Wrong voltage on speedball big board [resolved]

[illusineer]

I redid all the joints on the big board and reattached it, readings are

OA - 119
OB - 218
G - 0
B+ - 227

and the same two LEDs aren't lighting up :/

 

[Paul Birkeland]

There's a broken wire from the board to the socket, or a miswire from the board to the rest of the circuit, or there's still a bad solder joint on the board itself.  The B+ being high and OB being high means that half of the board just isn't allowing half of the 6080 to draw current.

 

[illusineer]

I looked at the board and the wiring back to the circuit and I can't see anything obviously wrong. It seemed so close to working now I'm not sure what to do

 

[Paul Birkeland]

You have a solder bridge across one of the LEDs on the bottom of the board.

 

[illusineer]

Wym where? All I see is the oily looking residue from soldering, is that stuff conductive?

 

[Paul Birkeland]

Solder bridge, iffy joint, and leads to be trimmed.

 

[illusineer]

How's that? Good to go back into the circuit?

 

[Paul Birkeland]

On each leg of each TIP50 transistor, I would hold your soldering iron on there and count to ten, then move on to the next leg.  You can do one leg of the A transistor, then a leg of the B transistor, then back to A, etc. switching off to keep the transistor from heating up too much.

It also looks like maybe one end of one of the 47K resistors needs some more heat and solder.

 

[illusineer]

OK, so I did all of that, put the leads on the board again, go to do resistance checks, then OB gives me around 4.6 megaohms, and OA doesn't read anything.

 

[Paul Birkeland]

As long as that's not close to 0, then you pass that resistance check.

 

[illusineer]

What about OA? It doesn't give any reading while the other one did give a reading on my meter.

 

[Paul Birkeland]

One side is reading 4,500,000 ohms and the other side is reading so high your meter can't determine what it is.

The manual says OA and OB on the big board should be several hundred thousand ohms at minimum.

I'm not seeing what the issue is here, I would move on to checking the voltages.

 

[illusineer]

OA - 120
OB - 221
G - 0
B+ 246

 

[Paul Birkeland]

Let's go back to pictures of the bottom of the board...

 

[illusineer]

Trying my best lol, this pcb soldering stuff isn't as easy as the stripping wires and soldering them to terminals sticking up ig...

 

[Paul Birkeland]

Can you send me a link to the solder and soldering iron you're using?

 

[illusineer]

I've been using this 60/40 rosin core radioshack solder that's been lying around in my garage forever throughout the whole amp, as for irons I've used this bluepoint soldering gun for about 95% of the time except for just recently with the pcbs I started using this weller iron (I like the soldering gun better).

 

[Paul Birkeland]

I would use the gun on your board.  You can leave the LED joints alone, but use the gun on everything else. 

 

[illusineer]

OK, reflowed with the gun put the board on again, resistance check, meter didn't read anything, voltage check

OA - 117
OB - 220
G - 0
B+ -226

 

[Paul Birkeland]

Well, at this point I would suggest sending it in for repair and I can fit a new board on it for you, or you can e-mail replacementparts to ask for a new board, or you can pull the LEDs and the two transistors on the bit that's not conducting and replace them.

Board rework isn't super easy though.

It's moderately probable that the big lump of solder that I saw bridging the connections on one of the LEDs would've required enough heat to melt the LED itself, and it may be damaged.  Some meters have a diode check setting that can allow you to measure them, and they should show about 1.57V in one direction and nothing in the other.