fault that is drawing too much current.
If the filters are after the standby switch then you flip that to operate and if the bulb flashes then goes to a low glow, then you should be ok to start populating the amp with tubes.

If the amp passes the 25 watt test then I usually put a 60 or 75 watt bulb in and test it with that.

Then I will install the rectifier, test again, and so on.

[Structo]

It looks like you have the insulated thing under the MOSFET, be sure that the tab isn't showing any connection to the chassis.

I can't make out the wiring very good on your power and standby switches.
Is that purple wire sufficiently large enough for the current it is handling?

[sliberty]

I don't think if the bolt touched the input jack it would blow a fuse.
There is no juice on that.

I agree, but since it was a concern of his, it is easy to test. On the other hand, the OP also indicated that he is not famiilar with amp internals, so I am hesitant to advise that he do any other sort of diagnostics.

[Cliff Schecht]

The first thing I do before powering on an amp is hook up the "hot" and "neutral" lines to a function generator and check all of the nodes for DC. Obviously the DC values will be much lower than the actual ones but this is a safe way to check if the rectifier and filter sections are working. Next step is to bring up the amp on a variac if you have one, or if you don't have one then plug it in and hold your breath .

[Gargoyle]

Thanks guys.
The amp has been running fine for some time now, up until yesterday that is. So I something has changed since Monday when I last fired it up.
The Mosfet does have a grey pad (similar in colour to chassis) between it and the chassis.
Checked the purple wire (attaching to the power lamp) seems ok in that there isn't any scorch marks or melting.
Did some more poking around (with chopstick) one of the connections to the power plug was wedged pretty tightly between the smaller of the 2 boards and the end of one of the mounting screws for the OT. Moved it aside, can't see any indication of problem with the insulation though.
I am off to the tech today, as I really have no idea what I'm doing.
Thanks again guys.
Cheers
Mick

[Cliff Schecht]

That's a good idea, you could easily hurt yourself debugging power problems.

[sst4270]

Regarding the Mosfet on the VVR. Even though there's a pad insulating it electrically from the chassis MAKE sure the screw isn't touching the back plate of the Mosfet as it goes through to the chassis. The Mosfet should have come with a small plastic insulating washer to prevent this. If you didn't use that washer it's possible the screw may have intermittently grounded the mosfet which could definitely blow a fuse and possibly the mosfet as well.

FWIW...

Steve

[Gargoyle]

Regarding the Mosfet on the VVR. Even though there's a pad insulating it electrically from the chassis MAKE sure the screw isn't touching the back plate of the Mosfet as it goes through to the chassis. The Mosfet should have come with a small plastic insulating washer to prevent this. If you didn't use that washer it's possible the screw may have intermittently grounded the mosfet which could definitely blow a fuse and possibly the mosfet as well.

FWIW...

Steve

Steve, I didn't fit the VVR a tech did it for me. The small nylon washer has been used against the nut on the upper side ofthe chassis. I used a multimeter to see if the screw is earthed, its not>. However the thin metal plate with the 3 legs that attaches the mosfet to the VVR board and sits on top of the grey insulation pad is earthed. After reading the instuction sheet from Dana, I don't think this is supposed to happen. Also I have found a very small amount of powery residue around one edge of the Mosfet that doesn't look to healthy. How do you tell if the Mosfet has had it? Just for my education, I'm booked in with the Tech on Friday>. I am glad I bought 2 VVR's from Dana.

[Cliff Schecht]

Usually you just assume that it's blown if you think it is. MOSFET's are cheap enough to replace if you have the skills to do so.

I recently fixed an Ampeg Rocket bass amp for a bandmate. He brought it in and it wasn't working at all because one of the small transistors that controls the current flowing into the power resistors was kaput. I changed out this transistor but because of the funky way they attached it to the heatsink, it got a chip in the transistor body and ended up blowing all four power Darlington's. There was no hesitation in my mind that the transistors were blown and this was verified after I cut out all four power transistors and the fuses stopped blowing. I replaced the five transistors and the amp fired right up to life; we got to use it in the studio recently and it's a damn fine sounding amp.

[sst4270]

Mick,

Unfortuntately I think that Dana should chime in here since he designed the VVR. I've installed several and the instructions are explicit in stating that the insulator (pad) MUST be used to keep the metal plate (heat sink) from coming in contact with the chassis. Though it's clear from your photos that it was used I don't think it should measure "earthed".
I know very little about mosfets (only the most basic stuff).
The one used in the VVR is expensive (as far as mosfets go).
I remember sourcing them and they cost around $10~$13 each.
I've purchased several substitutes, but I have yet to try any of them out because I had a small supply of the originals.
Testing the mosfet usually (if not always) requires that it be out of the circuit.

You might want to PM Dana and take it off-line.

Good Luck Sir.

[UR12]

Interesting. The mosfet usually doesn't have a ground on any of the pins or the back so other than having a faulty insulator I don't see how that can happen. It may be that you have another short somewhere from B+ to ground, like a filter cap before the VVR that is shorted or something. It would ber easy to remove the screw holding the mosfet down and lift the mosfet a little off of it's insulator and see if you are getting the same reading (short to ground) that would at least tell you if it was the Mosfet or something else.

[Ron Worley]

Guys,
Need some feedback from the Trainwreck collective brain.

Just plugged the amp in (Chassis only) had it sitting on the cab,Cheers
Mick

You did have the amp connected to a speaker, didn't you??