No good after speedball upgrade

[hardisondan]

How about the other pairs of terminals on the power transformer?

So Paul do you mean like between 3 & 4, 5 & 6 etc? or what?

[hardisondan]

V AC Between:

3 & 4:  5.8
5 & 6: 168.4
7 & 8: 168.1

[Paul Birkeland]

What DC voltage do you get between terminals 20 and 21?

[hardisondan]

What DC voltage do you get between terminals 20 and 21?

239

[Paul Birkeland]

How about between 14 and 15?

[hardisondan]

How about between 14 and 15?

7.2 mV

[Paul Birkeland]

OK, despite it looking OK, it seems very, very likely that the 270 ohm resistor that runs across the power transformer is blown open.  It's either that, or one end of that resistor is not soldered, or the black wire that runs next to it is not well connected.

With that resistor open, that first 220uF power supply cap will stay charged up.  You can use a screw driver with a well insulated handle to short its terminals to remove its charge, or check out a YouTube video on discharging a capacitor with a resistor. 

-PB

[hardisondan]

OK, despite it looking OK, it seems very, very likely that the 270 ohm resistor that runs across the power transformer is blown open.  It's either that, or one end of that resistor is not soldered, or the black wire that runs next to it is not well connected.

With that resistor open, that first 220uF power supply cap will stay charged up.  You can use a screw driver with a well insulated handle to short its terminals to remove its charge, or check out a YouTube video on discharging a capacitor with a resistor. 

-PB

"Dan's memorial service will be held at...  "   

So you mean the one in the attached photo? So I need to short between 20U and 21U?

[Paul Birkeland]

Yes, precisely.  This is not a lethal amount of energy thankfully.  When the cap is discharged, you can use your meter to check the resistance of that 270 ohm resistor.

[hardisondan]

OK. I did that. Even after a few hours, there was still over 200V in that capacitor. How long will they hold charge like that? (just out of curiosity).

I got a huge spark which scared the crap out of me, but then I tested it and it was almost completely drained. I couldn't seem to get it to zero. Even with more shorting it still had a volt or two.

I measured the resistance across the 270 ohm resistor and it was 0L.

I also noticed it now has a hairline crack right across it. I'm not sure if that was there before. See attached.

So what now?

[hardisondan]

I just noticed the Forum clock is wrong. It says 5:55, and it's 8:55 there isn't it?

[Paul Birkeland]

Yes, the charge will stay there for a long time, that's why you use the screwdriver to drain out the charge.

If you started with a perfectly working Crack, then put the Speedball in, then cooked that resistor, then the TIP50 transistors are unlikely to be properly mounted.  I know I've mentioned this before, but it's really important and if it's still an issue when you put the new resistor in, then you'll just get to do this all over again.

[hardisondan]

OK. Thanks Paul. So do you think I should remove the TIP50s and re-solder them?

Someone else mentioned a while back they thought it looked like centre pins on some other transistors were not soldered. Are there any other resistance or voltage checks I can do to make sure the speedball boards are assembled correctly?

As you say, and I keep thinking this, the Crack was working perfectly, so it must be something I did in the speedball installation.

[Paul Birkeland]

The TIP50 issue isn't a soldering issue, it's an issue where the hardware used to mount the TIP50s to the heatsinks isn't properly installed. 

[diynewbie]

Just in case you've lost track of things.  The TIP50's need to be insulated from the heat sinks.  Earlier in the thread he provided one way to test this  by checking the resistance from the metal tab on the TIP50 to a ground point such as terminal 12.  It may also work by checking the resistance from the metal tab to the heat sink itself.