SOLVED: MC2105 Mono Re-Wire; Also, Terminal Block Notes

[Johnny_Law]

A previous owner (likely a recording studio) re-wired my MC2105's Input Coupling switch, I can't fathom why.

Any ideas?

The input blocking capacitors (C1, C2 0.22uF mylar) are removed.
In "normal", the input signal passes directly to the input board (the wires from the input jacks are soldered to the wires to the input board, directly on the switch, as seen)
In "direct", two things happen: (1) L&R inputs are shorted together, and (2) L&R 4ohm autoformer outputs are shorted together (note the red wires).



(Note they also soldered the ground & 8 ohm outputs directly to speaker wire ...)



For reference, here's how it's supposed to look (credit to this YouTube video):



And here's the original circuit:

 

[Billa]

Sounds like they created a bridged mono out of it. Check the schematic for the MC2100 mono switch.

 

[Johnny_Law]

Looks like exactly what it is. Thanks!

 

[damacman]

Oft discussed, rarely seen ... now here it is.

 

[SaturationPt]

Sounds like they created a bridged mono out of it. Check the schematic for the MC2100 mono switch.

Not bridged, parallel.

This would double the available current.

 

[Billa]

Not bridged, parallel.

This would double the available current.

Oops...thanks. I was a little casual with my response. Correct...parallel.

 

[Johnny_Law]

To be specific - when I said "looks like exactly what it is" I meant it matches the 2100 circuit exactly, not that it's a bridged mono.

Suppose I'll leave it this way, it may come in handy.

 

[Johnny_Law]

I left the mono-switch as-is (what if I find another MC2105 someday ).

I re-wired the autoformer outputs to brand new terminal blocks, replacing the originals which were in bad shape. I thought I would share my lessons in this regard:

The original terminal blocks are 8-position (also called 2-row 4 position), with 0.375" spacing, open back. There are pins held in place with 6-32 screws. The pins extend back through the other row, extend through the frame, and are soldered to the autoformer wires. The original terminal blocks were made by Kulka.





If you need individual replacement pins, you can find these on Mouser, part # 538-Y140.



If you want to replace the original 2-row terminal blocks with new 2-row terminal blocks, the closest I could find are Mouser 504-TB100-04 which have a closed back. You would need to drill through the terminal block in one row, and install the aforementioned solder pins.



I decided to use a single-row equivalent, which fits perfectly and does not use those right-angle solder pins. It's a Molex part, Mouser 538-38710-6204. The pins are built into the terminal block. However, the pins are shorter, and you must be careful that the soldered autoformer leads don't short to the frame when installed.

I didn't take pics before installing, so here they are installed. Note that the original 6-32 machine mounting screws are too short, since the mounting flange is thicker than original. I used 3/4" long 6-32 screws from Ace, and took the opportunity to switch to Philips head from slotted to match the new terminal screws.

An additional note, I've found there are two types of the gold banana-adapters seen below. Some are thin (which fit within the original terminal block spacing), some are thick (which just barely don't fit). These terminal blocks have a hair more room, so they will accept the thicker adapters which is what you see installed.