ezP-1A woes

I built a pair of AML ezP-1A's a while ago and one of them has pretty much stopped working and I have no clue where to start.

Here's the patient's symptoms, nr. 1 works fine, thus the comparison:

• Nr. 2 is about 12dB louder even when eq is out
• Nr. 2 bass boost does not work
• Nr. 2 bass cut works, but slightly less than Nr. 1
• Nr. 2 top boost works, but significantly less than Nr. 1, it barely does anything really and very broad at that
• Nr. 2 top attenuation does not work

If anybody has an idea what could cause this, I'd be very grateful for any hints.

Can we assume these are passive eqs?  If so my first poke would be for a short that is bypassing signal around the filter circuit. Makeup gain stage is likely good... Maybe messy soldering or stray strAnds of wire

Yes, it's a passive eq: http://www.audiomaintenance.com/acatalog/kits.html

As ignorant as I am about the circuit and how to trouble shoot it, I'm quite experienced in the soldering department and the PCB is very spacious and practically all wires are using connectors, so I doubt it's either of those. But thanks for the suggestion anyway!

I can't find it now, but I remember in the back of my mind somewhere that with this particular kit one has to be careful to not shorten the output transformer when screwing it onto the PCB. Would that explain this behaviour? That's easy enough to check though, so I will do that next.

you have compounding issues. I would suggest going over all the solder points. You may have made a couple of bad ones. Check for cold joints, especially in the fact the low boost and high boost are not working.  As for the level discrepancies,  check and measure.

This is where an o-scope is handy and test tone... To see at what part in the circuit the volume does not match the good one.

Thanks guys! I started swapping PCBs and it turns out the top PCB with the boost and cut pots and the high cut selector switch was causing the trouble. I couldn't find any shorts, so I just cleaned it thoroughly and put it back in with the result that everything works now except the high attenuation. The frequency switch has no effect and the attenuation pot actually increases the output by about 12dB when fully open, but across the entire frequency spectrum.

At least now I know what to look really close at.

OK, this is a bit weird. I took the PCB in question out again, made really sure there are no shorts on it (the only two possible locations are the attenuation frequency switch or the molex connector, everything else is way too far apart from each other), checked the switch with a multimeter and put it back in.

The error I still have which is probably still the same as in the beginning, is that the EQ works fine until I use the high attenuation. Once I do all the symptoms mentioned above start happening, however I can 'reset' it of some sorts by turning the high attenuation all the way counter clock wise and switch through the 20k, 10k and 5k frequencies and back up to 20k. Then everything works again until I touch the high attenuation pot.

What could cause this? I mean I've really cleaned the pcb twice now, so there's no flux left and I checked all adjacent solder joints for continuity. I'm a bit at loss. At least I can use the thing as long as I don't touch the high attenuation which I barely used anyway.

are you certain you have the right value pot in there?

check your grounds,

I have completed quite a few of these for builders.

1. Pads around switches are often shorted.

2. Terminals on the grounds are tricky, make sure they are connecting properly.

And since you have a working EQ, you could go to work with the multimeter and compare!?

Gustav

Thanks for all your suggestions. There are no shorts around the switch, I triple checked. It seems the attenuation pot is defective as turning that up causes said behaviour. So I cleaned it and it's a little better now, almost useable, sometimes it works, but sometimes it behaves as if it had a switch built in and while turning it toggles between correct attenuation and said behaviour, which is likely caused by a short as the consensus is here. But there are def no soldiering shorts around the pot, the contacts are far away from each other, so it must be an internal thing.

I'll replace the pot then. If my logic is flawed, please enlighten me, but I can't see how this toggling between and short and no short when turning the pot can be caused by anything else but the pot itself?

Did you ever fix this?
I'm having a similar behavior in one of my units, it boosts by 15dB and if i wiggle the high attenuation pot i can make it stop boosting. Seems like some kind of intermittent problem. The question is, is it the pot or my soldering?‚