Acoustat Answer Man is here

[mr_ro_co]

Attached is the schematic for the MK121C. Note there is nothing different about the schematic for the Medallion transformers - they are different internally than the earlier non-Medallion transformer but are connected the same. However, the "C" designation indicates the improved HF primary circuit, which is shown on the schematic. The schematic covers all the models that used the MK121 interface. For your 2-panel Model 1+1, you'll want the LF transformer tap set to the RED position. Other positions (for 3 and 4 panel systems) will offer less bass boost, which you may prefer. There's no harm in trying other positions.

Failure of the connection at the pin plugs and hook terminals is not uncommon after many years of handling. After resoldering, a reinforcement of heat shrink tubing will help ensure the integrity of the connection.

Likewise, corrosion on the exposed winding of the HF Balance resistor is not uncommon. It can be cleaned with some fine emery paper.

Andy,

Excellent, thank you for providing this. Thanks for the circuit walk through, very helpful and interesting. Should be able to work on these this afternoon.

Best regards,

Steve

 

[mr_ro_co]

Vacuumed the socks, took them off (those staples are a real joy) to wash them. With the socks off, I did a close inspection of the panels themselves and their wiring. They seemed to be in good shape - no obvious physical damage could be seen anywhere, and everything was quite clean (the socks however were pretty discolored and dusty). So anyway, the panels and their wiring look good.

I inspected the interfaces but could see no obvious physical indicators of serious degradation. As described in an earlier post, when I disconnected one speaker from its interface, the conductors to the bias voltage pin-and-sleeve connection pin simply broke off right at the pin. In the process of desoldering the pin to prep it for re-termination to those bias voltage wires, a bunch of green oxidation debris came out of the pin’s socket area. I cleaned it up with some flux, cut back the diaphragm conductors .25" to ensure clean copper, stripped, tined and landed them in the cleaned up pin. I then repeated this procedure on the other speaker for good measure to keep them matched.

One small difference discovered when inspecting the interfaces was the audio input fuses. These were different, and not of the type that Roy Esposito recommends. I replaced these with the correct versions.

Oh, one more small change: I loosened the tap location on the HF balance resistor so I could clean up that connection on both interfaces.

I put the speakers back together and fired them up. They seem fine now! I cannot perceive any difference in sound between any of the panels. The speakers image correctly and the sound field is uniform and consistent.

So I think it was due to the badly oxidized connection in the pin that connects the diaphragm wiring to the bias voltage supply board that was causing a difference in biasing voltage to one panel, causing it to sound muted compared to the other three.

Right now I’m running them without the socks. Not looking forward to putting them back on. Are there any parties that make replacement socks out of spandex or something similar?

Finally, how long can I run them without socks? I live in New Mexico. Dry and dusty most of the time.

Steve in NM