Strange Foreplay III issue

[bk121]

Last night i tightened the 0D3 tube socket down a bit since i noticed it was a bit loose.  Afterwards, i plugged in my foreplay and the 0D3 tube no longer lit up.  within 30 - 45 seconds, I noticed a burning smell and a bit of smoke.  i immediately cut power to the unit. i waited a day or so and I poked around with a chopstick to ensure nothing was touching where it shouldnt, and re-heated some solder joints around those power supply caps for good measure.  I tried plugging it in, this time waiting about 30 seconds and with the unit flipped so i can check to see what is going in.  the 12au7 tubes seem to light up no problem, however the LED's do not. the OD3 still does not light up and seems to not be getting any power.  Anyone have any idea of what the root may be?  ive taken resistance readings and it looks like all the readings look to be fine.  i am a bit concerned about taking voltage readings since ive not left it on for more than 30 seconds since it started smoking and just worried i may do more damage by leaving it turned on more than that.

thanks!

 

[Grainger49]

I would check the three PS caps.  That is two 220uF@250V and the 47uF@450V.  The ends might be swollen, that is not flat.  If so, that is your toast.

I couldn't even guess as to the root cause of the failure but those are the first things to look at.  If they are damaged the two UF4007 diodes might be damaged also.  Then the OD3 and 0.1uF@630V might be damaged.  I have no idea how to identify if the last two are damaged except obvious smoke or heat damage.

 

[Doc B]

Investigate very closely around the 0D3 socket. If that was what was disturbed, that is probably where the problem originated. The most likely scenario is that something shifted and two wires that should not be touching are shorted.

 

[bk121]

Thanks for the Tips.  I would bet money that the issue was around the OD3 socket with either a stray wire hair or possibly the 0.1uF@630V hitting another tube pin.  should have mentioned that, but cant be sure since i straigtened that cap out before looking at it close enough.  i did also notice either a hair or wire(very thin) around the base and thought that could be one of the culprits.  i did blow some compressed ait to clear everything out and have since made sure everything is where it should be.  that is when i decided to try the second time, but still nothing and did not keep it on long enough to see if anything smokes.  thought i would check here first to see if i can narrow down a few things instead of replacing all the power supply caps and have it be a blown tube instead.  the caps do look fine but after the initial smoke, that is exatly where the smell was comming from so seems pretty obvious. so it sounds like, if i replace all the ps supply caps, the od3 cap, tube, and two UF4007 diodes I should narrow it down?  i was wondering about the shottky diodes could of been affected as well. 

Oddly enough ive renewed my interest in this project lately so was planning on doing upgrades of the output caps and the od3 cap as well.  i initally put this kit together wth a radioshack 20 watt iron and a cheap o wire stripper which usually ended up cutting off the drain wire every time i stripped it.  ive since upgraded tools and look forward to getting this thing done right.

thanks again. 

 

[2wo]

This looks like a good plan.  If you have to tear it down, change all of the, not-so-expensive-parts, now.

If not, there is a 30-50% chance you're going back in.

The Shottky's and the rest of the filament circuit should be fine, as you say the 12AU7 lights up...John      

 

[Doc B]

Check the uf4007s before you pull any parts out. I described how to do it on of the forums last week. Basically you measure the resistance across each diode and it should read infinity in one direction and around 500 to 2000 ohms in the other direction. If you get a very low reading like less than 100 ohm it's blown and needs to be replaced.

 

[Paul Joppa]

No LEDs and no glow suggest no voltage getting to the 0D3. Smoke suggests too much current somewhere. So a short to ground, across the 0D3 or upstream (caps for instance), are likely causes. Too much current might have damaged the series resistor in the power supply (2.7K/5W) or one or both diodes. Check the 2.7K and 10K (twin 20K) resistors. In fact, how about the resistance checks?

 

[bk121]

Thanks again for the info.  i think im narrowing it down and learning a few things on the way.  I took Docs advice first first and measured the first uf4700 on the exterior of the tag strip.  one side measured infintiy, the other a bit high at around 2.8k.  i get pretty much the same on the inside, just with the leads reversed.  not exactly sure why but seems to be right and that they are working. per pauls advice i took resitance checks as per the instruction manual and it looked fine.  one thing i think may have smoked was that 2.7k 5 watt resistor though, i cant seem to get an accurate reaking off it, i was thinking it may be the caps making it move, so not sure if that is the case but just comapring it to the other 5 watt near the 10000uf cap visually leads me to belive this was where the smoke came from.  this would also be obvious since ive already ordered pretty much replacements from mouser for everything else connected to that circuit except for that resistor!  anyways, before ripping everything out, i think that is going to be replaces first.  my next question would be how important would be the 2.7k rating and could i use something lower around 1k or 500 ohms as it appears i have a few of those laying around.  thansk agian for all the help, cant wait until i get this thing up and running again to ask the rest of the questions ive got! 

 

[Paul Joppa]

A smaller value will work as a test, but it will provide less filtering so should not stay there permanently.

To check the 2.7K resistor, unsolder one end before measuring the resistance - that way the capacitors won't confuse the reading. Sorry, I should have mentioned that!

 

[bk121]

Im happy to report that my forplay is back up and running.  it was the 2.7k resistor that had smoked.  It was pretty obvious after i took it out and noticed the underside cracked and brown....ive got a 1.5k in there right now and it fired right up.  Thansk again for all the help, i think this was the first time i actually looked at the circuit and was able to seperate out the stages.  it cost me over 100 bucks but well worth it as i did end up ordering some new old stock 12au7s to replace the output tubes and some of those dale military spec resistors to replace the selector imput selector switch which i had saw you mention on a old post.  ive even got some auricaps on the way to replace those two 2.2uf caps.....fun times!  thanks again everyone.

 

[JC]

Just for grins, when you've replaced the 2.7k resistor with the proper value, you may want to measure across it again.  It sounds like you got a funky measurement the first time with a resistor that was likely open; it may be a good point of reference to see how your meter reacts with a good resistor in there.