Pioneer CT-S550S not happy to record/play Dolby-C anymore

Hello all. I have had this Pioneer deck for a few months and I've been using it heavily ever since I got it, and overall I really liked the audio quality I was able to get out of it considering how much I paid for it.

Right after I got it I made a few Dolby-S recordings to see what it was capable of (and I was thoroughly impressed), but since I wanted to record tapes I could listen on my portables, I switched to recording on Dolby-C only. And that was it for all these months.

However this week while recording some Type-IV tapes I noticed how terrible/muffled they were sounding, especially quieter parts like just voice and guitar. I went through the usual (demagnetize and clean heads, re-calibrate the auto-bias) but nothing seems to fix it. However, if I record in Dolby-S or in Dolby-B, the tapes sound just fine! I then did some test recordings in my WM-D6C and played them on the Pioneer, and the results are identical: Dolby-B tapes sound fine, but Dolby-C tapes sound muffled.

That has really puzzled me. I've heard of how old circuits going bad can affect Dolby NR performance, but I was expecting it to affect all Dolby modes somewhat equally (after all it's using all the same capacitors). Has anyone here had a similar issue with an older (not so great) deck? Should I dare go fiddle with Bias/Level adjustments with an oscilloscope? Thanks!

 

Hi ... looking at the wiring diagram there are 2 circuits that control the dolby ... one in external input and one in the reading phase from the head + a separate circuit for the dolby S .... probably from your description the problem it is only in the reading circuit of the head ... I would start from there to look for the problem (green) the scheme can be found on the net (hifi engine)

 

Thank you Silver965! I haven't yet poked inside with my oscilloscope, but your suggestion gave me some ideas. It seems it is recording in Dolby-B even when Dolby-C is selected, because if I play the bad Dolby-C tape with Dolby-B selected it sounds right (or at least it sounds better), and if I turn Dolby off, the highs sound very high-pitched, so there's definitely some Dolby there. I'll keep investigating and post once I figure out more.

 

Sorry for the very late reply, but I wanted to give an update.

TLDR: This was all probably a SNAFU on my part. I remember trying to fix the record level channel imbalance right around when this issue began and I probably messed up the pots so bad that Dolby didn't work anymore. Good thing this isn't an expensive unit, though, and I'm glad I was able to learn more about this stuff.

Long version:

Since I made this post my understanding of tape equipment maintenance has improved a little, I've gotten another tape "deck" (a WM-D6C), and I learned a bit more about what all those measurement units mean. With all this at hand, I started investigating this issue again today, and I think I was able to understand what's going on.

I started the whole calibration routine from the service manual from the start. Right from the beginning I could tell something is off with this deck, because when I get a level measurement of the input signal in my oscilloscope, the deck's level indicator is off by a few dB between channels. Nothing I tried was able to fix this, so I'm shrugging it off as an aging input path and moving on.

Once I had the playback level set to a good enough value, I moved on to the record level adjustments. Right from the bat I found an issue on the service manual. It tells you to check the level of the input signal to -11.2 dBV on the TP3 and TP4 test points. However, these points have 0V on them when the deck is set to input monitor mode and recording is stopped. Makes sense, this are the playback dolby output paths, and there's no signal there if a tape isn't being played/recorded. You have to measure -11.2dBV on TP1 and TP2 test points instead, which are in the record dolby output paths. Once those are checked to be at the right level, then you can use TP3 and TP4 to ensure record level is set correctly during recording.

Another thing I learned by observing my oscilloscope while the auto-bias ran is that it uses the right channel only to perform the tests. Since I have a channel imbalance, I used the right channel as a reference for all the measurements where the channels differed, and adjusted the left channel to match.

I did not bother with bias calibration, because I haven't experienced issues with bias and didn't wanna risk messing it up.

After this I did some tests recording to type I, II, and IV tapes, in Dolby B, C, and S, and they all seem to sound good again. Some tapes I have recorded in the past sound significantly louder than they should, but since they were recorded on this same deck, I'm not sure if I can trust this to be a sign of anything. I used my WM-D6C to test the recordings in Dolby B and C and both sound just fine.