Fixing a noisy rf modulator - Page 1
Forum section: electronics / repair

author: fabiodl
author_rank: Frequent Contributor
timestamp: October 16, 2020, 04:09:18 pm
content: I am trying to repair a sega sg-1000 II which has a noisy modulator. It is not the inherently low quality of the RF signal, because another sg-1000 II on the same TV, with the same modulator provides a much better signal.I attach some photos of such modulator. I tried tuning the variable inductor and changing the electrolitic caps, but that made no difference. I also tried to slightly straighten the vertical wire wound inductor, which appears to have become loose from the epoxy, but that gave no improvements.Are there any common faults of RF modulators worth checking?(I know I can just buffer the composite video and audio, but I'd like to keep it original).

author: bob91343
author_rank: Super Contributor
timestamp: October 16, 2020, 08:41:05 pm
content: It's hard to help when you don't describe the noise or when it appears. Often it's caused by poor solder joints, oxidized connectors, and occasionally a bad component.

author: Fraser
author_rank: Super Contributor
timestamp: October 16, 2020, 09:11:28 pm
content: These very simple modulators are just a VHF or UHF oscillator that is amplitude modulated by the composite video signal. Crude, but they worked.As the modulator is an Amplitude Modulated oscillator it is important that the power supply to it is not too noisy. The fault with your Sega could actually be outside the modulator module and on the power supply to it. Noise on the power supply can get superimposed on the output of the modulator. Check the Sega power rails for unexpected levels of noise. You have another Sega Machine as a reference to compare against. Look for failed capacitors on the Sega’s power supply circuits.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: October 16, 2020, 09:23:48 pm
content: You might also want to check the video signal produced by the TMS9918 where it enters the modulator to ensure that the signal is being correctly produced ready for up conversion to the TV signal compatible output.All,of this is assuming that we are discussing a video signal issue and not audio as you have not stated the exact nature f the noise.http://www43.tok2.com/home/cmpslv/Sg1000/EnrSG.htmFraser

author: fabiodl
author_rank: Frequent Contributor
timestamp: October 17, 2020, 04:20:13 am
content: Thank you everybody.This is indeed a video noise problem. I attach the comparison. Left, good sg1000, right, sg1000 of this topic.I'll reflow the video related path (this version actually has the tms9918 integrated with the vdp ram in a sega custom chip), but to visual inspection they seem completely fine. I previously checked the 7805 output with the oscilloscope and saw no noise, but I'll try to replace it and its input/output caps.I'll also buffer and check the composite video itself, to see if the problem lies before or at the modulator.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 17, 2020, 01:43:34 pm
content: Although I wouldn't blame it on the modulator itself I'd still check Melf solders in there if any.Usually it isn't epoxy over the air inductors but paraffin wax, straightening or making them look neater isn't always a good idea as they may have been spaced open for tuning.I'd check the CVBS before modulation as suggested then possibly PS noise at the Sega video ASIC itself, replace decoupling electrolytics accordingly.

author: fabiodl
author_rank: Frequent Contributor
timestamp: October 18, 2020, 03:48:58 am
content: The composite (on a second TV) is fine. And even the RF, on this second TV, is ok. There must something funny which is filtered out by the second tv but not the first.

author: Fraser
author_rank: Super Contributor
timestamp: October 18, 2020, 10:55:31 am
content: Likely a dirty output RF spectrum that the first TV IF stage filtering cannot deal with. The second TV likely has better IF filtering. Not all TV’s like to work well with these simple modulators even when operating correctly. To see what is happening at the output of your modulator you really need a spectrum analyzer to look for signal spurs and mixing products.Fraser

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 18, 2020, 09:51:46 pm
content: What's the audio like?Maybe just too much output, if you don't have a S.A. around you could at least try an attenuator see if that helps.

author: fabiodl
author_rank: Frequent Contributor
timestamp: October 19, 2020, 05:55:03 am
content: Quote from: shakalnokturn on October 18, 2020, 09:51:46 pmUnluckily I don't own a spectrum analyzer. Audio is fine. I also tried to replace the crystal oscillator, suspecting the second tv was better at dealing with drifts in the color carrier, but that made no difference. And, to closer inspection, I would expect many more rainbows here and there if it was a problem of the modulating freq.What kind of attenuator are you referring to?

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 29, 2020, 03:36:29 pm
content: Resistive attenuator between modulator output and TV tuner input.

author: fabiodl
author_rank: Frequent Contributor
timestamp: November 01, 2020, 08:44:32 am
content: do you mean 75 ohm/ 330 ohm impedance adaptation? or just to try a resistive voltage divider? what kind of ratio do you suggest?