Studiomaster Mixdown PSU bypassing, recapping & mods help

[Philbo King]

I'll chime in a little. I did avionics design work for 27 years, on aircraft designed to last >50 years. The required design standard for all power and bypass caps was to use 2x voltage rating; if the part is exposed to 25V, use a 50V part.

On a repair/refurbish job like this the rule can be bent a bit if the new part physically doesn't fit well. But if it does fit, I'd say go with the higher voltage.

Oops, just noticed this is a necro/zombie thread. 15 years old! Still, I'll leave the post for future reference.

[Arpangel]

The original Mixdown also has extremely quiet mic preamps so I can't see any need to modify them.

Those preamps are legendary, amazing, I made a lot of recordings using a Studiomaster Mixdown 24-4-2 and the sound of vocals on those desks was beyond, totally. I think I’ve got a track I can post, I’ll dig it out.

[James Perrett]

The original Mixdown also has extremely quiet mic preamps so I can't see any need to modify them.

Those preamps are legendary, amazing, I made a lot of recordings using a Studiomaster Mixdown 24-4-2 and the sound of vocals on those desks was beyond, totally. I think I’ve got a track I can post, I’ll dig it out.

I recorded hundreds of songs on ours although not many of them have made it to the major streaming sites. Here's one...

https://open.spotify.com/track/0Z3ac3uLiJGsgWkqjX9VyO

[Howard_P]

...The required design standard for all power and bypass caps was to use 2x voltage rating; if the part is exposed to 25V, use a 50V part. ...

Oops, just noticed this is a necro/zombie thread. 15 years old! Still, I'll leave the post for future reference.

Well, unless you shoot them, Zombies never die

This is exactly the kind of info I'm looking for because I inherited a TASCAM 1600-24 console (late 1990's I think). It is in an unknown condition. Not sure when it was last powered up, etc. To be cautious, I was planning on yanking the power-supply feeds to all the boards, then test the supply first, and work my way downstream, etc. Surprisingly, to me at least, TEAC had all the manuals and schematics.

I'm unclear about what caps really should be replaced versus what is an just an improvement. Yes, this should prolly be its own thread, like, "how to recap a console" ... still searching... suggestions very welcome.

Thanks - HP in FL

[James Perrett]

I'm not a fan of blanket recapping unless there is a good reason. Certain types of capacitors are known to fail but others will work for many years at full performance. Heat is the biggest enemy of otherwise good capacitors. If your desk has been out in the Florida sun for a length of time then it is more likely that capacitors will need to be replaced but if it has been in an air conditioned room for all its life you may not need to replace any of them.

Your plan of checking the power supply first is sensible. Personally I would power it up using a variac or lamp limiter first to reduce the possibility of damage if there is a short circuit somewhere.

[Howard_P]

Thanks for the prompt reply, James.

Good - I was hoping to hear this. (These 'all in one' decks look like a pita ... unless you like having a shop full of loose knobs. ).

Yes, I was wondering about using my variac for that exact reason, but wasn't sure it was a good idea. Then again, I can't see how a transistor or an op-amp could be damaged by undervolting. (?) In any case, I have a funny set up with a bunch of different glass fuses downstream from the variac that I can switch between. (Having inherited a box of several thousand of them, I had to use them for something. LOL)

And, as you know, a good visual inspection reveals a lot. So, if an electrolytic cap is bulging or about to blow its lid - it's out. Burn / heat marks, etc. = other causes.

I don't have any experience with TASCAM products, but the schematics seem to show a good design for a low-to-mid level product. I won't call it 'pro' simply because it is an all-in-one. The op-amps seem a bit obscure, but maybe they weren't at the time (1990's).

Do you think replacing some of those with modern versions would be worth the trouble?

thanks again for your thoughts - I really appreciate it.
- HP

[James Perrett]

Quite a few bits of kit use similar JRC op amps although usually the 4560 or 4580. I'm not sure that you'll gain much by changing them as most of them aren't being used close to their limits in a mixer. Possibly the mic amps might benefit from something better though you really need to analyse the design carefully before replacing anything.

[Howard_P]

yes, good call - they're 4565's and a smattering of discrete transistors. In fact, most all of the many op-amps in the thing are 4565's ... guess that made it easier to design. : ))... Ok, nice, no reason really to fiddle with them. thx again - HP

[Howard_P]

[Hope it's ok to continue asking generic questions about recapping here. A bit of thread drift, but still fits broader aspects of OP's question, IMO.]

Was cleaning out an old storage area and discovered a rack-mount Behringer, single-purpose compressor/limiter. We bought in about 1998 but used it only long enough to make sure it works and fiddle with it. In other words - aside from the manufacturer's burn-in tests (assuming they do them) - it's been juiced all of a whopping 3 hours.

Considering this thread's topic, you probably see where this is headed ...

Does a long-idled* piece of gear potentially need any recapping at all?
[*In this specific instance, it was stored in a stable, climate-controlled location.]

The underlying questions really are:

• Do caps dry out just sitting on a shelf?

Also, I've heard that many audiophiles turn off their amps partly for that reason (and not to waste power), so,

• Is a cap's lifespan directly proportional to how long it's been energized? (Assume use within specs, of course.)

And one I thought I knew already, but in thinking about all this cap stuff, I wonder now:

• Is it only electrolytics that need replacing (due to wear, not damage)?

thanks all - HP

[James Perrett]

It is a big subject...

First - any gear that has been stored for years needs to be carefully powered on. You've mentioned having a variac which is a good tool for this. I use a cruder lamp limiter which should prevent any serious damage.

Electrolytics seem to last longest if powered up every so often. This allows them to stay "formed". Capacitors that haven't been charged for a long while tend to deform - they lose their polar properties. Old capacitors can sometimes be reformed by gently charging them up over a period of time.

Some people suggest switching the device on for a few seconds at a time will work as an alternative way of reforming any capacitors that may need to be reformed.

Unfortunately there's no real way of telling whether the capacitors in your Behringer are OK or not without taking each of them out of circuit and testing them. Switching on via a lamp limiter is as good a way to check as any in my opinion.

Electrolytics are the most common type of capacitor to give problems but there are certain other types that can also give problems. The Rifa X type capacitors that are used to reduce arcing when switching on and off will often crack when they get old and start to smoke. X type capacitors are self healing so they may go short circuit for a short while but they will then recover. I've had a piece of gear switch itself on and then off again thanks to a faulty Rifa suppression capacitor across the power switch. I was wondering about paranormal activity until I smelt that characteristic burned component smell.

In reasonably recent gear most film type capacitors don't really age. In old gear from the 60s and earlier there may be capacitors that need replacing - especially in valve gear that gets warm but there won't be anything like that in a Behringer compressor. An electrolytic capacitor's lifespan depends on how warm it has been - most are specified as having a life of 1000 hours at 85 degrees C but the lifespan increases dramatically at lower temperatures.

[Wonks]

Electrolytic capacitors in valve amps often have quite hard times, and although Janes mentions the 60s and before, a lot of modern mass-produced guitar and bass amps are made with low-cost electrolytic capacitors that only last a few years before they start to degrade to the point where they aren’t doing the job they are supposed to.

Fender persist in using the IC brand of electrolytics, but a lot of these will drop under spec and stop filtering out hum within three years. This is often made worse by the positioning of capacitors next to underrated (and so very hot) resistors, so it’s not always the capacitor’s fault.

Quality brands of electrolytic capacitor will last for many years, but even they will ‘fail’ quickly if used in too hot an environment.

[Howard_P]

I've had a piece of gear switch itself on and then off again [...] I was wondering about paranormal activity until [...]

Thanks for the thoughtful reply, James.
Nothing like a haunted piece of gear to make for an interesting session.

Quality brands of electrolytic capacitor will last for many years, but [...]

Thanks, Wonks. Interesting observations.

Although I've done electronics off and on for decades, I still couldn't tell you which are the quality brands. I've seen Nichicon mentioned a lot. What would an experienced audio tech say is the best for lower voltage (non valve/tube) work?

[James Perrett]

I'd be happy with most of the big brands that reputable distributors like Farnell or Mouser sell. Panasonic probably give you the biggest choice but Nichicon, Rubycon and Vishay are all fine. Even Farnell's own brand, Multicomp, have always been fine for me.

I try to use 105 degree rated capacitors where possible for longer life. Some gear also requires the use of capacitors with a low ESR or high ripple current rating in some places so you need to be aware of what sort of spec is needed.

[Howard_P]

ok great - I've been using all you mention, except Farnell's multicomp. Unintentionally avoided Farnell because their US distributor, Newark, is generally more expensive than Mouser, Digikey, or even Jameco here.

Nice coincidence: Checking Amazon US for multicomp lead me to a USB O-scope I've been looking for.

Any feedback on Farnell/Multicomp's test equipment?
- Cheers, H

[James Perrett]

Over here I usually use CPC who are part of Farnell but often cheaper (they're £30 cheaper on the basic USB 'scope). They don't sell the whole Farnell component range but, on the other hand, they sell more music related gear and general tools so it is easier to get the order above their free postage threshold.

I've not had any problems with any Multicomp items that I've bought (mainly components) but haven't bought any of their test gear. However, before they started using the Multicomp brand, Farnell used to sell their own brand test gear. We had some of their power supplies and signal generators in the lab which always seemed to work fine.