SF 1969 Champ, what is causing "chirping" ?

[Shontsy]

Hello All,

I've got a 1969 SF Champ, unmodded so far, that "chirps" like a bird when sitting at idle. It does not chirp very loud, but it IS readily apparent (not like some figment of my imagination).

It does not do it 100% of the time, it is an intermittent problem. Sometimes I turn it on and it's OK, other times it chirps. Does not seem to be any obvious thing that makes it chirp or not.

It sounds like it might be 60Hz. Can anyone tell me what components are suspect and how to go about fixing (if it's not just measure, find the component that's not in spec, and replace)?

Oddly enough, I had a VibroChamp last year that did exactly the same thing, but I sold it before I fixed it. I plan to keep this new one quite awhile, so I want to fix this one.

Thanks in advance,
Shontsy

[Enzo]

60Hz is the sound that occurs when you touch the end of your guitar cord. I would not describe that as chirp, myself.

ANy chance your cell phone is sitting on top of the amp or next to it?

DO any of the controls affect this chirp in any way? Its volume or tone or frequency?

[Dave Curtis, dB AudioTech]

Perhaps by "60 hz" he means "once per second". If that's so, I'd suspect the PS caps. Although 'motorboating' isn't normally associated with a chirp, it is a low frequency oscillation. Recap the power supply after you rule out tubes.

[Shontsy]

Yes, 1 Hz, not 60 Hz - Oops!

Yes, you are right. It's about 1/sec, not 60/sec. My bad. It's definitely not a buzz or motorboating sort of sound.

I just did the 3 prong cord mod. Let me play with it a bit more, and we'll see if it's still doing it. The VibroChamp I used to have had a 3 prong cord on it too, but I don't think I did it quite right. I didn't remove the cap from the fuse to ground, so who knows what else I might have missed on that one.

If I can get it to chirp again, I'll try the controls to see if that changes anything, including tone. That might help narrow down what part of the circuit it's coming from.

Thanks Guys,
Shontsy

[Dave Curtis, dB AudioTech]

Because it's low frequency (1Hz) and the caps are nearly 30 years old, they are highly suspect. Chirping or thumping, IMO it's still called motorboating.

BTW, did you do the 3-wire correctly?
http://www.rru.com/~meo/Guitar/Amps/...Mods/safe.html
Note the neutral goes directly to the transformer and the hot goes through the fuse, then the switch.

[Shontsy]

3 prong plug mod was done right

Yes, the 3 prong conversion was definitely done right, BUT I did it wrong the first time. I had the neutral and hot swapped the first time. As soon as I flipped the switch, the GFI in my socket tripped, but not until after it fried the power surge power strip I had it plugged into. The surge breaker won't reset anymore, Doh! It never did trip the breaker though, so I guess those GFI's really work!

Say what?! I rewired it just the way I unwired it, white to fuse, black to switch, plus green to chassis. Unfortunately it was backwards from Fender. I guess that explains why it was shocking me constantly with the 2 prong plug.

I had never heard the turn the plug around trick. I'm still under the misconception that since it's AC and the current is constantly changing direction anyway, why the heck does polarity matter?! I should learn more about that! This experience finally killed that dangerous notion with me for good.

After the lights went out, I searched on-line and found several posts showing the right way to do the 3 prong conversion, including putting both the switch and the fuse before the transformer, and removing the .047 uF cap from fuse to ground. The amp works fine now, and nothing seems to have been damaged from the mishap, except maybe the power switch but it was already bad anyway.

I have not had much chance to mess with it since I fixed the 3 prong wiring, but I have not heard it chirp since. That's either because the correct 3 prong wiring also fixed the chirp, or because I haven't had it turned on long enough for it to start up again.

Just to confirm, this Champ amp DID chirp with the factory original 2 prong wiring, even before I started messing with it.

The VibroChamp I had before likely did not have the 3 prong conversion done right (because I did it), so the fact that that one chirped too may just mean it was sort of acting like a factory 2 prong wiring due to the conversion not being done correclty. I DO remember the VC chirped both when I first got it AND after I did the 3 prong conversion. I think the chirping was one of the reasons I did the conversion, in attempt to ground it better in hopes of getting rid of the chirp.

BTW, this chirping is not a cell phone. I've heard that before also, and that's not what it does. That sound is easy to pick out because it sort of sounds like a fax machine communication. This chirping is a steady 1 Hz cricket sort of chirp.

I should have more time tonight to mess with it. I'll try to get the chirp to come back. I'm thinking if it does not reappear after some time operating (30 minutes), I might try reattaching the cap from the fuse to ground. Not sure why that would do it, but just want to check that off the list of possibilities.

Thanks for the replies!

[Dave Curtis, dB AudioTech]

Maybe you had an actual cricket in there that got mad when you powered up?

[Rob Mercure]

First of even if you "chirp" only happens once per second it's still has a frequency of it's on or it simply would be a "thud." A single 1 Hz pulse is different from, say, a 400 Hz pulse once per second with exactly 400 cycles during that second and none until the next second so there is still some sort of oscillation that's pulsed.

Secondly if you blew a GFCI with the cap from one side of the mains to ground you're likely to do so again. The GGFI doesn't give a rodent's rectum whether you've got three conductors or only two - what it does it compare the amount of current flowing in the hot (black, narrow plug prong) wire to that flowing in the neutral (white, wide prong) and if they differ more than a certain amount it interrupts the circuit. So obviously the cap is unbalancing that amount by letting some current return via the grounding connection (green, round prong). One analogy I use to describe grounding to non technical folks (I also do house wiring) is that it's sort of like a floor drain in a bathroom. As long as everything is going well water which comes out the tap (hot wire) returns to the earth via the sink drain (neutral) but if something interrupts that then the floor drain (ground) provides a safe path for the water (current). While this unidirectional flow is really more DC it helps folks visualize why there is a grounding scheme in household electrical systems.

So I believe that you understand that the switch and fuse should be in the black coded circuit - don't install the "death cap" unless you want to trip your GFCIs - it's totally unnecessary in a properly grounded system and a potential hazard if one of your, or someone else's, outlets are improperly wired.

Rob

[Dave Curtis, dB AudioTech]

The GGFI doesn't give a rodent's rectum whether you've got three conductors or only two - what it does it compare the amount of current flowing in the hot (black, narrow plug prong) wire to that flowing in the neutral (white, wide prong) and if they differ more than a certain amount it interrupts the circuit.

The 'certain amount' is 5mA, IIRC. I've had leaky death caps trip the bench GFCI (ELCB) on more than one occasion. I think the link I posted earlier mentioned removing the deathcap; not sure.

DC
Master Electrician

[Rob Mercure]

DC,

The comment was directed at shontsy who was contemplating restoring the death cap - as I read the post. And while the 5 mA is a hard standard someone did some testing a while back (yeah, I'm as vague as hell) of GFCIs and found a fairly wide range of actual trip differences - but I really can't recall the citation but remember that it was online. If you know it I'd love to have it.

Rob

[Shontsy]

Fixed is fixed. Thanks for the help!

Well, I tried removing the green wire from the chassis, no chirp. Jumpered the cap back into place (before I read the death cap posts), no chirp.

I figure it's too much trouble to unsolder/resolder everything back the way it was, just to go LOOKING for trouble (the chirp), so screw it. Fixed is fixed.

All I can say is, it WAS doing it, and both Champs I had did the same thing. If it ever comes back, I'll worry about it then.

It must have been some wierd oscillation being caused by improper wiring of the plug. I just think it's very strange that both of my champs were unaltered from Fender when I got them, and both of them chirped, AND no one else has ever heard it before but me. Oh well!

Need to learn more about that Death Cap thing! I'll go searching...

[Enzo]

I had never heard the turn the plug around trick. I'm still under the misconception that since it's AC and the current is constantly changing direction anyway, why the heck does polarity matter?! I should learn more about that! This experience finally killed that dangerous notion with me for good.

AC means the polarity changes 120 times a second, but that has nothing to do with the difference between hot and ground. If the 120v mains peak to about 170v each cycle, does it really matter what polarity it is? You still get a shock.

In the 2-wire amp, the leakage to the chassis will be from one side of the mains or the other. In US 120VAC wiring, one side is neutral - which should be at more or less ground potential - and the other side is hot. By turning the plug, you determine which side of the mains will link to the chassis.

If all the leakage is through the "death cap" then flipping the polarity switch (which decides which side of the mains to connect the chassis to with the cap) or turning the plug will accomplish about the same thing. But from other sources of leakage, one combination of polarity switch and plug orientation might work a little better than the other combination.

[Shontsy]

In my world view of AC, using a DC analogy, the power plant is the battery, and the battery changes polarity at 60 Hz. As far as I know, the Neutral wire goes all the way back to the power plant (like the Neg. wire in a DC circuit goes to the Neg. side of the battery), which means 1/2 the time, the Neutral is basically the Hot wire.

But after what you've said, and thinking about what I've read about panel boxes on the Death Cap threads, maybe the HOT is ALWAYS where the power comes from (even though that current is changing direction), and the neutral and ground are dumping to the ground rod outside my house?! (Since the ground and neutral bus are one in the same in the panel, and they are both also connected to the ground rod outside. That means there IS no return wire back to the power plant, like in my DC analogy?!

If that's not it, where the heck is the power GOING? It's gotta go somewhere to keep flowing, doesn't it? Or NO, BECAUSE it IS changing directions? What completes the circuit back to the power plant? Anything?

I hate AC, it confuses the crap out of me. DC I get. We didn't get into much AC in Mechanical Eng.! And what we did get, I promptly forgot, because at the time it was just information jamming to me.

[Enzo]

Imagine you have a battery and a light bulb. The bulb lights. Nothing is connected to ground. If you wanted, you could connect any one point in the circuit to ground, doesn;t matter where. Ground then is not the same thing as common, though they most often are connected together.

Now let us look at some circuit where one side of the bulb connects to ground - in this case make it the chassis of something. And the other side of the bulb has a wire sticking out for us to make use of. Now take the battery, and touch one end to chassis and the other end to the wire. That is a DC circuit and the bulb lights. Now invert the battery. We now have a DC circuit still and the bulb lights still, but the polarity is reversed. Of we call the chassis ground and the wire hot, we see that hot can be either positive or negative depending on the orientation of th battery.

Now continually invert the battery really really fast. The bulb lights and we now have an AC circuit. We still have the same hot and ground. From ground we see the hot wire alternates from pos to neg and back over and over.

Note that in all these examples, the ground wire is always ground, and the hot wire has the voltage. POlarity is not a determinant of ground or hot.

In powerline distribution, we call what I have referred to as ground "neutral." Ground is a local reference for neutral. The power company sends hot and neutral to you, and you connect neutral to ground at your house. In some absolute sense, the ground at your house and the ground at the power plant might not be the same voltage, but each is connected to the loceal earth ground and bonded to the neutral there. SO if your house and mine are three volts apart ground-wise, then there will be some small current flowing through the earth. Look up "stray voltage" sometime and see what can happen when it gets excessive. Farmers sometimes have a problem with it bothering their cattle.

In the USA we use 240vAC for most house wiring. It is center tapped, and that center tap is neutral. MY above description was for a single 120v power line, which would be a rarity, but it served to explain. Your electrical service then splits the 240 into two 120v sides at the service panel. The neutral center tap is grounded.

In any case, the neutral is at approximate ground and the hot alternates pos to neg to pos etc. Neutral does not take turns being hot, it takes turns being positive and negative... with respect to the hot wire.

Let's go back to my original single 120v distribution for its simplicity. It starts at the pole outside your house. The transformer there steps the high voltage from the power plant down to 120v for your house. At the transformer, the two wires have 120VAC between them, and when one swings pos, the other swings neg. They do this dance endlessly. At this point, there is no neutral or ground. Like my very first circuit, you could ground either one - I'm sorry, you can make either one neutral. This is exaclty like a winding of a transformer in an amp, either end of a winding could be grounded.

But you could think of that transformer on the pole as if it were the generating plant itself I guess. Your neutral wire extends to the nearest step down pol,e transformer, not all teh way to the plant, but it could reach to the plant if we were not concerned with efficiency.

Sorry for rambling. Your neutral wire completes the circuit, and yes it contiues all the way back to the current source, be it transformer on the pole outside or the plant itself across town. But though the neutral takes turns being positive then negative, it is always at ground potential or close. It goes pos or neg WITH RESPECT TO the hot, not with respect to...well, ground. It remains grounded.

I hope I have now nade this abundantly murky.

[Shontsy]

Interpretation of AC

So basically, the two 120 sides are like high potential, and the Neutral is the lower (near zero) potential. The electricity always has to flow from the higher potential to the lower, regardless of the polarity of that higher potential?

That's why it is possible to wire the amp backwards, because if the high potential is connected to the chassis that is supposed to be neutral and ground, the potential starts running the wrong direction is some parts of the circuits, and zapping you since chassis is hot and you can be ground?

Not to metion you're shorting right to ground via the green wire grounding the chassis if the black (high potential / hot) is wired to where the neutral should be?

And of course, the green wire is making sure there is no potential difference between white (neutral) and actual ground in your local vicinity, which keeps you from getting zapped due to stray potential difference between ground and neutral?

Am I missing anything? Thanks for the help!