Siemens W293 HPF issue

Hey all! So, I have a pair of Siemens W293s, that, aside from one having what I suspect to be a failing IC (Motorola SC5141G) which is difficult to find, I have noticed the other one has a resonant spike which I assume to be an issue of (internal) load  impedance being off?

Can anyone suggest where I might starting looking for the fault? The Schematic is slightly different to the layout of my units, I have changed two electrolytic caps in both (C25 and C26, with just one other remaining; C27 @ 6.8ohm / 25v which I don't have an equivalent for yet)

Schematic attached, and two basic sweeps to show the peak

Also, any insight on replacing the IC, which a round can with 10 pins, GREATLY appreciated, as I haven't seen any success stories as of yet :-s

image of resonant bump

image of intended slope

Internal view of unit

adamasnan said:

Can anyone suggest where I might starting looking for the fault?

Click to expand...

It looks like the second coil has the wrong value. Are the three coils supposed to be identical?
Anyway, you can compare with the working unit.

Also, any insight on replacing the IC, which a round can with 10 pins, GREATLY appreciated, as I haven't seen any success stories as of yet :-s

Click to expand...

I'm afraid these were custom IC's. They don't seem to be identical.  One is non-inverting, the other is inverting.
I woud suggest you make an adapter on a small PCB and use a 5532 or a more modern alternative.
Another option is googling for NOS or cannibalizing a non-working unit.

That’s a weird unit. I’ve never seen metal can IC’s in any Neumann EQ. Every time I have seen a metal can it has been a LM709.

An LM709 would be if the time period.  The LME49710 is also available as a metal can. 

abbey road d enfer said:

It looks like the second coil has the wrong value. Are the three coils supposed to be identical?
Anyway, you can compare with the working unit.

Click to expand...

Thanks, but I do not really understand; the value of the 2nd coil is off according to the schematic? How should I test or check this on the units?

I'm afraid these were custom IC's. They don't seem to be identical.  One is non-inverting, the other is inverting.
I woud suggest you make an adapter on a small PCB and use a 5532 or a more modern alternative.
Another option is googling for NOS or cannibalizing a non-working unit.

Click to expand...

I read elsewhere (here: https://www.tapatalk.com/groups/prorecordingworkshop/siemens-w293-hipass-opamp-problem-t4284275.html) that some extra components may be needed to use a modern/conventional opamp to replace them. It must have been done by someone somewhere..

Gold said:

That’s a weird unit. I’ve never seen metal can IC’s in any Neumann EQ. Every time I have seen a metal can it has been a LM709.

An LM709 would be if the time period.  The LME49710 is also available as a metal can. 

Click to expand...

This is a SIEMENS unit from the M444 Sitral console, circa 1969>early 70s

adamasnan said:

This is a SIEMENS unit from the M444 Sitral console, circa 1969>early 70s

Click to expand...

A W293 is the same circuit no matter what it says in the front. The details may be different which may account for the opamp used.

After looking at the schematic I don’t think an LM709 would be good. They don’t have good drive capability.  In the 293 the opamps  are line drivers.  They do make metal can to 8DIP adaptors.

So, for now I can bypass the output stage and I found it's only minimal insertion loss, so that's fine.

What I really now want to solve is why I am getting this resonant peak in that band. I thought I might trade some of MKL caps, but the production trend here is to bend the pins of any contact over on to the trace, it makes desoldering a bit tricky, as when bending the pins back for removal the traces are sometimes pulled too. Would be good to get a feeling for where to look exactly.

The coils all seem intact and pretty fine, I don't see how I would be able to test or fix anything there.

abbey road d enfer said:

It looks like the second coil has the wrong value. Are the three coils supposed to be identical?

Click to expand...

Please can you explain? how is this evident? what can i do to rectify?

adamasnan said:

Thanks, but I do not really understand; the value of the 2nd coil is off according to the schematic?

Click to expand...

The glitch in the response  is a symptom of the 2nd inductor being off.

How should I test or check this on the units?

Click to expand...

You must disconnect it and one from the working unit, then compare them. It can be done with an LC meter (many multimeters now can do LC measurements. They are not very accurate but OK for comparing). There's another method that involves some equipment (see attached).

I read elsewhere (here: https://www.tapatalk.com/groups/prorecordingworkshop/siemens-w293-hipass-opamp-problem-t4284275.html) that some extra components may be needed to use a modern/conventional opamp to replace them. It must have been done by someone somewhere..

Click to expand...

One of the amps is non-inverting unity-gain and the other inverting unity gain. Both topologies are very common and can be found in any cookbook.

OK, haven't gotten around to this yet - but if/when I find the difference between the second coils, how might I rectify? It is a case of adding/reducing inductance for every tap/contact to or from that coil?

Alternatively, any ideas as to how/where I could just get a replacement? or are the very special custom pieces for Siemens only?

Thanks so much so far!!

adamasnan said:

OK, haven't gotten around to this yet - but if/when I find the difference between the second coils, how might I rectify? It is a case of adding/reducing inductance for every tap/contact to or from that coil?

Alternatively, any ideas as to how/where I could just get a replacement? or are the very special custom pieces for Siemens only?

Thanks so much so far!!

Click to expand...

I would suspect either shorted turns, which could be diagnosed by measuring DC resistance and comparing against working unit, which can be done without disconnecting anything, or broken core, which could probably be assessed by examining thoroughfully the inductor. Without a picture, it's difficult to be more pertinent.

Gold said:

After looking at the schematic I don’t think an LM709 would be good. They don’t have good drive capability.  In the 293 the opamps  are line drivers.  They do make metal can to 8DIP adaptors.

Click to expand...

The only common point between a 709 and any of these custom IC's is the metal can. The 709 is a 8-pin, these are 10-pin, and clearly include some of the associated resistors for the NFB loop.