Studiomaster Mixdown PSU bypassing, recapping & mods help

[PaulRich]

Hi, so Ive just picked up an old Studiomaster 16-4-8 in great condition. Its just a bit noisy, Id like to recap it and mod it.
I have some info such as:
I went through a few of these around 20 years ago. Installing local .1 uf bypass caps on the power rails does help. All feedback loops need to have a small cap across, most stages had them. Use Wima FKP-2 or MKP-2 polyprop caps in the EQ, especially the hi mid and hi shelf. Opamps can be the older OPA 2134 BurrBrown stuff, the LT1358, the OP-275, the AD 8599 (with a brown Dog adaptor) or the National LME49720/LME49860, balanced outputs can use the National LM6172 for great output drive. Also check out the new AD8510 series, a BurrBrown fet input sub for the OPA134 series. Mic pre transistors can be 2SA1316 or 2SA1083's to lower front end noise and add some HF air. A nice big Power One HDD-15-5A will be a good power supply if you add the small 1/2 amp 48 volt supply for phantom. Then you can play with opamps all day long. The summing amps in the master should be the National LME part due to it's large open loop gain, = a clearer mix buss.
Jim Williams

and

I started by the power supply caps; major improvement in noise and hum. I found out that most original caps were rated low, like 16 volts for a 15 volt power supply, so I used 25 volts throughout (except where the design called for higher voltage).

and

Replace with the same type of capacitor to the one you remove. So, if you remove a 10uF or 22uF tantalum capacitor, you do NOT replace it with an aluminium electrolytic!
2. Neve designers were restricted back then on the size of capacitors and, in the case of the power decoupling to the 183/283 (fitted to the frame of the module), fitted the largest value that could fit in there... usually 640uF. I personally recommend fitting the largest capacitor that you can now fit in there... like a 4,700uF which may be the same size now as the original.
There is nothing to be gained (and, indeed factors may go against you in extreme cases) by using a much higher voltage electrolytic capacitor. You should fit one as close to the operating voltage as you can. 35v is probably a good, top limit, value.
3. As for capacitors on circuit boards... remembering the warning about substituting tantalums for other types... there's no harm in fitting bigger value capacitors if they will fit. If it's decoupling the power rail, it will decouple slightly better. If it's decoupling between two amp stages or an emitter bypass, it will improve the bass response marginally.
Geoff Tanner

Here are the links to pics:
http://i390.photobucket.com/albums/oo345/paulrichards7/PSU16-4-8.jpg
http://i390.photobucket.com/albums/oo345/paulrichards7/PSUschem16-8-16.jpg
http://i390.photobucket.com/albums/oo345/paulrichards7/PSUtracks.jpg
http://i390.photobucket.com/albums/oo345/paulrichards7/PSU.jpg

Now as I dont know anything about electronics and gonna have to rely on your guys's knowledge. I have attached the schematics for the PSU, a pic of the front of the PSU and one of the PCB track. If you could point out which caps should be replaced and where the bypassing should go, I will break out my soldering iron and do the hard work.
Thanks so much for this
Paul

[Adam Inglis]

This guy has some interesting views on re-capping old audio gear - he seems to disagree with those you mention regarding replacing tantalums.

[ef37a]

As a long time electronics technician (not with synths but practically everything else!) I have always believed that tantalum capacitors had two main advantages. They could give a higher capacitance in a smaller space for a given WV. They had a very low leakage current.

In the domestic electronics trade, TV's Hi Fi, the Blue Tant'was notorious in fact you could almost guarantee it would be short or leaky! Modern ally electro's are very good and there will be very few places they will not be a better replacement for a tantalum.

'Ave 'em out, don't mess about.'Ave 'em out!

Dave.

[ef37a]

Just had a look at the pikkies Paul.

Those caps look as flat topped and clean as the day they went in. Personally I would not touch any of that circuitry. Check the op volts and check the ripple voltages under load if you have a spec' and a 'scope.

One area of suspicion if you do get hum is the reverse biased diodes across each regulator chip. These have been known to go leaky putting ripple on the line. The 48v PPR is the most likely to suffer.

Dave.

[dmills]

My instinctive response to having any mixing desk power supply on the bench is to upgrade the bridge rectifiers to higher current examples (been burned by that far too often), and to fit 220n caps across each leg of each bridge (helps with rfi).

Those knee jerk reactions are however driven by work on much bigger desk supplies and by work in rf heavy environments, so may or may not be relevant to your situation.

Regards, Dan.

[ef37a]

Fair points Dan and the heatsink clips on the rec's are a little worrying!

But there is no sign (on the pikkies anyway) of excessive heat from the bridges so maybe all is ok this time? A careful finger test after the unit has been on load 1/2hr or so might be prudent. If you cannot touch them indefinately (assuming not kinky mazzo!)Yes upgrade. Cheap as chips and fit a heatclip as well.

Dave.

[James Perrett]

I seem to remember that those power supplies are designed to supply up to 24 input channels so a 16 channel desk won't be stressing them too much.

Can you identify which part of the circuit the noise is coming from? In my experience, the Mixdown is a fairly quiet desk - even with the mic gain turned up fairly high. Is the noise mainly hum or is it more of a hiss?

The audio circuits are mainly based around TL072's with a pair of transistors on the mic input. They're fairly simple but work well. Don't be fooled by the XLR connectors on the output - the outputs are actually unbalanced.

Cheers

James.

[PaulRich]

Thanks for the experience guys
As of the noise, it wasnt hum, more of hissy
Mind you that was when i tested it at the guys house i bought it from
Right now it is totally stripped down. Hopefully today Im gonna lube the whole thing up
I didnt want to start putting her back together if i was gonna mod the PSU with new caps and bypass caps
As i said in my post about the only thing ive soldered are all my own cables so i dont know too much about the electronics side
i was hoping someone coupld show me soldering by numbers on the pics ive posted
thanks once again
paul

[James Perrett]

The PSU will probably require new electrolytic caps in a few years time but, given your level of knowledge, I'd advise you to get the thing going as it is and forget about doing too many mods. The PSU is easy to get to so it won't take much effort to open up the desk and modify it when you've gained more knowledge.

Cheers

James.

[PaulRich]

The Latest
So Ive just put her back together and did a bit of testing
There only seems to be 2 channels that are quiet
The rest are either noisy, not working and the scratchy pots seem worse off then when i first got it
I took it all apart and lubed everything usun Deoxit faderlube
There is a hum coming out the left hand side of the headphones
All the channels right/left configuration seems to be backwards
Is there any way I can take out each channel and check it with some kind of scope to see where the problems are based
I have a laptop running Tru RTA which has a scope and frequency sweeps etc
I gotta get this baby working. Soon
Thanks
PAul

[James Perrett]

Sounds like you may have put the master channels back the wrong way round - I seem to remember that there is a link on each that controls whether they are left or right.

The controls may simply be worn out. I would also check the insert sockets on each channel - they will almost certainly need cleaning.

Cheers

James.

[PaulRich]

Thanks for that James, I will double check the order even though i marked them
Maybe they were in the wrong order when i got it
As of the channel inputs, everything on the desk has been cleaned.
Everything
Some said maybe flooding each pot, what do you think
And what about the hum
Thanks again
Paul

[PaulRich]

Latest(17/10/08)
After drowning all pots and switches in Servisol Super 10 I have gotten rid of intermittentness on the channels i have tested
I am still getting, what sounds like to be white noise and maybe 50hz spikes at 50, 150, 200, 250, 350, 450 & 650hz
Ive knocked up a crystal earpiece contraption my friend told me about and on 2 of the channels ive traced the noise, which starts at the collector of the 1st transistor(http://www.toshiba.com/taec/components2 ... 0/6451.pdf) from the line input. My friend said that I should check the caps around it to see if thats the problem as the transistor should either work or not. No in between.
Paul

[ef37a]

Yes Paul, check the caps but I fear your friend knows jack about transistors. Silcon transistors can go intermittant all ways up from christmas, noisy? You would not believe how noisy they can go!

Germanium transistors are inventions of Beezlebub, horrid things. Get a hair dryer and a can of freezer spray.

Dave.

[PaulRich]

Hey ef37a,
hope youre having a good weekend
Get a hair dryer and a can of freezer spray?
Is this some kinky game thatll coax it into sounding nice
Just kidding
How does that work?
thanks
Paul

[ef37a]

Thermal stress laughing boy, thermal stress!

You warm the general component area up and this often initiates or worsens the symptom. Zap each "infallable" transistor or susupect cap' in turn.

Sometimes it works better the other way. Cool something down then warm it up (you CAN use a solder iron but takes years of practice not to bugger everything you touch!).

Happy thermal stressing!

Dave.

[PaulRich]

Hey ef37a,
not quite sure i understand
so i power up channel, spray freeze at component(transistor, cap), then what am i doing, looking for, testing?
thanks
paul

[mu7et]

Hi! It sounds like you have a good idea of what you want to do with your Studiomaster mixer. Recapping and modifying it can certainly improve its performance and reduce noise. Here are some additional tips and suggestions:

1. When you're recapping, make sure to use high-quality capacitors with a low ESR (Equivalent Series Resistance) and a high voltage rating. This will help ensure that the capacitors perform reliably and don't introduce additional noise into the circuit.

2. In addition to adding bypass capacitors to the power rails, you may also want to consider adding a separate regulator circuit to provide clean, stable power to the audio circuits. This can help further reduce noise and improve performance.

3. Upgrading the opamps can make a significant difference in the sound quality of your mixer. The ones you listed are all good choices, but there are many other options available as well. You may want to experiment with different opamps to find the ones that work best for your particular application.

4. When upgrading the mic preamp transistors, make sure to use high-quality, low-noise transistors. The 2SA1316 and 2SA1083 are good choices, but there are also other options available.

5. When modifying the EQ, you may want to consider adding additional frequency bands or changing the frequency response curves to better suit your needs. Just make sure that you are careful not to introduce additional noise or distortion in the process.

6. Finally, be sure to take your time and carefully document your modifications.

[Mike Stranks]

Welcome mu7et!

I suspect the person with the problem has long ago fixed it, binned it or moved on...

... the thread is 15 years old...

[Wonks]

And I have no idea how you’d manage to add extra EQ controls to a mixer that’s already got as many controls on its fascia as it’s possible to cram in.

[James Perrett]

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

[Hugh Robjohns]

I suspect mu7et was really only posting to promote his own website... a link I removed.

[zenguitar]

I suspect mu7et was really only posting to promote his own website... a link I removed.

Ahh... And I thought that his post reeked of ChatGPT too.

Andy

[Hugh Robjohns]

Yes, it does seem to repeat a lot of earlier comments in a GPT kind of way.