Noisy Alice 828S

[Dave Austen]

Hi Ted,

Hope you can help with this one …

I provide engineering for a community radio station (Forest FM in Dorset) and we use a couple of Alice 828S mixers, one on-air and one for production. Before we started broadcasting three years ago I gave them both a service (mainly changing faders and pots where necessary). Apart for the usual legend-wear, both have performed faultlessly until recently. The on-air 828S has now developed popcorn noise on mic/mono channel 3, which is most noticable on line input and very noticable on PFL.

I investigated the fault and found that the circuits I have for the mixer are of an earlier version than the problem unit. My circuits show discrete transistors, whereas the mixer has ICs on the mic/mono channels. Physically, the noise is affected by slight flexing of the PCB near the input end, so appears to be mechanically-related (although I could be wrong!). I have tried cleaner on the mic/line switch and have re-seated all the ICs but to no avail. The problem is not being caused by any of the pots being noisy.

I wonder if you have the correct circuits for these boards (board number 578-1082) or have any ideas about the cause of the fault? Duff chips? Instability? Dry joints?

Thanks for your help.

Dave Austen

[Ted Fletcher]

Hmmmm a bit of a problem here….. I would suggest the quickest way is to replace the chips in the offending channel; it’s doubtful if its mechanical. If the crackles vary with input gain control, then it’s the front-end chip, and that’s the most likely. Replace with the same type, or if it’s a dual, I suggest using MC33378 as an alternative to TL072. It’s best not to go for the lower noise modern equivalents as you could run into stability issues.
I’m sorry I don’t have circuits for the later 828S mixers.
As a last resort, I can probably fix it for you, but sending a mixer to Torquay is fraught with risks.

[Dave Austen]

Thanks Ted,

I’ll try the replacement you suggest. In fact I think the first IC was an NE5534, whilst all others were TL071s (or possibly duals), so instability may be the cause. Definitely worth a try! I’m sure I’ve got plenty of the beasties stashed away somewhere.

Cheers, Dave

[Ted Fletcher]

BTW, I gave you a wrong type number….. the one I meant was MC33078 This is a good sounding dual that’s very forgiving.

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