Question about output of pin 6 on IC6

[rube the noob]

Hi guys!

I am working my way through the Hammerhead, and I am a bit stuck. Carl's instructions state that I should be getting -1.6v to 1.6v on pin 6 of IC6, variable on the position of R18. I am getting 4.36v with no effect from R18. Sadly, I really don't know what I am doing yet, so it is hard to troubleshoot, but here are some suspicions.

I am getting -.7v on pin 2. Is that wrong?

Does R5 need to be installed?

I used 22ohm resistors in place of 27's called for in R20 and R21. Could that be a problem?

The potentiometer for R18 is a trimpot. Is that ok?

This was my first circuit board, both etching and soldering, so there are a million ways I could have goofed something up. Does anyone have other pin values on Q1-Q3 so I can see if I fried them? I got good values for pins 7 and 4, by the way.


Any ideas? I will post a picture...

Thanks!!

[rube the noob]

A couple other things:

For R3 I have a 20k pot, where a 10k is called for. I was out, but have some ordered. I figured if I kept it in half it's range I should be ok.

Also I am using a 25v capacitor for C27 and 16v for C14 and C15. That shouldn't make a difference, should it?

Thanks

[Reg]

Hi Rube,

I will try to help by answering some of your questions.

.7V on pin 2 is not correct. You need to find out why. Make sure you have the two 1N4148 diodes wired correctly. It is possible one isn't connected properly.

R 20 and R 21 can be any value from 1 ohm to 27 ohm and not make that much difference. It is a good idea to make sure they are the same value though. So, your 22 ohm resistance is fine.

Using a trim pot for R 18 is fine. That is the most logical setup.

I would not think R 5 would have to be installed to get a basic adjustment. If it is not installed, then the FET should be turned off. If the FET is good, then there should be no voltage from the pulse circuitry. However, it is a good idea to be pulsing to get the best dynamic adjustment once all is working fine.

The reason for R 18 is to balance the output out of the preamp and adjust for any small offset voltage. This can be off a little and still work, but not off as far as yours.

The 4+ voltage on pin 6 would indicate the opamp is in saturation and being driven there. Make sure you do have both power supply voltages at the proper pins on the preamp. By this I mean you have a +5V on pin 7 and a -5 on pin 4. Double check that pin 3 is tied to the board common. Once again, double check the two diodes connected to pin 2 and make sure that both are connected properly. If R 5 is not in, then you should have 0V at TP 2. Any voltage there will cause a problem. If this voltage is greater than the -.7V, then the FET is conducting and that shouldn't be happening if R 5 is out. If the FET is conducting, then check the voltage at the gate of the FET. This should be a -12V if R 5 is out. If it is a -12V, then it is possible your FET is bad. (This is assuming you are using a 12V power supply).

If it is not a -12V at the gate of the FET, then you need to find out why. Double check to make sure you have Q2 and /orQ1 mounted correctly and that the pc board is ok. With R 5 out, you should have -12V on the collector of Q 1. This will cause a -12V on the base of Q2 which turns that transistor off and this will mean there should be a -12V on the gate of the FET. With a -12 on the gate of the FET, the FET will be turned off and there should be 0V at the collector of the FET. Now, if you have a coil connected, the resistance should be very low and the voltage should be very close to 0V even with a -.7V on pin 2. If you disconnect the coil, then the voltage at the FET might increase a little but still should be less than .7V. For testing, if you have R 5 out, then you could short the two coil leads together so there is 0V at the collector of the FET and see what happens. If the coil leads are shorted together, this will eliminate the FET and associated circuitry as the problem If you do short the leads, make sure that nothing gets hot. If you have a bad transistor or one in incorrectly, then something could get warm very quickly.

Now, if you do short the coil leads together and you still have 4.36 volts on the output of the preamp (pin 6), then your problem is around the preamp somewhere. You will just have to double check everything and make sure there are no solder bridges, no bad or improper traces, and components are installed correctly.

The different voltage ratings of the capacitors will not make a difference. Make sure they are installed properly and the polarity is correct.

Hopefully, the above info will get you on the right track.

Reg

[rube the noob]

hmmmm. I am supposed to be getting -12v at the gate of the FET and I am not...

(I sense this is going to be a really dumb question)

I don't have anything connected to gate (pin 1) of the FET. On the pc board layout, there is a rectangle which is not marked, which encloses the gate of the FET. Am I supposed to have something here?

I guess pin 1 should be hooked to something, lol. Then again, maybe not. I will attempt to educate myself on Mosfet's...

[rube the noob]

OK, I think I see now. I need to choose between two options and put a jumper in depending on which way I decide to go. grrrr. I gotta learn to RTFM

Thanks a LOT for your help!!!

[Reg]

Hi Rube,

You need to look at the schematic and also the pc board layout. There should be a connection to the gate of the FET and that connection should go to the collector of Q 2 and to R 9. Now, on the later design, these connections are done with two jumpers. So, it appears that you may have not installed the necessary jumpers needed. One jumper connects the collector of Q 2 to R 9 and the other jumper connects that tie point to the gate of the FET.

Reg

[rube the noob]

Thanks Reg,

All is good on Pin 6. There is an option for "passive pulldown" and "active pulldown", and I hadn't made the proper jumper connection to choose between the two. I decided on the passive route, so I pulled and shorted R7 and R8, and jumpered to the emitter of Q3.

Thanks again

Ruben