Fuse blowing in 300b

[justin brown]

I hope this isn't cheeky, Richard, but I was wondering if you could try uploading my photo. I spent an hour and a half (in between cooking), and I want to be able to show the other forum members that the problem -at least at this stage seems a little obscure. And maybe encourage more feedback.
There are certainly no exploded diodes, Philip. I made sure I found all of the nuts from the TXs, it's just that I did panic a bit, wondering why this has happened. I am curious to learn what people think, but feel that perhaps the only way forward is by testing with a multimeter. One thing that I am concerned about is that we need to sing from the same hymn sheet. The configuration of my 300b is significantly different to Richard's photo, so, naturally, I am a bit dubious about poking around when my knowledge and experience is so limited. At this point I'd like to say that the fault isn't because of a poor build, as John Caswell built it for me.
I apologise if this is a bit tedious, but I am just trying to go through a process of elimination.

[Richard]

Ah, tagboards, C9 and 10 will be the 2 large cans to the left of the tagboards.

First thing I'd do (after checking voltage is discharged!) is to gently straighten up the HT diodes, they are the 4 squarish ones covered in heatshrink by the look of it to the left of the top board. Stand them up and check their legs aren't touching where they shouldn't and note that their metal tags which may be exposed at the top are live too so must be isolated.

Do you think you may have squashed that area fitting the new TX covers? Straighten up the 2 green resistors next to the HT diodes too and have a good look around there for shorts and parts touching that shouldn't

[justin brown]

Hi Richard,
Ok, when I get in from work (about 8.30), I'll have a good look.
I didn't realise the square shaped components were the HT diodes. That makes sense now. Up till now, I was only familiar with ordinary diodes, so I assumed they were some sort of cap. The heat shrink sleeves don't help with identification, either...

Thank you,

Justin.

[justin brown]

Hi All,
Richard has come to the rescue! Unbelievably, all it was were the HT diodes had been pushed over (most probably by me, without realising), and needed to be lifted up/straightened out, so that the air could circulate around them. Simples!
At least I now know what HT diodes look like now.
A big thank you to everyone for their input, especially Richard.

Justin.

[Richard]

Great news Justin, I thought they looked a bit squashed and that John would have stood them up neatly for ventilation and electrical isolation.

Check for 0V then have a last check around that no bare wires are touching each other, or onto tags, or component legs etc.

This type of construction relies on the mechanical strength of the tags, wires, and component leads to keep things apart. There should only be a connection where there's a solder joint so give everything else a little space

[justin brown]

I know this may sound daft, but how should I check for 0v? Where should I hold the probes?

Thanks,

Justin.