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Old 31st Mar 2023, 9:09 am   #21
beery
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

Hi Rob,

I once restored a Sony CV2000 that had a similar issue.
The problem with closed loop servo systems is that the faults can be both mechanical and electronic, but it can be hard to determine which is which.

Check the mechanical side first.

Does the frequency of the picture shift vary in time with the take up or supply reels?

How is the pinch roller?

How is the braking material on the tension band?

What condition are the friction is the take up friction clutch in?

Is the tape sticking to itself?

Are there any distorted parts on the belts?

Failing that, I would then re-cap the servo circuit.

When I restored the CV2000, I had a similar problem. I re-ground the pinch roller, re-lined the tension band and even skimmed the tension braking surface of the take up reel on a lathe to make it run truer than ever.
However, the route cause of all the problems were the electrolytics on the servo board. I could have saved myself a lot mechanical work if I had realised this earlier.
BTW, I ended up with a really good CV2000, which I have since sold.

Cheers
Andy
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Old 31st Mar 2023, 1:08 pm   #22
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

Quote:
Originally Posted by TIMTAPE View Post
I said that this recording, even though below broadcast quality (so were Quads if we want to be fussy) is possibly better quality than whatever BBC copies survive.
I'm intrigued by your statement, which I've highlighted - what makes you believe that?
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Old 31st Mar 2023, 1:52 pm   #23
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

I was just about to ask the same question of TIMTAPE.

Are you saying that during the 60's & 70's Quadruplex VTR's did not produce broadcast quality video & audio?

If however you are comparing them with todays technology then quite naturally they would be found wanting, as would Logie Bairds mechanical scanning system.
But they were the best we had at the time.
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Old 31st Mar 2023, 2:07 pm   #24
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

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Originally Posted by Amtec123 View Post
I was just about to ask the same question of TIMTAPE.

Are you saying that during the 60's & 70's Quadruplex VTR's did not produce broadcast quality video & audio?

If however you are comparing them with todays technology then quite naturally they would be found wanting, as would Logie Bairds mechanical scanning system.
But they were the best we had at the time.
To my eyes at least, when correctly set up a Quad produced better pictures (IMO) than some C-format machines.
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Old 31st Mar 2023, 2:46 pm   #25
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

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Originally Posted by Cathovisor View Post
Quote:
Originally Posted by Amtec123 View Post
I was just about to ask the same question of TIMTAPE.

Are you saying that during the 60's & 70's Quadruplex VTR's did not produce broadcast quality video & audio?

If however you are comparing them with todays technology then quite naturally they would be found wanting, as would Logie Bairds mechanical scanning system.
But they were the best we had at the time.
To my eyes at least, when correctly set up a Quad produced better pictures (IMO) than some C-format machines.
I agree entirely, having become accustomed to first generation VR2000 quad recordings, 'C' format when it arrived was a disappointment. Its saving grace was flexibility as a production tool, slo-mo, freeze, even pictures in shuttle, pure luxury!

As you said earlier prior to domestic cassette formats the BBC used Shibaden. At Thames we used Sony 2100 1/2" open reel m/c's for production staff viewings. These were only ever used to save 'main area' VTR time. Otherwise their playback would tie up one of our five quad VTR's
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Old 31st Mar 2023, 3:05 pm   #26
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

I guess it depends on how we specify broadcast quality, both in picture and in sound. My understanding is the live studio broadcast was or could be of a higher standard in picture and sound than the Quad playback at best, but by how much higher and in what departments might be a longer discussion. My understanding was that the average viewer might not tell the difference between live and Quad playback but a trained eye could. I'm more an audio guy. I know a Quad's audio signal noise and distortion was significantly worse than could be had with say a U67 studio microphone and preamp, and it introduced some wow and flutter. But again what was the standard by which the Quad was measured? One for live or one for the Quad? Did they dumb down the standards for VCRs so the VCRs could meet them?

Last edited by TIMTAPE; 31st Mar 2023 at 3:13 pm.
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Old 31st Mar 2023, 3:37 pm   #27
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

If you are comparing 'Live' with any form of recording it follows that the recording by nature will be of lower quality. As for audio a PAL 625 quad had a linear tape speed of 15.625 IPS. So should be capable of high quality, however the limiting factor was not the VTR but the magnetic tape. It was manufactured with transverse particle orientation to achieve best video s/n. Even the later generation of digital broadcast VTR's which at first glance looked excellent did produce artifacts.
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Old 31st Mar 2023, 5:41 pm   #28
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

There is a Shibaden SV700 VCR upstairs above the shop. It hasn't been used for at least twenty years but I do remember it played back stable 625 line recordings with no disturbance at the top of the picture.
In it's present state the machine is modified for 525/60 operation.
There is also a Sony VO1800 VCR keeping company with the Shibaden.

DFWB.
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Old 6th Apr 2023, 9:45 am   #29
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

Hi everyone. Another update on this story

So.. I checked
- the 12V power - all ok (11.97Vdc)
- changed the pinch roller and pinch roller arm assembly (from a junker SV700) - no change to picture wobble or sound wow/flutter
- cleaned the control head and tape path - no change
- tried other tapes - all suffer from same issue. None appear "sticky"
- checked the capstan belt. Nice and tight, very thick although there is a small kink

Finally I recapped the DSC (Disc Servo) PCB. The elcos were wildly out of spec - many with very high ESR. Whilst not good enough, I think the results have improved a little

Post servo board recap.
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Old 8th Apr 2023, 6:46 pm   #30
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

Back to Quads and broadcast quality...

It is true that they had worse bandwidth, SNR, DP/DG, K factor than other contemporary signal sources (see IBA Code of Practice), but they were definitely THE broadcast standard until the early 80s. Artefacts (e.g. banding) could be an issue but were improved by later developments such as line by line autochroma and super high band (which was never really used due to the advent of one inch).

Early one inch stock was not good, but later stock much better. I would put a BVH-2000 or VPR-3 with good tape up against a quad. No chance of banding with C format.

The sound on quad was marred by the video being transversely recorded on top of the longitudinal audio. They needed a notch filter in the audio replay path.

Jonathan
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Old 8th Apr 2023, 10:44 pm   #31
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

If it has improved it doesn't sound or look by much. Have you checked caps in the deck's power supply? The servo board can only compensate for a weak power supply up to a point.
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Old 9th Apr 2023, 10:05 am   #32
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

VTRestorer,
Whilst I broadly agree with most of your comments, I feel your last sentences do not tell the full story. It is indeed true that early Ampex quad machines VR1000, 2000, etc did record video rf across the full width of the tape. However further downstream came the audio and cue head assembly. This had a convensional hf erase stack which erased the audio area of the video signal, and was followed by a record / replay head. This combined function also contributed to the less than perfect audio quality, but was necessary in order to maintain audio sync. There was one time when an audible 'buzz' could be heard, and that was if a 'video only' edit had been performed, the new video rf would cut across the existing audio.
Staying with Ampex, move on a generation to the AVR-2. This machine removed the problem by simply turning on and off the video record rf when it wasn't needed.
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Old 30th Jan 2024, 2:28 pm   #33
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Default Shibaden SV700 Open Reel VTR - Wobbly picture (revisted)

Hi all,

Last year I posted about my renovation of the above (a late 60s open reel VTR - a Shibaden SV700).

With the help of a friend, revisited the machine and cleaned the drive wheel assemblies, replaced the capstan belt and tried again. Still not great (video here) ), but noticed two things

1) When the picture looses tracking, there is a distinct change in sound in one of the motors as if the servo system is "hunting" for the right speed. I suspect this is the capstan motor based on the fact the picture is still present. (My understanding is the picture will disappear if the head motor is not at speed)

2) When the recording on the tape finishes (and the screen turns to snow), the hunting noise stops.

Any ideas?

Last edited by Station X; 30th Jan 2024 at 2:53 pm. Reason: Threads merged.
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Old 31st Jan 2024, 10:56 am   #34
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

Hello again,
Seeing your latest link tells me the capstan servo is failing to lock the control track pulses to the off tape video. Was the drive belt a genuine one, or just of a similar size. If you gently apply pressure (damping) via your hand to the belt does the servo become more stable ?

The "hunting" motor noise stops at the end of the recording due to the fact the servo has nothing to compare.

Good luck
Les
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Old 31st Jan 2024, 11:04 am   #35
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

Quote:
Originally Posted by Amtec123 View Post
Quote:
Originally Posted by Cathovisor View Post
Quote:
Originally Posted by Amtec123 View Post
I was just about to ask the same question of TIMTAPE.

Are you saying that during the 60's & 70's Quadruplex VTR's did not produce broadcast quality video & audio?

If however you are comparing them with todays technology then quite naturally they would be found wanting, as would Logie Bairds mechanical scanning system.
But they were the best we had at the time.
To my eyes at least, when correctly set up a Quad produced better pictures (IMO) than some C-format machines.
I agree entirely, having become accustomed to first generation VR2000 quad recordings, 'C' format when it arrived was a disappointment. Its saving grace was flexibility as a production tool, slo-mo, freeze, even pictures in shuttle, pure luxury!

As you said earlier prior to domestic cassette formats the BBC used Shibaden. At Thames we used Sony 2100 1/2" open reel m/c's for production staff viewings. These were only ever used to save 'main area' VTR time. Otherwise their playback would tie up one of our five quad VTR's
Offline editing at the BBC used to be known as "Shibadening" because of the use of their machines!
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Old 31st Jan 2024, 8:32 pm   #36
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

Looking back at your post #1 link the servo was at least locking, all be it unstable. However now it is not acheiving lock at all, is the belt the only thing that you have changed if so maybe try the original belt again,
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Old 1st Feb 2024, 4:47 pm   #37
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

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Originally Posted by Amtec123 View Post
Looking back at your post #1 link the servo was at least locking, all be it unstable. However now it is not acheiving lock at all, is the belt the only thing that you have changed if so maybe try the original belt again,
Yes - this is odd. I did have to remove the capstan motor to replace the belt as I thought the belt tension might be part of the problem. I may have made things worse if now their is no tape servo lock.

I'll take another look.
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Old 2nd Feb 2024, 3:34 pm   #38
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

Okay. So I've concluded I hate this hobby !

Took the capstan motor out and cleaned/oiled. Then noticed the capstan belt had a sticky patch on it which had deposited "goo" on the motor shaft. I thought I was onto the cause.

Replaced the belt and put back together.

New video of the Shibaden playing

IT'S EVEN WORSE !

If it's the capstan motor/belt struggling to respond to servo instructions (hence the hunting noise), why would a clean, new belt cause the symptom to get worse?


Really struggling on this one.
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Old 2nd Feb 2024, 6:20 pm   #39
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

Finding that "goo" on the motor shaft would not have helped in acheiving servo lock. Your latest link shows the motor speed has changed i.e. more rapid passes through correct tracking. What did you clean the belt with? Sorry if you have mentioned before but does the tracking control have any effect, and did you try slowing the motor by appling light hand pressure. As pointed out in an earlier post these sort of problems are usually mechanical rather than electrical.

Do you have circuits / description of the VTR, it would also be useful to check with an oscilloscope that the servo is getting both control track pulses and off tape vertical pulses. If either is missing then it will never acheive lock.

Just have a break, and come back fresh to it.
You'll get there.
Les
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Old 2nd Feb 2024, 8:24 pm   #40
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Default Re: Shibaden SV700 Open Reel VTR - Wobbly picture?

Quote:
Originally Posted by Amtec123 View Post
Finding that "goo" on the motor shaft would not have helped in acheiving servo lock. Your latest link shows the motor speed has changed i.e. more rapid passes through correct tracking. What did you clean the belt with? Sorry if you have mentioned before but does the tracking control have any effect, and did you try slowing the motor by appling light hand pressure. As pointed out in an earlier post these sort of problems are usually mechanical rather than electrical.

Do you have circuits / description of the VTR, it would also be useful to check with an oscilloscope that the servo is getting both control track pulses and off tape vertical pulses. If either is missing then it will never acheive lock.

Just have a break, and come back fresh to it.
You'll get there.
Les
Hi Les!

I didn't clean the belt - I replaced it with something similar. And cleaned the goo off both the motor shaft and the capstan that the belt connects to.

I did try slowing down the motor. Tricky as there's not much room to get a finger in, but as I slowed the motor, the frequency of the "hunting" increased.

Does this imply the capstan isn't turning fast enough? Perhaps the belts I am using are not taut/tight enough?

I'm away for the weekend, but I agree the next steps is to try and examine the waveforms in the servo. I have the service manual now. The fact the "hunting noise" only starts when a recording starts on the tape and the hunting stops when the recording finishes (and before the tape runs out), to me implies the control pulses are being received and speed up/slow down voltages applied to the capstan motor - is this sound logic?

Not really being a video engineer, I am starting to doubt myself!
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