Help with Pearl compressor from 80's

[nsbluesman]

Hi All:

http://www.geocities.com/SunsetStrip...rl_co04scm.jpg

I have a Pearl co-04 compressor from the eighties in great shape, except the attack control doesn't seem to work, it sounds like it's on full all the time. I have the schematic and was hoping one of you could offer some insight as to where to start. I have checked the value on the pot that adjusts the attack and it's fine. the pcb looks like it's never been touched. Any thoughts? the schematic is linked above.

Thanks

Colin

[Mark Hammer]

First off, while I don't wish to underestimate your knowledge, have you actually used the pedal when it was presumably "working" properly, and do you know what the Attack control is supposed to do? I ask because a great many folks expect wonders of the attack pot (which technically isn't one) and it is one of the subtler types of control out there whose audibility depends considerably on your picking style.

Though there are a couple of differences between the sidechain in the Pearl pedal and in the Keeley/Dynacomp/Ross, CS-2 and Nobels, the Attack control works essentially the same way. The 10uf cap labelled C9 provides some smoothing of the envelope coming via D3/D4. Technically, R23 is the "Attack" control since it determines how quickly the cap will charge up. The resistance formed by VR1 and R24 determines how quickly the cap will discharge, by providing a variable path to ground. Hence, although is says Attack on the chassis and in the schematic, it is actually a decay/recovery control.

Keep in mind that the compressor functions by turning down the gain on IC2 in response to peaks. How much it turns down the gain is a function of the envelope signal amplitude that comes via D3/D4 and gets stored in C9, but how long it gets turned down for depends on how long it takes for C9 to discharge. If the resistance of VR1 and R24 is low enough, then C9 discharges quickly and the gain is only turned down momentarily, like a peak limiter. If C9 is only allowed to discharge slowly, then gain will creep up more slowly, providing more of a sustain function since slow gain recovery provides more constant volume over a longer interval. In the other compressors mentioned, the total resistance range is a 10k fixed resistor in series with a 150k pot, with 10k representing the fastest recovery time, and 160k the slowest. Here, also with a 10uf cap to charge up and drain off, the resistance range goes from a min of 33k to a max of 133k, which is a smaller overall range of control compared to the other pedals.

What is the audible consequence of varying the Attack control? When notes are spaced farther apart, the cap has sufficient time to drain off (though in the case of the other pedals noted, it is a cquestion of the cap charging up rather thandraining off, but it is still a question of time to accomplish the mission), restoring gain fully. When the cap is not allowed to drain off fully, the initial peaks of any notes played immediately after the first note are not heard because, like a turtle, the CO-4 has pulled its head into its shell and is still coming out. If the Attack is set to fastest recovery time, then initial transients are heard on notes that arrive in rapid succession, but each initial peak is quickly constrained.

The bottom line is that settings of the "Attack" control on ANY similar stompbox compressor will matter most when players have a tendency to strum/pick hard and quick. If one has a leisurely blues style of playing where notes are spaced and held, variations in Attack will be largely inaudible unless compression is at full tilt and the compressor is fed with a hot signal. If one is a fast country picker, then variations in Attack will be more audible since notes N+1, N+2, etc from your Tele bridge pickup will lack the initial bite while the gain is temporarily suppressed by the first note you played.

Make sense?

A more audible "Attack" control on the Pearl unit might involve replacing R24 with a 10k-22k resistor, and replacing the 100k pot with either a custom 150k from a place like Robert Keeley, or with a more commonly available 250k unit in parallel with a 330k-390k fixed resistor. That would provide both faster and slower recovery times than a stock Pearl unit.

Of course, it is always possible that variations of the Attack control are not audible because the pot itself is dirty or some contact has fractured. Never hurts to clean the pots or verify their continuity.

[nsbluesman]

Hey Mark,

Thanks for the crash course on compressors , it's great to get a response like that.

Actually, in re-reading my original post I should have said it sounds as though it is on full all the time, If I pick a single note it sounds like someone playing a bass by popping the strings?If I strum a chord hard it distorts and for lack of a better term sounds "blatty". I made the assumption it was the attack control, since it didn't seem to do anything sound wise... As far as experience with pedals, i have modded a few overdrive type pedals, and replaced some components in others, BUT I am much more comfortable working on amps. So back to the original issue, I guess I am looking for some hints on where to start. I am comparing this compressor to the only other one I used which was an Ibanez potato bug one years ago. Other than that I only have the one on my DG Stomp. I found the Pearl in a pawn shop and couldn't say no. The level knob , sustain knob and the tone knob act as I would expect them to. I also hooked a mm up to the attack pot and turned it through its cycle, seemed OK.

Any help you could offer is greatly appreciated.

Thanks

Colin

[cjlectronics]

Hi Colin,

Have you checked the +9v filter cap and the zener voltage of 5.1v? I try to not use a battery and use a power supply when troubleshooting. This way I can tell if something is loading the power.


I am currently working on a Pearl OC-07 Octaver. My symptoms are slightly different but I'm interested in the outcome... could you please keep me posted?

CJ

[nsbluesman]

Haven't checked, I will when I get home tonight. They're interesting old pedals. I will let you know what I find.

Colin

[nsbluesman]

Voltages

Still nowhere, voltages at the zener diode were the same as at the 9v filter cap, not sure if that should be, I am a little lost with pedals, I much prefer tube amps and those little cramped circuit boards drive me nuts.

It appears as though I don't have any control over the effect, almost as though it's on full bore all the time. Any ideas or suggestions would be greatly appreciated.

Colin

[Mark Hammer]

Compressors are always on full-tilt in the absence of any signal from the sidechain. That's one reason why you need to feed them with the cleanest possible signal you can, since they will only amplify residual hiss on your input signal if left to their own devices. Feed it an audio signal and it then *reduces* gain from that maximum, depending on how hot that input signal is. Hotter signals result in greater envelope-signal amplitude at the junction of D3/D4, which is then subtracted from the default gain.

What you describe suggests the "problem" lies somewhere between C7 and the Sustain pot. The first place to look for issues is at the wiper of the trimpot. That ought to show an AC voltage that corresponds to picking strength and input signal level, probably somewhere in the neighbourhood of under 1volt AC. The op-amp immediately after it has a gain of x213, so the AC voltage you read at pin 1 of that op-amp should be much higher.

Check first to see if what is arriving at the D3/D4 junction corresponds to picking strength.

[nsbluesman]

Thanks Mark,

I will check it tonight, so the trimpot you are referring to is labled as avr-1 on the schematic with one side going to C7, right? and the junction of d3/d4 would be right before R23? So if I am seeing a low AC voltage on the wiper that fluctuates when playing I should see a higher voltage at the junction of d3/d4 with corresponding fluctuations?

Colin

[Mark Hammer]

Yep.