EDEN WT-800 Xover Mode Problem, Green corrosion on IC leads

[nevetslab]

One of my steady clients brought another of their Eden WT-800 amps. Later generation model, having Neutrik NL4 output connectors, 3-pairs of outputs, chassis mounted IC 15V regulators. Complaint was fuzzy sound in HF horn. Amp wasn't set for Bi-AMP mode though. I powered it up, with just the LF/Right Ch woofer connected. Cracking the Master Volume, I heard the HF Sweep EQ band control was oscillating, with the Freq Sweep control setting the pitch of the oscillation. Amp was in Normal 2-ch mode. I powered down, disconnected cables/AC Mains. Already had the top cover removed.

I removed the FP control knobs, unplugged the harness connectors from the preamp board including the cables from the Preamp motherboard to the Sweep EQ board. Then, removed both boards for close inspection, changing to my surgical loupes and headlight.

First, I found Lead-Free solder in use, and the right-angle 4mm spaced -.062" square headers on the Sweep Frequency pots board had a lot of solder joint fractures on the headers, so using solder wick braid, de-soldered the pins and re-soldered with 60/40 Kester solder, now having clean connections. Looked at the bottom of the main PCB's connections, seeing I had previously had to restore circuit connections in correcting earlier solder joint problems. Touched up some solder joints. Then turned the board over, and my eye caught green liquid on the JRC 072D Bi-Fet Op amps plugged into tin-plated IC sockets. What's the green stuff? I haven't seen that sort of green liquid except from some cable insulation, that seeping out of the copper wire. I unplugged six of the op amps, and applied Caig Contact cleaner, brushed onto the IC sockets. On the IC leads, there was a build-up of corrosion on both sides of the leads from the base where they exit the IC body. I wire-brushed the outsides, as well as the insides, then after cleaning them with denatured alcohol, I took my X-Acto knife with #11 blade and scraped away the oxidation from both sides of the leads, brushed it off, cleaned it again with alcohol, then brushed on Caig DeOxit, and plugged them back into the 'treated' IC sockets. I've not seen this sort of corrosion on IC leads/sockets before. Used to seeing Black oxidation on silver plated leads on aged TI IC's.

I didn't think to stop and take photos of all this, now wishing I had. After putting the amp back together, I powered it up, thinking what I had encountered, that fuzzy sound they were getting fin the HF horn was no doubt from both the corrosion as well as the solder joints interfacing the Sweep EQ controls. I only connected the woofer, again to the Right Ch, still in Dual Mono mode. Turned up the master, played with the HF Sweep EQ boost and frequency control, and no more oscillation. That I expected. Applied pink noise, heard what sounded stable, and the Sweep EQ all sounded nominal.

I switched into Bi-AMP Mode, which came on with a loud Pop in the speaker, and a nasty hum that sounded like the amp was ready to fail, though no difference in the AC Mains current/wattage readings (under 600mA/60W. I immediately switched back out of Bi-Amp mode, all sounding normal again. Moved the speaker cable to the Left Ch, tried again. Same result, sounding like it's ready to fail. I haven't yet connected any instrumentation to the amp, power supply connections to see what I'm dealing with. Usually when you hear something like that, there's a huge change in the AC Mains current flow.

So....pondering the problem in Bi-Amp mode and this nasty Green corrosion found on the IC leads. It wasn't on all of them, just on six of them. First time I've seen that on the Eden amps. Client has several of these amps.

WT800C schematic.pdf

[J M Fahey]

The green corrosion on a relatively new amp, and so localized may only mean some kind of bug pee.

*Personally*, IŽd remove corroded IC *and* socket, I donŽt trust them any more, cleaning doesnŽt mean much once surface protection has been lost, it will definitely NOT last, once away IŽd clean area with alternating acid (diluted white vinegar) and alkaline (baking soda in water) washes so as to neutralize whatever was absorbed by PCB itself ending with cotton balls wet in clean wate, various times, drying with dry cotton balls (or paper napkins)

I am not worried about water but *salts* so no hot air drying (which leaves them behind) until the very last cleaning operation.

Solder with your preferred stuff, maybe squirt some plain old WD40 or similar, including socket, an oily film is not the main problem here.

Maybe IŽm paranoid/OCD but Buenos Aires is a very humid city, half surrounded by a HUGE (largest in the World) warm water river (it flows from Brazil sub tropical areas) so corrosion is easy.

>2 y.o. parts pins become hard to solder, nowadays wirebrushing/deoxidizing/tinning a box full of 1M pots which work fine but will NOT take solder, go figure.

[Jon Snell]

Copper goes green when oxydised or the technical term of combining copper with oxygen. Commonly caused by liquid contamination.
If it has been repaired before and the 'tech' didn't use self cleaning flux or cleaned the work afterwards, that will cause oxydisation.
Remove the IC and socket, clean the board with a fibre glass pen, wash with Isopropy alcohol. use green conformal coating to protect the board; Chemtronics CW3300G.
That will complete the reliability as it seals the pcboard from moisture.
Fit new TL072s WITHOUT the sockets and there is no point is fitting expensive over rated ICs in their place because we don't need HF anywhere near 50kHZ, it just causes troubles down the line.
Job done.


I have a WT800, (under the wings now of Marshall Amplification), not a bad little amplifier for its time. Mine has NL4s as I thought most have.

[nevetslab]

Having no success trouble-shooting this Crossover, and kept getting high DC offsets on IC7 and IC8 with new chips, and IC9, which must be IC1 on the schematic, as there is NO IC9 anywhere in their documents, and IC1 is used in the front end. I had unplugged the two power amps so they weren't involved, but finally pulled the preamp board out again to tempt fate, knowing the board is built using Lead-Free solder. I already had problems with traces lifting on it before, and now, having to get IC sockets off the board.....

Started with solder wick, sucked up as much as I could get out of the socket holes, then re-soldered with 60/40 Kester solder. Then, with a fresh tip and clear plumbing air-flow in my Pace SX70 De-Soldering iron, I sucked out the solder again, this time succeeding on two of the three IC's. I had to stop to clear the dross chamber, then changed to a finer tip (0.030" ID), and got the rest out of the last socket, cleaned up the board/holes, and installed new machine-pin sockets and fresh TL072's. Powered up the board from my Tek PS503A dual tracking supply, set for +/- 15VDC. Finally, no DC offset. Followed the schematic, injecting signal into the top of the Master Volume pot thru P2 header, and collected the signal off of P3, so I could verify the Xover was working. All checked out, apart from having
some hum from the board's pots not being grounded as would be in the chassis.



So, hopefully this will work still when I put it back together.

[nevetslab]

OK, I've now mounted the PCB assemblies back into the chassis.





With all but the power amp modules connected, I powered up the chassis again, and checked the voltages at IC7 Pin 7 and IC8 Pin 7. I now have DC offset just as I had when I started. How delightful. I like solutions that only work when pulled apart and sitting on the bench separated from the chassis and the rest of the system.

I removed the left hand power amp module (Right Ch), to see if there's any active circuits on the rear panel PCB's.



Unless there's active circuitry hidden from view back there, it looks like all the trouble is sitting on the preamp board, which apart from the chassis....all seems to work just fine.

So, what now? I didn't find any open resistors in the circuitry of the Xover (around IC7, IC8 and IC9A) when I had the preamp board on the bench. At least I now know what the power amps sound lilke when presented with full level DC offset, though that's feeding the next AC-coupled stage, and not the inputs to the power amps directly.

What would cause the circuits around IC7 thru IC9 to change from no offset (powered via +/-15VDC on the bench) by itself to now having near-rail DC offset on IC7 Pin 7 (-13.7VDC) and IC8 pin 7 (+14VDC), and IC8 Pin 1 (+0.23VDC) powered from the chassis's regulated +/- 15VDC supplies, with the outputs from the preamp board connectable to the power amps. I intentionally disconnected them, so they weren't receiving any of this corrupted signal. First time I heard the amp groan like that, I thought for sure there would have been high current flowing thru the AC Mains, and getting ready to blow smoke and shrapnel around.

[nevetslab]

What would cause the circuits around IC7 thru IC9 to change from no offset (powered via +/-15VDC on the bench) by itself to now having near-rail DC offset on IC7 Pin 7 (-13.7VDC) and IC8 pin 7 (+14VDC), and IC8 Pin 1 (+0.23VDC) powered from the chassis's regulated +/- 15VDC supplies?

I'm not in the shop at the moment, but looking at the images of the preamp board and the schematic posted above, I'm not seeing any power supply ceramic bypass caps in place around the TL072's up and down the long board. Also wondering if I changed IC7, IC8 and IC9 to a non biFET dual Op Amp, would there be any less inclination to latch up as we're seeing when I move the board back into the chassis? I've never seen this behavior before. When it was in the chassis, I had swapped out the JRC 072D BiFET's with new TI TL072's, and all latched up in those positions. With the board out on the bench, powered externally, I began with new yet-unused TI TL072's, and no latch-up until moving it back into the chassis.

[J M Fahey]

Can you explain me the airflow?
I see a fan on the back panel and vertical fin heatsinks at one end, pointing outwards, not sure how they work together, if at all.
Logic says fins should be in front of fan to get full flow and hot air exiting chassis through slots "somewhere else", I see nothing of that there.

[Jazz P Bass]

There are slots underneath each heatsink at the fins.
Presumably to let air flow in and across the fins.

[nevetslab]

The top cover has vents above each heat sink on the sides, but is only vented with three rows of vents that span 0.80" from the top of the 2.8" tall heat sinks. There's a pair of stud-standoffs under the heat sink to mount it to the chassis floor. Fans are only turned on when either/both heat sinks' COR thermal switches close, and then it runs at full speed. Fan is 12VDC, model PL80512M, but no mfgr name. Just a capital M above the model number, so I can't look up what the CFM rating is. I'm guessing only around 30CFM, and no second speed. With rack mounting these, and equipment placed directly above it, without no gap, that cuts off more than 50% of the air intake on each side. I can't say I've ever been overly impressed with Eden's cooling design.

I thought I had some photos of the earlier generation Eden chassis to see if there were vents below the heat sinks, but I don't recall there being any there.

[nevetslab]

Seems like I've spent all day removing, making repairs, re-installing the preamp board into this chassis. I found the Dual-Gang Reverse Log 100k Frequency pots were intermittent, so that seems to have been the cause of the DC latch-up. I did install 0.1uF Monolythic Ceramic caps around IC7, IC8 & IC9, and DID find 100uF/35V Electrolytic Caps C38 & C40, and two 0.1uF Film Caps C39 & C41 adjacent to P3 Header. I didn't find that Frequency Pot issue until the third time putting the PCB back in, finding the crossover working, then not working. So, out came the PCB again. I luckily had a couple of those pots in my Eden pots bag, so had to roll the dice getting that dual gang pot off of the Lead-Free soldered PCB. I did get it off, while the top side solder pads NOT attached to anything but the plate-thru peeled off with the leads of the pot. Got the new pot installed, and put it back together. Now I had NO DC OFFSET and a working crossover. But, now I didn't have any BALANCE function! Yanking on that pot shaft, I found I could get it to work or not work, so had to pull the PCB out again and de-soldered, then re-soldered it, as well as sprayed contact cleaner into it. Verified I had continuity from the wiper of the Dual-Gang Master to both sides of the balance control, and ground continuity. While the board was out....from repeated removals/re-insertions, the pots tend to get flexed from such tight quarters for the PCB assy and fighting the harness/connectors that only want to be in the way at all times. So, I nearly had the LF Sweep EQ pot break off. Managed to repair that.

I finally have the board back into place, and all functions working. There was a continual flow of nuts or washers leaving my fingers to the floor to bounce to places unknown to add more thrill in the hunt! Grrrrrhhh!