Vox Berkeley II

[lowell]

Cool. Geofex is down? RED ALERT!!

[R.G.]

OK, they fixed it. Geofex is back up. Something about fungus in the application pool, bit rot, or isothermal sysocrachy.

[lowell]

Glitch in the matrix

[lowell]

Ok so I got all the electrolytics replaced in this thing. There is still a good amount of noise. It sounds like johnson noise, not really hum or static but just airiness. It's definitely loud enough to be annoying when the amp isn't played. I BELIEVE I've found the source but not totally sure. With my signal probe I'm only getting it post Q7 - on the primary of the driver transformer, but not on the base of Q7. I replaced R30 emitter resistor for Q7 and the noise remains. I'm guessing that it's the germanium Q7. Thoughts? If it needs replacing, what device shall I use?

[R.G.]

Q7 isn't germanium, it's silicon. It can be replaced with a 2SC2073. That's what I use in the Cambridge/Berk II replacement boards and it seems to work pretty well. A small bolt-on channel heat sink or metal tab will be all the heat sinking needed. Different package, so you have to bend the leads to match the PCB holes to get it right.

It is possible that Q7 has gone noisy, as it's under a fair amount of both voltage and thermal stress, but it's odd that a transistor that late in the amp would cause noise problems. Generally the biggest noise contributors are the input transistors on the guitar and reverb input jacks.

It is possible that the hiss is actually RF noise being heterodyned back down to audio by the output stage oscillating. Non-fatal and non-all-consuming RF oscillation will sometimes cause this. Takes an o'scope to tell what's going on in that case.

[lowell]

I have a scope. What am I looking for?

[R.G.]

Continuous oscillation above the audio band. I'd monitor the collector of Q7 directly. I've seen these things scream at about 100kHz at several volts before.

[lowell]

Will do thanks!

[lowell]

Looks like 2ms (120hz?) at around 20pmv p-p. Not sure I'm seeing the hiss or oscillations. I put the scope on 10us (100k) and just see a fuzzy line. Thoughts?

[lowell]

Here's the noise

[R.G.]

Looks like 2ms (120hz?) at around 20pmv p-p. Not sure I'm seeing the hiss or oscillations. I put the scope on 10us (100k) and just see a fuzzy line. Thoughts?

Another good theory all shot to blazes. Nope, that's good old audio hiss, not heterodyned ultrasonic or RF.

If it were me...

I would put new 2N5088's in for the input transistor at the input jacks and at the reverb return and listen to it before replacing Q7. They're pretty sure not to make it noisier, and they might even help.

After that, you might want to look at Q7.

It's just that Q7 is an unlikely (but possible...) source for annoying noise.

[loudthud]

Does the amp go silent when the Volume control is all the way down? Reverb control have any effect?

[lowell]

Another good theory all shot to blazes. Nope, that's good old audio hiss, not heterodyned ultrasonic or RF.

If it were me...

I would put new 2N5088's in for the input transistor at the input jacks and at the reverb return and listen to it before replacing Q7. They're pretty sure not to make it noisier, and they might even help.

After that, you might want to look at Q7.

It's just that Q7 is an unlikely (but possible...) source for annoying noise.

Ok I'll do it... any guesses on what WOULD be the source of the hiss? Only thing I can think to do is start replacing more carbon comp resistors. We'll see what happens after these transistors though.

Does the amp go silent when the Volume control is all the way down? Reverb control have any effect?

This noise is there regardless of those controls. There is additional, different sounding noise, when those controls are increased. But it's of no concern as it's minimal.

[R.G.]

Ok I'll do it... any guesses on what WOULD be the source of the hiss? Only thing I can think to do is start replacing more carbon comp resistors. We'll see what happens after these transistors though.
...
This noise is there regardless of those controls. There is additional, different sounding noise, when those controls are increased. But it's of no concern as it's minimal.

Hmmm. That's a slightly different colored kettle of fish. The additional, different sounding noise when the controls are increased is the noise that would be fixed with new input transistors. Don't change those resistors yet. It won't hurt, but it won't fix what you're trying to run down now.

I once debugged the noise in a hissy Beatle down to one resistor with a BFC on clip leads. I connected the negative lead to ground and the positive lead to various transistor bases until I found the one transistor that made the noise get really, really small. That got me into the section of the circuit (tremolo in that case) and then to the individual resistor. The cap shunts signal and noise and all to ground and forces quiet. When the noise changes a lot, it's shunting a lot of noise.

You might try this on the bases of Q3, Q6 and Q7. 100uF to 470uF would be good. Place the cap with the power off, power on, opine about noise, then power off before (re)moving the cap. No signal will get through with this in place, so don't feed it a test signal.

[lowell]

What's a BFC? I already replaced both input transistors and the transistor before the verb driver. I did however disconnect the 4uf cap feeding the driver transistor and the noise went away. Boy oh boy am I wasting hours and hours trying to find this hiss. Also, theres no signal after replacing the transistors. I think the 2n5088s are EBC and the TV ones are ECB? Gotta find out what I screwed up there. Not worried about that...just need to find this dang hiss. So at this point it doesn't seem as thought the driver is the problem. Again whats a BFC? Bipolar filter cap?