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-   -   Send and return on Bozak CMA 10 2DL (http://www.wavemusic.com/community/showthread.php?t=15138)

Oki 12-29-2015 02:01 PM

Send and return on Bozak CMA 10 2DL

After years of searching for a reasonably priced Bozak, I have finally purchased a CMA 10 2DL! Iím not sure on the condition yet, but it looks like it hasnít been touched. Iíll find out next week when it arrives in the UK.

Iíd be really grateful if anyone could help answer a few questions. For reference, Iím pretty good with a soldering iron and Iím very keen for a project! Iím sorry if any of these have been answered before.

1. Iíd really like a send and return on each channel but Iíd also like to keep the mixer as original as possible. Looking at the schematics on the Urei 1620 and CMA 10 2DL, a send and return seems to just intercept the line level signal. As Iíd like to use the original Bozak phono cards (and not external phono pre-amps), could I theoretically put in a send and return point after each phono card but before the signal reaches the summing amp?

2. The front panel is in pretty poor condition. As Iím planning on some custom amends Iíll most likely press up a new panel. Does anyone know what font is used on the CMA 10 2DL or have a high quality version of the Boak logo?

Thank you so much. After reading the forums for so long, Iím really excited to get started!


Richi 12-30-2015 07:34 AM

You would need to isolate the send pots from the summing buss by using a basic emitter follower or an op amp buffer, you would ideally also isolate the output with another buffer. Each send channel will also need to be summed before leaving the mixer.

So you will need a reasonable knowledge of these circuits to attempt it properly.

vinyl_junkie 12-30-2015 02:54 PM


Originally Posted by Richi
You would need to isolate the send pots from the summing buss by using a basic emitter follower or an op amp buffer, you would ideally also isolate the output with another buffer. Each send channel will also need to be summed before leaving the mixer.

So you will need a reasonable knowledge of these circuits to attempt it properly.


Also from memory the original FX loop on the Bozak suffers from a impedance miss match, if you engage it you will loose some HF.
Search the forums, it's all here and been written about in detail.
The Urei's FX loop from memory is buffered.

Personally I wouldn't bother, it's a classic mixer. I kept mine as close to original as I could. But then I have the same views with regards to classic cars ;)

Re front panel I think few people on here should have either new ones or old stock they could sell you.

Oki 01-02-2016 03:29 AM

Thanks, Richi and and Vinyl. I suspected it wouldn't be as straight forward as hoped. The more I think about it, the more I am tempted to keep it as original as possible. I've sent Justin an email...

Thanks again!

Richi 01-02-2016 09:59 PM

It would be a straightforward job for Justin to complete though is the cash outlay worth it, if it does need work then it would be a good time to do it.

vinyl_junkie 01-03-2016 11:42 AM

If you have the adequate soldering skills why not just do any of the work that may be required on the mixer yourself?

It's fun and interesting at the same time whilst not being too difficult, I wrote a little article on this here


Scotty would be proud :D

Oki 01-14-2016 07:19 AM

Sorry for the lack of reply. I didn't get an alert to say the thread had been updated.

I met Justin (top guy) and we discussed options. The mixer has a few wider issues around the summing amp so I'll not be doing the work myself. I think my project will be to build an external power supply. That way I'm not messing with the mixer too much but can still get hands on.

I'd also not seen your article on your Bozak rebuild, Vinyl. Just wow. That's super impressive.

I'll post pics in the next few weeks.

Thanks all once again!

vinyl_junkie 01-14-2016 01:51 PM

Cool, what's up with the summing amp or the mixer in general?
Funny story when I got mine I got so excited I plugged it in before I even checked it. Keep in mind it came all the way from America via plane to the UK. So I was getting sound in the headphones but no output on the main and I was like da fuck is this shit? Open it up and the summing amp came out and was loose.. FAIL! lol

FunkSoulLover 01-14-2016 06:04 PM

On a related note, the output on my Bozak is really unbalanced with the right being much louder than the left. This is uniform on all channels (phono and line) and can be rectified with the the balance pots.

Is there some thing inside that needs to be adjusted to fix this?

Oki 01-14-2016 06:19 PM

@Vinyl - that's far too familiar! I did exactly the same thing. Was four weeks in transit from from the US to the UK and I felt like I was refreshing the courier page hourly! I was so excited to get it here that I plugged in immediately but got nothing. Then I opened it up and found that all the cards were loose - I guess from transit. Unfortunately, when I reseated them all, I've still got problems.

Everything seems ok through headphones, although there is a little cross channel interference. I think that's fairly typical though. I'm also getting nothing out the master (although the pot is really loose, so could be a mechanical issue) and there are left/right balance issues through the tape out. Also, all the volume pots have been fitted with linear 50k rather than log 50k which is very weird. Otherwise though thankfully it feel like it hasn't really been touched inside. There was a nice bonus of an extra phono card though!

Luckily none of the problems seem insurmountable. And I got it for a great price so can't complain. As a minimum, the issues are being fixed and it;s getting new pots and caps throughout. At the same time though I'm also considering either a minimal version - bypassing the tone and balance, or a version with send / return on each channel. Again it's a toss up between keeping it original vs something genuinely useful for me.

@Funk - I just saw your reply as I was about to submit, but the problems sound quite similar. I'll ask Justin what the issue was when he's done some more investigation and will report back.

vinyl_junkie 01-15-2016 02:09 PM

Re channel imbalance most of the time it's the pots. With age they generally degrade and will drift. You will get channel imbalance over time especially if it's still got them A&B pots, ALPS generally tend to fare better I think.
The A&B pots on mine were totally shagged.

It could also be the resistors around the balance pots. Balance pots are set up as voltage dividers from memory.

Re no output, check the pins on both the mainboard and the summing card.
Are you getting voltage there? Is audio getting there?

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