Easy fix for QU-32 faders not finding position/ dirty faders

[log2Participant]

So I had an issue with a QU-32. After a hot dusty outdoor festival the faders would not always make it to the correct position when changing layers, fader flip etc. It seemed to affect odd and even faders differently. I took it to a A&H service center for diagnosis. Tech said the faders were dirty and worn and needed replaced for aprox $1000. The diagnosis didn’t seem correct to me. So I decided not to have it serviced. I know a lot of users have had their faders replaced for similar symptoms. It seemed like a waste if it was only a small problem or component on the board. I removed the first bank of 16 faders and thoroughly cleaned them and lubed them. This did not solve the issue. After some probing I found the problem. Turns out it was some PTC fuses on the fader boards. Each board has four fuses and they where all damaged. My guess is, the dirty faders cuased more current to be pulled through the fuses. This combined with the hot air temp cuased the fuses to fail. PTC fuses don’t open like a regular fuse they go to a higher resistance. With only one or two faders moving in any bank it could still reach position. If three faders needed to move per bank the high resistance of the fuses made the motors move too slow. After replacing the PTC’s for about $4 and cleaning the other faders everything works perfect.

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[GeddonArchonParticipant]

Awesome work and thanks for documenting. I’m not that great with electronics so how did you determine the fuses were shot? Also could you link the replacements you used?

[log2Participant]

I determined the fader motors were driven with 5volts. I put a oscope on the 5v supply and changed layers with all the faders set to move from top to bottom. I saw that the 5v supply stayed solid at 5v. I just kept tracing the 5v power path until I found the point where the supply would sag on a heavy fader move. I did this from the bottom of the board as the faders cover up most of top side of the board. After I found the sag I flipped the board and found the tiny green PTC fuses. I ohm metered the fuses they were all bettween 2-5 ohms. They should be .2-.3 ohms. They are surface mount so it really takes two soldering irons to heat up both sides and pull off the compents like chop sticks. Its helpfull to have solder flux to solder the new part on, but not necessary if you have flux core solder. Also keep in mind if you over heat the pars soldering them on they will “trip” and take a few minutes to cool off and return to low ohms. And too much heat could damage them.

[GigaParticipant]

Great job !

Giga

[Tom IernaParticipant]

Log2, what was the replacement part you went with?

Looking at DigiKey and Mouser show quite a lot of PTC Fuses available with a variety of specifications.

[Tom IernaParticipant]

On our Qu-32, the 5vdc rail on either fader board of ours drops to 4.5vdc any time there is a fader move, so I had high hopes this would fix our issue.

There were actually five PTC fuses on the board with the master fader, and four on the board that has faders 1-16.

I probed each of them and they rung out at about .5 ohm each, with one coming in at .7 ohm, so maybe we aren’t having the same problem.

In any case, before I went to the trouble of replacing the PTCs, I just bypassed all of them with some thin wire.

Unfortunately, bypassing them didn’t fix our issue.

When I was taking voltage readings from these boards, I noticed they are being fed with 12VDC, so the buck converters to get it down to 5vdc must be on each board.

The 12vdc feeds did not drop in voltage with a fader move, so I figure some component of the buck converters has deteriorated.

There are several electrolytic caps on these boards, some near the 12vdc feeds, so these may be the issue. None are discolored or swollen.

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