Acoustat interface mods & upgrades?

I’m restoring my M3s and next up will be repairs and upgrades to the MK 121-2A interfaces. I’ve seen lots of references to components to replace and modify but I haven’t found any specifics or schematics. What should I be looking to replace and how? If anyone has published a good reference please share it!

Also wondering what I should expect from the mods. I’m having trouble developing full volume, I’ve been blaming the amp for being underpowered (H/K Citation Sixteen, 250 wpc @4ohms) but maybe it’s the interface?
 
Replace the High voltage diodes, high voltage caps ( the ceramic ones) and the 500 Meg resistor. The speakers may work but are low in efficiency because of worn out high voltage components.

Also perform the C mod with polypropylene capacitors.
 
Good advice, but the 500 Mohm resistor rarely goes bad. Save your money on that part. Bias supply parts (diodes and capacitors) can go bad and will reduce the sensitivity of the speaker, which means low volume for a given input.

The C-Mod is also recommended but will not solve any of your complaints so far. The C-Mod is an incremental improvement in sound quality: don't bother with it until you are otherwise satisfied with the speakers' performance.
 
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I'm restoring my M3s and next up will be repairs and upgrades to the MK 121-2A interfaces. I've seen lots of references to components to replace and modify but I haven't found any specifics or schematics.

You are welcome to make use of the report I posted to the Planars & Exotics forum , 'Acoustat Magne-Kinetic Interface MK-121-2 - A Successful Restoration'.
https://www.diyaudio.com/community/threads/acoustat-magne-kinetic-interface-mk-121-2-a-successful-restoration.404764

In this report I documented the circuit diagrams and individual component part numbers as the restoration progressed from original configuration purchased new in 1980, up to final restoration status completed in 2023.

Please see:
Restoration of High-Voltage Board
Figure 4. High-voltage circuit, original as purchased new - 1980.
Figure 6. Step 1, high-voltage ladder components replaced during rebuild (replaced components coloured in blue).
Figure 7. Step-2, high-voltage polypropylene capacitor added to high-voltage ladder (new component coloured in green).
Figure 8. Step-3, high-voltage polypropylene capacitors added to high-voltage ladder (new components coloured in green).

Restoration of Audio Circuit
Figure 11. Audio circuit, original as purchased new - 1980.
Figure 12. High-frequency input enhanced with added capacitors (blue colour) - 1982.
Figure 13. Final configuration of audio circuit, 2023 (component changes coloured in blue).

With a total of 31 figures in this report, regret file size restrictions on the diyAudio site prevented me from posting separate individual images, but everything is there in the full report.

It's been 2 years since I completed this restoration and pleased to report both MK-121-2 interfaces have been operating without issue, the Model 2's sound is amazing.

While the priority should be the restoration, if interested, there's a couple of other reports I posted related to the MK 121-A interface.
- Acoustat Interface MK-121-2 - Minimizing the Bias Transformer Hum Noise
https://www.diyaudio.com/community/threads/acoustat-interface-mk-121-2-minimizing-the-bias-transformer-hum-noise.412612
- Acoustat Interface MK-121-2: Example of Measuring the DC High Voltage
https://www.diyaudio.com/community/threads/acoustat-interface-mk-121-2-example-of-measuring-the-dc-high-voltage.411370

Thank you,
John