Crack & Speedball - Seeking advice re: no signal when powered on

[rburrows]

Hello again,

After some tube rolling I have found that the left channel hiss is always there, but its volume is subtly changed by different tube combos. I haven't investigated any further on this front.

As for the LEDs, I performed some diode checks on all of them to get a sense of their relative behavior.

On the big speedball board: the LEDs on the edge of the board do not light, but read .955. the inner ones light but give no readings.
On the two smaller boards: the LEDs closer to the black MJE350 unit read 1.2 with no light, and other ones light but give no readings.
On the input tube:  A3 LED reads .245. If I reverse the probes (ground on the non-metallic side on the LED) I get .228. For A8, I get the audible tone from the multimeter, and nothing when reversed. Given the cramped layout here, I'm measuring at the terminals and not directly on the LED leads. Not sure if that matters. No lights in any case.

All speedball LEDs light up when the amp is on. It is just the input tube LEDs that do not.

To get more reference points, I also tried Hopeful's method of using the low ohmeter settings, but I couldn't get any LEDs to light up or report values this way.

 

[Doc B]

Not sure if this was mentioned before, but check the value of the R1 resistors on the small PC boards.

 

[rburrows]

reading 235 ohm for each R1 resistor on the small boards, so those seem OK.

Does it make sense that if the bias LEDs were dead and/or improperly soldered, the voltages could be mostly right and the amp could pass a good signal (but perhaps not its best) to the headphones? I recently re-measured voltage with different tubes, and they are closer to the manual now than my previous listing, though still a bit low:

1:  55
2:  171
3:  0
4:  171
5:  64
6:  0
7:  90
8:  0
9:  95
10: 0
11: 0
12: 0
13: 173
14: 0
15: 191

20: 0
21: 210

a1: 63
a2: 0
a3: .572
a4: 0
a5: 0
a6: 54
a7: 0
a8: .243
a9: 0

b1: 54
b2: 173
b3: 90
b4: 63
b5: 173
b6: 95
b7: 0
b8: 0

Thanks,
Robert

 

[Doc B]

The LEDs usually either work right or don't work at all, but I suppose they could be partially functional. I would double check that the small metal can transistors on the small PC boards are 2n2907s and the ones on the big board are 2n222as.  If nothing else is obviously wrong I would maybe go ahead and change the LEDs on the A socket to see if it makes a difference. I might also ask the seller to verify that he saw the proper voltages at pins A6 and A1 and A8 and A3 when he built the unit with the stock tubes. It would help to know if the deviant measurements have always been there or if they developed after shipping.

 

[rburrows]

So I replaced the bias LEDs on the input tube. It was a little tricky with all the other circuitry in place, but with patience I got everything set. Now they light and the voltage readings are in line with the manual.

The sound quality has bumped up a couple notches. It was good even with the broken LEDs but I sensed it wasn't running at peak performance. The left channel hiss is also gone

 

[Laudanum]

Excellent!  Glad to read that you've got it sorted out.  Enjoy.

 

[hopeful]

Very happy to hear you got her up and running!