diy univibe no sound

[plen]

i changed Q2 transistor and a 220k resistor at the bottom of the board. the transistor now reads.   base .638 collector 3.75 emitter 1.515  the resistor was a 220k one. it was reading 47k in circuit, the new one is readung 72 in circuit. there still is no sound. i read all the resistors on the left side of the board, they are all reading the same as in my sons working clone univibe.

[Marcos - Munky]

You shoudn't measure resistors in a circuit, because they interact with other parts and you'll get the wrong reading.

Check the resistors for their color codes.

I can't clearly see from the photos you posted, but the 3,3K resistor should be orange-orange-red. I can clearly see orange-orange, but can't see the other color.

Also, what exactly did you removed from the board?

[plen]

the colour of the 3.3k resistor is orange orange red. it reads 2.9k in circuit. i am checking my resistor readings with my sons working clone univibe. all the resistors are the right value.

[R.G.]

Marcos is giving you solid advice. You cannot reliably read resistances in circuit. The parallel parts affect the reading. He's also correct - the first three transistors form a preamp and phase splitter. Yours is hosed.

Your power supply is giving you about 15.5VDC for power to the preamp and phase stages. The way the preamp works is that Q3 has to turn out equal-and-opposite signals at collector and emitter. It does this with approximately equal collector and emitter resistances. They are set up so that ideally, the collector will be sitting at 3/4 of the power supply, and the emitter will be at 1/4 of the power supply. This is approximate, but I would expect about 4V on the emitter and ~11V on the collector.

With 4V on the emitter of Q3, the emitter of Q1 is held at 1.2/(1.2+3.9)= 0.235 times the 4V, or about 1V. The base of Q1 then has to be 0.5V or so above that. So the actual base of Q1 has to be at about 1.4-1.5V. It's using very, very little base current, which it gets through that 1.2M resistor to the emitter of Q2.

The voltage drop across that resistor is unknown without knowing the hfe of Q1, but since Q1 can't conduct more than 15V/1.3M = 12uA, the base current can't be much more than 1/100 of that, or about 120nA. That, through is about 0.14V.

So Q2's emitter needs to sit at 1.4 + 0.14 = 1.54 to 1.64V. This, through the 6800 ohm emitter resistor, is 227uA roughly. That much current has to flow through the 47K collector resistor. That's 10.665V, which is down from the 15V power supply or 4.33V.

We're nearly back to where we started - with 4V on the Q3 emitter. We have come back to the base of Q3 being at 4.33V. That's 0.33V above Q3's emitter. It really ought to be 0.5V; however, its >>really, really<< close, and it's close enough to be auto-corrected by the circuit itself. So the estimations of circuit voltages seem to be reasonable. Actual circuits do seem to work this way (by measurement). The voltage on the base of Q1 is hard to measure accurately because the impedances are so high there that a meter will load it down and it will measure low. But the other voltages on Q1, Q2, and Q3 do in fact measure this way.

This long wandering ramble through how it works is just to say - yep, something is very wrong in the preamp. Since there are only three transistors there and a few capacitors, it might make sense to just clip out all three transistors, remove the electro caps; then check the resistor values >visually<, replace any wrong values, and then put in new transistors and caps, carefully watching polarity.

[plen]

RG i have changed Q1 and Q2  i will change Q3. I was taking resistor readings from another working univibe to see if mine were the same. if they have to come out i will take them out.

[plen]

RG  the last reading i gave for Q3 was collector 11.05  base 3.77 emitter 3.51 is that within spec.

[R.G.]

Well, there isn't any specification for this thing, but that's certainly better than the first ones.

So what are your voltages now?

[plen]

Q1 base .009 collector 1.327 emitter 14.40  Q2 base 2.108 collector 3.73 emitter 1.523

[Marcos - Munky]

Measure Q1 again. You can't have 14V at the emitter. Emitter voltage should be lower than collector voltage.

Think about this: if you check the univibe schematic (or any npn transistor circuit), you have the collector connected to the power supply voltage via a resistor, while the emitter is connected to ground via another resistor. So the collector is more close to the maximum voltage, while the emitter is more close of the zero voltage. So a working circuit will have the collector voltage higher than the emitter voltage.

[plen]

you are right it is 14.40 on the collector.

[R.G.]

OK. Q1 has 14+ volts on the collector.  The schematic shows Q2 base wired directly to the collector of Q1. How can they have different voltages?

[antonis]

Verify Q1 outline package type..
It might be TO-39(A) with case tab pointing Collector (instead of Emitter..)
(although its size seems to be TO-18 but you never know..)

[Marcos - Munky]

Also, what exactly did you removed from the board?

[plen]

antonis it is a bc 109 it has it written on the side. i also checked the pins with a peak dca55 transistor meter. the tab is emitter middle leg is base and third leg is collector.

[plen]

marcos - munky  when i built it about 6 years ago i left it till a couple of months ago. when i opened it there were parts missing. i must of used them for other pedals. i replaced the parts which were the diodes and a 10uf capacitor. i cant remember if there were anymore. last weekend i took all the capacitors and transisters out one by one and tested them with an atlas esr meter and a peak dca55 transistor meter.

[antonis]

antonis it is a bc 109 it has it written on the side. i also checked the pins with a peak dca55 transistor meter. the tab is emitter middle leg is base and third leg is collector.

OK but then you have to explain what R.G. asked you:

Q1 has 14+ volts on the collector.  The schematic shows Q2 base wired directly to the collector of Q1. How can they have different voltages?

[plen]

antonis i cant check that at the moment as i have taken the three transistors and capacitors out of the board to read them. i dont know if anyone wants to check any of them.

[plen]

after taking the capacitors and transistors out to read them and read the the resistors there was a 47k resistor next to the ldrs cover which was reading 4.5 i took it out and put it back in, then it read 47k. after soldering everything back in and trying the univibe i now get sound. the effect is very weak.

[R.G.]

OK, so it's better. From your description, I think you corrected a bad solder joint.
In any case, what are the transistor voltages now?
I'm harping on this because it's a deadly effective way to find problems.

[plen]

RG my power supply is not working. i am using a different 19v power supply. i dont know if it will give the wrong readings.  Q1 base 1.299 collector 2.093 emitter 920  Q2 base 2.097 collector 3.72 emmiter 1.503   Q3 base 3.72 collector 11.14 emmiter 3.102  i took them out of circuit and read them with a peak dca55. i dont know if the readings are any good for you.