Measuring voltages in PNP circuit (negative ground again)

[zjokka]

hi all,

i cooked up a basic fuzz face pnp negative ground board yesterday morning and have been testing different transistors I had lying around. initially i used two AC128s with gains 90 and 135. this sounded pretty good, no oscillations or RF problems. I did put in the 220 pF capacitors between the emitters as suggested.

I'm trying to tweak the circuit by playing with the 100k biasing resistor, which works just fine.

But I don't seem to be able to check the voltages on the circuit. If I power up, keep the black probe to ground and connect the red probe to the transistor pins I only get readings of approximately 8.7 V anywhere. There must be something I'm doing wrong.

I've read all the post of people not being able to get decent results from their PNP negative ground circuits, and all suggestions to use postive ground circuit.

Mine's working all right, I just want to tweak. Is there a different way of checking the voltages.

PS: I also seemt o have burnt a perfectly fine AC128. When I was testing the voltages in the way described above, I noticed that Q1 had gotten very very hot. Whereas it had about 90 Hfe before, it now measures 400 Hfe. Is this because of wrong way of testing?

thanks for reading this
Johan

[petemoore]

  WIerd, that's about battery voltage.
  Somehow, a whole lot of current is going through if a Q fried, 8.7's on all pins is not a good sign, Q1's emitter is to be grounded. I would figure something out because if it's now at 8.7v and then you ground it you might see sparks.
   That board...hmmm...maybe go through and get familiar with the DMM's resistor resistance checker, then test for shorts between the transistor pins.

[zjokka]

  WIerd, that's about battery voltage.

hi pete, it IS battery voltage. Am using a multiple 9V supply (Dinosaur) that, I am sad to see only delivers about that.

  Somehow, a whole lot of current is going through if a Q fried, 8.7's on all pins is not a good sign, Q1's emitter is to be grounded. I would figure something out because if it's now at 8.7v and then you ground it you might see sparks.

I simply don't understand how the circuit worked perfectly for an half an hour. Then I started checking the voltages and I noticed the that Q1 was scalding hot. Like it was something I did with the multimeter. I haven't put the ground probe in the wrong socket of my DMM? After it got hot, I didn't try it again, I measured Hfe of over 400.

That board...hmmm...maybe go through and get familiar with the DMM's resistor resistance checker, then test for shorts between the transistor pins.

I would have done all that -- but I'm not troubleshooting. The circuit's working. I feel I'm doing something wrong with the measuring still. Will check for shorts later today.

thanks!
zj

[petemoore]

  Check GEO for bias info.
  the transistors can't be amplifying unless bias voltage is seen on their pins.
  I dis-recommend trying another transistor in the position the last one got hot, you can check the diode junction of the transistor that was a heating element with the DMM's --->l Diode checker, look for the B/E diode.
  ....Connect B/E to DMM, reverse and repeat, readings of good Ge transistor will look like readings of Ge diode, reading the foreward threshold one way, nothing when inverted.

[zjokka]

thanks for your reply, pete, but..

I've all the GEOFEX info printed out on my table, circuit plugged in and fuzzing away in every way imaginable. Still I'm reading 8.9V   or so on all transistor pins. The transistors must be good, else I wouldn't have any gain at all right. I hold the black probe to ground point (connected to battery ground and ground on jacks) and the red probe on a transistor leg.

I could bias this circuit by ear, it's on breadboard and a resistor is easily swapped. I had no trouble measuring voltages on my NPN Silicon Fuzz project. I even tried a different DMM.

This is driving me crazy. but stil 

zj