speedball voltages [resolved]

[Jsmooth65]

Installed the speedball today after getting my crack running nicely.

All LED's turn on.  Constant left channel hiss, regardless of volume.

voltages also dont match the manual

Right channel I = 180v (ok)
Right channel O = 77V (ok)

Left channel I = 180v (ok)
Left channel O = 32v (too low!)

B+ on big board = 180v (ok)
O on big board = 108v one side and 77v on the other side (manual says 90-100)

I can't find any obvious solder issues, and I've resoldered several points, like middle of the MJE350's with no change.

It sounds like a ground issue.

Any help?

 

[Paul Birkeland]

I would be curious if both LED's on the 9 pin socket are illuminated?  (Don't mess with them either way, they aren't the problem, but potentially a source of additional information).

Resolving the small board voltage will most likely resolve the big board voltage, so it is unlikely that you need to troubleshoot the big board until your "O" voltage is up.

One situation that would cause your exact problem would be if there was a short on the 9 pin socket between either pins 7 and 8 or pins 2 and 3, which would ground out the LED and reduce the plate voltage to about 30V.

 

[Jsmooth65]

I'm pretty sure both 9 socket LEDs lit up, but I will check again.  Thanks for the lead on where to look.  It took some effort to unjumble some of the wires near the screws to get the nylon standoffs to squeeze in, so that's got to be a good spot to check.  I thought about swapping little boards to see if the hiss swaps channels, too.

 

[Paul Birkeland]

You could swap the little boards, but I'm reasonably certain that the voltage will stay put. 

-PB

 

[Jsmooth65]

the LED on pin 8 lights bright, and the LED on in 3 lights, but is very dim.  I don't measure any shorts between the pins 7-8 and pins 2-3 though.

8 socket pin 4, terminal 5, and 9 socket pin 1 voltage (which feeds the O on the small board) are also confirmed at 30v.

 

[Paul Birkeland]

For the sake of experimentation, try removing the 6080 and running the amp with just the 12AU7 to see if you can get that voltage to pop up.

Also, can you very carefully measure the voltage at pin A3?

 

[Jsmooth65]

with the 6080 removed, still getting 30v on those points.

voltage at A3 is 1.5V

 

[Paul Birkeland]

Let's try swapping the small boards, but actually, before doing that, also verify that the voltage on A2 is still 0V?

Low light output from that LED compared to the other one is strongly pointing to the small board having an issue.

-PB

 

[Jsmooth65]

A2 still 0V.

swapping small boards now.

 

[Jsmooth65]

swapped boards.

hiss is now on other channel.

A3 LED lights up bright.  A8 LED is either not on or very very dim.

8 socket pin 1 is now 30v, along with terminal 1 and the small board O on that side.

8 socket pin 4, terminal 5 and the O on the small board on that side are all back to normal range.

 

[Paul Birkeland]

Well, it looks like you've isolated the problem.

Ultra low current from the board could be from an R1 that's very high in value, or more than likely from a cold solder joint that's putting the brakes on things.

Beyond that, you can contact replacementparts(at)bottlehead(dot)com to get a replacement small PC board and parts if those efforts do not fix the issue.

 

[Jsmooth65]

Thank you.

I'll disassemble and reassemble the small board and see what I get!

 

[Jsmooth65]

Its a Christmas miracle.

I removed the MJE and stuck it back on and resoldered all other pieces, and the hiss is gone, the LEDs on the 9 socket are on, and we're golden.  When doing this work, I didn't secure the small boards down to the standoffs, and when I thumped the problem board with my finger, you could hear it in the headphones, while the other board was silent when I thumped it.  SO its still not perfect.