HT Fuse Mystery - Standby Switch Problem?

[jasonguitar]

Marshall JCM 800 2203 100 W lead re-issue head. Brought to me because tubes light up but no sound. 1A Slow Blow HT fuse was blown. HT fuse blows with all tubes removed and when standby switch is flipped to play mode. Fuse does not blow with lightbulb limiter connected to mains. When standby switch is flipped to play mode with lightbulb limiter connected, bulb flashes quickly and then dims quickly - fuse does not blow. When amp is brought up to full voltage slowly on variac in play mode with no light bulb limiter - fuse does not blow. With ammeter setting on Fluke DVM connected in place of fuse, peak reading never exceeds 1A when standby is flipped to play mode albeit the meter is probably too slow to read a peak happening that fast. So there are no apparent shorts as the amp runs with or without tubes when brought up slowly but blows fuse whenever it is switched to play mode, with or without tubes. What gives? Could a Standby switch somehow cause this?

[Jazz P Bass]

When you do get it running(however that may be) check the power supply caps for excess ripple.

That may be a clue that a cap may be bad.

[jasonguitar]

It powers up fine and works/sounds great if I turn it on full mains voltage with the standby switch already in play mode. It’s not humming or anything. There are 4.9VAC on that first cap connected to the HTfuse

[mozz]

Check the sockets for arcing.

[jasonguitar]

Check the sockets for arcing.

No arcing! Besides, if it were arcing it wouldn't be running when turned on in play mode.

[trobbins]

Are the heaters at 6.3V (ie. is the mains voltage appropriate) ?

Can you link to a schematic?

What is the idle current through the fuse, and can you tell what that increases to when the amp is cranked?

Are you using a quality branded fuse whose purchase history is known ?

[Enzo]

I too question the fuses, are they really slow blow (timed) fuses in your box?

[jasonguitar]

Idle current is under 100ma and never exceeds 500ma. Doesn’t exceed 200ma when standby is flipped to play mode. Gone through all my fuses now. I’ll try a bus type 1A clipped in.

[jasonguitar]

I too question the fuses, are they really slow blow (timed) fuses in your box?

Gone through all my GMA 1A fuses now. Slow blows typically have the spiral wire wrapped around white material don’t they? That’s what the ones I used had. I’ll try a bus type 1A clipped in that is definitely slow blow.

[olddawg]

I suspect the Standby Switch is arcing internally from a carbon buildup when engage. Try spraying a little Craig into it if you can.

[The Dude]

They don't all have the spiral wire, but the fuse should be labeled to indicate that it is slow blow- T1A or something of the sort.

[g1]

Doesn’t exceed 200ma when standby is flipped to play mode. Gone through all my fuses now. I’ll try a bus type 1A clipped in.

Does your meter have a peak current mode? Otherwise I don't expect it will be quick enough to catch the current surge when standby switch is thrown.

[pdf64]

Link to accurate schematic? Point being that the dreaded standby can be implemented various ways, and when things go wacky, such detail may be significant.

[jasonguitar]

Link to accurate schematic? Point being that the dreaded standby can be implemented various ways, and when things go wacky, such detail may be significant.

https://www.thetubestore.com/lib/the...-Schematic.pdf

[trobbins]

Hmmm, I'd suggest the fuse type needs to be confirmed to be UL284 compliant time delay from a respectable manufacturer.

The difference between your fuses blowing when standby was in or out would indicate a higher initial peak surge as the transformer is already on line. If the transformer was also being energised at the same time as the secondary caps, then that can limit the peak surge current available to the caps in the first half-cycles of mains AC connection, due to transformer primary winding in-rush current, and the extra peak loading on the heater winding from cold heaters.

I'm guessing you only have one applicable and available primary voltage tapping on the transformer, ie. 110V. If you did have a significantly higher mains AC voltage, then the capacitor inrush current (energy) would be significantly higher than 'rated', as the energy needed to charger up the caps is proportional to voltage squared, and so that may also influence your fuse hassles.