My AquaBoy works with MN3208 but not MN3205

[BillyBoy]

Hi - Just build my second AquaBoy.  First one was an MN3005 build - it's excellent!

This one is a 9v build to try MN3205s.  I'm powering it with a RoadRage board (LT1054) and a 9v regulator to get nice clean power to it.  Stock build with jumpers horizontal for 9v build.  R13, R14 are 10K.  R27 was 470K, but is currently a jumper.  R55 was 100K but is also currently a jumper.  R37 is 20K.  R56 is 470K.  R58 is a jumper.  D3 is empty.  MN3102 clock.  NE570.

As it sits, I have it running OK with MN3208's.  I cannot get any of my (4) MN3205's to pass any signal on pin 3/4 when only 1 (or both) are socketed.  If I leave a working MN3208 in place for the first BBD and add any of my MN3205's in the second position, not only does the MN3205 not pass any signal, but the signal is completely trashed coming out of the first BBD.  Even the input to the first BBD (pin 7) becomes just loud noise.  Take out the MN3205, and the single MN3208 does what it should.  None of the MN3205's I have will pass signal in either BBD position.  I've learned that sometimes you cannot get them to bias without having both socketed, but even with both socketed, I can't get any signal through them.

Only other oddity I haven't addressed is that the feedback runs away with the knob at about noon, even if you don't play a note.  This happens with the T1 trimmer set to kill all feedback (full CCW).  I expect my signal is too hot somewhere.  Maybe the R27 and R55 jumpers need to be resistors again to knock a little off

It is possible that the MN3205s are all bad, of course.  But before I conclude that, is there anything unique to the MN3205 build that I should try?  It isn't mentioned much in the instructions.  Since I can run with a pair of MN3208, the build itself is probably OK.  It also runs with a pair of BL3208.

[Scruffie]

Fakes are rife, where did you get them, ebay? If so that's probably your issue, that or you managed to ESD damage all of them (cold day, got the central heating on and no windows open? possible) there's no reason if the circuit works with the 3208s it wont work with the 3205s.

You can post photos of your chips, sometimes by eye it's possible to spot a suspected fake (although impossible to be 100%).

The symptoms you describe sound like dead chips regardless.

[marmaliser]

Scruffie, what ESD precautions do you take / should we take when installing BBD's? 

[Scruffie]

You can buy an anti-static wrist band (they're pretty cheap) or at the very least ground your self on to a bit of bare metal of something earthed before starting (radiator?) you can use a humidifier if you have one or turn off the A/C & Central Heating and open a window, some people have even suggest running the bath hot and doing stuff in there although that sounds a little inconvenient to me.... avoid wearing synthetic materials and no big wooly jumpers.

And of course, store your chips in something anti-static, either the foam or anti-static bags and don't use sticky tape unless it's anti-static.

[marmaliser]

Thanks scruffie, good info for all.  Personally i have the wristband ..so far so good

[BillyBoy]

I got them off ebay, but from a seller that I've used many times and never had issues with.  I've purchased a few other BBDs there and they have been OK. 

I'm careful about static electricity and grounding myself.  I've been testing quite a few BBDs, and haven't fried any of the others, so I'm guessing I just have a bad batch. 

I guess I'll find out now how good the seller really is.  I just wanted to make sure I hadn't overlooked anything about the circuit that might need to be tweaked specifically for the MN3205s before I try getting them replaced.

Thanks!

[Scruffie]

That's the problem with ebay that's happened time and time again, you get a few good ones, the supplier runs out of good chips or for a certain type never had them in the first place and instead you get fakes. You can't rely on 'they were real last time I bought' when it comes to ebay BBDs, sad but true.

That's why those threads where someone says they got real MN3005 or whatever always head south, eventually due to supply and demand the prices rise and then people start getting fakes as whatever small batch of real chips die out.

So... i'd probably just try and return them, count your losses and use the MN3208 or buy some v3205 than get replacements as it'll probably just mean it'll end in a paypal dispute when the replacement batch don't work.

[BillyBoy]

Yeah, I knew all that going in, but thought I'd give it a try as I didn't find any anywhere else at the time.  I ended up with some good MN3005s out of it, as well as several other good BBDs, so even if I get burned on the (4) MN3205s, it's not a disaster. 

I'm looking forward to the upcoming DirtBag revision.  I'm hoping to get quad 3008s to rival my dual 3005.  I'm using the AquaBoy builds (15v and 9v) to figure out which BBDs sound best to me.  Still auditioning other chips in my 9v build - BL3208, V3205, MN3208, in addition to the MN3205 (if I get them replaced OK).  I think I have an incurable analog problem.  Nothing sounds as good to me as a nice dual 3005 delay. 

I am also going to do a ZeroPoint build soon and try to convince myself that digital is OK.