Ampex MX-35 / MX-10 recap

(I am now retired and no longer accepting work.  This page is left up to help those doing DIY repairs.)

In my opinion, the MX mixers are best used unmodded. Well, at least without significant modification. (Please see the MX-35 section of this page for more) I relocate filter caps and upgrade their values some when recapping an MX. Getting the filter caps closer to where they are used can reduce noise more efficiently. I also replace the potentially hazardous Selenium rectifier with Silicon diodes and all the electrolytic capacitors circuit-wide. Most of the 2 watt resistors in the power supply section have been replaced with 1% 2W metal film resistors. Those not replaced were checked for drift. If I have to heat up (or generally mess with) a carbon resistor when soldering new components, I replace them. The big can capacitors were left in place for cosmetic reasons. They are not connected to the circuit.

In the purple circle you see the new B+ resistors for the 4 channels.

In the red circles are additional filter caps for each channel.

In the yellow circles are the new cathode bypass caps (Nichicon PW series 105)

In the blue circles you see the new Nichicon 105 1000 uF filter caps for the DC filament power. The original design had to use a single 2000 uF cap at one end of the unit. With modern caps I could upgrade this to 3000 uF total among 3 caps and place them in more effective locations across the circuit. In this case, more capacitance means quieter operation.

Here are some of the little things I do when working on these units.
Switches before cleaning:

Switches being cleaned and treated against corrosion with Deoxit:

After cleaning:

Using Deoxit and sticking an old tube in and out of tube sockets to clean and treat them. (Deoxit leaves a protective coating to prevent corrosion.)

This unit's power cord had broken outer insulation. This is very common since the plastic gets brittle with age and this is where the cord gets bent the most. I used black liquid vinyl to repair the cord and keep it original. Note: This is only a good idea if the inner insulation is in good shape.

Some mods to consider:
The mylar coupling caps are good quality. They are not as good as Auricaps though. Upgrading the coupling caps is an option but I usually recommend leaving them because of cost/benefit analysis.

Replacing a couple input transformers would be something I would be interested in personally, but it is expensive. It would provide the ability to use mics and mic techniques that would overload the original Beyers. (Did I hear someone say "pad?") I also like the sound of many other transformers over the Beyers, like the Ampex octal, UTC A-10, A-11, HA100x, LS-10, Altec 4722, Jensen 115, etc. The Beyers are excellent for their size and cost. The others are just plain excellent; no qualifiers needed. One last thing about the Beyers: according to those who would know, they are better for other purposes than as a mic input transformer. They don't really shine in this application, but they work.