High speed (double speed) VHS scope

[RajkoBizjak]

Hi!
I hope someone can help me. Years ago (in 80's) I try to improve VHS quality by doubling the tape speed. Our service guy add something in my scopes and add an extra switcher so I can play the tape in double speed and make gap between the traces on the tape. Then I sell the scopes and now I find some recorded tapes on this double speed with precious memories I could not play on any machine.
Anyone knows how to prepare scope to double the tape speed in the scope (make 90 min tape from 180 min)? I think it must be simple because It did not cost too much then and was done on my 3 machines that all work compatible.
Thank you for your help-

Rajko

[dellsam34]

What's a VHS scope? Are you translating from a another language?

[FLP437]

It seems a hardware Mod the vcr hardware have been modified to allow an extra record mode speed. Instead of SP, LP, and EP they do have another record mode at 2x SP.

[dellsam34]

I think he means by "my scopes" is my VCR's, In French VCR is called Magnetoscope and maybe some other languages too, So it came out in the translation as Scope which we all know in English is an Oscilloscope. So I guess he is asking if anyone knows how to make that mod again, So, unless someone from that era still has the schematic I highly doubt that this will have a happy ending.
Basically the mod has to alter both capstan and video drum servos, The bandwidth will be doubled so the VCR guts has to be bypassed and work from the head RF signal which is basically just like the VHS-decode project, plus the extra motors' servos modification.

[RajkoBizjak]

H! to all.
Thank you for your effort. Yes, I mean VCR. I had JVC BR8600U and portable JVC HR2200. And the service guy in Zagreb change me the VCRs in one afternoon, and he was not the JVC service. HE could do that from any VCR, so I think this should be something very simple. And the VCR were working perfectly synchronous, because I can play on BR8600 all the tapes recorded in HR2200 without problem. I think it simply double the motor speed and with one switcher I can select between normal and double speed. If I try to play this tapes on normal speed, I got simply the noise, without any signal. The modification makes a gap between recorded slices, so they were not recorded side by side but due to the higher contact speed between head and tape maybe also change the recording frequency.
Maybe there was simple the bridge that double the power on motor so it rotates twice as fast?
Thank you

Rajko

[dellsam34]

Oh, I get it. So he only modified the capstan speed, So the video tracks will be slightly longer, but since the drum speed is unchanged it will result in a space between the video tracks which slightly eliminate cross talk, although double azimuth should've already taken care of that but maybe there is a very slight interference between video tracks, But the major benefit is the longer tracks which supposedly produces better S/N ratio especially for chroma.

Well this indeed can be done if someone knows how to double the synchro frequency for the capstan motor, Someone who can read the schematic and understands how the capstan motor is timed and double that timing frequency for 2x speed.

JVC decks with Dynamic drum system do have 2x SP playback speed but they are optimized for normal video tracks, so if the tracks are longer than what the drum tilt can cover it may not produce a stable frame, plus there might be no Hi-Fi audio.

[RajkoBizjak]

Yes, this could be.
Do you think it is easy to make in any VCR? I do not care about Hifi audio, only picture interested me.

Rajko

[dellsam34]

I don't know, Maybe some members from the VHS-decode project have the skills to modify the capstan motor control, I know it uses some kind of BLDC motor control with hall effect sensors for positioning, Whether it uses trapezoidal with PWM or sinusoidal scheme, that I don't know.