Problem with P1800 amp kit build - actually build thread

AustrianJohn

I just started a P1800 build. When I switched on having completed the power supply the indicator light came on briefly, then the fuse blew.

I have checked the components and wiring and can't see any obvious problem. I'd appreciate it if anyone who has built this kit could take a look and see if there is a mistake.

Thanks!

Danny1969

I can't see C15 to C18 in your pic, have you checked you have em the right way round ?

AustrianJohn

Thanks for your suggestion Danny, but C15 to C18 are not part of the power supply so I have not yet installed these.

Nomad

Does the fuse blow if the transformer brown & white are disconnected from the board?

AustrianJohn

Nice thought - that's my first test for tomorrow.

ICBM

I can't see anything obviously wrong, so do that first. Also check the rectifier diodes for shorts with a multimeter set to beep test.

Just as a matter of being picky, you have the mains fuseholder connected the wrong way round - the incoming live should always go to the inner end connection not the outer ring. This is because the way you have it, if you put a new fuse in with the amp powered up, as the fuse passes the ring it can make contact and the outer end of it will then become live.

AustrianJohn

Having disconnected various components it became clear that the MOSFET was blown, so I'm waiting for a new one. In the meantime I connected the valve heating circuit.

Note the missing MOSFET in this picture:




The very satisfying first glow of the valves...

AustrianJohn

More pics coming soon, but I'd just like to say a big thank you to @dern for his build pictures that helped me avoid some dumb mistakes.

AustrianJohn

Almost done. Just the input wiring to finish plus connections from the output transformer primary to do.





Next time I'm reincarnated I'm going to ask for long thin fingers instead of short stumpy ones.

Chalky

Your wiring looks better than mine! I don't know how people get that perfect straight line and 90 degree corners in their wiring without making the signal path several time longer!

AustrianJohn

The circuit board that was looking neat and tidy a few days ago now looks like a bowl of coloured spaghetti. Today the input jacks and the 4 way output control have been wired up.

One comment in the excellent instructions caught my eye: "If you look at the numbering shown on the schematic, you should be able to work out how the wires are connected." Layout diagrams I understand, but the schematic is beyond me. I spent a few minutes this morning trying to work out what the round brown thing in the schematic was, until I realised it was a coffee stain :-S

4114Effects

Great job, so satisfying when you've wired one and it fires up!

Couple of minor points if I may be so bold:

The mains IEC connector should really have it's own earth point, as close to the IEC inlet as possible and on a completely separate eyelet and chassis bolt from any other earth reference. It should also be secured with a locking nylock nut or two normal nuts - this is a safety thing, if this earth comes loose it can be potentially fatal. Yours shares an eyelet with the power transformer centre tap and heater centre tap as it stands.

You ideally want to keep the heater wiring twisted right up to the socket. If possible it should pass over the centre of the socket to pin9, rather than looping around it. You might not get hum as it is now, but it's best practise to do this from the off just in case. Here's an image from another site illustrating what I mean.

hope this helps

AustrianJohn

@4114Effects Thanks for the comments. As an amp building newbie I appreciate your input.

4114Effects

No problem, happy to help. I'm sure I've made just about every mistake possible in my amp building journey, including destroying an OT and PT in a very dramatic puff of smoke ... :-)

AustrianJohn

I finished the wiring yesterday, all test passed and it sounds wonderful (although I'll have to wait for my wife to go out before I can really wind it up). Even in single ended mode (5W) it is nice and loud.

While I'm waiting for the cabinet (from Chris Uff) I'll tidy up the wiring, and fix the one problem that has come to light. The Hot input jack works, but the Normal one doesn't.

Thanks to all for advice along the way.

Fatfingers

You want to get the Normal input working - it's the better sounding one in my humble opinion. And I like crunch.

AustrianJohn

@Fatfingers Yes, I'll rewire that before I put it in the cab.

AustrianJohn

Normal input problem fixed. It was a simple wiring error - the sort that any fool could have made.

NPP

AustrianJohn said:

Normal input problem fixed. It was a simple wiring error - the sort that any fool could have made.

I remember rewiring almost an entire Strat pickguard chasing down a fault before I realised that I had wired the jack the wrong way around

Fatfingers

I found the jack socket working the only confusing part of the instructions.

MtB

I had exactly this problem with my build, and flagged the problem up to Barry when I did my build a year or so ago - I suggested that his instructions for this part of the wiring needed photos rather than a diagram.