EH vintage blackfinger

[lowell]

hey all,
got a blackfinger here w/ 748ic. it's actually the metal can type opamp in the particular pedal i have. replaced that w/ an 1171. I'm not familiar w/ compressor circuits, or at least this one. I can see what's going on in a dynacomp or ross, but not this black finger. If anyone could shed some light on the signal flow here and how this pedal works that'd be great. The pedal is not working... but i've diode tested all transistors they are fine. The opamp has no signal on the output pin, but if I disconnect the wire from Q6 to the 27k resistor the opamp works.

[52 Bill]

The opamp has no signal on the output pin, but if I disconnect the wire from Q6 to the 27k resistor the opamp works.

Check the number of the transistor Q2 is that an FET? In any case, the signal is probably being squashed at Q2. The voltage from Q6 is supposed to be in proportion to the signal coming out of the opamp.

[lowell]

No it's not an FET, just an npn transistor, i replaced it with a new one and it's ok. A few other transistors were bad and I replaced them. Q9 I just tested and was bad, but have not another to replace with. It is a 2N5087. I put a 2n4122 in there and it doesn't work so maybe not a suitable replacement?

[Mark Hammer]

The circuit would seem to be a "high-falutin' " version of the Orange Squeezer...or at least apply some of the same basic principles.

Q2 and the 100k resistor at its collector form a kind of virtual attenuator. With no signal input, Q2 presents a high-impedance path to ground. As the input signal is increased, voltage is applied to the base of Q2, reducing its resistance. As that value descends below, say, around 1M, you start to get some attenuation of the input signal that is fed to the LM748. In pedals like the Dynacomp and CS-2/CS-3, increases in signal level result in the gain being reduced. Here the gain of the op-amp is fixed, but the level of what it amplifies is turned down. The net result is essentially the same.

Where the Blackfinger improves beyond the Orange Squeezer is that it provides variable sensitivity, an improved rectifier/envelope-detector, and a tone control for adjusting the amount of treble in the output signal.

[lowell]

Thanks Mark,
Would be able to explain what's happening around Q8 Q9? Is that a phase inverter? Also, Q6 Q7 look similar to a differential pair... right? Finally Q4 Q5... what's goin' on there?

[Mark Hammer]

There is, unfortunately, too much discrete transistor stuff going on there for this tiny brain. However, the near symmetry of the Q8/Q9 pair suggests to me it is used to provide full wave rectification. Looking at the schematic for what I gather is a different issue of the Black Finger (using a 3080 OTA in the feedback loop of a 741 op-amp) there is a similar configuration that performs that role.

[lowell]

so I recently got this thing working... only it seems a bit off still. The "sustain" or "compression" effect is there, and the thing is definitely boosting the signal. I have replaced almost every xistor and tested ones that I did not replace, also tested and replaced and bad resistors. I also swapped out the "sustain" pot w/ no difference. The only problem to me is that I need to set the "sustain" knob at about 75% for it to work well, any further up on the knob and it dies away. Another symptom is that the pedal seems to effect the noise floor more than the signal. anyone gather anything from this?

[lowell]

correction... the pedal sounds great! except that the sustain knob needs to be at 7 or 8 and above for it to work. FYI the further up the sustain knob, the less of that 100k resistance is there. In fact I measured it and if the resistance of the sustain knob exceeds 6k weird things happen: The pedal because somewhat of an antenna with crazy high pitch "radio noise", I even heard some talking from a radio channel at one point! Does this point anyone in the right direction as to what's wrong here?