neovibe debugging

[jeff g]

I just finished the GGG layout Neovibe and can't get it working.  I've gone through debugging and found some problems but don't know how to fix them.

No parts substitutions - except I swapped a 3DPT using corresponding outside lugs

Didn't connect offboard points at I, X and O

Used 2N088 for all transistors except a 3904 for the lamp and a 78L15

Rechecked solder joints, continuity and bridges - no visible problems

Rechecked parts values - OK

With power applied, the lamp glows and the speed and depth controls seem to work based on changes to lamp intensity and rate of flashing,

But - no sound at all, and a constant hum

There are several transistors with voltage problems, but I don't know what to do about it.  Here's the voltage readings

T1  E  -1.0
      B  -0.85
      C  -2.10

T2  E  -1.5
      B  -2.10
      C  -4.34

T3  E  -3.80
      B  -4.34
      C  -11.03

T4  E  -4.35
      B  -4.27
      C  -15.02

T5  E  -3.77
      B  -4.34
      C  -11.19

T6  E  -4.30
      B  -4.23
      C  -15.02

T7  E  -3.73
      B  -4.24
      C  -11.21

T8  E  -4.23
      B  -4.28
      C  -15.02

T9  E  -3.75
      B  -4.33
      C  -11.23

T10  E  -5.52
       B  -5.42
       C  -15.02

T11  E  fluctuates between approx. -8 and -10
       B  -3.10
       C  -16.77

T12  E  fluctuates between approx. -8 and -10
       B  fluctuates between approx. -8 and -10
       C  -16.78

lamp trans E -1.8
                B  fluctuates between -2.3 and -2.5
                C  fluctuates between -13.3 and -13.7

78L15   O  -16.7
            G   0
            I  -15.02

Where should I start?

Thanks
         

[R.G.]

An NPN transistor can only be operating as a linear amplifier if all of some conditions are met.
1. The base must be about one diode drop (0.5 to 0.7V for silicon) above the emitter. This shows that some base current is flowing.
2. The collector voltage must be higher than either the base or the emitter.
If these are not true, it's not amplifying.

In all of the following, I'm assuming that you either measured with the positive lead on ground and therefore read negative voltages erroneously, or that you used a hyphen in front of the voltages when you typed, and that the votlages on the board are not really negative. That seems to fit the data best.

T1  E  -1.0
      B  -0.85
      C  -2.10

Here's a problem. The base is lower than the emitter. Is the first capacitor shorted? Or installed backwards?
Mr. Simulator says that the voltages for a dead nominal T1 will be
E =  0.96V
B = 1.51V
C = 2.18V
Strange, yes? How could the collector voltage come out right if T1 is not biased?

I'm inclined to think there's a measurement error there. Can you measure again?

T2  E  -1.5
      B  -2.10
      C  -4.34

OK, base is about 0.6V positive with respect to the emitter. Collector to emitter is about 3V, could be amplifying.

T3  E  -3.80
      B  -4.34
      C  -11.03

This one has base higher than emitter by about 0.54V. It's possibly working.

From all that, I'd say that the first three transistors may be working OK, except for that funny business for Q1. Can you measure that again?

T4  E  -4.35
      B  -4.27
      C  -15.02

Unless there's a measurement error, this one's not passing any signal. Do you have a base-emitter short? shorted or backwards cap on the emitter bootstrapping cap?

T6  E  -4.30
      B  -4.23
      C  -15.02

Unless there's a measurement error, this one's not passing any signal. Do you have a base-emitter short? shorted or backwards cap on the emitter?

T8  E  -4.23
      B  -4.28
      C  -15.02

Unless there's a measurement error, this one's not passing any signal. Do you have a base-emitter short? shorted or backwards cap on the emitter?

T10  E  -5.52
       B  -5.42
       C  -15.02

Unless there's a measurement error, this one's not passing any signal. Do you have a base-emitter short? shorted or backwards cap on the emitter?

Seeing the consistent results I'd say to check the orientation of the electro caps which bootstrap the emitters of the phase stages back to the inputs. If those were backwards and conducting, they would hold the first transistor off and might cause this.

Let me know what you find.

[jeff g]

Thanks for the suggestions RG, but I still haven't got it working

I've checked and rechecked parts values, cap orientation, shorts, bridges etc. but can't seem to narrow down the problem. I've also resoldered a few suspect joints

But, I'm right back where I started.  All voltages previously posted are the same (all positive).

Any suggestions on where to go?  Off board wiring?

[R.G.]

I just got in from loading 100 bales of hay into a trailer by myself and I'm a bit tired for any mental work. I'll catch you in the morning when I'm less likely to make mistakes.

[R.G.]

OK, that's better. Coffee is a wonder drug.

Some questions.
- you say "GGG layout". Did you buy a board or etch your own?
- you mention "2N088". I took that as 2N5088. Is that correct?
- did you check the datasheet and ensure that the transistors are turned the right way round?
- is this thing in a box yet? Did you use a power supply jack that connects one side of the power to the metal box?
- Did I suppose correctly that the voltages you list are actually positive voltages, not negative?

[jeff g]

Loading hay, you say - I could probably do with a bit of that to take out some frustrations!

GGG layout - etched my own board, but it turned out really good, with no visible bridges or gaps (and I tested continuity where I thought it might be suspect

Yes, I used 2N5088 trans

I used the layout diagram with 2N5088 pinout - and the diagram showing flat side

It's not boxed yet

I used the Smallbear 3 lug  plastic DC power jack - maybe I need just the 2 lug version? Although I wouldn't have thought powering was a problem since I had lamp power and pot control?

Yes, positive voltages - the first time, I had switched around the leads from my DMM

Next, I'll try the audio probe, assuming I can figure out how it works

[R.G.]

All of the voltage defects I mentioned would lead to no audio coming through, including the one on the first transistor.

Can you do this: measure the voltage directly at the base-emitter junctions of the transistors I mentioned as looking bad? If the measurements were correct, there would be no sound past Q1.

An "audio probe" is just a cord into  your amplifier with a capacitor on the free end of it. You connect ground to the equipment being tested and probe with the capacitor at different places on the circuit to see if you can hear any signal there. It's a go-no-go test for the presence of audio.

[jeff g]

Voltages at base - emitter junctions fluctuate between the following values:

Q4-Q5: 3.75 - 4

Q6-Q7: 4.25 - 4.35

Q8-Q9: 4.30 - 4.35

Q11-Q12: 8.20 - 10.20

[R.G.]

Sorry - I wasn't clear.

Can you measure the voltage between the base and the emitter? Black lead on the emitter and red on the base.

[jeff g]

Yeah, I wondered if I misunderstood.  Here they are:

Q1   .49
Q2   .58
Q3   .61
Q4   .42
Q5   .60
Q6   .40
Q7   .80
Q8   .42
Q9   .60
Q10  .57
Q11  .45
Q12  .62

Q13(lamp)  .68 and feels hot