Sony TC-378 Reel won't record

rocknroller

Active Member
Have this reel play but it won't record the audio in record mode, though does erase what is previously on the tape. Meters shows audio (when viewing source) The record head has 2 two wire pairs going to it. There is audio on one wire of each pair. I don't know what the other wire of each pair is used for and the service manual (a tc-377 model which apparently has the same innards) was no help to me. Seems unlikely that the record head itself went bad on both channels. More likely it's not being properly "turned on" (guessing in some fashion via the other two wires that go to it) though I'm not clear what to look for. Thoughts/things to check?
 
Thoroughly clean the record slide switches on the pc board inside. They are located on the record pc board and connected by linkages to the two record buttons. They have a bunch of little contact fingers on them, spray contact cleaner into them and work the record buttons back and forth a bunch of times. Follow it up with a blast of compressed air to help clean and dry it all out then see if record function returns.
 
Thanks will do that. Can you help me understand why there would be audio present at the record head if the those switches were bad/dirty? Or do they control something else that connects to the head via the other two unknown wires?
 
Those switches are doing many things at once and all signals pass through them at one point or another (hence the many many contacts on them). They provide ground paths for certain circuits at certain times, allow signals to pass through and even lift grounds. They provide power to the oscillator circuit, power up the indicator lights, ground out the incoming record signal in the off mode when record function isn't selected, you name it. The oscillator may only be partially functioning and the bias may not be getting to the record heads, who knows. I wouldn't overthink it, but clean those slider switches and also clean the bias selector switches too while you're at it, see how it goes.
 
Cleaned all switches, including the tape speed swicth, which was a bear to take apart/resemble. No change. Or rather I did notice a difference that I hadn't before, as this time i happened to have the receiver volume up more than I did yesterday. Turns out is it actually recording - just barely audible when playing back. Culprit is not the playback head or circuit - since when it get to the pre recorded tape portion everything is fine. When recording, levels show full up to 0dB and even higher for tests, but no (0) level on playback for that recorded portion. So it would seem whatever controls the recording level (which is not the input level control, since that is working fine) would seem to be the problem.And since it both channel the same it's not likely a specific left or right channel component - more likely something central to both of them, like maybe a power component?
 
You are monitoring the recording as you are making it, right? Monitoring the tape is necessary to properly troubleshoot problems like this and to make good recordings with accurate levels once the machine is operating properly. If you don't have a monitor loop in your preamp / receiver you can monitor the recording through headphones plugged directly into the tape deck.

While monitoring the recording try working all of the switches and (carefully and gently) pushing the components on the record board with an insulated stick such as a pencil or tongue depressor, see if anything springs to life (may be a bad solder joint). The record slider switches and bias selector switch are still primary suspects and it often takes multiple attempts at cleaning to get them working again. They must be vigorously operated and re-sprayed over and over again to clear the corrosion away from the contacts.

If the recorder is laying down a quiet and distorted / garbled version of your recording then the recording bias is not making it to the record heads. This could be the high / low bias selector switch, the EQ selector switch, the speed EQ portion of the speed selector switch or the record slider switches. If the erase head is working then the power supply to the bias oscillator is probably ok, so it's going to take some detective work on your part to find out where you are losing it if that's what it is doing.
 
Before I do this I want to answer you question about monitoring. I have the monitor switch set to SOURCE for the Meters to show activity when recording. I have the switch set to TAPE when playing that content back (which shows no activity for the newly recorded content).
 
Record portion of the schematic. When you say "the record bis may not be getting to the head" what is that exactly? A AC voltage? DC Voltage? roughly how much should it be? Is it on the same line as the audio on the head, or the other line of the pair?
 
There is approx 9.4 Ohms of resistance between the wire pairs at the head (both sides similar). the tape is making full contact to the recording head. The recording head audio present confirmed with an audio tester and the head is very clean. It's simply not laying down a strong signal. This is irrespective of the the tape type selection, the bias selection or the speed selection. I have tried all three in every possible combination and there is no change. the Tape bias and tape type switches have been thoroughly cleaned and I am convinced they are not the problem. The Tape speed switch is more difficult to clean though i have sprayed it many time and worked it and taken apart once. It might be easier for me to confirm via circuit readings if it is making proper contact - If I knew where to look.
The record head does lay a signal down, but is almost inaudible on playback with no meter indication of volume on playback when monitoring the tape on playback. When monitoring the source on recording it is showing full volume. . So whatever the head needs to record this signal properly is not present, or is not present in sufficient quantities. I don't understand how the bias works or what it actually does so I could use some clarification of that.
 
AHHHHH- I just tried another reel of tape and it record to it fine!!!! How is it possible that the original reel of tape holds a previous recording but will not record on it???
 
It happens, I've even had tape that only records on one side and not the other which is really weird to me. Glad you found it, that's a pretty nice recorder and one of the nicest versions of the old TC-366 "Wedgie" decks they made (the last one being the TC-399).

BTW - by monitoring I mean that whenever you have a three head tape recorder you should always set it to Tape when you are recording so that you can see and hear the actual recording as it's being made (well, a split second after it is made anyway). That way you can set the bias and record volumes for the tape while it is recording as it plays back to make sure they are right. Source is just for reference volume setting, it is just the signal coming into the deck from your amplifier.
 
Record portion of the schematic. When you say "the record bis may not be getting to the head" what is that exactly? A AC voltage? DC Voltage? roughly how much should it be? Is it on the same line as the audio on the head, or the other line of the pair?

Moot point since you now know it was a defective tape causing your issue but to answer your questions:

The record bias is a high frequency AC voltage that's applied to the record signal before it goes into the record head. It provides an appropriate magnetic field for the record head to lay the signal down onto the tape. On that Sony deck I think it's supposed to be something like 14 or 15 volts AC @ 160khz but it changes depending on speed and bias settings. You can see the basic block diagram of the bias oscillator circuit and where it connects to the record and erase heads in that picture you posted.

If you have ever seen one of those older Tandberg or Akai "Cross Field" decks they are kind of interesting in that they use a separate head to lay the record bias down on the tape instead of sending it through the record head along with the signal. They have a bias head that moves in directly across from the record head in the record mode and it sets up the bias field for the record head to lay the signal down in. When the oscillator fails your recording will be a garbled and distorted mess, that's why I was asking if it sounded that way.
 
Not defective tape - Tape is mounted on the reel upside down but WHY is a complete mystery.
Definitely was not a twist in the tape I was using. Ran through the entire tape
checking with my finger the whole way and no twist and I concluded it was simply mounted on upside down. Put an intentional twist in the tape and ran off to another reel, then retried to record and now of course works fine.

To make things even more ironic, I pulled a tape from a stack completely unrelated to this player. Tape had handwritten notes on it (on the reel itself,not the box) as to what had been recorded on it. Put the tape on - would not record and did not have the material (any material) present on it. My first thought was - deck is doing it again. Went back to one of my previous sets that I had just tried successfully. Worked. back to this tape. Doesn't record. With the assumption the tape was on the reel incorrectly also, I put an intentional twist on it and now records fine. These are not my tapes and I have been using reels for decades and know how to load tapes properly (just to rule out the idiot factor). Someone STORED this tape this way. As was the original tape that had been giving me grief from the beginning.

So the million dollar question - Why would anyone do that?? And how would they even put it on that way? You can't run the tape through the machine upside down.
 
Some old decks has their heads mounted like on a cassette recorder, with the oxide facing outwards - a lot of Telefunken Magnetophones used that method up until the early 70's, even on their top tier mastering decks.

Sometimes people just behaved silly and winded the tape on with a twist somewhere in the path, I have a stack of BASF LPR35 that have all been recorded from the back coated side :dunno:
 
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