ARP Solina String Ensemble - low frequency popping on output

[tired eyes]

Hi there,

I'm an indpendent tech and had this ARP Solina come in with a tough issue. Before I start pulling my hair out I thought I'd hit up this board to see if any other techs out there have dealt with this before.

There is a popping noise on the output whenever the modulation is engaged. It is quiet, but well above the noise floor of the unit. To be precise, it is about 40mV peak to peak. The noise floor is about 10mV with modulation on, 5mV without. I attached a picture of the scope of the output, set to 100mV scale, 20ms/div sweep. What you see there is one of the pops captured. Also atttached the schematic.

The modulation works by using 3 LFO signals, 120 degrees apart in phase, to modulate the clock of its 3 BBD delay sections fed by the same audio signal, and combining the outputs. The LFO signals themselves consist of a 1hz sine wave (the tremolo oscillator) superimposed on a 20hz sine wave. The popping noise is cyclic and it follows the tremolo oscillator, making 5 to 7 pops per second, or per oscillation. These circuits, referred to as the Control Circuit and Modulator Circuit, are on page 10 of the schematic.

Troubleshooting so far, I have tried disconnecting the audio out of each of the 3 modulator boards to inspect the output of each. It appears that each of their outputs is carrying the noise. When scoped or connected to the output independently, each modulator board produces 2 or 3 pops per second. I then tried disconnecting the LFO signal from the modulators. This eliminated the noise (and the modulation) and again, with the LFO connected to either one of the boards a component of the noise returned. This leads me to believe that the issue is in the tremolo oscillator or associated low pass filter, rather than the BBD circuits.

The LFO's are generated by square wave oscillators smoothed by RC networks, so I imagine some transient components are leaking through at the rise and fall of the square wave, pushing their way through the low pass filter and the BBD output, through to the audio.

As for solutions, nothing jumps out at me as an obvious suspect. I thought of replacing the electrolytics on the control board (cheap) and replacing all the 741 opamps with lower noise chips with identical pinout. Anything that may lower the noise floor I would not consider a waste of time, and I'm considering replacing a lot of these 741s. This customer is a professional who makes world renowned records, and he likes his signal to noise ratios.

I'm also thinking to check all the values of resistors and caps in the smoothing network on the LFO's, and if everything tests good maybe messing with the values a little.

I'm hoping someone else might have one of these boards or experience working on one, and can tell me at least that tThanhis issue is not inherent in the design and is actually a problem, and maybe some advice of where to look. In the meantime, at least I can say I did not underquote the project!


Thanks in advance for any insight or assistance.

[Dr Kaos]

I would certainly start with the caps on the supply rails i.e. 55, 53 & 56.
Changing the 741s might not help much the noisy part will most likely be the BBD.

Kaos

[Enzo]

I have not studied the schematic yet, but I'd highly suggest solving this problem first, and THEN upgrading your op amps and whatnot. I recall learning that lesson on some Marshall solid state head a long time ago. Had a bad op amp, MC1458, I replaced it with the "more modern and better" (at the time) 4558. The amp was unstable. Eventually I called someone. They told me to get some 1458s and replace the old IC with one of those. SOmething about the 4558 had too much gain and the circuit was barely marginal even with the 1458s.

SO with some odd little glitch, changing a bunch of op amps would not be a good idea.

You scoped the audio out to show your glitch, but you only described the LFO I think. You said disconnecting the LFO stopped the popping, but then if the popping is on the LFO driven circuit, that would be expected. You imagined maybe some transient component of the waveform might... Well, don;t imagine, find out. scope the LFO circuits, anything funny on them? You mentioned wveforms were square than shaped. A spurious double pulse or something could indeed cause trouble, but ought to be visible on a scope.

I suggest not throwing parts at it. It may need new caps, and it may just love newer op amps, but while those upgrades may be great, and you certainly should scope any filter caps and decoupling caps on the power supply, your problem could also be some circuit needing a trim. If your glitch is riding a power supply rail, it will be hard for the circuits to ignore it.

[Enzo]

I see the three phased outputs from the 741s, one runs up to each modulator. Scope each, I'd expect them to all look the same. And verify their power supplies. I could be wrong, but it looks like the 15v rails for those 741s are made on the little board? Voltage divided from 25v rails?

Those LFOs go to the three modulators, where they appear to trigger flip flops made of a couple transistors. And those in turn clock the TCA350s. I did advise against wholesale cap swapping, but the LFO does feed through a cap into the flip flops, so scope each modulator LFO input on both sides of each ones cap. Scope the flip flop outs by looking at pins 3 and 5 on the 350 ICs. DO all three have similar looking waveforms? Down by the LFOs are those voltage dividers I mentioned. There is also a -3.5v divider off the -15 one. That -3.5 serves the 350 chips somehow, so make sure it is good and present at all three chips.

This is a single sided circuit, so the signal path is not at zero DC, it looks to be up to maybe 10v? But off that board, it looks to me like there are blocking caps. SO look to see if a leaky cap there is calusing DC level troubles.

There are three modulators, and their outputs all merge with their 10-4 pins wired together. But you can scope the output of each modulator father back, perhaps at cap 42, post A on the board edge. Look to see if one of the modulators is creating the problem or all three.


And lastly, this is a great forum, lot of smart people here. But this is a bit out of our main stream, other than a few of us. YOu might also post this at a vintage synth forum. No I don;t have a link to any specific one. But if you do, please let us know what you find out.

[timpotent]

Tired eyes, did you solve this issue ?
I have the same popping ...