Buick (I mean) Randall Century 170 II LOW output and no drive.

[Helmholtz]

The input signal goes to the first jfet of the normal channel as well as the input jfet of the gain channel.
From your symptoms either one of those fets might be shorted.
What fet type?

[Indyryder02]

Yeah, I'm sorry.
What is the voltage at S Q1 ?

about 10mv

[Indyryder02]

The input signal goes to the first jfet of the normal channel as well as the input jfet of the gain channel.
From your symptoms either one of those fets might be shorted.
What fet type?

2N5484 N CH.

I actually did suspect Q1 and Q5 as being bad earlier, due to some odd diode tests I got from them.
When I went to pull them, it looked as those two of the pins were tied together underneath the board.. (which I’ve seen in some cases and figured was normal and the cause of my strange diode test results, strange by showing the same voltage both ways. But not emitting a beep sound as when I normally find a shorted diode / resistor. This has fooled me before only to pull the components and have them be fine.. but I just looked again and I was wrong. It was the D pins tied to the resistors underneath. So now I’m gonna pull those two and test them out of the board. Hopefully that’s it!

or it’s the parts in circuit fooling me. I hate that! Haha

[x-pro]

about 10mv

No. Constant voltage?

[Indyryder02]

No. Constant voltage?

No. it appears that Q1 and Q5 are both faulty.
see last reply above.

[x-pro]

Field J-fet can be tested without special instrumentation only for short circuits

[Indyryder02]

Pulled Q1 and Q5 due to their strange diode tests in circuit, unlike all the rest. My transistor tester, shows them as faulty. I tested one of the others to be sure. And it shows it as an N ch JFet. So looks like they are indeed failed.

So apparently the diode tests don’t always have a prolonged “beep” sound for a shorted transistor.
is that because it’s a JFet maybe? It shows roughly the same voltage both ways between pins. Like a short. But only a single beep. Not a steady beep for a short. Usually when I’ve ran into that, it was the part being tested in circuit fooling me.. and once removed the parts tested good. I hate getting tricked like that!
when I first looked under the board to remove them.. it looked as though two pins were tied. (Which I’ve seen before, and looked like a short of course) so I thought it was the reason for odd test. But I looked again and it’s the D pins that were tied to the resistors next to them.

So likely. The source of the problem!

I was halfway on the right path then! I just got tricked and looked at the bottom of the board wrong. I was even looking for replacements in my stash before creating this thread as I suspected them.

You guys are really amazing at being able to diagnose this stuff with just the schematic and some readings here and there. I’m always very impressed!!

My next question, besides why my diode tests does a steady beep for some shorter transistors and just a single for others…. Is what could have caused them to both fail in the first place? As I don’t want it to happen to the replacements!!

Thank you both so much!

[x-pro]

By their nature, field-effect transistors are very sensitive to large signals. A random disturbance induced in the cable is enough. For example, during a thunderstorm, from a powerful radio transmitter, a hungry neutrino flying through the Earth and biting off a J-fet crystal.

[Indyryder02]

By their nature, field-effect transistors are very sensitive to large signals. A random disturbance induced in the cable is enough. For example, during a thunderstorm, from a powerful radio transmitter, a hungry neutrino flying through the Earth and biting off a J-fet crystal.

Ha! Well makes sense with them being at the very beginning.

Getting some replacements coming. Hopefully that will be the end of its issues!

Thanks again! I really appreciate the guidance!
Without experienced techs like yourselves sharing information and know how. It’d be a lost art for sure!
I keep trying to learn as much as I can. Which will never end! And incidents like this always teach me a lot.

THANK YOU!

[x-pro]

There in the schematic, Q3 is redundant. You can put it in place of Q1

[Helmholtz]

Q3 is redundant. You can put it in place of Q1

How this?
And how do you know, what is Q1,Q3, Q5?

[x-pro]

The schematic says all j-fet 2N5484.
Look at the inclusion of Q3. Why is it there?
Really, going by the diagram from the first post, it's not superfluous, that's what it means to falsely represent schematics. Although you can do without it.

[x-pro]

that's where it's redundant: https://music-electronics-forum.com/...tch?id=1004701

[g1]

There in the schematic, Q3 is redundant. You can put it in place of Q1

Which schematic ? The schematics in post #1 and post #13 are different. #13 has been modified.
I'm not sure if either one is exactly correct for the Century II .

[Helmholtz]

that's where it's redundant: https://music-electronics-forum.com/...tch?id=1004701

Oh well..
I don't think this last schematic is correct.
A Jfet typically doesn't really completely open (infinite resistance) nor does it completely short (zero resistance).
So for best attenuation typically there is a series Jfet followed by a shunt Jfet.
Even more important, the second (shunt) fet shorts the input of the following stage and thus prevents noise.