Smoking crack after speedball

[RenEH01]

I assembled the base crack a week ago and it ran well. I gave it a full week of burn in.

Today I upgraded with speedball and the following happened
1) Got horrible cracking and low level hum
2) Unplugging the unit, the volume pot shocked me.
3) Diagnosed a loose 3U wire due to moving things around to install the nylon washers. I soldered it back in place.
4) Crack was working fine for 30 seconds - 1 minute when transformer started smoking. I turned off the power bar asap.

Any ideas? Pictures in the morning as I’m too exhausted.

 

[RenEH01]

Here's the photos. Please let me know if additional photo of particular areas are required.

 

[Thermioniclife]

The thing that jumps out to me on your first pic is the untrimmed leads on terminal 20 and 21 shorting out the cap in that position, this will cause mayhem. on your last pic it seems that those leads have been trimmed. Check the fuse and trim all long leads and procede cautiously.

 

[RenEH01]

Thanks for the help. The picture is a bit odd, the cap isn’t shorted. I did solder the caps a bit odd there though.

 

[Paul Birkeland]

The Speedball manual instructs you to test the installation of the small PC board and to check those voltages before building and installing the big PC board, were you able to complete those checks successfully?

If the power transformer was that unhappy, you may have two green wires touching each other or one green wire touching the chassis or a ground wire, which would cause unlimited current to be drawn from the heater winding without blowing the fuse.

 

[RenEH01]

Small board passed checks fine. It also passed big board checks when the ground was loose too. Voltage was 165 Volts, but I was using a not stock tube. OA/OB resistance was around 32 MegaOhm though.

I'll re-check wiring, thanks.

 

[RenEH01]

Green wires are showing a short to ground. Is this normal behavior? Does it matter which green wire goes to which terminal after the twist?

 

[Thermioniclife]

Twisted green wires can go any way that is convenient. You stated you were not using a stock tube so you might want to remove the tubes and check or shorts and visually inspect for shorts.

 

[RenEH01]

Green wire short to ground is present with no tubes in at all.

 

[Paul Birkeland]

Yes, the green wire short to ground is normal. If you would like to disrupt that connection, the black wire from the power transformer to #8 solder tab by the IEC power entry module is where that connection is made.

 

[RenEH01]

I removed the ground from terminal 8 of the transformer and there is no longer a short to ground. Any other ideas? Can a bad tube cause this?

 

[Paul Birkeland]

You can run the amp without the tubes to see if the issue is still there. 

 

[RenEH01]

I put in tubes as you suggested, and with terminal 8 still floating there is not short to ground. Could this have been an issue of cables? Left or right channel shorting to ground?

 

[Paul Birkeland]

I'm saying that the short to ground from the heater wiring is normal for the amp.  If you put the wire back on terminal 8 of the power transformer, the ground reference will be restored.

If you run the amps without tubes, that takes the C4S boards themselves mostly out of operation, so if you still have operating issues, then the problem is more than likely something fundamental to the stock build.

 

[RenEH01]

Thanks, I’ll try it without tubes.

 

[RenEH01]

I haven’t have plugged it in yet, but I went over ever single solder point again and I noticed A6 and A7 could have intermittently touched after the nylon washer was installed. The solder tabs technically had enough give they could have touched, but there was no continuity when I checked it.  I cleaned it up and will hope for no smoke in the morning with fresh eyes.

 

[Paul Birkeland]

A6 and A7 touching won't cause the kind of mayhem you're experiencing.

 

[RenEH01]

I tried to run the amp for 5 minutes with no tubes and it had no issues after 5 minutes. Transformer was cool and belll cap was also cool after 5 minutes. I had no headphones or dac plugged in when this was attempted.

Should I give it a try running it with tubes?

 

[Paul Birkeland]

Repeat the experiment with just the 12AU7.

 

[RenEH01]

Experiment passed with just the 12au7. I used the stock tube. I think it’s an RCA clear top, I’m not sure, but it lit up nicely