I just wanted to post information about my experience in modifying my marantz cd400 here, so that others can have this information if it helps.
I had a tech do the modification of adding the trimpots, and calibrating according to the instructions of the modifications, so I didn't do the calibration myself and can't speak to what exactly happened with that process, but I've known the tech to do good work and feel he should have been able to follow the instructions properly. What I've noticed ever since the modification though is that, while adjusting the demodulator to have very discreet rear channels, the fronts have been seeming to get some bleedthrough. I've also noticed that much of the bleedthrough can be cancelled out by mixing in the rears at -4dB...but not always, sometimes it adds bleedthrough of things not in the front. Really odd. I wasn't sure if maybe things were mixed that way, so I pulled up some of my past conversions, and I confirmed my fears...there was rear info bleeding into the front currently, which hadn't in the past. I also ran the channel id portion of a test record and found there was definitely something wrong.
Ugh...
When I first had my suspicions, since I don't have the test equipment required, I played with the trimpots with the test tones to adjust the separation knobs, and I did find that the spot the trimpots were adjusted to allowed the most discreet rears, once I had adjusted the separation knobs. I also found adjusting the trimpots did nothing for the fronts.
What to do?
My biggest struggle was a lack of clear back information...calibration records give you test tones in the front to adjust the rears to be silent, but not the opposite. So, I went to ebay and found one of those Project 3 test records for cd4, the one that came out in all 3 formats. I know from my SQ copy that it has a portion where it puts pink noise alternating in the front and rear, to help balance your front and rear channels. I've not gotten this yet, but I think this will help me zero in on the best setting.
But, I wanted to play around some more, so I opened the thing up again, and went looking for a record that would have a simple mix with stable parts, so I can listen to the front and hear those parts cancel out as I adjust. I went with Average White Band. So, I cue up pick up the pieces, listen to the fronts only, and hear the guitars bleeding into the fronts. Adjusting the added trimpots did nothing. But, turning up the separation knobs on the back, made the guitars drop out. Continuing to turn up the knobs didn't result in the guitar coming back up, it wasn't a null point. So, I figured, ok, probably best to just turn it up to the point where it's fully dropped, but not further, so I went back and did that. Then, I put the test record back on, put on the test tones, and of course I had more sound in the rear than before on the test tones. But, adjusting the added trim pots allowed me to find a null point on those. Was it as quiet as before? I don't know....I guess I should have recorded all this testing.
Anyways, after this, I was left with a setup that was performing right when playing the channel id portion of the record. I'm not sure how scientific it is, or if it's fine tuned as accurately as possible, but now I know I'm on the right track. I also don't know if this is the norm for this modification, and if the calibration procedures in the instructions are flawed, or my tech goofed something...but I'd suggest anyone doing this mod pay close attention to their front channels, and have a project 3 record on hand. I plan on doing some more thorough calibrating once my project 3 record shows up. I'm thinking what I found above is the best procedure to calibrate with added trimpots....adjust the front to have the rear info just cancelled out with the separation adjustments, then find the null point on the rears on the trimpots.
Just wanted to share this because, with so few people doing this, it really helps for those of us playing around with these things to post our findings and experiences. I know it would have helped me greatly if while i was going a bit out of my mind wondering if I had a problem or not if I found a post of someone having a similar experience.