Paramount Terminals 16U and 17U Sparking, Popping, Caught on Fire

[burgerbassist]

Hello All,

I came across an unbuilt Paramount 300B kit for sale recently, and thought I'd take the plunge.  I've built a Quickie and a Reduction, so I figured I'd try my hand at some 300B monoblocks to go with my Heresy IIIs.

I built the first monoblock, checked resistances, and then fired it up to check voltages, and it warmed up for about 20s, at which point there was a loud POP and a spark, and a fire stared between terminals 16U and 17U.  I walked away for a bit and inspected my work, and I thought I'd found a short (between a wire and resistor on the 9-pin socket), which I fixed, but when I fired the amp up again I got three more loud pops with sparks right away.

I've gone back and checked all of my connections and I can't see any more shorts or anything miswired.  Does anyone have any idea what to look for as the possible cause of these pops and sparks?

I know this is an old kit, and I'm sure no one remembers what terminals 16U and 17U are off-hand, so if anyone needs me to upload anything from the manual, or pictures, etc, just let me know.

Thanks guys!

[Paul Birkeland]

16U is the incoming high voltage from the power supply board and 17U is ground.  There are no components between them that could catch on fire.  If there is damage in this area, I would suspect debris from the build or a short from something like improper flux and/or solder used.

You could certainly post photos of the damage and we might get a better idea of what's going on.

-PB

[burgerbassist]

I've attached two pics here - one of each side of the affected area.  I've already cleaned off some excess flux, which I assume was the issue.  I'm now looking for other places I may have additional flux.

[Doc B.]

Are the LEDs on the board under the 5670 intact? The description reminds me of the zener string and LEDS getting blown on those boards when they have been miswired.

[Paul Birkeland]

What kind of solder are you using? 

[burgerbassist]

Thanks for the reply, Doc!

The LEDs are intact.

After a good cleaning, everything seems to be in working order.  Voltages check out and I fired up the first monoblock last night and got sound out of it.  I think I just had some flux that had dripped between those two connections on the terminal strip and caused a short.

Hopefully I won't be too annoying jumping off-topic here, but with the absence of the Foreplay in your product line now, is there a relatively low-priced preamp you guys know of that works particularly well with these 300b Paramounts?  I'm sure I'll eventually move to the BeePre, but I need to save some money first, so something a little less expensive would be good for the time being.  This is my first foray into monoblocks (I've always used integrated amps) so any guidance would be appreciated.

Also, I'd be happy to create a new thread for the preamp question if that's preferred.

Thanks again!
Dan

[burgerbassist]

Paul,

I'm using 60% tin 40% lead rosin flux core solder.  It was labelled as "fine electrical repair solder" so it seemed like a good fit.

Thanks!
Dan

[Paul Birkeland]

Did you use any additional flux other than what came with the rosin core solder?  There are fluxes out there that are conductive and will do exactly what happened to that terminal strip, but rosin won't.

[burgerbassist]

Yes, I did use additional flux. I went through and cleaned up a bunch of it, which I believe fixed my issue.

[Doc B.]

We're seeing more cases of builders applying extra flux when working with flux core solder lately. Don't do this, the flux in the solder is more than enough. Additional flux quite often creates the problem described here. Inadequate soldering iron heat is more likely the problem than lack of flux if joints don't flow well.