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

[Jeff eelcr]

Record/playback switch assy was a problem with these units years ago and I would think they still are.
Jeff

[Xena E]

This is not the problem here, however anyone wishing to check the continuity of an audio tape head can do so with a DVM set to a reasonably high ohms range, say 2M. As the most likely fault will be an open circuit, an accurate reading isn't really necessary... the current in the head done this way will be in the order of a couple of microamps only, and any residual magnetism won't cause permanent damage.

Do not however use low ohms, or continuity ranges as that can cause permanent damage to the head.

Demagnetizing the head, guides, and capstan, regularly, particularly in high end equipment is always worthwhile.

X

[bostonman]

Is demagnetizing only needed for cassette heads or would it be necessary for floppy disk drives such as the Commodore?

Recently I measured the resistance on a floppy head and didn't know about demagnetizing, however, I also didn't experience any issues.

[cincin]

Ok, I checked a few things:

- There is continuity across the head. And I checked in continuity mode, before knowing I shouldn't, and the head appears to have survived.

- The bias is 24.2V p-p at 57.8 Khz (which I believe is good?). A nice sine wave.

- The grounding of the heads is questionable in rec mode, the signal side of the active track is 57 Ohm from chassis ground, and the ground side is 29 Ohm from chassis ground. So maybe the switch is still a problem and the resistance to ground is preventing the head from doing its thing?... I gave it a more thorough deoxit flush.

- The signal is also getting very weak and lame after passing through the C31 / R47 combo (not much of a "filter" after all, not sure what that's about). I popped both out to check for failure, and the cap was actually a 2.7n that read more like 3.1n, as opposed to the 4.7n from the schematics. I replaced it with a 4.7n, but I don't think that was the problem.

Wiggled the switch some more... and tada, I can record, on both tracks.

I'm guessing it was the oxidized switch.

I have another one of those TC-105 that also can't record. I suspect a similar problem. But I might be back!

Thanks!

[wasedadoc]

1. Damaging a head by measuring its resistance isn't the issue. The direct current leaves it slightly magnetised. That has two bad effects. It slightly erases a recording each time it is replayed and it increases replay noise.

2.  A head has relatively low impedance. The other components in series in record mode result in current drive rather than voltage drive. Hence the apparent small audio voltge across the head itself.

[jzx]

I have read in old books that the bias signal acts as degaussing when is powered off (the bias fades slowly). Normally you cannot magnetize the head enough to erase the tape, because coercivity of the tape, and because the core of the head is ferromagnetically "soft", but when recording, a "dc" component is recorded and this increases the noise (because when recording you have the bias).

Crappy cassette recorders from the 80-90s have dc bias, and they recorded very low quality.

[cincin]

The unit records with good enough quality as is, but I will demagnetize it to be sure.