The background: just (partially) rebuilt my ST70, and have installed my VTA board for a second time. I ended up preferring the stock board last time I did a head to head between the two, but this time I've used better quality film caps on the VTA board (Clarity Caps). Also bought 3 matched Mullard CV4024 valves so this should give the board a chance to shine this time.
All parts in the Dynaco were new when I first built it around 2 years back, except for the output transformers which are genuine Dynaco. Power transformer is the PA-521-S 240V, from Triode.
Had checked and triple checked and then checked again all the wiring against the VTA diagrams.
From the smoke that came out when I turned it on I thought the damage would be horrendous. However when I looked inside there was just a nasty smell and a little fluid around one of the caps: bias capacitor C17. It's bulging slightly, and was VERY hot.
This is the same 100uF 100V standard grade Black Gate cap I had used in the amp before (but then using a new Dynakits 7199 driver board). The other cap (C18) is fine. I'm assuming I either got unlucky and the re-used cap had a failure, or there is a more serious blunder with my work.
I've built kit amps before, but am still a bit of a novice when it comes to knowing what to measure. First of all, what's the most likely cause of failure for Cap C17 on the VTA driver board? I checked for shorts and polarity and all OK. Ditto the soldering too.
I have some other 100uF/100V caps I can use, but don't want to try those before I know what caused this.
- John (UK).