Capacitor questions (re-capping old delay unit)

[Adam Inglis]

I'm re-capping the electrolytics around the A/D and D/A areas of my Korg SDD-3000 dig.delay hoping to reduce some effect signal noise. There are several 47uF that are visibly leaking and swollen. There are also a couple of NP (non-polarised or bipolar) 'lytics that look affected, and a couple of 0.1 uF ceramic discs also.
Question: what can you substitute for the NPs in these applications? I can't seem to find ANY NPs in my usual local emporia. Do they need to be electrolytics or can you use something more commonly available?

cheers

[cfb4]

You'll need to use NP electrolytics (non-polarised electrolytics) There is a way to make them from regular caps but not worth it as RS and Farnell carry every value and voltage rating of these. The uF value should be the same (or quite close) - but the voltage rating can be the same or higher (but never lower)

[James Perrett]

but not worth it as RS and Farnell carry every value and voltage rating of these.

I don't think RS or Farnell have actually made it to Australia yet - but I'd guess that there is an Australian equivalent.

It might be worth trying a loudspeaker repairer as non polar electrolytics are often found in loudspeaker crossovers.

Cheers

James.

[Ted Kendall]

If all else fails, use two electrolytics back-to-back (that is with positives connected together) and bias the junction with 100k or so to the positive rail of the circuit. The value of each electrolytic should be twice that of the one you are replacing, since the replacements are in series.

[ef37a]

What value in NP's?

I have shedloads in 1.8mfd @250V NP and foil.

Dave.

[IvanSC]

Had a look at the IC`s to see if there might be modern lower noise versions available? Might do more for the S/N ratio than the caps....

[cfb4]

Heh, should have looked at the location
I, too have, a Raaco box full of NPs - declare thy values and see if I have them here

[Adam Inglis]

It might be worth trying a loudspeaker repairer as non polar electrolytics are often found in loudspeaker crossovers.

Ah! That was a clue! Found them at a shop selling car kits and crossovers - cheers!

Ivan - there are several TL072 op amps at this stage (not socketed of course) but I thought I'd start with the obvious first, and my desoldering skills are not very time-efficient so I'd like to keep the replacements to a minimum! Apparently the caps on this unit are notorious for this, although I've never heard of ceramics "going bad".

[IvanSC]

Didnt bother googling it so I dont know if it is Surface Mount or not.
If not the desoldering bit is pretty easy with a pump and a decent iron.
DOn`t sell yourself shor t- if you are good enough with an iron to remove the caps you can do the IC`s.
Just remember to socket the new ones if you have room!
Good luck with it.

[ef37a]

Morning Adam,

I have no hands on knowledge of the SDD-3000 but can I give a few general observations?
Re noise. The unit has a S/N of 88dB Effect On (I have found a rather scruffy service manual)which suggests to me a 16bit device and so you are never going to better that. It is provided with 3 operating levels which probably means you need to run it with close regard to signal levels to get the best noise performance? A really low noise pre-amp (NE5534 type) could lift by say 10dB and improve things a lot.

The TL072 might be long in the tooth now but it is still pretty good, replacing the ic's en mass would not I think improve things much. If it MEASURES out of spec' a can of freezer spray can isolate guilty components. In any event you stand a good chance of damaging print unless you make up a "form tool" and attach it to a solder iron. If you have some brass stock and access to a drill press they are easy enough to make.

Re caps. I note there are two Tantalum components,(1mfd@25V, [email].22@35V)[/email] these were infamous for going leaky/short especially if they are blue!I would simply relace them with foil types if there is room. Ceramics certainly can give trouble! Leaky, temp' sensitive, microphonic even.
N.P's. I would not worry too much if you cannot find the value/size/voltage you need, modern ally caps have very low leakage and can cope with zero polarization voltage. If you are worried, "pre-form" them for a minute on a PP3 and/or check the residual voltage on the existing cap and fit the right way round ( the offset can make the ic output + or - of zero V).This does NOT apply to power applications like speaker crossovers!

Have fun,
Dave.

[Adam Inglis]

I really appreciate the input people - there's no substitute for experience!

This unit has an "analogue" board (preamp, filters) and a "digital" board (cpu, multiplexers, A/D,D/A etc). The preamp is based on a couple of TL072s and sounds sweet and quiet on its own direct output (some guitar DIYers have copied this circuit as a standalone, cos the Edge uses it apparently). The noise is in the effected signal, a crunchy, cyclical noise with high freq. aliasing audible.
The digital board has thirteen 1-bit RAM chips on it, and indeed Korg say it uses "Proprietary 13 bit technology". This board is double sided so I have to be really careful and going has been slow! The corrosion on the cap leads makes the joints extremely resistant to melting.

There are also a LOT of 0.1 uF ceramic discs bypassing the ICs, and there looks like quite a few blue polarised caps I assume to be tantalum associated with the RAM chips.

Might have bitten off more than I could chew...DOH!

[grab]

If you don't care about being able to reuse the TL072 (or any other IC for that matter), a pair of cutters is your friend. Just snip the legs off it, and then desoldering becomes a one-pin-at-a-time job instead of having to make sure all 8 pins are free. In fact you fairly often find that the cut-off bits of leg get hoovered into the solder-sucker, which makes life even easier.

I presume that if you're looking to reduce noise, you're replacing the TL072 with something a bit more recent and lower-noise, instead of just doing a like-for-like change? Plenty of dual op-amps around with the same pinout.

[James Perrett]

I suspect that you won't improve things much by replacing caps and won't improve things at all by replacing IC's. Old digital gear was noisy, even when new and working perfectly. I remember having to gate the output of delays and reverbs if I was mixing a quiet song.

cheers

James.

[Jim Y]

It is possible that replacing the amps with cleaner & faster ones will only make it better at reproducing inherent noise and digital grunge!

As to ceramic caps - without seeing them, I couldn't be definite, but they often have contamination of some kind on them. Flux residue, wax, mould release agent, especially if they are of the raw, matt finish ceramic type. It's not necessarily a problem.

Jim

[Adam Inglis]

Well I replaced the obviously leaky caps, also all the P.S. electrolytics as well, and I cured some anomalous behaviour of the meter, but it's hard to say if there has been a significant improvement in effect noise. I went thru the circuit with the scope - I really cant convince myself that the op amps are contributing much noise, it's restricted to the feedback circuit. There are a lot of multiplexers (4053 chips) dealing with the feedback signal, and I don't really understand what's going on!
It is probably going to be diminishing returns from here though.. Luckily the feedback path has hi and lo filters which remove a good deal of the noise (but affect the fidelity of course). The great thing about this unit is it's envelope CV and external CV control of sampling frequency - mental stuff!

[Adam Inglis]

Would those in the know like to comment about these claims on this site ??

Quote "We have the ADC and DAC in stock for these units. They were discontinued years ago but we found a batch!. If you have an SDD3000 that has clicks, pops or distortion in the delay signal we can fix it!
We have recently discovered a long lost batch of delay memory chips. These chips are used to keep the delay signal clean! If you've got ghosts in the machine. we can clean them out! " Unquote

Are these chips likely to "go bad"?

I have found some info from the old crow about chips changing over time...

"All 4000-series chips made in
6um process suffer from 'metal migration', a slow reaction where ions from
the metallized gate material of each MOSFET (metal-oxide semiconductor
field-effect transistor) in the chip flow down to the substrate of the
chip over a period of years. The result is a short-circuit from one or
more MOSFET gates to the substrate layer.."

There are a lot of 4000 chips on this board!

[ef37a]

The term "barg pole" comes to mind!

Now my workshops over the years have never been minmalist icons of purity but I would at least tidy them up for pictures!

I am unfamiliar with US wiring and practice but the part about 240V and phasing/12V ground differentials is bollocks over here.

Run the whole thing over to Korg but I would not employ them to put a 6.3V lamp in one of our amps!

Dave.

[Jim Y]

It's hard to predict the life of chips - especially plastic encapsulated ones because daft as it sounds, it is not a perfect hermetic seal against moisture. That's why Military spec' chips are in the ceramic package.

I can't imagine what is meant by special delay line chips in a digital delay.

4000 series analog switches are not kind to the signals they pass, but you have to spend a lot more money to get any better and they're usually acceptable in FX.

Jim

[funkyant]

FYI Both Farnell and RS have been present in Australia for many years.

[Adam Inglis]

And just as a further note to the subject of 4000 series chips going bad - I've just had to repair a voice board on my Jupiter 8 where the chip that selects the VCO waveform, a 4053, refused to switch in the triangle and sawtooth channels, always bouncing back to the pulse or square. Replacing the chip sorted the problem. I found that quite strange to get such specific erroneous behaviour, rather than a general failure!