Studiomaster 16-4-8 restore

Hi, so I’ve 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:

16 Replies to “Studiomaster 16-4-8 restore”

  1. Hi Paul

    I’ve checked your posts and the pix.
    I agree with some of what’s said. Basically;
    The electrolytic caps look okay, so leave alone for now
    Leave the thing running for 30mins, connected to the mixer with the connector, preferably with some music going through into a normal output, say tape deck/PA etc.
    Then put your finger on the black part of each of the voltage regulator chips. They should be hot, but not too hot. If you can’t touch one, either replace with a higher capacity component and/or gently raise it and try some new heat sink compound. The stuff doesn’t last for ever, especially if it’s been continuously hot. What you want is all the heat from the IC to end up in the metal bracket! The transformer is pop riveted to that as well so will warm it up also.
    Now do the same test with the PSU open circuit i.e. switched on but not connected to the mixer. Depending on the design, some chips are hotter in open circuit condition as they effectively take all the power!
    Your power supply is already decoupled! That’s what the 100nF capacitors are doing across each input and output rail to the Voltage Regulator ICs! All they are doing is to short rf frequencies to earth.
    I don’t know the purpose to the reverse biased diodes across each chip. They look to be a kind of fail safe protection. I’m pretty sure that they are not on the IC manufacturer’s data sheet and suggested usage.
    If it’s hiss you’re getting, you should be able to nail it down to a channel or buss, except if the gain was right up somewhere and you didn’t spot it. If you can nail it to a channel or buss, it’s most likely coming from an op-amp! It may need re-soldering to correct a dry joint, or replacing.
    Dry joints you test by pushing on each joint with a small screwdriver or something while the thing is on and listening to the output. Because the PCB flexes, it’s quite hard to actually really zero in on the problem, and I’ve always just retouched a block of joints one at a time with a dab of new solder at the same time to trap the offender.
    Dry joints are a real pain.
    If the hiss is on channels equally, it’s the output stage(s). If it’s the same L & R I don’t know! Maybe it’s just like that. Depends on the gain.
    OR it could be active gain on the final stages as opposed to a simple earthed fader on the output.
    Check all the pads on each channel are at minimum gain and have everything output set to zero with EQ centred. Is it still hissy?
    I’ve no opinion on electrolytic/tantalum. Go by the manufacturer’s data sheets to fit the specs you need. Basically, they shouldn’t be in the direct signal path. Either of them.
    Always use an electrolytic with the same or greater voltage rating. Apart from that there is no difference. ALL capacitors (and especially electrolytic) have a huge variation in actual Farad value c.f. what’s written on the tin! E.g. 100uF could be anywhere between 50-200 for an electrolytic! – which is why they are not in tuned circuits but are usually only for power supply smoothing and things like that.

  2. The Latest
    So I’ve 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 using 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

  3. You should have used Servisol! It’s made in Bridgwater at Ambersil’s http://www.crcind.com/csp/web/Countrypage.CSP?country=GB&lng=3 ha ha!

    Hum mean bad earth. Check the solder or clamping joints. Squirt a joint with a freezer spray (also made in Bridgwater at Ambersil’s) to see if the hum changes or whack with a screwdriver. Sometimes just touching a dry joint picks up taxis etc so helps tell you which are the dodgy ones.
    If ALL channels are L-R swapped then it’s the wires to the outputs that need swapping round!
    Sometimes you need to really flood the pots with Servisol. A bit like washing grit or chemicals from your eyes. Use servisol – it’s cheap and plentiful and Canford use it so it must be good.
    The channels are probably powered in groups by circuit board say in 2’s, 4’s or something. Disconnect the power supply to all but one board and do a board at a time. You’ll only need to unplug a terminal or unsolder a few main +ve and –ve supply wires. Then test just the one part at a time. It helps to zero in on a problem.

    Rees

  4. I’ve checked their whole web site and there doesn’t seem to be a product for lubing pots?
    Ill do some other checks on mixer tomorrow to see if main outputs mirror headphones
    There is a ribbon cable that connects all the channel boards together, but there is just one earth to ground(mixer chassis) coming off the main out, that I would assume is grounding all channels(as obviously they are all connected
    The hum problem is why Jim Williams recommended putting caps on PSU, but you said they were on the PSU board
    I’ve tried to figure out the schematics from the component side and from the trace side, but I just don’t get it?
    Thanks
    Paul

  5. Servisol super 10 & servisol aero klene
    http://www.crcind.com/csp/web/functdisp.csp?lng=3&country=GB&brand=SERVISOL

    The freeze spray is on the same page. Most electrical/electronic shops have it on the back shelves. Unfortunately, I don’t know anyone who works there now – I used to and they gave me some fancy glues!

    They also sell tins of compressed air to blast shit out!

    You shouldn’t assume that the only earth is from the socket. There’s probably one near the PSU and it will have something to do with the transformer as well, probably, to set the 0volt between the two voltage regulator chips. On the circuit it runs from the connector to the centre taps and zeroes the PSU regulator chips. It’s drawn heavier on the circuit.
    What usually happens is that all the earths try to meet at one place – the star pattern. But usually, this is a theoretical point. Quite often the point stretches out into a bus, like a rocket on bonfire night. The key point is to avoid a loop. If you have a desk that’s 10 feet wide, it’s impractical to have a point for the earth. There will be a long line of earth star points, connected in series.

    I’ve checked what you mean by the Jim Williams (who he?) quote. There are 100nF caps across the power rails on the psu board – they are the green shiny blobs. 100nF = 0.1uF. So what he’s recommending is to stick them on each board/channel. Another guy said to use 200uF. It’s all the same really, given cap tolerances.
    I don’t know how many channels you have on a board. (16 twit – ed) Anyway, they wouldn’t harm as the long power leads act like little aerials.
    However, it does seem odd that a major manufacturer didn’t think it fit to add them in the first place. They don’t cost much, only pence….
    It depends what you are hearing as noise/hum. I can’t tell from here. I think the source lies somewhere else if it’s that intrusive to you – even considering your ears are used to v.good quality preamps and digital working.
    • My gut feeling on any old stuff like this is the joints. It may not be the normal solder ones either…
    • How are the op-amps attached to the boards? Are they soldered or in sockets?
    • Check punch-in switches, pads, switchable EQ; all that sort of thing.
    • Check resistors across sockets etc.
    • If you have an old tape head demagnetiser, waft it around the circuit boards. It may trip a hummy bit into overdrive that will help trace the source.
    The Jim bloke also recommends ditching the PSU and replacing it with a two new PSUs, a big main one and a tiddly one for the 48V phantom supply – this is one solution I suggested for Dave’s Seck, if you recall.
    I’ve just read your posting(s) again….

    I’d follow James Perretts words. Don’t do too much!
    To recap (no pun intended)…

    We know the PSU works.
    We know 2 channels are clean sounding but the rest aren’t…

    Therefore….

    It’s not the output stages, it’s the remaining 14 input stages! They ALL need every single contact and switch in the signal path cleaning! Perrett is right – check the inserts as well! All of them. Check ALL connections are sound, unplug and re-seat EVERY connection and component that can possibly unplug. Squirt copiously with Servisol cleaner to blast the oxide away. Afterwards mop up the residue or clean it away with another solvent. It’s surprising how much you can use. I used half a tin once on one pot because I couldn’t be bothered to re-solder a new one in. It was used in a harsh environment though – beer, fags, crisps, tea, sweets, back of van etc. But I just kept flushing and jiggling to wash out the gunge.

    The L-R on the outputs seem to be a little switch according to Perrett, else swap the wires like I said. You’ve put something in back to front.

    If you can’t match the schematic to the actual, well tough! Some people can’t read maps for toffee but they can play a mean fiddle. If you’ve tried for a few hours and get stuck on an easy bit like the PSU in your pictures, you’ll never sort the real guts out. It IS hard, especially when the wires and bits are small and scrunched into a tiny space. Just be patient. Only you know if you’re capable but if it really is too hard after several hours struggle, then move on. At least you’ve got two channels!

    As another thought, you could whip the PSU out and use it on the Seck! There were more working channels on that!

    The voltage rails are exactly the same.

    Rees

  6. Hey man, thanks for that
    I think I may pick up a can of it and flood the pots
    I came across this video last night https://youtu.be/R_5QvAUkA4A and some of my pots are acting the same
    I think this is why its been advised to change caps as they are probably old and dried out
    Can you explain what he is doing to the caps? Is he just jiggling them to find the faulty one?
    Also I found another post http://www.tangible-technology.com/audiobasics/upgrades/upgrade1_opamp_cap.htm where the guy uses a 500hz square wave to check for problems
    A friend of mine is gonna lend me his oscilloscope, which should be interesting
    In the meantime I can use this software https://www.trueaudio.com/ which has various sweeps and an oscilloscope too
    The only problem is, how do i connect it? Do i just use a 1/4 jack in and out? but how do i check various points on the channel card to find the specific problem
    I have taken multiple pictures of the board of which i can post on photobucket
    Jim Williams is the main man from Audio Upgrades, he does all kind of upgrades to the old school mixers
    He posts now and again at Tape Op forum

    I’ve checked what you mean by the Jim Williams (who he?) quote
    Everyone talks of him very highly, the only problem is he’s in the States which doesn’t help me

    I don’t know how many channels you have on a board.
    I have 16 channels, 4 buses , a left and right Master fader

    How are the op-amps attached to the boards?
    They are in sockets(which could be great for later upgrades?)

    My gut feeling on any old stuff like this is the joints.
    I’ve looked at all the soldering on the track side of the channels and all seems OK
    I’ve lubed every switch and clicked many times
    or Do you mean something else?

    Check resistors across sockets etc.
    What am I looking for?

    If you have an old tape head demagnetiser
    I don’t have an old tape magnetizer

    They ALL need every single contact and switch in the signal path cleaning!
    Not quite sure how i go about cleaning the contacts. If you mean 1/4 sockets etc, they’ve all been cleaned with isopropyl.

    Check ALL connections are sound, unplug and re-seat EVERY connection and component that can possibly unplug.
    Do you mean the Op amps also?

    Squirt copiously with Servisol cleaner to blast the oxide away. Afterwards mop up the residue or clean it away with another solvent. It’s surprising how much you can use. I used half a tin once on one pot because I couldn’t be bothered to re-solder a new one in.
    How do you know how much is enough?

    Luckily I have been lent a Mackie desk for a bit so at least the studio isnt completely down
    But I AM DETERMINED to get this Studiomaster desk up and running, so Ill keep plugging away at it until I do
    Thanks again for all the help
    Let me know when your free and well figure out a visit for you
    Have a great one
    Me

  7. Hi dude.
    Like I say, get every contact in the signal PATH cleaned – so op-amps if they are in sockets and all the switches and insert points. He does a good thing with the inserts which is what I was trying to explain. The sockets are exposed to a lot of physical abuse and dirt so anything that touches them will be too. Hear what he said – a good clean with spray then 25 insertions! This is an idea of how much cleaning and jiggling is necessary on your pots. Squirt and jiggle until they stop cracking or until they wear out and need replacing!
    Isopropyl isn’t enough! It just takes the grease away. You need Servisol or similar. It leaves a kind of slime on the contacts to prevent oxide build-up. It’s electrically neutral but can leave the circuit board messy which picks up fluff so you need isopropyl alcohol or meths to flush the crap off the board afterwards and then put a small amount of Servisol on the contact points again afterwards to prevent oxidation.
    When you first squirt the servisol into the pots, lots of black stuff will come out – this is dirt, spiders, crumbs AND bits that have worn off the carbon tracks. Remember, it’s a mechanical device with one thing rubbing on another. By design, bits wear off just like anything that rubs. A bit like brushes on an electric motor. All this builds up so flush it out.

    He’s jiggling and gently pulling the capacitors. Because the physical dimensions between insulators and conductors are so tiny both inside components and with the external connections we make, movement emphasises any problems with physical coherence and oxide build-up giving intermittent diode-like contact.

    If you scope test, there are two basic waveforms that you trace with – sine and square.
    Sine sounds like a flute, square like a fuzz box, and they are used differently. Basically, sine tests the purity of your signal chain – sine in, sine out!
    Square tests the transient response of your system. The edge of the square should be sharp in, sharp out!
    Anything that deviates from these is called distortion!

    The signal generator/scope combination is good but not necessary. You can do most stuff with common sense and your ears. Remember your mixer has two good channels. This is important and means your output stages are okay and all faults lie somewhere else! You see what I mean by the common sense point? With a (Hi-Z) voltmeter you can compare voltages between the good channels and duff ones. That’s another bit of common sense.

    After that, it’s a process of do the work and eliminate potential problems methodically till you end up with the thing that’s duff.

    To do all this, you need a couple of probes (not anal) to check the voltages and signals at the various points in the signal path. This is where the ability to mate up a circuit diagram to the physical reality of components on a board comes in. Most stuff, is related to 0volts or earth. They are not always the same.

    The true audio thing – I dunno. It depends where it’s connected and what it’s inputs are like. A scope usually has a valve or FET input offering really high impedance to minimise loading. Textronix are a good make. I’m not sure that the thing you mention is a true scope.

    It seems a lot of bother and overkill to what should really be a straightforward clean and resolder job, replacing duff bits as they are found. The duff things are the ones that don’t respond to cleaning and/or make bad noises when jiggled. Remember, all mechanical things wear out eventually – even jack sockets!

    Rees

  8. Hey Paul.
    I’ve just thought, that you’ve come a long way since you first asked about passive mixers… It all seemed so simple back then.. (casts mind wistfully back into a forgotten, misty, Nirvana, free of op-amp and capacitor worries)
    Your studio is turning into a bit of a parts store or graveyard for old mixers ;-) It’s ironic in a way, given the wealth of low-noise kit you’ve got!

  9. The freeze spray is just another way to jiggle components mechanically. Because everything except water contracts as it gets gold, the cold is stressing the mechanical joints and contacts as they all contract at different rates.

    To get the op-amp and other chips out depends on how much space there is and how dexterous you are… I’ve always managed to grab them by the sides. NEVER grab by the pins as you can bend them a bit and then it’s quite tricky to get them all lined up nicely to go back in the socket. (They only need to be a fraction of a millimetre out and then, oops, they’re not lined up – then you can damage the pins really easily as you try to push the chip home.)
    You can get little plastic clamp-type grabbers, called IC Insertion Tools quite cheap. Except; I never used one!

    If it’s been in a while, it’ll be a bit sticky and may need levering up with a screwdriver or something – trying to keep the chip pins straight all the time.

    A loose connection is as bad as a rusty one! Bear that in mind as well.

  10. Hey you wrote about the summing amps to get a clearer mix bus. So, I have 5532 on the whole board apparently (which aren’t too bad in my opinion). Opend the board, but can’t figure out which ICs on the master channels are the summing amps to replace with better Opamps, any ideas or schematics?
    And does it make sense to replace the lytics in the PSU. Didn’t get any hum or noise so far and for better channel separation I would have to replace the local decoupling on every PCB I think…

    Thanks

  11. Not sure if Paul still has his machine, but here’s http://www.ti.com/product/ne5532 with the maker’s tech data with image of the chip.  I’ve found a Series II manual which probably covers your machine.  However, replacing the summing op-amps with better won’t make that much difference to noise or headroom.  You may get better CMRR though and slew rate. Stuff like that.

    If the PSU is okay, then it’s okay. Ideally to remove any influence of the PSU, take it from the box and use a shielded external power supply. By all means use the most expensive and fattest capacitors you can afford…. after a time you hit the law of diminishing returns.
    Decoupling puts two capacitors in series and parallel with the signal path and across the power lines and earth. The low value capacitor in parallel shorts high frequency interference away and the high value one blocks DC and low frequency when in series with the path. If they’re duff they’ll affect the signal.
    Channel separation reduces due to cross talk from signal tracks in close proximity. When all the circuits are stuffed into a box you’re kind-of stuck with that design. The manufacturer will have provided shielding where he could to minimise this as well as interference from the PSU and the local taxi firm, but it’s the nature of a mixing desk to have lots of signals running around inside the box that are in close physical proximity to each other. It’s just the way it is.
    Clarity and noise reduction at the first input stage is everything, since the background mush is mixed into your final mix in direct proportion to how much you started with. Once there, it’s nigh-on impossible to get rid of.

    To reduce noise (assuming the capacitors and pots are working correctly, that is), always look at the first steps in the circuit.  Use balanced mikes if you’ve got them. Then check the connections, then the very first input resistors and the first amplifying stage.  Of course, with 16 channels, that could be a lot of replacing… ;-)  The mic circuit produces most noise as that’s the one that’s had the most amplification.  The circuit below shows discrete transistors at this point with 220R & 1k0 resistors in a switch-able input impedance setup. 
    These resistors must be the lowest noise and probably most expensive resistors you’ll ever come across if you want to minimise hiss and crackle.
    Soundmaster and other good brands used good ones to start with, but it’s worth checking. You’ll need resistor charts to figure out what you’ve got – it’s all in the colours if it’s not written on the side.

    To answer your question about locating the summing chips and in the absence of any circuit diagram, follow the busses from each input channel to each master bus and the chip should leap out at you on each board.  There will be 16 wires all rolling into one point!  Since each chip has two op-amps they may have it configured to use one chip on each channel, using the extra amp for buffering say; or they may have one chip handling two channels with each op-amp doing the full work that’s intended.  Similarly, the final 2 masters will have something much like this.

    This block group/mix stage shows this.  After the switching selections, all the signals are pulled together and will go into the inverting input of the mixing op-amp and then to an output, in this case, the tape sends etc.

    I notice that the op-amps in these circuits are nearly all TL071/2/4 which isn’t yours, so perhaps it’s from a different year.  However, the principle is the same.

    So if you can’t get a circuit or plan, follow the wires and circuit tracks.  It’s tedious sometimes and a bit like following a hosepipe to find the tap end, but worth it, and at least you’ll be certain that that’s where the signals are going!

    I got these circuits from dancetech.com – you could too since it’s a free sign up.

  12. Hello can anyone please tell me exactly what the round black pieces are in the power supply? There are 3 of them 2 of which have a black star shaped plastic piece around them. They Look like caps but they are not. I have a burnt one and need to replace. The numbers on the top say W0-02 where could I find replacements? Thanks in advance

  13. They are full wave bridge rectifiers Jay. WO2 in a WOB (aka RB-15) case style. Very widely available and cheap….
    https://www.google.co.uk/#newwindow=1&q=W02m+bridge+rectifier
    Maplin sell them for 9p – http://www.maplin.co.uk/p/w005-15a-silicon-bridge-rectifier-aq94c

    The 2 in the code refers to the max peak voltage handling of 200V. Here’s one of the many datasheets on the part – https://www.digchip.com/datasheets/parts/datasheet/2136/W02.php

    Since you’re in the USA, this is a RS link to the thing – http://www.alliedelec.com/hvca-w02m/70016126/

    You should be able to use a higher voltage rating with no ill-effects, e,g use a WO5 for 500V rating. Sometimes it’s more cost effective. Just don’t use a lower rated component.

  14. Hello

    i found this here now, cause i`m rearching arround to find info about my 16-4-2 studiomaster, i took it appart some weeks ago, to clean it and recap etc. . i did some pics on my phone, but lucky as i am, my phone got stolen so i lost them. if anybody of you got some pics from inside it would be amazing if you could send me some. its all about wirering, cause i removed all the connectors and so on. ( I#m not able to read it out of the shematics ) thänx in advance

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