Rocktave Motorboating

Jaicen_solo · July 14, 2007, 03:14:33 PM

I just finished up building my Rocktave from GGG, but surprise surprise, it doesn't work. I get motorboating which changes when any of the Fuzz, -1 or -2 knobs are turned up. Any ideas??

oskar · July 14, 2007, 04:45:12 PM

http://ampage.org/hammer/files/rocktave.pdf

It sounds like the compander vca (ic2b) is pumping...
Do you get a good clean signal vith the three fx volumes all the way down?
What does the clean signal from the divider sound like?

D1 polarity?
R23 solder joints?

oskar

Jaicen_solo · July 14, 2007, 05:02:53 PM

I get a good strong clean sound, so the input and output op-amp stages are working.
I've got D1 in a socket, so I can experiment with different diodes. R23 is good as far as i'm able to tell.
I'm pretty sure the problem is somehow related to the compander, because nothing that goes through it works properly. The NE570 I used was a NOS, still in its tube from way back. Should I look at getting another, cos they're expensive! The problem wouldn't be caused by wiring would it? I used multi-core cable for the pots.

oskar · July 14, 2007, 05:25:25 PM

No, the IC is most likely OK.
It is the compander.
What does the output from IC2A sound like?
C20 value?

Jaicen_solo · July 15, 2007, 01:48:25 AM

I've not had time to probe it yet but there's definitely some kind of output from IC2b, because the tone control affects the sound of the motorboating. Where's C20??

oskar · July 15, 2007, 05:00:19 AM

Quote from: Jaicen_solo on July 15, 2007, 01:48:25 AM
I've not had time to probe it yet but there's definitely some kind of output from IC2b, because the tone control affects the sound of the motorboating. Where's C20??

The one from IC2 leg 1 to ground. You can't miss it it's marked... C10...

Jaicen_solo · July 15, 2007, 05:21:22 AM

Quote from: oskar on July 15, 2007, 05:00:19 AM
Quote from: Jaicen_solo on July 15, 2007, 01:48:25 AM
I've not had time to probe it yet but there's definitely some kind of output from IC2b, because the tone control affects the sound of the motorboating. Where's C20??
The one from IC2 leg 1 to ground. You can't miss it it's marked... C10...

Heheh!
I thought that's what you meant. I've got a sneaky suspicion it may be in backwards, but i'll have to check when I get home.

Jaicen_solo · July 16, 2007, 06:07:14 AM

Ok, i've been over my board and as far as I can tell, nothing is in the wrong way around, and all the parts values are correct.
What would cause the NE570 to be pumping?

Paul Perry (Frostwave) · July 16, 2007, 07:02:09 AM

When you say "motorboating" if you mean a kind of repetitive thumping, it could be ultrasonic feedback from havign the wires bundled together.
**NOTE this is entirely theoretical - I havn't built one of these - but, I have had cases where ultrasonic oscillation via input-output wire coupling gave a LFO frequency swooshing pattern in similar gear.

oskar · July 16, 2007, 07:03:33 AM

I'ld try lifting the 4013 out of the socket and leave it out till the rest works. It is easy to do
and the clean and fuzz parts works without it.
IC2A is an inverting OP-stage. The other functions are disabled (pin 2,3 to ground ).
I think one of the capacitors are misbehaving, but it could be a faulty component value.
Other than that... you have some meassures to do... voltages on the caps around IC2B
and the transistors.

oskar · July 16, 2007, 07:20:40 AM

I think C11, the one on pin14/ IC2B, is bad...
What is the voltage on C11?

oskar · July 16, 2007, 07:28:03 AM

Quote from: Paul Perry (Frostwave) on July 16, 2007, 07:02:09 AM
When you say "motorboating" if you mean a kind of repetitive thumping, it could be ultrasonic feedback from havign the wires bundled together.
**NOTE this is entirely theoretical - I havn't built one of these - but, I have had cases where ultrasonic oscillation via input-output wire coupling gave a LFO frequency swooshing pattern in similar gear.

Would the frequency then be dependent on the levels and which of the signals are mixed out?

Jaicen_solo · July 16, 2007, 09:00:31 AM

I think the problem is likely related to the fact that I used mulit-core wire. The Rocktave is being built into a guitar, so to get it all to fit, I used a length of 9-conductor wire, with a socket on the end, connected to another pair of 4-conductor wires that are soldered to the pots.
As an experiment, I tried unpluggin all but the two fuzz level wires, and also removed Q2. For a while, this just gives the clean buffered signal, but then that would fade out and a nasty spluterring fuzz can be heard, at about half the volume. It's got a nasty choppy almost trem sound to it, and only works on the lower strings. I noticesd that if I touch the bare ends of any of the other wires, I get my clean buffer signal back for a minute or two, before it all repeats again!
So, what's going on? That sounds like behaviour indicative of a backwards cap somewhere, but as far as I can tell, they're all correct. Can anyone confirm to me which direction the polar caps should be oriented in on the layout?

oskar · July 16, 2007, 11:29:28 AM

But is it shielded in any way?

Do you have something like aluminum foil around the cavity where the pcb is stuck?

I have seen many good machines work with nonshielded multicable but surely they were
all in a metal box of some kind.

Unfortunately I would only be like a parrot if I gave you advice on shielding!

Jaicen_solo · July 16, 2007, 08:53:08 PM

The multicores are shielded, but only around the outside of the cable, there's no individual shield for each conductor.
I tried taking out the 4013 and 4017, but it was still doing that nasty trem thing, so I don't know what is going on really! I might try re-wiring it anyway, just to see if it is the problem.

Jaicen_solo · July 30, 2007, 01:13:10 PM

Ok, so i've not had much joy with this thing.
Would it be worthwhile to disconnect all but say, the fuzz volume, and see if the NE570 is ok with that.
The thing is, I think the NE570 is NOS, but I can't be sure it's working fine. If this works, how would I go about stopping the motorboating?
Could I for instance add a 22pF cap to ground at the connectors of each wire?? That would be much easier than re-wiring!

oskar · July 30, 2007, 01:23:49 PM

Quote from: Jaicen_solo on July 30, 2007, 01:13:10 PM
Could I for instance add a 22pF cap to ground at the connectors of each wire?? That would be much easier than re-wiring!

Try it!

Quote from: Jaicen_solo on July 30, 2007, 01:13:10 PM
Would it be worthwhile to disconnect all but say, the fuzz volume, and see if the NE570 is ok with that.

Yes, when I have stuff that don't work I strip it completely. Actually usually build things in steps and don't go ahead until I get that part working.

Quote from: Jaicen_solo on July 30, 2007, 01:13:10 PM
The thing is, I think the NE570 is NOS, but I can't be sure it's working fine. If this works, how would I go about stopping the motorboating?

Well, I don't think any of the chips are bad...

Quote from: Jaicen_solo on July 16, 2007, 08:53:08 PM
The multicores are shielded, but only around the outside of the cable, there's no individual shield for each conductor.
I tried taking out the 4013 and 4017, but it was still doing that nasty trem thing, so I don't know what is going on really! I might try re-wiring it anyway, just to see if it is the problem.

Is the shield properly grounded?

The thing is still in a guitar and it isn't really in a proper enclosure (metal/ grounded)?

Little caps on pots sound fine to me...

oskar · July 30, 2007, 01:33:02 PM

Ok, something about the ground!
On the pcb the ground is made like a closed circle around the board.
I think my CAD teacher mentioned something about not making your traces like this. It's like the opposite to stargrounding.
Perhaps if you cut the ground somewhere on the board, it would make it less sensitive (if that's what it is), but add the cap's first...

oskar

Jaicen_solo · December 04, 2009, 04:50:01 PM

Hey, me again guys! I'm still not having any joy whatsoever with this thing. I've completely re-wired the thing as per a normal stompbox, and swapped out all of the IC's.
Unfortunately it's still making a horrible thumping sound. I can get the octave down sound (-1 and -2), but only on the low E string.
When I hook it up to my (new!

) oscilloscope with a tone generator, it all scopes out just fine. I'm totall tearing out my hair with this one. Does anybody have any further suggestions?
I'm sure it's got something to do with the NE570, but I get the same result with all three of the fresh chips. I've checked all the connections to the 570, so i'm all out of ideas!
I'm using TL074 and TL072's by the way, not sure that makes a difference?
EDIT: I forgot to mention, the thumping stops when I pull Q2 out, not sure if that's important.

jimbeaux · December 05, 2009, 12:02:29 PM

I'll try to help although my troubleshooting skills may not be that good.

I built the Rocktave from the PAIA kit & added most of the mod's (including the extra divisions) as mentioned in the DIY Projects for Guitarists book - and it worked first time I plugged it in (this was a first for me considering it is a complex build).

I'm a little curious - you mention using a TL074 & a TL072 - the schematic only shows 4 op-amps in an LM4136 PDIP package.

The TL074 + TL072 would give you 6 op-amps total.

You may know this already - but I just thought I'd point out that although the TL074 & LM4136 do have 4 op-amps each (and are for the most part electronically compatible) - they're not pin-for-pin compatible.

Don't know if this helps?

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Rocktave Motorboating