Resistance Trouble

[Karl5150]

As I said in my other thread the PS & SR tests were good. Not so much the resistance checks. The positions that are on are on, almost no variation from stated. The ones that are off are all high, somewhat over the 15% allowance, the same and very balanced both channels.
I went with Clarity and Audyn Caps as the only deviations, they all spent about a week in-line on the positive speaker outs of the garage stereo, one pair at a time.
Term            SB                  IS
2 / 14          71K        85.5K / 85.3K
4 / 12          60K          75K / 74.8K
10 / 11        39K        46.7K / 46.6K
Sockets
A6 / B6        60K        75K / 74.8K
C1 / C6        39K      46.8K / 46.7K
C2 / C7        60K      75.2K / 75K 

As always, thanks for any insight.
Karl

 

[Paul Birkeland]

I would go ahead and fire up your amp, then check DC voltages.  None of those are so far off that operation should be affected, and often times we find that different meters read resistances differently when there are capacitors in the path of measurement.

 

[Karl5150]

Thanks Paul, I'll check it later.

 

[Karl5150]

Voltage checks all within spec. Some intermittent loud noises. First left, then both and then right, during the 1st half-hour or so. I played 3 LPs at low volume and it seemed to have sorted itself out. I'm hoping it was just the tubes settling, speaker drivers got some serious exercise. Musical impressions and photos to follow.
Thanks again Paul,
Karl

 

[Paul Birkeland]

The Eros is also somewhat sensitive to the RF vomit that comes out of some cell phones and wifi routers.  If the noise persists, you may want to try powering down each device and seeing if that influences what you're hearing. 

-PB

 

[Karl5150]

So I'm pretty sure it wasn't outside noise creeping in.
After 3 or 4 trouble free hours over the last couple of days I noticed a tiny blue flash from the vents to the left of the 12BH7 when I switched it on today. I immediately shut it down, took it to the bench, looked it over and ran through the resistance and voltage checks. All good.
No blue flash at restart but the growling in the sub was back for a few seconds before it settled down.
I just happened to be adjusting the volume when I heard a pop, saw smoke and it went quiet. The transformer was hot, not too hot to touch, but hot. The 12BH7 continued glowing for the few seconds it took to switch it off
Back to the bench. No components look obviously smoked. The only thing I see is the wire that goes from D6 to IA on the C4S board is scorched. Not at the connection, at one of the bends visible in the photo.
The wire had gotten behind the standoff when installing the SR board and I had snaked it around pretty severely to get it headed back where it belonged.
Does a compromised or broken internal wire here explain the noise, the heat and the sudden quiet? Should I keep looking up stream?
TIA, Karl

 

[Paul Birkeland]

Yeah, if that connection isn't there, you'll sure get some noise!  I would just replace that piece of wire.

 

[Karl5150]

Paul, I replaced the wire, but I think it's something else.
The soot wiped right off the insulation on the original piece.
I had trouble mounting the big ClarityCaps on the SR board due to the thick leads. The solder joints are ugly and look like they took their toll on the board.
The 2N2907 transistors that came with the kit were pretty corroded, I didn't even look in my parts bag for replacements, I just soldered them in.
Maybe a new board and smaller caps are in order.
Thanks as always for your input,
Karl

 

[Karl5150]

Well the what is solved. The photo shows the trace from the +reg to the Rc exposed and the 110K R4 resistor joint smoked. I realized when I built the board that I soldered in a hole closer to the b reg, but looking at the trace I thought it was ok as the path was the same. Too much voltage or too hot a solder joint close to the +reg /Rc trace? Maybe that's the why? I guess I'll be talking to the Queen about buying a new board.
Karl
PS. The long end on the Rc in the red oval isn't touching the open hole leading to the LED, it's just the view.

 

[JeffYoung]

Post a picture of the top of the smoked hole (it's hiding under the resistor in the top-view you already posted).


Any chance you forgot to solder that joint?  That could cause the arcing, and the arcing (and higher resistance of a bad connection) will generate a lot of heat.


Cheers,
Jeff.

 

[Karl5150]

Jeff,
It was soldered, my bad was soldering it in the wrong hole of the board trace, too close to the bottom layer trace.

 

[JeffYoung]

While the wider holes might have helped, I don't think there's anything wrong with the holes you used.  (They appear to be there in case you want to use a thick-film resistor.)


Before ordering a new board you could put the resistor into the other holes, remove the damaged portion of the trace on the bottom (with an x-acto knife, for instance), and then solder a wire between the two holes at either end of the trace.


Cheers,
Jeff.