Weird Tape Deck Problem

I have been working on a long term project to convert my (and my Father’s) old reel-to-reel tapes to digital. As part of that I have been working on a Teac X3mkii deck. Skipping all the mechanical fun, at this point it is pulling tape very nicely (for a consumer deck) and I was impressed with the 8 KHz stability coming off of the MRL alignment tape. The purpose of this post to search for an answer to an electrical problem with the deck. The problem is ion both channels, but much more pronounced in the right. There is an about 18KHz tone present. See the following spectrum.

This the right channel, the left also has the problem, but much less so. Here the peak is about 40dB above the noise floor, in the left channel it is only about 15dB above the noise floor. Ignore the spikes of hum at the low frequencies, those are easy to fix in the computer, the spike at about 18KHz is not. It is not a pure tone and it is not constant. See the spectrograph below.

Note that the 18KHz pulses on and off.

I have traced it down to the playback head amplifier, see the schematic below. Thanks to HiFi Engine for the X300 Service Manual (close enough to the X3Mkii).

U101 is a 4558 dual op-amp and I tried replacing it with a 5532 with no change to the problem tone. I also looked at the power supply and replaced C306 the decoupling cap and added a 0.47uF right at pin 8 of the DIP to ground, the power supply noise did go down, but the tone is still there. I tried replacing C105 and C107 thinking that they may have gotten old and tired, no change in the 18KHz tone. I even replaced C102 thinking that perhaps the fake DC reference for pin 5 was getting contaminated, but no change in the 18KHz. I am out of ideas. The tone is not visible at pin 3 of U101 but it is there at pin 7. The FETs (Q101 – Q104) are for changing playback EQ based on the settings of two switches (LH1/LH2 and LH/EE), changing these switches does not change the 18KHz tone, but I am starting to think the FET switches are somehow involved.

Why am I obsessing over this 18KHz, not terribly audible tone? Well, even though this is a consumer deck (1/4 track running at 7.5ips), there is material at this frequency coming off of the tapes. See the following spectrograph of one of the tapes. There is clearly energy above 20KHz and I don’t want to lose it by trying to filter out the 18KHz tone (I have tried). The tone at 36KHz (twice the 18KHz) doesn’t worry me as there is no program material there and I can effectively filter it out.

If you have any ideas what I am running into here, please post a followup comment below.

Full size copies of the images above are located here (much easier to see than the small WordPress images above).

P.S. Thanks to iZotope’s RX for showing me the problem and helping me fix many others.

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