Vox Berkeley II

[loudthud]

BFC is Big Effin Capacitor.

OK, here's where I'm at. I replaced all the electrolytics on the PCB and the three 500uF on terminal strips. The 1uF NP (it lives on the bass and treble controls) was replaced with a film cap. I actually found a 4uF non-polar electrolytic cap to replace C19. At first I didn't replace the main filter cap. The amp still had a small problem. When the Volume is up past about 5, low notes cause a brief farting sound. I connected a big filter cap across the exsisting one with no change.

Q6 was replaced before I got the amp. That stage looked like it wasn't operating in the right current range with only about 1 Volt at it's emitter so I swapped Q11 into that position. Before I soldered that transistor in place, I measured the beta and it was about 300 at 1mA. The 2N4400 that was in the Q6 position only had a beta of about 200. I replaced Q11 with a 2N4301 which had a beta of about 300. I replaced the main filter cap with a 4700uF 50V mounted on a small chunk of FR4. Now the amp is starting to sound right. I still need to revise the AC wiring to add a grounded cord and nix the death cap. Finally hooked up the reverb tank and there is an oscillation. Dang!

[lowell]

OK! So BFC'ing the base of Q3 kills it! If I touch before the coupling cap feeding Q3 it does NOT kill it. So it's in the Q3 stage. Am I correct in guessing that Re would be the most likely culprit for the hiss? My train of thought here stems from tubes, cathode resistors can cause this type of noise.

[R.G.]

What's a BFC?

Yeah, loudthud has the correct interpretation. Big sucker, compared to what is in the circuit at that point. Intended to dramatically shunt all signal and all noise/hiss to ground at the point it's clipped in. The idea is like turning down the volume control to eliminate hiss from before the volume control, but with the ability to apply it anywhere. It being a cap, it does not upset DC conditions and should in most cases only kill signal and hiss where you clip it. So any noise comes from later on in the signal chain.

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.

AHAH! Noise is before the driver transistor. Opening a coupling cap is even more effective than shunting signal to ground with a BFC - it's just a bigger PITA. And on the Beatle, I was really worried about wire breakage, so I didn't want to flex the board up and down to open a cap. That's a much smaller problem with the Cambridge/Berkeley.

Boy oh boy am I wasting hours and hours trying to find this hiss.

There was a full-page ad in the real estate section of the local newspaper back in the early 80s - when there was a FURIOUS real estate boom on here and before the internet killed newspapers. It was a picture of a rail-thin lady in evening clothes, with pearls no less, in her late 50s or early 60s, sitting at a grand piano, with a big fancy house and a to-the-horizon view out the floor-to-ceiling windows behind her. The caption read "Hours spent in the casual pursuit of your interests are part of the good life". I still chuckle at the hubris.

But more to the point, hours spent in the frenzied pursuit of a quieter amp are part of the good life.

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.

Yep, different pinouts. 2N5088s do work in those circuits. It's probably a pinout issue. Or a broken wire.

OK! So BFC'ing the base of Q3 kills it! If I touch before the coupling cap feeding Q3 it does NOT kill it. So it's in the Q3 stage. Am I correct in guessing that Re would be the most likely culprit for the hiss? My train of thought here stems from tubes, cathode resistors can cause this type of noise.

Getting closer! Q3 and Q4 are a nice, tight feedback pair driving the reverb tank The signal tapped off the emitter of Q3 drives the cold side of the reverb blend, and the reverb return after Q5 drives the hot side of the reverb blend. It's definitely associated with the Q3/Q4 pair.

Just to check, with Q3's base shunted, does moving the reverb control from cold to hot change the "quiet"?

Hmmm. One other thought comes to mind. Try this test: pull out the reverb tank cables and short the RCA connectors. Does it hiss? This does two things. It shorts the input to Q5, forcing the wet side of the blend to have a quiet input, so noise on the wet side is Q5 noise, not reverb-ed noise from the reverb front end, and it shorts the output of the driver transformer, forcing the primary of the transformer on Q4's collector to be an AC short (kind of...) which mucks with the feedback pair's ability to oscillate or pick up hiss from a leaky transformer. This makes the sound at the dry side of the blend pot be Q3 emitter noise. Might be instructive.

It could actually be any one of the resistors, caps or transistors there. It's about here that my desire to be doing something else would overwhelm my desire to know exactly what's going on, and I'd notice that there are only - what? five resistors, three caps, and two transistors? I'm pretty sure I had a shotgun somewhere here. In lieu of more hours, what I would do is replace parts one by one in order of my suspicion that they're the problem, and chalk the time up to a laboratory lesson in circuit theory when I found the one part that fixed it. I'd say that R17, R16, and R15 are possibilities, in that order, as well as Q3. But the other parts are possibilities.

[R.G.]

BFC is Big Effin Capacitor.

I still need to revise the AC wiring to add a grounded cord and nix the death cap.

Be sure that when you do that, the fuse and power switch both go in the line side of the AC line, neither one in the neutral side. The old un-polarized plugs usually put the fuse on one side, the switch on the other. That's explicitly wrong where you know which side is hot and which is neutral.

Finally hooked up the reverb tank and there is an oscillation. Dang!

DANG.

[lowell]

Fixed the no-signal issue. I had the transistors in correctly. I had misplaced a Rc on Q2. Now back to replacing resistors.... The good life.

[lowell]

Welp, all surrounding components for Q3 and Q4 have been replaced. Still the hiss. So must be Q4! Reverb driver. I don't see anything listed on your Geofex transistor document RG. What would you use for this?

[lowell]

Guess I didn't read far enough down on the document. You say to use Toshiba 2SC4793 or 2SC5171.

[R.G.]

Actually, I've been using 2SC2073 with good results. Well, KSC2073 now that Toshiba has EOL'd the 2SC part. They're $0.65 or so at Mouser.

Mouser has 2500 of the Toshiba 2SC4793 for $0.87, but they're leftovers. Both the 4793 and 5171 are EOL'd too.

[lowell]

Cool I ordered the 4793 yesterday. Thanks

[lowell]

Ok, well I thought I have a big improvement here after replacing Q4 reverb driver but now the frying egg is creeping back up again as the amp warms up. Dang. Just gonna double check that it is still coming from there. It's possible this noise could be from multiple components so lets poke around....more.

[lowell]

ehhhhhh! Repalcing Q6 helped a TON! Still a tiny bit of sizzle but mostly gone! Gonna replace the surrounding resistors/caps as well. Amp is sounding great. There is also a good amount of hum as well so I'll see if adding extra filtering helps with that.

[Enzo]

If the reverb driver is noisy, then the noise coming out the speaker will be reverberated. If it is not, then it is after the reverb or not related to it.

[lowell]

Good point. I've had the reverb down mostly the whole time. BUT, there is interplay between Q3/4 cause of the feedback resistor R17. So I think Q4 was causing some noise, granted far less than Q6.

[lowell]

I tried redoing the grounding with alligator clips. No change. Ended up reassembling the thing and all the hum went away. Guess the front panel of the chassis needed to be screwed on to maximize noise rejection. I had had it clipped to ground while working on it but that was allowing it to hum. Amp is VERY quiet now. Thanks for all the help. Glad I stuck with it. Now, on to charging 20% of what one should be paid for so many hrs. Part of the job sometimes.

[R.G.]

Congrats!!

The B2 is a very nice amp when it's all repaired and running like it's supposed to.