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The smell of resistors in the evening

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moggi1964

moggi1964

Member
Joined 2010
So I've been getting some terrific one on one support from one of the members here but I feel it unfair to keep pestering him so:

I have progressed through the checkout to where all tubes are in and I have leads across R18 and R29.

(I had previously set the output grid to its most negative (-325v more or less))

I get 424V B+

I get -365V on B-

I have both input tubes at 175V.

I get no reading at all from R18 and R29 even if I turn the trimpots (R12 & R23) all the way from one side to the other. No reading AT ALL.

I see the 300B's are glowing (one more than the other).

I have the Output Transformers hooked up and an 8ohm load attached (speakers).

All solder joints appear to be testing fine and the resistors test ok.
 
moggi1964

moggi1964

Member
Joined 2010
Even' I made that error at the beginning but changed it to a more sensitive setting and still no luck.

Swapped in a new jumper and fired it up. Started getting a reading on R18 and R29 then smelled and saw smoke from near the rectifier. Switched off quickly but before I could establish which component.

Went back to square one with no tubes.

B+ -30V (and falling towards zero)
B- -285V

Put in a known good rectifier and got

B+ 450V
B- -92V

Noticed a multimeter lead had a melted section and wondered if the smoke had come from that resting against a tube? Or maybe against an overheating resistor?

Before I start a lot of desoldering and testing, is there an obvious candidate for the B- drop? B- wasn't as high as I would expect in the first place but the drop was huge!

My son says he wants his Netbook back, the one I borrowed 6 months ago for "a month or so while I build my amplifier"! Oops!

Sometimes it really is on step forward ............ :(
 
moggi1964

moggi1964

Member
Joined 2010
So I took everything from R2 to R8 out and checked them. I replaced R7 because I had a spare.

I powered up without a rectifier and got the expected figures across R7 and R30.

I put in a rectifier and the figures went in the right direction until the rectifier warmed up and then B- plummeted from over -300 to -60. I tried three different rectifier tubes and all displayed the same behaviour.

What am I missing? Is this Fred related and where can I find him to give him a good talking to? :)

I'm at the point of buying a new board and new board parts and starting from scratch.

Thanks.
 
moggi1964

moggi1964

Member
Joined 2010
jpjones3318 said:
I don't know that there's much to be gained by throwing parts at it, unless the current board is damaged. How about some pics of the current board?
Click to expand...
I'll grab some pics tonight JP and post them.

I'm not too concerned about starting from scratch as I would like to build another board for the friend who has been helping me.

If I can get to the bottom of this current issue that will of course make that desire more easy.

Thanks for the continued support.
 
moggi1964

moggi1964

Member
Joined 2010
As requested
 

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