Quasar VR-1000 - Page 5 - Videokarma.org TV - Video

starbond · #61 10-31-2019, 02:00 PM

Another update, i finally got my hands on the frequency counter, and have been testing out the various crystal frequencies. Looks like my problem spot is the 4.27mhz crystal on the color/rec board. its pretty badly out of spec, and no amount of screw adjustment will get it close. But it seems nobody makes that frequency anymore.. so am I out of luck?

Electronic M · #62 10-31-2019, 02:29 PM

Quote:

Originally Posted by starbond

Another update, i finally got my hands on the frequency counter, and have been testing out the various crystal frequencies. Looks like my problem spot is the 4.27mhz crystal on the color/rec board. its pretty badly out of spec, and no amount of screw adjustment will get it close. But it seems nobody makes that frequency anymore.. so am I out of luck?

The ham radio community often has need for uncommon or unique crystal frequencies, and there are custom crystal makers that cater to them....they can probably help you.

old_tv_nut · #63 10-31-2019, 03:09 PM

I see the exact frequency is specified in the setup instructions as 4.267200 MHz. If you do order one, be sure to specify the full accuracy and not just 4.27.

Can you post a clear pic of the schematic of the circuit that uses this? Curious as to what it does with such an oddball frequency.

Also strange that your reading is off by almost exactly 1 Mhz. Can you find the crystal and read what frequency is marked on it?

old_tv_nut · #64 10-31-2019, 03:18 PM

Actually, it's probably the oscillator that beats the color subcarrier down to a lower frequency (~690 kHz) for recording, but why it is critical in the input monitoring mode beats me, unless this machine operates the full record and playback electronics end to end all the time.

starbond · #65 10-31-2019, 06:01 PM

Quote:

Originally Posted by old_tv_nut

Actually, it's probably the oscillator that beats the color subcarrier down to a lower frequency (~690 kHz) for recording, but why it is critical in the input monitoring mode beats me, unless this machine operates the full record and playback electronics end to end all the time.

At this point I dont know what else could be causing my weird banding on the picture, so far this is the only thing I've measured with the frequency counter that's out of spec. I've even replaced the bad 47uf cap, and the 2SC828 transistor as a precaution, neither which had any effect on the crystal. In fact I went to measure it again just now, and its reading 2.8 mhz!

old_tv_nut · #66 10-31-2019, 10:11 PM

Again, can you post a clear pic of the schematic of the 4.27 MHz oscillator, and indicate where you are connecting the counter? Putting a 0.001 disc cap in series with the counter lead might be a good thing to make sure you aren't loading the oscillator.

starbond · #67 10-31-2019, 11:33 PM

Quote:

Originally Posted by old_tv_nut

Again, can you post a clear pic of the schematic of the 4.27 MHz oscillator, and indicate where you are connecting the counter? Putting a 0.001 disc cap in series with the counter lead might be a good thing to make sure you aren't loading the oscillator.

I posted the schematic in the last pic but I'll isolate it down. Unfortunately whoever scanned this schematic did so at a terrible resolution, and I haven't been able to find a better one since.

Frequency counter is attached to TP204,the 4.27mhz crystal is X201. The adjustment screw is C219.

old_tv_nut · #68 11-01-2019, 10:20 AM

OK thanks. The counter should not be loading that test point significantly, so it seems the crystal/oscillator really is the problem.

starbond · #69 11-01-2019, 10:35 AM

Quote:

Originally Posted by old_tv_nut

OK thanks. The counter should not be loading that test point significantly, so it seems the crystal/oscillator really is the problem.

Thanks for verifying. I am having a hard time finding a crystal with the correct frequency, unless I want to special order and pay $$$$

My question (in general) is that if this quartz crystal was so important to color syncing in vcrs and tvs, why is it not as common as the 3.57mhz crystal?

old_tv_nut · #70 11-01-2019, 10:50 AM

I believe different VCR systems used somewhat different color-under frequencies. VHS crystals would be the most common. Also not 100% sure, but likely that different frequencies were needed for PAL and NTSC.

starbond · #71 11-01-2019, 01:33 PM

Quote:

Originally Posted by old_tv_nut

I believe different VCR systems used somewhat different color-under frequencies. VHS crystals would be the most common. Also not 100% sure, but likely that different frequencies were needed for PAL and NTSC.

Thanks TV Nut. The closest crystal i could find is 4.2944mhz. Not great. Do you think the little 30pf cap could tweak it down to 4.2672?

https://www.mouser.com/ProductDetail...XTAL035831BULK

old_tv_nut · #72 11-01-2019, 01:46 PM

Quote:

Originally Posted by starbond

Thanks TV Nut. The closest crystal i could find is 4.2944mhz. Not great. Do you think the little 30pf cap could tweak it down to 4.2672?

https://www.mouser.com/ProductDetail...XTAL035831BULK

NO. A crystal oscillator that could be detuned that far wouldn't really be dominated by the crystal resonance, and might oscillate at an unstable crystal mode as well. An extreme value for detuning might be 1 KHz, and that much would never be designed into an oscillator meant to be a reference, only into a voltage controlled one meant to track a broadcast or tape playback signal.

starbond · #73 11-01-2019, 02:14 PM

Quote:

Originally Posted by old_tv_nut

NO. A crystal oscillator that could be detuned that far wouldn't really be dominated by the crystal resonance, and might oscillate at an unstable crystal mode as well. An extreme value for detuning might be 1 KHz, and that much would never be designed into an oscillator meant to be a reference, only into a voltage controlled one meant to track a broadcast or tape playback signal.

OK thanks for that. I will go back to the hunt. Are you saying ONLY a 4.267200 crystal EXACTLY will work, and nothing out of that range will be acceptable?

old_tv_nut · #74 11-01-2019, 05:38 PM

Quote:

Originally Posted by starbond

... Are you saying ONLY a 4.267200 crystal EXACTLY will work, and nothing out of that range will be acceptable?

Yes. There's a outside possibility that an off-frequency crystal would allow the VCR to record and play back its own tapes , if the under-color frequency is all it affects; BUT even then the VCR would not play or record tapes compatible with VCRs using the standard crystal.

starbond · #75 11-04-2019, 09:24 AM

So I have some great news. Firstly it turns out the 4.27mhz crystal WAS OKAY. The counter i was using was giving bad values. So the error was somewhere between the color circuits and the color output.
I hooked up my scope leads 1.) to where the recombined NTSC color was coming out of the color circut and 2.) the INPUT where the signal generator was coming to the unit. Both should be the same waveform and it was..so I started looking at the EV color play board.

Due to some dumb luck I started tweaking one of the screw adjustments (L509) all to the way to max level, and suddenly BOOM, color! Its not perfect, as when I twist L509 to make color on the video tapes, the MONITOR colors are still wrong, but when I twist l509 to make the color bars correct, the tape color goes back to rainbowing. So tonight I have some work to do on the adjustments, but man it feels good seeing color on these tapes (even if they are taped in the 80s from a source with high generation copies)