Sony TC-105 Reel to Reel that won't record, yet has signal to the head.

[cincin]

I have a Sony TC-105 reel to reel from the 60's that's in good shape, but won't record. At all. The tape is blank after attempting to record to it (the erase head works!)

Attached are the schematics with my quick tracing of the ground plane, some of the playback signal path, and some of the record signal path.

I traced the signal around while recording, and it's still there all the way to the head, though it gets pretty thin and weak after the C31 / R43 filter.

This is my first try at repairing a tape deck, and I did hear something to the effect of record heads needing bias, potentially a high frequency AC bias. This is new stuff for me and I wouldn't be able to recognize an AC bias setup if it hit me in the face.

So here I am.

Anyone can help shed some light as to why it won't record even though the signal makes it to the head?

Is the C31 / R43 filter just bad, causing too weak a signal?

Where's that AC bias and how do I make sure it's righteous?

Thanks!

[tooki]

My first thought was AC bias. This is critical to how high quality tape recording works, so I highly suggest reading up on it. But in a nutshell, you need a certain amplitude of signal on the head to “mobilize” the magnetism on the tape. But obviously you can’t just have your audio signal at full amplitude all the time, because then it would barely be audio. So you instead use an AC signal well outside the audible range (which also makes it too high a frequency to record), which keeps the overall signal level extremely high, onto which you mix your much smaller audio signal. When the tape moves away from the head during recording, the AC bias can’t be recorded, but the audio signal is left behind.

So what you should see at the recording head — even with no signal on your audio input — is a signal at a pretty high amplitude, at many tens of KHz. If not, then your AC bias oscillator isn’t working, or it’s not getting mixed with the audio signal.

(I suck at transistor circuits, so I can’t really say where the oscillator here is.)

Also, just to be sure: playback fundamentally works, right? As in, other recordings will play back properly? (Just to make sure it’s not actually recording properly and just failing to play back.)

[Xena E]

Not enough information.

Does this machine play pre recorded tape?

Do the record/replay heads have continuity?

Does the switching all function?

The bias circuit is outlined in red on the attached schematic.
The function of the bias is to "lift" the signal level into the linear portion of the magnetic characteristic of the tape. It also provides an HF signal to the erase heads.

If the bias is not working I would expect the machine to still record, just poorly with high distortion.

If the tape is being erased, for sure the bias circuit is working.

That filter circuit as you describe it, combined with s4 and s5
, C27, and C28, are for selection of different grades of tape.

Regards X

[wasedadoc]

1. If the tape is being erased the bias oscillator is working. That alone does not guarantee that bias signal is reaching the record head. There will be some adjustable component (usually a variable series resistor) to set the level to the head. Two if a stereo machine. In this machine the level for both channels is set by choosing a tap on the bias transformer.

2. Bias frequency is typically between 60kHz and 80kHz.

3. Do not use a multimeter to test head winding continuity unless you have a demagnetiser to demagnetise the head afterwards.

4. C27 and C28 are for different tape speeds, not tape grades.

[Xena E]

1. If the tape is being erased the bias oscillator is working. That alone does not guarantee that bias signal is reaching the record head.

The Audio passes through the selected bias oscillator secondary tap. So, if the Audio is getting to the head...

In this machine the level for both channels is set by choosing a tap on the bias transformer.

Not channels, tracks, (if we're being pedantic), this model is a mono machine with switchable ¼ tracks to double tape capacity.

If the amp is basically working, just signal trace through the switching is a good place to start.

X

[jzx]

I traced the signal around while recording, and it's still there all the way to the head, though it gets pretty thin and weak after the C31 / R43 filter.

Have you checked if the gnd side of the head is well connected? (S1-2)

Often the problem are the switches, if unused for some time tarnishe or become dirt.

[cincin]

Thank you all for the valuable pointers!

Great explanation of bias. Makes perfect sense.

- Playback does work on the machine. It plays pre-recorded tapes, and erases them with nothing when I try to record over.

- This is indeed a mono machine with 2 tracks per side with a selector switch. The track switch works well during playback, so that's probably not the issue.

The main switch for the record / playback function is a long sliding stick-like device on the PCB, that changes contacts at many points (see S1 - 1 to 9). I gave it a good deoxit, but that may not have been enough. Though, since the signal is present at R47 / C31 my understanding of this schematic (only partial understanding) suggests that S1 - 9 works. (doesn't mean the other contacts all do)

Continuity of the heads, I'm not sure. But now I'm not sure I want to check, I don't have a demagnetizer (aren't those devices often unnecessary and over-hyped? I know nothing, just what I heard)

So... if the audio goes through the bias secondary tap with the bias, and the audio makes it to the head, then I'm guessing the bias does too. Yet it won't record. Signal too weak, bias too weak, both, head not grounded at S1-2...

I will go check the bias on the scope, head grounding, and check the S1 switching.

Cheers!

[Messtechniker]

After completing work on any tape machine,
demagnetizing it is a standard procedure.
Keep any tapes well way from the demagnetizer.
Did in my time ruin some valuable test tapes by forgetting to do so. 

If possible, take the tape machine outdoors
for demagetizing.

[cincin]

I'll have to make a demagnetizer then I guess!
Cheers!

[jzx]

You can measure R47. This network is for equalization, and for control the level of audio in head.
But if you have seen the audio in the head, probably is another problem. The bias signal have to be much bigger, and also, being higher frequency, you need an extra of voltage to have enough current, in the head you need current to make the magnetic field.

If it erases tapes, the oscillator must be working, and normally, if you have not bias, the recording is very distorted, and with low level, but something is recorded, not full silence.

If you want to demagnetize, you need an head demagnetizer like this:
https://www.amazon.com/Universal-DEMAGNETIZER-Cassette-Recorder-Track/dp/B003ZKLP4W
(I bought one totally alike some 30 years ago  )

not an crt demagnetizer, that are much more powerful.

[Messtechniker]

If you want to demagnetize, you need an head demagnetizer like this:
https://www.amazon.com/Universal-DEMAGNETIZER-Cassette-Recorder-Track/dp/B003ZKLP4W
(I bought one totally alike some 30 years ago  )

Using exactly the same. Except for the mains plug.
Was quite standard at the time.

How to operate:
1. Keep the demagnetizer some 1 to 2 meters way from the tape machine.
2. Power demagnetizer up.
3. Slowly move demagnetizer towards the tape head just not touching the tape head.
4. Slowly retract demagnetizer  1 to 2 meters from the tape head.
5. Power demagnetizer off.

Repeat for the second tape head, if present.
Repeat for any metal (tape guides/rollers) in contact or close to the tape path.

Its important to do this slowly.

To bystanders this looked like black magic