Shibaden SV700 Open Reel VTR - Wobbly picture?

[Amtec123]

That is an interesting observation. The fact that the "hunting" frequency increases when you slightly slow the capstan motor implies to me that indeed it is running slow, possibly due to slipage. It might be me but I thought the audio sounded slow in your last link. As has been said before the belt condition / tension is very critical on these machines.

In the Sony 1/2" VTR I had servo lock was maintained by using the error voltage to control a dynamic brake. Now you have a manual it will be interesting to see if Shibaden use a similar idea.

Enjoy you weekend.

[TIMTAPE]

VCR's are my strong suit but isnt the VCR supposed to have a free running (capstan?) speed a little faster than the normal, which allows it to slow down a little for the regulated speed dictated by the control track? I seem to recall a previous upload seeming to run slower than a YT upload of the same concert piece. When regulated shouldnt the two uploads be the same speed?

[collisr]

OK. So I've solved one problem - the tape speed. As noted by some previous comments, the audio was indeed running slow. That was because a spacer over the capstan motor was upside down and shortening the motor-to-capstan orbit. I also swapped in a slightly tighter belt. I think this is better from an audio perspective but the "wobbly picture" original fault remains

Latest update

So.. started to try and understand the service manual. Am I right to conclude from the attached pics that the capstan runs at a constant speed and is not affected by the tape control pulses on playback? I was assuming the "hunting" noise I can hear during playback was from the speeding up/slowing down of the capstan motor, but my interpretation of the service manual is the hunting must be the head-drum motor - is that correct?

Thus, is it also correct to assume the issue then is either erroneous electronics in the servo board telling the head drum to alter speed incorrectly OR the head drum motor is struggling to do what its asked to do by the servo board?

There is a beafy start-motor capacitor on both the motors - would these be worth swapping out?

Determined to fix this but quite lost on this old beast.

[Amtec123]

Well that information makes things clearer, the servo system is somewhat different from the Sony version I had.

Yes, the capstan motor is locked only to the mains frequency.
Servo control uses a positional tachometer pulse derived from the video head drum, this is compared to control track pulses using a ramp and sample circuit. Assuming a locked condition optimum tracking is adjusted by slight changes to pulse timing.

Assuming the capstan speed is correct, It's worth checking the pulses used in the ramp and sample circuit.

[collisr]

Originally Posted by Amtec123

Assuming the capstan speed is correct, It's worth checking the pulses used in the ramp and sample circuit.

Thanks Amtec. Based on the Disc Servo physical circuit (attached), it is correct to interpret:

- TestPoint 601 (left, input) is the video head drum pulses
- TP 607 is the tape control pulses ?

I assume by ramp/sample you mean what Shibaden call the Disc Servo Control (DSC) board?

I have the waveforms listed against each test point so can test all this.

What I don't understand is what the output from the sample/compare is - is it the output (right-hand-side-of-diagram) labelled "MDT1/2" ? These are meant to be 16Vp-p square waves. How would these speed up/slow down the head drum?

Sorry if this is obvious to many, but whilst I understand the fundamentals of modern VCRs, this is looking quite alien to me.

[Amtec123]

Thanks for the servo circuit, unfortunately it is almost unreadable. Maybe my eyes.
However looking at the block diagram the ramp (sawtooth) is generated by the head drum tach signal, the control track pulses are used to generate an error voltage. This error signal controls an oscillator which in turn controls the speed / phase of the head drum motor.

I would try shorting TL606 and TL607 and see what effect VR601 has.

Also look closely at the pinch roller, as previously mentioned this can be a cause of instability.

[collisr]

Thanks Amtec!

So a few updates. I *think* I have both disc head pulses AND control head pulses but they clearly aren't right. (The shifting left and right of the Disc head pulses and the output square wave are in sync with the picture wobbles on output)

Scope measurements cf service manual

Attached below is a clearer version of the Servo schematic as the last one wasn't really readable was it?

Any observations? I can't tell what is cause and what is effect from these readings.

Originally Posted by Amtec123

I would try shorting TL606 and TL607 and see what effect VR601 has.

I will do this. What is this procedure doing? Does VR601 adjust the "free run speed" of the head drum? Does shorting these point disengage the drum pulse/ctrl pulse comparison so it won't try and "hunt" while you are adjusting the free-run speed?

Originally Posted by Amtec123

Also look closely at the pinch roller, as previously mentioned this can be a cause of instability.

Understood. Have checked the roller and seems in good shape. Not hard or shiny, no pits or cracks. I would swap it out for a "junker" but it does seem a beast to remove.


Thanks again for your help so far. I can't let this one beat me as I have a stack of old BBC dubs I would love to digitise.

[Amtec123]

That circuit diagram is much clearer, thanks.

Once TL606 & 607 are shorted then VR601 should indeed set the head drum free running speed. I expect the manual says something like adjust for closest free running drum speed. Once the shorting link is removed you should have a locked servo, the "tracking" control should then be able to maximise the off tape rf envelope.

One thing that concerns me is that the control track pulses are not stable. Given the capstan motor is mains locked I would expect them to be pretty steady. If your scope is looking at Q601 then there could still be a problem with the belt / pinch roller / motor, its also possible that the recording was made on machine which was unstable. Have you tried other tapes?

Good luck

[collisr]

Originally Posted by Amtec123

Once TL606 & 607 are shorted then VR601 should indeed set the head drum free running speed.

Amtec/All,

I think we have a root cause! After adjusting the variable resistor (that the Shibaden calls "Freq") on the Disc Servo Control (DSC) board, I now have a lock on the Head Motor and a reasonably stable picture & sound. It didn't need much of a turn but it did need adjusting. (And a spray of contact cleaner). Tracking now works too

Fault Root Cause Found !

There still may be room for further improvement but it was a head drum running too slow for the servo feedback system to cope with.

Thanks again to everyone for their help. It took over a year to solve but ho-hum.

[Amtec123]

Congratulations, looking good.
Les