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Old 6th Aug 2022, 2:55 pm   #1
ButchJames
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Default Sony TC-645 1/4 Track R2R Erase Head & Issues

Hello everyone.

Recently bought this R2R machine to restore, the jobs done so far, and that need doing are too many to mention. However, I'm interested in you guys who have had this and other similar Sony machines (like TC-399 etc), and in particular the raising of the tape away from the heads in RW/FF modes. You see, on this deck at least there is some contact between the high speed tape movement and the erase head!? Is this a common design flaw on these Sonys?

Here is a shot of before and after lapping of the erase head.

Any thoughts?
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Old 6th Aug 2022, 3:59 pm   #2
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

The TC645 has 3 motors unlike the others e.g. Akai 4000DS where the supply spool is braked, so the wear is down to the voltage on the left spool motor.

If the tape is very abrasive, you could bulk erase the reel and stick some polyester tape
on the erase head. I used to do sound-on-sound experiments with my Akai by sticking
Dymotape on the erase head !
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Old 6th Aug 2022, 7:35 pm   #3
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

My thoughts ...

Here's a shot of the TC-645 head assembly - you can see the bar that moves the tape away from the heads during RW/FF.

IMO, that bar is probably a little too narrow - it only needs to be 2mm or so wider in diameter to allow for better clearance. The role of the tape guides would be undisturbed.

Of interest too, the angle of movement of the said bar cannot be widened, nor can the erase head be re-positioned.
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Old 7th Aug 2022, 1:43 am   #4
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

Quote:
Originally Posted by ButchJames View Post

Here is a shot of before and after lapping of the erase head.

Any thoughts?
I believe that ferrite heads like that one can be expertly relapped but without that expertise, I wouldnt try it.

As for the tape lifter not lifting enough, a picture of the tape as it passes the erase head in fast wind might help.
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Old 7th Aug 2022, 7:05 am   #5
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

Quote:
I believe that ferrite heads like that one can be expertly relapped but without that expertise, I wouldnt try it.

As for the tape lifter not lifting enough, a picture of the tape as it passes the erase head in fast wind might help.
I have absolutely no intention of attempting to lap the F&F heads. The erase head was easy - I did this because erasure is not 100% guaranteed, as it was one potential source of the problem. The bias voltage at the erase head is 100v peak, according to the service manual it should be 80v (rms?), which is about 112v/113v peak. The bias oscillator is working, but perhaps seemingly not inducing enough EMF, so I'll open it up later.
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Old 7th Aug 2022, 5:58 pm   #6
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

Judging by the low response on this issue, I can only assume few in here have a TC-645, or even a TC-399? Does the TC-377 have a similar head assembly I wonder? I can't believe nobody has not noticed this 'rubbing' on the erase head? I don't think my machine is any different - but may be it is?

Anyway, here is a shot taken in FF mode, the tape is clearly under 'high' tension. Note the tape's partial 'wrap' on the erase head.
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Old 7th Aug 2022, 6:17 pm   #7
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

Just for completeness, the TC645 head assembly during Play.
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Old 8th Aug 2022, 1:52 am   #8
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

Quote:
Originally Posted by ButchJames View Post
Judging by the low response on this issue, I can only assume few in here have a TC-645, or even a TC-399? Does the TC-377 have a similar head assembly I wonder? I can't believe nobody has not noticed this 'rubbing' on the erase head? I don't think my machine is any different - but may be it is?

Anyway, here is a shot taken in FF mode, the tape is clearly under 'high' tension. Note the tape's partial 'wrap' on the erase head.
I only have a TC 377 here. Photo attached in FF.

In mine the lifter is held only by spring tension so any excess back tension from either reel might push the arm up. There is a plastic or rubber bush at the base of the arm (just visible in my photo) which I guess should contact the end of the slot in REW and FF.

As per the photo there is about 1mm clearance between the tape and erase head at the left edge. Maybe in your machine's previous life the servo back tension was too high, perhaps due to a dirty drum or felt band, pushing the arm up. Maybe old grease on the reel table spindles contributed to high tape tension in all modes.

Also in your photo of the head pre lapping it looks from the wear pattern that the tape had been riding a little high. There are two separate wear lines at the lower edge, suggesting the tape height had been correct originally but at some stage it rode high, creating the higher wear line at the top, and perhaps polishing out the original correct wear line. Hard to be sure from the photo but normally it would be lower, close to the top edge of the track 1 pole piece. A tape height error there could explain poor erasure.
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Old 8th Aug 2022, 5:39 am   #9
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

Sorry mods, double posted. Post #8 can be deleted.
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Old 8th Aug 2022, 8:40 am   #10
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

Now that's very interesting! - I'm now comparing your machine with mine. Thanks for the photos, and explanation, I'm looking into it.
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Old 8th Aug 2022, 9:27 am   #11
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

Quote:
Originally Posted by ButchJames View Post
Judging by the low response on this issue, I can only assume few in here have a TC-645, or even a TC-399? Does the TC-377 have a similar head assembly I wonder? I can't believe nobody has not noticed this 'rubbing' on the erase head? I don't think my machine is any different - but may be it is?

Anyway, here is a shot taken in FF mode, the tape is clearly under 'high' tension. Note the tape's partial 'wrap' on the erase head.
The tape shifter doesn't look to be fully out in the slot, I would make sure the back tension is within spec. (40~80 g.cm in FF and REW) if it is then check the shifter mech.

When I worked for Sony I serviced lot's of Sony reel to reel machines that had that tape shifter mech, I don't remember erase head wear being a common problem due to the shifter.

Lawrence.
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Old 8th Aug 2022, 9:59 am   #12
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

The shifter mech won't move out any further, and doesn't 'fall back' even slightly under FF/RW spool tensions. I have attempted with my fingers to pull it further out - but it cannot. Examining the mech underneath, there is a limited amount of outward swing that is permitted, it's simply blocked from going further.

I will return to this issue later on, and re-examine. In the mean time I'm concentrating on some of the electronics.

Many thanks for you feedback, appreciated.
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Old 8th Aug 2022, 12:26 pm   #13
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

Strange, this deck (TC-6650) seems to be the same in FF & RW (1:39 in):

https://www.youtube.com/watch?v=mfen4Ue708g

Lawrence.
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Old 8th Aug 2022, 12:36 pm   #14
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

Hi Lawrence,

Yes, I've seen this video before I bought the 'TC-645', this one is a Japanese version I believe. Thanks for spotting that one for me. I didn't think about any rubbing with the erase head then of course!

It's just catching the tape to the left side of the erase head. So I'll later look into fitting a broader tape lifter with hard wearing characteristics etc. Only needs to be a few mm wider diameter.

An oversight by Sony?
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Old 8th Aug 2022, 12:44 pm   #15
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

At this moment, I'm looking at the oscillator, which I've removed and examined the cct within. So far the bias cct is working, but I sense that the amplitude of the bias 'carrier' is not high enough, as both (~ 160Khz) erase bias and recording bias amplitudes are lower than expected, especially at the recording head. Currenly: Erase ~ 100v peak (70.7v rms), and at the Rec head, it's only 10v peak, or 7.07v rms!? And bias adjustment is allowable only up to 14v peak .... strange.

Incidentally, the service manual and the actual (physical) circuit components do *not* match either.

I'm examining this cct 'off line' on the bench.

Eric.
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Old 8th Aug 2022, 1:55 pm   #16
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

I had a weak erasure problem in an old TC270, and found that the tilt of one of the heads (forget which one) was critical, probably due to wear and age. All I remember is a minor adjustment cured it.
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Old 8th Aug 2022, 3:57 pm   #17
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

In Timtape's picture of the 377 it looks like there is a small guide pin just to the left of the erase head which is just far enough forward in the tape path to lift the tape off the erase head when the main tape lifter is engaged. On the OS's 645 there doesn't seem to be one.

Is it missing or did Sony at some point conclude that the wear on the erase head was negligible and consequently omit it from later designs? After all, the wear wouldn't take place in the actual gap area but slightly left of it.
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Old 8th Aug 2022, 4:34 pm   #18
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

Quote:
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In Timtape's picture of the 377 it looks like there is a small guide pin just to the left of the erase head which is just far enough forward in the tape path to lift the tape off the erase head when the main tape lifter is engaged. On the OS's 645 there doesn't seem to be one.

Is it missing or did Sony at some point conclude that the wear on the erase head was negligible and consequently omit it from later designs? After all, the wear wouldn't take place in the actual gap area but slightly left of it.
That's the motor auto shut off in the TC-377, the motors auto shut off isn't fitted in the TC-645 on that side, the TC-645's FF & RW motors auto shut off switch is linked to the tension arm on the right hand side of the tape path.

Lawrence.
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Old 8th Aug 2022, 5:01 pm   #19
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

The erasure problem has now been identified on my TC-645. Basically, it was the supply side tape guide setting. I strongly suspect someone in the past has been messing about with this? I have taken a shot of the setting; the blue spot identifies where the screw slot 'matches' its previous position - this gives me +- 180 degrees of 'play'. I've turned the screw just 60 degrees clockwise, and the erasure issues completely disappears. For now, this will do.

Last night, and today I even took out the bias oscillator and worked on it. Racapped most of the capacitors in there, and replaced the two transistors - the C1364s replaced with C1815-GRs. Although replacements were probably not needed, it's not a bad idea to do this as this deck is ~45+ years old.

Oscillator working well 'offline' and in circuit.
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Old 8th Aug 2022, 5:08 pm   #20
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Default Re: Sony TC-645 1/4 Track R2R Erase Head & Issues

I forgot to mention that the oscillator was encased in a shielded 'box'. I had to desolder around the rim to get access to the circuit board. Desoldering with copper braid and dabbing flux with a Q-tip did the trick!

I shall do all those calibration refinements later once I'm happy that I've got the deck fully on the right track! At this moment I'm making good progress. Must have spent about 20-25 hours work since last Wednesday when i got it. I bet the seller thought he/she fobbed me off with a loser ....ha ha .... no chance!!!

Enjoying *all* your comments gents!, thank you.
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