I have too many questions Re: Rustbucket, ne OG Vintage EHX Attack Decay pedal!

[Carlsoti]

   Hello, all. My name is Tim, and I have a problem. It's a lack of confidence in myself and my abilities that some friends around me can't understand. Because of that, I've been "burdened" with the task of repairing a pile of vintage EHX pedals. Among them are some not so vintage and not so rare items; a BassBalls and MX-3M were in and out with very little fuss. There's a Rhythm-12 I'm waiting on parts for and might get stuck on, once they arrive. Not a lot of readily available info for repair of those beat-boxes.

Then, there are the "Big Three." These are all big-box, 2-prong AC powered units made in the late '70s or early 80's.

   The first I tackled was the PolyChorus, the value and visibility of which has skyrocketed due to it's use by Kurt Cobain and Adrian Belew years before Nirvana. As it sits, I'm waiting on new knobs from EHX for that one. I had to fly-wire two new pots, as I couldn't source the stand-off legged CTS pots to replace the ones that had the D-shaft sheared off. I contacted Ron at Ronsound, but he didn't have any D-shafts available. The original pots work, and will go back to the owner should the proper d-shaft plastics be located.
   Next up was a Bass Micro Synth that didn't need much work. A new "trig" fader was sourced from Small Bear, as the original had the slider-shaft broken off. The gain-trim pot doesn't line up with the hole on the back, and I suspect a previous owner inadvertently turned the squelch trim after opening up the back of the pedal, making the pedal "dead." (I have short videos of these two now-working pedals up on facebook; Later today, I'll take the time to figure out how to do pic and vid uploads that won't burden the server here.)

   That brings me to the topic of this thread, a pedal I had no prior knowledge of, and therefore had put on the back burner while I attended to the others, the gen-u-ine, O.G., vintage EHX Attack Decay pedal! This board is marked as EH2500A, making me believe it's either a test-bed or the earliest rev. The only other pics I've found for a OG PCB is for a EH2500B, and there are some notable differences, specifically in the power section. They weren't made for very long, so I'm not sure what to make of those differences. I have copies of the original schematics, re-drawn schematics, and RustBucket schematics, but I'm stuck because of how rare this pedal is. There's not a lot of direct info that I can find, mostly just references to re-issues and clones. The designer, Howard "Mick" Davis, has been the go-to guy for these for quite some time, but has recently gone incomunicado. I hope he's just busy with other new projects.

   As it sits, direct sound is fine, and un-effected sound comes through, but when I start to crank the blend in, it hums. I presume that's because no one taught it the words.
I haven't measured the frequency, but I'd guess it's 60 or 120Hz. Since this is one of the "holy-grails" of EHX pedal collecting, I'd like to try to keep it as original as possible. Obviously for a collector, an all-original working pedal will have more value than a non-working, pre-bodged pedal. Someone has definitely been in this one before. One of the x-istors was replaced with a MPSA56, and there's a few newer looking e-caps, along with some messed up traces. I'm trying to find the balance between "working" and original, as I have no other options, at this point. So for starters, I'll ask a few general questions:

1. What ICs lend most to the "sound" of this pedal? Or to put it another way, which of the IC's would it be best to source "original" parts for, as opposed to re-pops? For example, Xvive makes MN3007 chips, but Panasonic MN3007s are also available. There are 9 different ICs in this pedal I have the same questions for.

2. Is it worth my time to build the appropriate test-jigs to individually test the ICs? Some of the chips are socketed, some are not. It would make my work easier to put sockets in all the positions, but would that negatively effect the value of this pedal?

3. Should I try to make this as close to the OG schematic as possible? It's currently got 2N5087 and 88 x-istors in it, where the OG schem shows BC309 and BC239s. In a few posts floating around the internet, HMD states that some of the original EHX schematics are not "Correct." Might this just be his way of saying "Send it to me. I'll make it "Right."?"

   I'll take some time to figure out how to link pics/vids properly, as I'm new to the forum. I've taken some voltages, and some are certainly out of whack. I'll figure out how to post that as well, as I know that'll be asked of me. I'll post one pic of the big three, just to let you all know I'm not B.S.-ing you.

Any info or direction you could provide would be greatly appreciated.
Tim C.
Valley of the Sunburn, AZ, USA

[Carlsoti]

One more question, for now. Are pictures or PDFs of schematics preferred on this forum?

Thanks,
Tim C.

[Scruffie]

First things first, post a full set of voltages for every chip, you either need to trace it and correspond the IC numbers to the rust bucket or photo it and annotate the image.

Second, the 2N5088 & 5087 are probably original, they were interchanged with BC239 & 309 @ EHX regularly.

Third, don't assume it's been worked on, EHX did a lot of in house modifications for when particular parts were out of stock or circuit updates/modifications etc. and their soldering could be... less than desirable.

Fourth, pictures suit me best.

If you wish to go the shotgun route, replace all the CD chips in sockets i.e. 4047, 4066 etc. (the MN3007 will almost certainly be fine) as those are likely culprits and have no bearing on the 'sound' of the effect.

Oh and I don't know which attack decay schematic you have, but the main 'original' one on the internet is very wrong.

[Carlsoti]

Any specific knob settings or signal inputs for the voltage readings, Scruffie?

[Scruffie]

Stick all controls CW, no need for an input signal.

[gordo]

I'm getting a comfy chair and getting a ring side seat for this.  I just love to see Scruff in his natural environment.

[Carlsoti]

   Thanks for stopping in, gordo. I've got a bunch of other stuff to work on, as well as trying to maintain a healthy home environment. I won't be able to get back to this until Sunday, at the earliest.
   I'm not exactly forlorn about tomorrow's day-trip to a former mining mountain-town; it's "inside season" in the Valley of the Sunburn and I think cabin-fever is setting in. In one sense, I'm putting off the AD to complete some easier tasks in order to get my head together. I "de-noised" a 90's JoeMeek SC2 earlier today, and I'm deep into shielding and re-wiring a '85/'86 MIJ Jazzmaster that's been autographed by Dick Dale for it's owner. It's been a good day and I'm in a much better head-space than I was at this time yesterday.
   I'm pretty frustrated with myself that I've had to resort to asking for help with this pedal. I feel it warrants it, though. It's not like there are many of these around, so it should be done right. To soothe those interested, here's a few more pics of the AD. Sorry to whomever maintains the server here. I haven't gotten around to learning the ropes as far as linking from hosting sites.

  The first three pics show how the unit arrived to me. Please note, in the "pots lifted" pic, I had switched two x-istors around, as I had inadvertently opened the wrong PDF, but caught it and and reverted them to their original config before re-applying power.

Also, here are FB links to the short videos I made after fixing the PolyChorus and BMS.
https://www.facebook.com/100074733483188/videos/1210476163033816/
https://www.facebook.com/100074733483188/videos/991333494877419/

I hope you all have a great weekend and I look forward to getting back to this repair. I've got some work to do.

Tim C in AZ

[Scruffie]

Well, you were definitely right about it having been worked on before, several new chips there.

I wonder if the same section might have failed again, if it has, an electrolytic/tantalum re-cap will be in order. There's not a lot of point keeping them original, their value has usually drifted horrendously over the years (some EHX I've repaired they've all been at 100% off tolerance) and all that'll happen is they'll eventually fail even if they haven't already so you're giving it a much longer potential life.

[Carlsoti]

Scruffie, I suppose I should re-cap it before bothering with new voltage checks? Haven't quite woken up yet, but my quick check is 8 electrolytics and 4 tantalums. I'm wondering about two others, though. There's a fat, light-brown chiclet-looking cap near the mica at the end of the CD4047 and a thick looking disk cap near the 741 IC. They don't appear to be tants, but sometimes packaging change.  Would it be worth while to upgrade the electros in the power supply to appropriate film caps, given all the extra space on that end of the power supply? Any reason to suspect the mica cap, as well? Any suggestions regarding "upgrades" to parts that must be changed before continuing would be greatly appreciated.

[Scruffie]

Those other caps are ceramic, wouldn't ever suspect a mica cap and any way, easy to test, if it had an issue the 4047 clock wouldn't work.

I'd just leave the power caps as electrolytic personally, another 40 years of service life and keeps it fairly original.

Just replacing the electro and tantalum is all I'd worry about.

[Carlsoti]

   I've pulled all the tantalum and electrolytic caps. At first, I thought my cheap meter's capacitance measuring circuit was dead, as all the 'lytics were reading in the nF range, but after the tants all read much closer to their specs, it seems all but two of the 'lytics are essentially dead or nearly 50% out of tolerance.
   In looking at the axial 3.3uF IC cap and seeing that the different schematics variously list that as a 2.2 or 3.3 uF part, it dawned on me that I should probably clarify which schematic I should be working from. More on this cap later.

   I've got the "original" schematic, hand drawn, with no notation of who drew it when, and known to be incorrect. A schematic drawn by Larry Leon, drawn in Dec of 2011. The Madbean Rustbucket docs dated 2015. Also, I have a layout drawing from Kaputepalla drawn in 2010, which seems to reflect the original schematic. This is only based on a cursory glance which shows a 4558 as the input IC. I've got to lean on the "original" schematic, just a bit, as it seems the closest representation of the power supply layout on this board.

Is it safe to assume that the madbean docs will be used going forward?

After pulling parts from the board, I found 2ea of the 1uF tants, one of which is in a position that I believe is noted as C15 in the madbean docs and C18 on the Leon schematic. I suspect it should be replaced with a 2.2uF tant, correct?

Regarding the axial cap, it appears to be non-polar. Should it be replaced with a 2.2uf or 3.3 non-polar axial electrolytic? I guess the answer will likely depend on the answer of which schematic I should be using.

While waiting for the new parts to arrive, should I pull the xistors and test them with my Atlas DCA55? 

Thanks again.

[Scruffie]

Yup, that sounds like old EHX electrolytics

Replace like for like, don't try and adjust values for the schematics, your unit is as intended. The schematic is purely for reference.

No, testing the resistors would be overkill, some unit component drift is part of its original sound, but a 5% drifted resistor isn't going to kill a unit (unless it's visibly black from burning out) unlike a dead capacitor. Very-very rarely they had a carbon composite resistor which can go brittle, that's about the only time I'd ever consider testing or replacing a resistor but I can't see any there.

[Carlsoti]

My apologies. I meant "should I test the transistors?" If caps were bad and the unit powered on for a while, couldn't that kill some of them? 

Additionally, the 33uf tantalum capacitor that was in here is rated at 6V. It's connected to the -15V rail. On my meter it reads 36.3 uF, so it's not dead. I ended up selecting a 16V cap for that replacement, just to be safe.

[Scruffie]

Transistors are pretty hardy unless they're reverse biased, I don't think any will have been harmed.

Yeah, upping that to 16V was a safe bet  Higher voltage caps would have both been more expensive and physically larger back then, probably why it was as low as could be gotten away with.

[Carlsoti]

Thanks, Scruffie. I ordered parts from Mouser yesterday. Aside from upping the voltage of that one tantalum, the only other changes on the new parts is that the axial non-polar cap will be radial. Supply chain issues, I figure. It'll be a few days before the parts arrive. In the meantime, I'm gonna take a look at the EHX Rhythm-12 and other projects I need to build some momentum on. Thanks again, for your for your help.