Filters don't work right

[pingosimon]

I have a MB-6582 stuffed with 8 6581 SIDs - all different types.

Today I went through all my SID chips (I have 13 6581 chips) and recorded their performance on waveform and filter tests. (They all pass the waveform test!)

My test was: turn on OSC 123 on SID 1 L. Test cutoff sweeps on LP, then BP, then HP filters. Repeat for SID 1 R. Repeat for SID 2 L, etc.

After switching out the first 8, with the next 5 SIDs, and recording the results, I noticed patterns on the placements of the SIDs. For example, both times, the chip in the SID 4 R slot had perfect filter tests. Both times the SID in SID 3 R slot failed LP and HP filters. If I took the "good" chip (from SID 4 R slot) and put it in the "bad" position (SID 3 R), it doesn't work!

So I tried switching out the filter capacitors - that didn't change anything.

I have tried putting one "bad" chip into a "good" slot, and it was still "bad."

Am I missing another factor here?

[Hawkeye]

Hi,

I´d say, there is probably a connectivity problem with the filter capacitors on your "bad" channels. Can you check all traces from the filter caps to the sid socket and reflow the solder spots there?

Many greets,

Peter

[NorthernLightX]

I have tried putting one "bad" chip into a "good" slot, and it was still "bad."

Logic dictates that you have at least one defective SID then. I would suggest picking one IC socket that you deem good, and test all your SIDs in this one socket. Place the bad SIDs aside.

Then test the other sockets with your "known good" SIDs, to check if all your sockets are OK. It's probably best do use one SID for the socket test, to rule out any differences between individual SIDs.

I have no experience with broken SIDs (fortunately), but from what I've read a SID with a defective filter does not pass the LP test but fail a HP test. It's either broken or it's not. You didn't bend any pins by chance?

What Hawkeye said is also a good tip; reflow solder points, clean flux, and check for unwanted shorts and damaged tracks, also on the top layer of the board.

[pingosimon]

Thanks guys!

I did a quick solder check...it seemed fine but I'll check again. I'll also do a more comprehensive chip/slot check.

Can anyone confirm that a the SID will either fail both LP and HP, or pass both? That would make things a lot easier!

[Sauraen]

I also don't have any experience with bad SIDs, but I did build a filter capacitor switching circuit (digitally choose 1 of 4 pairs of capacitiors) for my "MIDIbox ASIDITY" project. At one point I accidentally disconnected one of the filter capacitors entirely (what would happen if you had a bad solder joint) while it was playing, and the SID made some noise but sweeping cutoff did nothing. If this is what you mean when you say "failed", then check the capacitors. My untested guess is that in this situation, changing filter types would change the sound, but sweeping the filter would not work in any of the filter modes, as both capacitors are necessary for all the modes--it's not that one is just for low pass and the other just for high pass.

The converse of what @NorthernLightX said is also valid: if you get one SID to work entirely in a certain socket, test all the sockets with that SID.

[pingosimon]

I found two 6581 chips that work, so I used those to test.

 

In the case of at least one slot, I had placed the jumper in the wrong spot (I was supplying the 6581 with 9v instead of 12v).  Putting the jumper on the correct voltage makes the filters work!

 

I'll report back if this doesn't fix all the slots.

 

 

EDIT: welp, that was the only problem.  Cool!

Edited December 22, 2013 by pingosimon