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Vintage threads from the first ten years
Re: Ultimate DIY debugger award
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 5/25/1999 10:55 AM |
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gfr |
Re: Ultimate DIY debugger award
Maybe these diodes help to avoid the "go-up-go-down" effect (the output going positive for a portion of the negative peak) when the input is very high (by clipping the input so that it can't be very high).
Would that nodes be the 220R resistor (to set the gain to one), the .0082 cap and 220K (to disable tone control) and the volume knob (so that it stops attenuating the output)? I hope this thing is not sold as "true-bypass" 
Can you give some details on these values? Did you find where the tone switch came from (or is it something "new")? | |||
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| 5/25/1999 11:50 AM |
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R.G. |
That's what I thought. Keeps the differential mode range from being exceeded.
Yes, except the drive pot node is lifted, not the 220R being broken - same thing, parts in series. Makes it into a follower with output impedance. There's a DPDT in there, but one half is used for an LED. No Millenium in there yet...
They're trivial, 0.047 for 0.05, that kind of thing.
I'm pretty sure it's in the Boss Tone; I'll do some more checking - I haven't been in a position to look at schemos. | ||||
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| 5/25/1999 11:57 AM |
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| aron |
>However, as to why you can't bias up your breadboard version, as GFR notes, the bottom zener is just a refernce voltage, so your 2.7V zeners should give you 2.7 and 5.4V at their cathodes OK, I know I have gotten these voltages before. I'm sure of that. >it is possible that some mishap has damaged the IC you're working with. I didn't consider this. In fact, I've been using the same IC over and over on all 3 circuits. >There is a pair of back to back diodes across the + and - inputs of the opamp But this is missing on the PCB right? Are the diodes there? OK, I'm going to be bummed if this whole thing was caused by a bad chip but I will check it out. I guess it sounds plausible. I could always get the transistor to kind of bias correctly voltage-wise but not the op amp. Pin 2 would frequently be too high etc... I never did get pins 2,3 and 6 to be close. Aron |
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| 5/25/1999 12:27 PM |
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| R.G. |
Having the +, - and output pins of an opamp not all be within a few millivolts is six out of the seven deadly signs of opamp death... |
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| 5/25/1999 12:48 PM |
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| GFR |
Well, not necessarily. You may be using it as a comparator (not the case of this circuit). The most obvious "deadly sign of opamp death" is that smell when you connect it to a power supply with the wrong polarity |
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| 5/25/1999 9:04 PM |
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| R.G. |
I did say six out of the seven *<
... and the smell is truly bad... | |
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| 5/25/1999 11:10 PM |
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| aron |
Re: Ultimate DIY debugger award/No Go GFR, R.G., The chip is not bad. I went out and got 2 ECG 857M (TL071/TL081) replacements. When the transistor is not in the circuit, I can get all the correct voltages 2.7v at the diode junctions, 5.4v at the top diode. I can play the circuit and it distorts fine. The pins on the IC without transistor: pin 2: 2.7 pin 3: 2.5.5 pin 6: 2.7 pin 5: ~165mv As soon as the transistor (2N3906) is inserted: The base is 5.4v the collector jumps to 6.2v and the emitter jumps to 6.2v Also pin 2 jumps to 6.2v and of course pins 5 & 6. pin 3 remains 2.5.5v. I tried at least 10 transistors, 4 different IC's. This circuit doesn't bias. Can someone tell me what's wrong? Thanks, Aron |
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