Bozak Refurb Process
So thought of posting this on here, a few years back I got a silver Bozak CMA 10-2DL from roughly 1976
It had 3x newer style phono boards brought from John K and also a booth output card but no actual booth out modification (also from John K) the previous owner told me.
The rest were all original old boards I.e. the summing amp, eq, 2x mic amps and main output boards.
All the pots were original Allen&Bradley but was told they were all pretty scratchy and the seller also sold me 8 NOS ALPS RK-40’s for a pretty good price (later Urei style pots)
The mixer got here in the UK all well and I have to say I was super pleased with FEDEX, the import duties were minimal.
The Bozak wasn’t in bad shape considering it’s age but it did need all the pots doing as mentioned and at least the PSU caps replacing just for peace of mind but I sat on it for ages without doing the work and just using it like it was mainly as I was too lazy :-D
So one day I managed to get in contact with Buzzy Beck who was absolutely brilliant in helping me out and here's what happened.
To replace all the pots, re-cap the PSU, clean the cue switch, replace the knob caps as they were beat up and looked bad and replace the pilot lamp with a newer style one.
I originally wanted balance pots with ones that had centre detents but Buzzy didn't recommend it and also recommended that I didn't by-pass the balance pots. Here is what he said on that;
“I do not recommend the centre detent for two reasons. Yes they have a stop for centre feeling but the stop when centre with the face plate at 12:00 may not actually be equal balance.
The balance pots should be electronically centred as we did with every mixer. This means the pot when looking from the rear of the face plate may not be equal looking on both sides physically.
I have installed this option for customers who want it done but to do it right and not just stick a pot in takes time. The centre detent if installed correct may end up with different value resistors on the left and right side of the pot. I have never seen anyone take the time to do this correct. I have received mixers that these were installed in and the balance as much as 15% off.
I never suggest eliminating the balance pots for two reasons. Mainly you loose the ability especially in vinyl to equal out the right and left channel on a recording when needed. You must also properly load the input internally with a 51-68k resistor that should be select fitted with a oscilloscope. “
I went with Vishay capacitors for the PSU, Buzzy also gave me a new rectifier.
4x A/B Mod Tone Pots
1x Later style pilot lamp
4x RK27's for the balance pots (Linear taper)
Knob caps, bushing adapters, ground bus, resistors, wire etc
Regarding re-capping the whole mixer Buzzy said
“I am not a replace all capacitor person. The original Frako brand were the Rolls Royce of the day and though some in the voltage filtering circuits may need attention I have found very few problems with them in the audio signal chain over the years. If the mixer sounds good chances are the audio signal capacitors are still working properly. I would suggest the power supply card being freshen up if you keep the original type power supply “
I also spoke to Geoff Frost of Sound Techniques fame and he was also of the same opinion regarding re-capping.
Notes on replacing the pots
One of the first issues some one may encounter when wanting to refurb a Bozak is replacing the pots.
This is because as I found out there is more than one kind of RK-40 and the ones fitted to Bozak's differed from the ones you might buy on ebay or for the Urei.
One of the most noticable differences is that the Bozak Rk-40's have eyelets instead of solid pins where you solder on to, this makes fitting them in the Bozak slightly more fidley but isn't a major issue.
Other small things are the a slightly different taper and cycle life but the most important one is the following one.
The Bozak RK-40's have a different bushing size to the others, to put it very simply the ones on the Bozak are bigger and thus the face pannel has larger holes cut out whilst the non Bozak ones are smaller so you won't get a propper fit.
The A/B and Bozak Alps pots have a .250 diameter shaft or 1/4 inch bushing and the UREI and later Alps have a .235 diameter shaft or 6 mm (M8x0.75)
Mostly everybody thinks that other than the solder pins instead of loops they are the same but they aren't.
Thankfully Buzzy supplied me with Bushing Adapters so all my pots would fit well.
And now a little history, here is a great email from Buzzy on the development of the RK-40 “Black Beauty”
“Forgot to tell you that back in 1978 Shelly who was the head design engineer went to Japan for three weeks and with Alps designers developed the RK40 for Bozak use. The first run was for 10,000 pieces and that was used up in about three years. Bozak owned the rights exclusively for five years and then left it expire even though they continued till 1987 to buy and produce Bozaks custom designed RK40 potentiometer. Around 1983 UREI made a few changes and picked up the design and put them as replacements in the production of the UREI 1620 mixer for the earlier Nobile brand they were using. And yes Rudy at one time had lots of stock in Allen Bradley that his wife retained but he was pretty much out of the company by 1975 or so no matter what you may read or see on the big so called Bozak web sites. The reason the pot was designed was because the mixers and mixer/amplifier combinations were designed mainly for studio microphone or broadcast use. When the D and DL came about for disco and club use the input pots were being turned several hundred times a night and they were designed originally as a premium control for less used microphone level adjustments. The problem was that the carbon wipers wore out very quickly. The mixer became somewhat a standard and something had to be done to eliminate so many units requiring potentiometer replacement. The pot Shelly had designed was an adaption to a military style Alps was producing for overseas use. All the misinformation that is out there from some of the so called large Bozak web sites would have you believe the Japan CMA designed everything but this is so far from the truth. The Japanese CMA may have helped designed early units but had nothing to do with the D or DL series and up production mixers. Ironically the company who was involved with the D and DL mixers had the same CMA initials and that is where confusion comes in. The other CMA that 75 year old Shelly and others besides myself remember was a small German manufacturer in Connecticut that helped with in house design and production. The CMA stood for Commercial Manufactured Audio and would more likely be accurate when you consider the mixer had Frako and so many other German engineered parts internally. Also Shelly became part owner with the first initial sale and was head electronics engineer prior to that. He like me get a laugh and will admit there are so many story's out there that have no factual background at all. They are exactly that just story's and it is a shame that so many people try to take credit for something they were not even involved in. Another fact is that as a young man graduating high school in the mid 1950s Shelly personally knew Rudy and the family and had no idea that 20 years or so later he would work for him and become partners in the initial buy out. I became involved with Shelly and the Bozak company a bit later and although age is creeping up we still talk to each other on a regular basis. Shelly is now head engineer for a company that produces and builds custom microphones and conference room equipment.”
Notes on the summing resistors
When replacing the 4x mix pots I also replaced the summing resistors, Buzzy supplied me with either 51k or the standard 47k resistors shown on the schematics.
Regarding this he said “for the last 15 years or so we decided to go with the 51k because they seem to sum a bit better and when using older and slightly used pots they help to keep channel bleeding down on rebuilds. We just use them as standard now but you are safe going either way with new pots. “
I went with the 51k's
I've seen on here people keeping the originals saying it changes the sound and not to change them for new metal film resistors.. Can't say me or Buzzy believe it much so I replaced them with fresh resistors.
Notes on wiring+soldering etc
It is worth mentioning none of this work should be carried out if you have never soldered before, the boards are very fragile and the tracks can be ripped off very easily if you don't know what you are doing.
Soldering the resistors and bits on the pots is tricky if you've never done this before and I wouldn't like to see any one ruin their mixer.
Regarding the wiring of the front pannel etc as Buzzy said just copy it, it's a good idea to take lots of pictures and also draw a wiring diagram so you don't get lost.
Replacing the knob caps is easy, I posted few years back how I did it on my Urei 1620LE here on wave.
Process of doing
Before doing the big repair i.e. replacing the main pots it's best to do the easy stuff.
That is replace the tone pots first as it's pretty straight forward, check everything is ok then re-cap the PSU making sure to check it's ok after doing it.
You can also replace the knob caps now albeit I left that last.
Once all the easy stuff is done you can go and start tearing the mixer down.
Can't think of anything to add at the moment.. So here are some pics
The OG Bozak as I got it..
The re-capped PSU
The newer style replacement tone control MOD pot next to the original one that I took out the mixer
One of the tone control boards with the pots removed
RK-27 and 40
RK-40 mix pots with the ground bus bar and summing resistors
The front panel with the original AB pots
Zzzzz slowly doing shit late into the night
Wooo wiring the RK-40's, this is the main out pot from memory
Fast forward... Getting everything mounted in now, pretty tight. No I didn't fully fasten the pots in yet hence why one of them looks wonky.
Oh hello it's done
Lets put the cards back in..
Power's on how does she sound?
Again I'd like to thank Buzzy for all the info, help and guidance whilst doing it! It sounds great and should provide more years of joy.
If any one has anything to add or anything I've missed please feel free to add
We want to hear it!
Amazing! You saw this through to the end. Great work VinylJunkie :)
Thanks guys, I'll upload some tracks recorded through it so you can hear what it sounds like.
I need to re-foam the LF drivers on my speakers now, never ending hahah
I temporally used my KEF 104aB's which I have in the studio to listen to the Bozak after doing it and I liked the sound.
Sounds great to my ears, lower noise floor than my previous Urei 1620LE and none of it's harshness.
To me it's very natural sounding, doesn't seem to distort or add anything to the music letting you enjoy the records. Oh yea the pots not being scratchy and having copious amounts of channel imbalance is a good thing too lol
Excellent work! Thanks for taking the time to document and write up the process. I'm sure that will help a lot of people.
Any plans to add the booth out?
Thanks, I thought about this and still not 100% decided but I'm leaning on the leave it as it is side.
Whilst this is a useful addition the mixer will only ever really be used in my home where it's less needed.
Also I actually like the power switch there and find putting a knob in it's place looks a bit cack handed plus I'd kinda like to keep it close to original.
I'll keep the booth out card should I ever sell the mixer I'm sure the person buying it off me would appreciate having it plus I may change my mind.
I have to admit the Urei was one of the nicest laid out mixers with great i/o but I love the sound of the Bozak and it's not that much of a sacrifice loosing a few inputs and outputs for me.
If I was using the mixer live frequently I'd install the true booth output.
Worth noting, with the card installed and no mod it acts as a tape out booster card as normally the tape output is rather week.
That is great stuff and very useful to me as an all-original DLA owner.
Quick question: did Buzzy go into more detail about the PSU, why he would re-cap that module in particular and how you know would know if/when it's appropriate to do so?
I am interested in this part of the mod but to be honest I am a poor solderer!
No he didn't, it was something I wanted done and requested. I did ask him if it would be a good idea to do the whole mixer but as mentioned earlier if there were no issues with the mixer just a re-fresh of the PSU would be good.
Electrolitc capacitors have a finite lifetime, large electrolytic capacitors in power supplies are more prone to fail over time which is why I wanted to freshen the PSU seeing as I was doing other major work on it.
Failure depends upon the capacitor type and environment (primarily voltage, heat and amount of ripple current)
As Buzzy said earlier the caps used in the Bozak were of very high quality and the ones in the audio signal chain should hold up quite well still.
I guess if the caps started to fail in your PSU you'd have hum/buzzing issues.
Here is a email from Geoff Frost regarding re-capping his mixers he built in the 60's and 70's at Sound Techniques. A lot of famous albums were recorded on these and those recordings are regarded by some as the finest at the time.
He's recently got back into it and managed to get hold of one of his old desks and we got chatting about the servicing.. Here is a extract
(The A Range desks were produced from 1964-1969, again high quality capacitors were used)
"Most old desks must be re-capped as the dialectric in the electrolytec capicators dries or leaks out. This means that it is no longer a capacitor but a resistor! The advice you've been given is quite correct. Faulty capacitors, by letting through the DC that they were intended to block, will affect the biasing of the next stage in an amplifier or cause faders to appear noisy.
In the case of the 'A' Range desks we used very high quality electrolytec capacitors made by Wima. (They are still making capacitors but not high value ones ) I have to admit my surprise when I found no sign of leaking in the amps I'd tested at the time of making the video. However since them I've found that about 5% are in fact leaking.
There are many theories about electrolytec caps in professional audio design. In general buy a few of the best you can afford and ones with the highest working temperature, at least 110 degrees C. Then keep the units COOL!"
Also on the subject of the PSU Buzzy added this
"Unlike many mixers that use a fixed voltage regulator the Bozak mixer has a variable regulated power supply.
Even though the mixer is very forgiving internally it does not hurt after many years to freshen up the power supply card.
The mixer is capable of proper operation even when the voltage drops below or exceed the proper rail voltage specification.
There are no components in the audio signal chain on the power supply card other than the physical path through the turn on relay. Because of this audio bridge it is a good practice to guaranty solid pull in voltage. Also the audio signal could suffer from deterioration in the filtered DC power supply."
You did a fantastic job on that Bozak. You must be pretty good with a soldering iron. That job is not something the average person can do. Buzzy Beck is one heck of a nice guy. We could use more people like him.
Thanks :) And agree
Whilst on the subject of smoothing caps in PSU's, here is a picture some one sent me of the PSU filter caps removed from a original 1620
The filter caps are right next to the vreg (too close) and get scorched, not nice!
Looks like you have done a good job of the rebuild, personally i would have replaced the resistors with MF especially the summing resistors where noise is being combined.
Just because I can, more pics hahah
It amazes me how well the silk screening stood up considering it's age.
The back is almost intact and the RCA sockets don't have any corrosion/oxidation albeit the pins on the XLR out are a quite oxidised.
Thought I might as well put these on here as I never did get round to it.
Here are some audio files recorded with the Bozak if any one wants to hear what the mixer sounds like.
It's quite funny as I kinda lost interest in DJ'ing lately so I rarely have a mix now days but I still love to listen to records on the Bozak and to my ears sounds nicer than my Audiolab 8000C preamp.
downloading the audio files now... what cart/stylus was used?
WOW, nice wide stereo image!
Stanton 680EL mk1, recorded from main output into Sony DTC-1000ES then transferred digitally from the DAT to the computer.
VInyl_junkie sounds very good. Was once told by Buzzy Beck that the phono cards were tailored to the Stanton 680 cartridges in the days of disco and the building of the D and DL series mixers. Could be why the Stanton's sound so good with your mixer. Did he ever give you any insight on that?
I asked him once if he knew the capacitive loading of the phono input because as far as I know for a MM cart to have the correct response the resistive load has to be correct which is usually standardised at 47k ohms and the capacitive load also need to be correct.
Usually the latter is a lot harder to find out as hardly any one publishes this figure any more. You also you have to know the capacitance of your TT interconnects i.e. from head shell to phono leads. I THINK for the 1200 it's 125pf
Some top end receivers back in the day even had variable capacitance for the phono inputs.
Here's an example of the affects capacitive loading has on a Moving Magnet cart
Any way I'll paste his reply...
"The phono card that is in the DL series mixers came from the 919 stereo home Hi Fi mixer. The original front end of the card was designed to accept a wide variety of cartridges with different capacitive ranges and the second stage adapts it to broadcast flat response. A couple resistor changes can be played with but you will not end up with a very much better response than the original card is capable of. I may have an old chart I can dig up showing the resistor/capacitor combination changes and some of the carts we experimented with and their values. The final tailoring of the front end of the card was aimed at the most popular Stanton Disco series at the time and those specifications still generally work well with most cartridges."
He also later said that a lot of customers switched to Shure in the 80's - 90's and added that they sound great with the mixer. If I am not mistaken Buzzy uses Shure's albeit I'm not certain on the model.
Personally I prefer Stanton, they've always sounded great on most systems I had and to me had the truest sound. Albeit I never tried vintage Shure's, from what I have read even the old USA made SC-35C sounds a lot better than the current made in Mexico SC-35C so who knows.
Shure like Stanton declined in quality as time went on albeit not sank to the murky depths that Stanton has. If I am not mistaken Stanton has stopped making all styluses since the Gibson buy out, I know for a fact their factory in USA has closed
|All times are GMT -4. The time now is 05:09 PM.|
Powered by vBulletin Version 3.0.0
Copyright ©2000 - 2020, Jelsoft Enterprises Ltd.
©2006 Wave Music