1968 Philips MD console, troubleshooting

[Baltazar]

I bought this old Philips MD 1968 console, loads of xformers and all discrete, VERY solid build around 90 kg for 12 channels…

I have some issues with it though.
At some point someone has replaced all the pots inside the "faders". Very nice, however this person replaced what should be 2.5k log pots with 5k log pots… only half way of the fader is "used".

2.5k log pots doesn't seem to be the most popular type, so i was wondering, what will happen if i replace them with 2.2k instead?
I've found this one as a 2.5k candidate, would this be a good one? (https://www.banzaimusic.com/CTS-270-2-5k-LOG.html)
And what in the circuit decides what value on a pot that should be used?
If I can just replace one resistor or something else instead, i might just do that?


I also have some issue with output bus 1 making a lot of noises and oscillates when moving around the fader above a certain point, sometimes even below. This can maybe have something to do with the wrong value on the pot, BUT the weird thing is that this only happens on bus 1 and when i depress the pre listen switch (SK9 on the attached schematic) , this disappears… Any thoughts?
These oscillations also spreads around to other buses...

I have some more issues with the console, but will try to work that out myself before i ask you .

Attached is schedule for the output bus.
I will upload some more pictures later..

 

[gyraf]

For the fader (R32), it's really not that important if it's 2k5 or 5K. It's an attenuator, looking into not-too-low input impedance.

For Gain in the individual stages (R4, R21, R41 Trimmers?), actual value matters.

C16, C27, C30 are local power supply decouplings - you may want to replace these in the channel acting up

And btw - you can ALWAYS use 2k2 for 2k5 pots, as standard pot tolerance is +/-20% anyway

Jakob E.

 

[Baltazar]

Wow, thank you for your super quick reply Jakob!

Will try to adjust the trimmers and see if that helps! But can't really get around my head why a 2k5 or 5k doesn't matter. Twice the value fits in so well with all the channels fader just operates half way?
Or is what you're saying that it might be calibrated for 2k5 and not re-calibrated when they where replaced with 5k?

Ah, local decouplings, been youtubing that all night to try to understand. Thanks for pointing out what capacitors that was in this circuit!

What i forgot to mention is that it has been fully recapped, but it could be a bad solder joint in that area, have found some capacitors facing the wrong way and a cold solder joint in the monitor amp.

What i forgot to mention number two is that i've located the self oscillations to the amplifier stage behind the fader. What goes into the fader sounds/looks fine, the output (from the fader already?) from the fader doesn't.

 

[DerEber]

Well in this case.

If you set the fader to, say, half way up, you will (simplyfied) get half of the Input.
No matter if it is 2,5k or 5k.
What counts is you get a fraction of the total. No matter what that is.

Ofcause, if you go way off, some other issues will come in. Like driving capability of the former stage and stuff like this.

Please describe what you mean by "only half way of the fader is "used""

 

[Baltazar]

Well in this case.

If you set the fader to, say, half way up, you will (simplyfied) get half of the Input.
No matter if it is 2,5k or 5k.
What counts is you get a fraction of the total. No matter what that is.

Ofcause, if you go way off, some other issues will come in. Like driving capability of the former stage and stuff like this.

Please describe what you mean by "only half way of the fader is "used""

Thanks for the explanation, it might be that i'm more used to how "active" faders work and react.
What i mean with "only half way of the fader is used" is that most of the action happens around lower 1/3 part of the fader. hard to make finer adjustments. Will calibrate the console and see if that helps.


Made some progress today, seems like this console at some point have been pretty heavily modified. Got rid of the worst oscillations in bus 1. Bus 2 (it didn't work before) output transformer was a mess in how it was connected between output from the pcb and transformer. Signals where going everywhere. All good now.

 

[gyraf]

most of the action happens around lower 1/3 part of the fader.

Could it be that they have been replaced with linear faders, where they should have been logarithmic?

Hint: set to mid position, and measure resistance from each end to centre/wiper. Are they roughly equal, the pot is linear. If one resistance is 10x the other, it's logarithmic...

Jakob E.

 

[abbey road d enfer]

For the fader (R32), it's really not that important if it's 2k5 or 5K. It's an attenuator, looking into not-too-low input impedance.

Indeed. But this circuit has very low input Z. It's a VE input. So the fader impedance is quite significant. I don't know why they did that...
I think we need the part of the schemo that's on the right to further investigate the issue.

 

[Jaadaaren]

I've got 12 channels of what seems to be the same desk (three modules of 4 strips) that I'm racking as soon as I have some more time on my hands. I have a copy of the manual and schematics. Let me know if you need some of it.

 

[abbey road d enfer]

I've got 12 channels of what seems to be the same desk (three modules of 4 strips) that I'm racking as soon as I have some more time on my hands. I have a copy of the manual and schematics. Let me know if you need some of it.

  Could you please post the complete schemo of the fader section? That would help understanding the problem. Thanks.

 

[Jaadaaren]

Hm, there seems to be no schematic of the fader section unfortunately. Sorry. I have the same schematic as you posted and the rest of the (quite thin) service manual is all about testing and calibrating.

 

[abbey road d enfer]

OK. I had only a fraction of the schemo.
Now, I found I misinterpreted the system.
The VE stage is a summing amp, so near-zero input Z is normal, and I can see the fader now, which is connected to a standard high-Z input, so , yes, the Jakob's comment stating that " it's really not that important if it's 2k5 or 5K. It's an attenuator, looking into not-too-low input impedance." is correct.
And "Could it be that they have been replaced with linear faders, where they should have been logarithmic?" seems like a very plausible explanation.
Sorry for the "hair-pulling" my comments may have caused.

 

[Baltazar]

I found a scanned version of the service manual here http://technicalaudio.com/pdf/Philips/Philips_MD16_BINDER.pdf skip the first 10 pages.
Output block on page 51
Input block on page 38.
I have the same fader problem on both input and output blocks. All of the are replaced with 5k, instead of 2k5.

Yeah i thought about the linear fader thing too, but already checked and that was unfortunately not the case.

I've placed a order for a few 2k5 log pots to see if that would change anything, some of the pots are scratchy anyway.

Also found out that the console currently is running on -34vdc?? According to the manual it should be -24vdc=
But according to the manual of the PSU it's only capable of sending out -24vdc?

Makes me very confused, everything is "working" but 10v off is pretty much?

 

[PRR]

> manual of the PSU it's only capable of sending out -24vdc?

When the manual was written, the power supply was working correctly.

It is very likely a 24V regulator which would usually be fed with about 35V raw DC. When the regulator fails, it can output nearly the full raw 35V.

 

[Baltazar]

Some progress, yes the PSU was not working, replaced it, voltage looks great now.

Also replaced the LOG pot with a LIN one and now it's working as it's supposed to. Very very strange.
Any suggestions on good quality manufacture of 2k5 lin pots?

 

[Baltazar]

More progress and need more help.

Have started to calibrate the console but I've had a feeling from start that there is something strange going on in the busses.
The output from the busses are to loud (3.5v instead of 1.5v).
Have tracked it down to this part of the circuit, marked in red.
Where i should read 35mV i have 96mV, where i should read 140mV i've got 360mV etc.
The trimmer r21 can't bring the signal any lower than this.

What in this circuit controls the amplification?

I have the same error in all of the 4 busses - so i would guess it's something with the circuit design or a replaced trimpot or capacitor?

 

[Baltazar]

More progress and need more help.

Have started to calibrate the console but I've had a feeling from start that there is something strange going on in the busses.
The output from the busses are to loud (3.5v instead of 1.5v).
Have tracked it down to this part of the circuit, marked in red.
Where i should read 35mV i have 96mV, where i should read 140mV i've got 360mV etc.
The trimmer r21 can't bring the signal any lower than this.

What in this circuit controls the amplification?

I have the same error in all of the 4 busses - so i would guess it's something with the circuit design or a replaced trimpot or capacitor?

To answer my own question, when i bypassed R20 everything works and all the voltages are correct…

 

[mike440]

Hi! Just picked up a 4 of the Philips md12 mic pres. I also got the monitor section and the master bus out transformers. Any idea if I could make a master bus with the cards out of the monitor section? Also anyone have any Philips md parts for sale? Would love to find more of the pres and eqs to complete the board. Thanks

 

[abbey road d enfer]

Hi! Just picked up a 4 of the Philips md12 mic pres. I also got the monitor section and the master bus out transformers. Any idea if I could make a master bus with the cards out of the monitor section?

The monitor section includes the PFL circuitry, which could be used for one output (not bus, neither buss)* You would need two monitor sections to constitute a stero output.


*a bus is a line that joins several modules. Most often bus (bus is the plural of bus) are power or mixing (summing) bus. However, for supply, we prefer the word "rails".
Using bus to describe a group is incorrect.
https://groupdiy.com/threads/is-it-bus-or-buss.85777/#post-1119102

 

[Baltazar]

Hi! Just picked up a 4 of the Philips md12 mic pres. I also got the monitor section and the master bus out transformers. Any idea if I could make a master bus with the cards out of the monitor section? Also anyone have any Philips md parts for sale? Would love to find more of the pres and eqs to complete the board. Thanks

The circuits are pretty simple so you could clone the master, especially if you have the (huge) output transformers.
If I recall correct from memory the pfl circuit is different than the masters.

Or you can add direct out to the channels, more useful than 4 channel summing perhaps. I have a schematic for that.


I’m looking for another 4 channels to make my desk 16ch. Actually already cloned the line inputs but the real deal would of course look better. So let me know if you want to sell .

 

[abbey road d enfer]

If I recall correct from memory the pfl circuit is different than the masters.

The main difference is there is no post insert buffer, but the summing amp and output stage are there.
But I agree it would be a lot of work, more than completely redesignaing groups.