xayo

Sennheiser M8 Broadcast Mixer
« on: January 17, 2021, 08:26:15 AM »
Hello everyone,
first post here so I thought I'd introduce myself: I'm Robin from Hamburg and I've been producing electronic music for 10 years+.
During my time doing this I realized that analogue is the way to go. Now I'm well in my 20's and finally have the means of making this a reality for me.

Against better judgement I recently bought an old broadcast mixer that's older than myself. It's the Sennheiser M8, 8 channels said to be built to the same specs as the Studer and Siemens Mixers (Braunbuch specs). I knew from the start that it needs some work, so I've been stocking up on the electronics side (soldering iron etc.) and reading a lot of stuff. I don't have much hands on experience, but I come from a family of electric nutjobs (my grandpa used to work at philips in the 60's, father is in the tv broadcast industry) so I have been around soldering irons and correlation meters quite a lot growing up. I am willing to listen and learn from you old heads, so please give me the benefit of doubt on this one :P.

The mixer is in relatively good shape, everything is built like a tank. I opened up one of the channels and it's all looking quite clean. I could not spot any leaky or blown caps for now (I referenced images from the internet but I might be wrong of course). The previous owner sold it as "serviced", but that could mean anything really. Sadly there is a slight communication barrier as he is a french gentleman in his 70s or 80s, so he gave me the schematics and I will take it from there. 

So now onto the problems:
1. The Pan-Pots seem to be "wired" the wrong way, I guess? When the signals are panned hard left or right the phase correlation on the master is normal (-45°, 45° on my meter). When I turn the Pan-Pot to the middle position though, the correlation goes near -90° (-100%) instead of  0° (100%). So it's like one channel has a flipped phase? This is my biggest problem as I can totally not use any of my mono instruments without creating "illegal" signals. I can only imagine that the previous owner probably only used stereo pairs of channels so he never noticed it or something. Interestingly the integrated 1khz tone gen also exhibits this behaviour, so it might as well be a problem on the master module? I already did a long search on this forum and on the web in general, but I just can't seem to hit the right search term.

2. The other problem is that the channels do not gain match. This is to be expected from component drift. My intuition tells me this won't be fixed unless I do a full recap on everything. Which is not something I want to do right now. I can live with some variation, as I'm using this as a production unit for music making and not mastering or broadcast applications, so I can always adjust gain as needed. I mean, i wanted analogue and I got analogue. But being a naturally curious person I would love to have some further info on where to start looking first.

I have the mixer schematics, phase correlation measurements, REW graphs and a cute photo attached below.
Thanks for taking your time reading this and maybe even replying :) . Looking forward to learn a lot in this place, I love a well cultured forum.

Thanks in advance,
Robin

Images and measurements:
https://imgur.com/a/qbmYN10
« Last Edit: January 17, 2021, 09:52:43 AM by xayo »


cyrano

Re: Sennheiser M8 Broadcast Mixer
« Reply #1 on: January 17, 2021, 10:29:46 AM »
Nice find!

I've been looking for one for many years. Came across a few, but they sold too fast for this slow bugger  ;D

I did have an M101 for years. That was in a bad state cosmetically, but turned out to be fully functional, except for one broken trace and bad connectors.

Expect things to be reversed. Starting with male connectors for inputs on the M101. And positive ground circuits.

As for the signal loss, check for loose connectors, broken traces (from oxidation) and so before the recap.

When I recapped my M101, most Cs were still OK. If there was drift, it was up. Leakage was minimal for it's age.

Sennheiser transformers, if you have those, are excellent, but a complete unknown, as far as the net is concerned. You'll find a lot about common mic transformers, but the ones in the M101 as well as those offered by Philips in their PRO12 tape recorder as an option, are unknown.
Why is it people love to believe and hate to know?

xayo

Re: Sennheiser M8 Broadcast Mixer
« Reply #2 on: January 17, 2021, 10:49:09 AM »
Nice find!

I've been looking for one for many years. Came across a few, but they sold too fast for this slow bugger  ;D

I did have an M101 for years. That was in a bad state cosmetically, but turned out to be fully functional, except for one broken trace and bad connectors.

Expect things to be reversed. Starting with male connectors for inputs on the M101. And positive ground circuits.

As for the signal loss, check for loose connectors, broken traces (from oxidation) and so before the recap.

When I recapped my M101, most Cs were still OK. If there was drift, it was up. Leakage was minimal for it's age.

Sennheiser transformers, if you have those, are excellent, but a complete unknown, as far as the net is concerned. You'll find a lot about common mic transformers, but the ones in the M101 as well as those offered by Philips in their PRO12 tape recorder as an option, are unknown.

Thanks! So you are saying that my weird phase issues could stem from using standard adapters and connectors with the non-standard mixer? That would be great as I only would have to solder custom cables instead of ripping the whole thing apart,right?  :D .

ruffrecords

Re: Sennheiser M8 Broadcast Mixer
« Reply #3 on: January 17, 2021, 11:22:10 AM »
Looking at the schematic, the pan circuit is 100% conventional so unless someone has modified it I think the phase problem itself is not due to the pan.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

xayo

Re: Sennheiser M8 Broadcast Mixer
« Reply #4 on: January 17, 2021, 11:52:02 AM »
Looking at the schematic, the pan circuit is 100% conventional so unless someone has modified it I think the phase problem itself is not due to the pan.

Cheers

Ian

Thanks for clearing that up :) Still have a hard time getting any value from reading a schematic. I will investigate more into the direction of connector issues for now.

abbey road d enfer

Re: Sennheiser M8 Broadcast Mixer
« Reply #5 on: January 17, 2021, 04:45:25 PM »
1. The Pan-Pots seem to be "wired" the wrong way, I guess? When the signals are panned hard left or right the phase correlation on the master is normal (-45°, 45° on my meter). When I turn the Pan-Pot to the middle position though, the correlation goes near -90° (-100%) instead of  0° (100%). So it's like one channel has a flipped phase?
Well it looks like something along the way between the input channel and the correlation indicator is polarity reversed. I don't think the problem is with the outputs, it's probably the connection between the outputs and the meter. Unfortunately, the  schemo doesn't show where the correlation meter is connected, though I suspect it's connected in the monitor module, so there is a large probability that a connection between the outputs and the master module is reversed.
Maybe it's simply one of the inputs to teh correlation meter that's reversed...

In order to know if the outputs are in or out of polarity, you just have to connect them in parallels. If they are of same polarity, the signal will be normal, if they are out, the level will be much smaller and sound weird.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

xayo

Re: Sennheiser M8 Broadcast Mixer
« Reply #6 on: January 18, 2021, 05:57:12 AM »
Well it looks like something along the way between the input channel and the correlation indicator is polarity reversed. I don't think the problem is with the outputs, it's probably the connection between the outputs and the meter. Unfortunately, the  schemo doesn't show where the correlation meter is connected, though I suspect it's connected in the monitor module, so there is a large probability that a connection between the outputs and the master module is reversed.
Maybe it's simply one of the inputs to teh correlation meter that's reversed...

In order to know if the outputs are in or out of polarity, you just have to connect them in parallels. If they are of same polarity, the signal will be normal, if they are out, the level will be much smaller and sound weird.

Thanks for the thoughts! I failed to mention that my correlation meter is an ITB Vst Plugin, so I will check the connectors first :)

Greetings
Robin

xayo

Re: Sennheiser M8 Broadcast Mixer
« Reply #7 on: March 08, 2021, 06:44:44 AM »
A little update on this mixer:

The phasing issue turned out to be a grounding issue and went away when I invested in balanced cabling. Sorry for wasting everyones time there. The mixer is working great, I really like the sound of it and even use it for routing my DAW through it for the EQ and subtle phase effects. I think the wider stereo image stems from the fact the EQs are never 100% matched in gain or frequency between the stereo channels which results in a warbled phase response left to right. I like that.

After extended use I noticed a fishy smell and started taking out channels one-by-one to isolate the smelly channel.
I opened up the channel and discovered some very strange veiny corrosion on the back of the board, but I couldn't isolate the area causing it.

Here are some pictures from different angles: https://imgur.com/a/9z93TLc
A few things that I noticed:
- weird white stuff on the plastic shielding
- some type of corrosion on the connections of the big metal box.
- the capacitors all looked okay to me, although there might be some I can't see without complete disassembly.
- the smell wasn't present when the channel is not running

The channel is measuring pretty ok and operates as it should, but the ever increasing smell is not something I want to be sleeping next to.

Thanks for reading and I hope anyone has some advice on how to go forward with this channel.

Greetings
Robin

abbey road d enfer

Re: Sennheiser M8 Broadcast Mixer
« Reply #8 on: March 08, 2021, 08:32:28 AM »
I opened up the channel and discovered some very strange veiny corrosion on the back of the board, but I couldn't isolate the area causing it.
Not corrosion. these ridges are the result of applying tin finish the old fashioned way. Nothing to worry about.

Quote
- the capacitors all looked okay to me, although there might be some I can't see without complete disassembly.
- the smell wasn't present when the channel is not running
I would strongly suspect the red electrolytics. They are known to fail.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

xayo

Re: Sennheiser M8 Broadcast Mixer
« Reply #9 on: March 09, 2021, 06:53:19 AM »
Not corrosion. these ridges are the result of applying tin finish the old fashioned way. Nothing to worry about.
 I would strongly suspect the red electrolytics. They are known to fail.

Thanks for the quick answer! Will investigate those red ones first.


Re: Sennheiser M8 Broadcast Mixer
« Reply #10 on: March 14, 2021, 12:42:22 PM »
That fishy smell could well be electrolytic fluid from somewhere. I'm recapping some old Frako caps in my EAB/Geiling Tre400, some of them just lose their legs in the process and the fluid comes out, and thats how it smells..

While we're on the subject of the M8, I have been asked to carry out a very simple direct out mod for a friend, that he discovered on GS.

The instructions can be found here:
https://www.gearslutz.com/board/showpost.php?p=12200196&postcount=13

Images and Schematics here:
https://www.dropbox.com/sh/ipbpqvn4d9968s6/AABDmMB05nplmD_b88eey86aa?dl=0

My question is, would a resistor (100ohm?)-capacitor (100uf?) network be advisable per channel?

Thanks!


 

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