Burns Orbit Three - Ticking

[Gahoo]

I've got a Burns Orbit Three amp needing service on my bench. Don't usually do too much solid-state work, especially not of this era.

Schematic:

The whole amp is poorly handwired and is nearly impossible to navigate.

Anyway, I received it because the reverb wasn't working; couple bad solder joints. Now however, there is this audible ticking, about 6Hz according to my scope. For the life of me I can't determine where it's coming from. Could a bad capacitor start oscillating like this?

Then I started thinking maybe it was the LFO from the Vibrato bleeding into the signal path. Not sure that's the case since adjusting the rate of the vibrato doesn't change the rate of the tick.

I know there are some users on here who have experience with Burns amps and this model in particular so any advice would be appreciated!

[Enzo]

Two ideas occur:
one is to try adjusting the speed of the tremolo/vibrato. Does that change the speed of your ticking? Oh wait, you did that, and it is not the issue.

The other is to first make sure the noise is coming from the amp. A lot of complaints about ticking in amps come from having set the cell phone on or next to the amp. Is there a cell phone within a couple feet? There are other noises in teh electrical environment. Rotate the amp 90 degrees on the bench, does this affect the amplitude of the ticking? ie if the unit is directional, then it may be picking it up from somewhere.

And if it IS coming from inside the amp, try to isolate the problem. The amp appears to have tone controls and volume controls. Do ANY of those controls affect the ticking in ANY way? The tone of it or the volume of it?

Scope the power supplies, is the ticking riding that, or is it only along the signal path. And along the signal path, is the noise signal present everywhere or only in some stages?

[Jazz P Bass]

See if you can isolate the tick to one supply.
It sounds like a decoupling issue.

I see, from right to left, a 49V, a 33V, a 12V & a 24V supply.

You may find that jumpering a suitable value/ voltage cap across existing caps will help to narrow it down.


Burns%20Orbit%20Three%203.pdf

[Gahoo]

The other is to first make sure the noise is coming from the amp. A lot of complaints about ticking in amps come from having set the cell phone on or next to the amp. Is there a cell phone within a couple feet? There are other noises in teh electrical environment. Rotate the amp 90 degrees on the bench, does this affect the amplitude of the ticking? ie if the unit is directional, then it may be picking it up from somewhere.

Unfortunately I ruled most of that out. Was ticking when I played it in the cabinet and then afterwards, on the bench in a separate room. And I don't wear a watch

And if it IS coming from inside the amp, try to isolate the problem. The amp appears to have tone controls and volume controls. Do ANY of those controls affect the ticking in ANY way? The tone of it or the volume of it?

The only knob that effects the ticking is the treble knob; cutting the highs with the knob attenuates the volume of the ticking.

Scope the power supplies, is the ticking riding that, or is it only along the signal path. And along the signal path, is the noise signal present everywhere or only in some stages?

See if you can isolate the tick to one supply.
It sounds like a decoupling issue.

Good call checking the power supply, I found the ticking right after R72, on the 24V line.


Getting close to isolating the problem?

Thanks for the responses!

[Jazz P Bass]

C44.

Jumper a good one across it. (in parallel)

[Gahoo]

Got her up and running! Turns out I had wired the negative side of C41 (in the reverb section) to the other side of R64 (the 33V line). They're right next to each other on this terminal board. Feelin' a little foolish for not realizing that earlier...

Thank you both for the responses and guidance!

[Enzo]

It happens. I had a customer who built a commercial kit amp, and just could not make it work. he spent hours looking at the circuit and the board. he sent it to me. I found that on his eyelet board, up in one corner he had swapped position between a resistor and a cap. So one triode had a cap in place of its 100k plate resistor, and a resistor from that plate to the next stage. All the parts were correct, just one pair out of position. it didn't LOOK wrong until you carefully looked part by part.

[Gahoo]

Alright I'm back. Put the chassis back in the cabinet, fired it up, and noticed a deep, bass hum. When I was working on the amp out of the cab I had it hooked up to just a little speaker which doesn't produce the hum (thinking it was too small to reproduce a frequency that low). Unfortunately this means I don't know when the humming started since I didn't notice it until after it was all assembled.

I do know however;

The hum isn't effected by any knob on the amp.
It hums when vibrato/reverb are both on or off.
It starts as soon as the amp is turned on.
Guitar can still be played through the amp, the effects/controls all work etc.

It looks like this.

Is that 60 cycle? My scope wouldn't reliably read the frequency.

Also thought it might be bad filter capacitors so I probed the 49, 33, and 24V lines. They look good, don't have that noise on them.

Anything I might be missing? I sincerely appreciate the advice I've been getting, thanks fellas!

[Dave H]

Is that 60 cycle? My scope wouldn't reliably read the frequency.

It looks to be just over 8mS between peaks so it's 120 cycle (120Hz).

[Enzo]

Here is a very simple test for 60Hz versus 120Hz.

Touch your finger to the scope probe tip. You will get an ugly waveform of 60Hz. Set the sweep so you get one complete cycle across the screen. Now leave the sweep speed alone, adjust the vertical as needed and probe the hum in your circuit. If it also has about one complete cycle across the screen, the hum s 60Hz. if you now have about two complete cycles across the screen, it is 120Hz. That takes longer to describe than to do. I never worry about milliseconds.

[Gahoo]

Ahhh good call. That's a neat trick! Using your body as an antenna. Meanwhile I'm thinking of ways to generate a 60Hz and 120Hz tone from my cellphone, scope them, save the waveforms, compare the waveforms etc..... Always overcomplicating things

I also realized that on the 33V line I AM getting that noise. Think it's that 1000uF filter cap? Am I being naive to assume that it's a filter cap causing this hum? Might be wishful thinking because I reaaaally don't want to try and source some old odd-ball transistor.


EDIT: I'm also getting the hum right at the rectifier. This is typical right? My thinking being that the rectifier is the genesis of the 120Hz signal to begin with...

I guess my question now is should I suspect the 2000uF cap right after the rectifier? Or the 1000uF cap after that?

[Gahoo]

In going through the power supply, checking the capacitors, I noticed the two zener diodes are reading as shorted. I can't find a datasheet for either of those part numbers. Are those being to regulate the voltage?

I don't have any zeners on hand, could I swap in 1N4001s in the meantime or is that too significantly different?

Thanks in advance!

[g1]

What part numbers? On the schematic or on the diodes themselves, please list.
For regulation, zeners are usually reverse biased, so they are drawn upside down. Check if that is a drawing error or if they are installed as drawn.

[Gahoo]

The schematic lists what looks to be Z818 (hard to tell) and Z815(?) as the two diodes. On the diodes themselves, the part numbers are RS 8ZX SIC 13 and RS 8ZX SIC 20. Pulled out of circuit, both read shorted.

They were installed correctly, I think they are reverse biased.

[Jazz P Bass]

In going through the power supply, checking the capacitors, I noticed the two zener diodes are reading as shorted. I can't find a datasheet for either of those part numbers. Are those being to regulate the voltage?

I don't have any zeners on hand, could I swap in 1N4001s in the meantime or is that too significantly different?

Thanks in advance!

Where are these zener diodes on the schematic?

And NO, you cannot replace them with a silicone diode. (rectifier)

You may want to look up 'zener diode'.
They work in the reverse breakdown region (exactly what you do not want a rectifier to do.)
They have a voltage rating that they breakdown at & a wattage rating.

This is the schematic symbol for a zener diode.