JVC CX-500ME repair [Ex: Troubleshooting/fixing analog crt tv receiver] - Page 1
Forum section: electronics / repair

author: Bendba
author_rank: Regular Contributor
timestamp: May 16, 2017, 08:52:42 am
content: Hi,I found not long ago, at the scrap yards, a JVC CX-500MEI opened it up before pluging in, not knowing what the condition was, if it had been wet. Looked alright so I plugged it in, the switches and potentiometers were a bit crusty, it took a bit of playing with them to get it working smooth.The radio part of it works perfect, I got no sound coming from the cassette deck but that shouldn't be too hard to fix.The string running around the knob, dial and potentiometer for the tuning of the tv needed fixing, done.I thought, sweet, a little monitor for my Commodore 64 as I am away from home all the time, a week/10 days at a time.Problem is I can't get an image on the screen, it looks like it is trying to tune but I can't get anything decent. The Commodore works well on the big tv at home, so that's not the issue.Any lead on a good resources on how to troubleshoot analog crt tv's? Or an idea of what could be wrong? I only know the basics of how it works but obviously not enough.I'll try to add photos, in case it could help.Thanks

author: Jeroen_Bezemer
author_rank: Contributor
timestamp: May 16, 2017, 08:56:45 am
content: You could make a composite or rgb connection to the exit of the tuner and bypass the modulator as well.Verstuurd vanaf mijn ONEPLUS A3003 met Tapatalk

author: Bendba
author_rank: Regular Contributor
timestamp: May 16, 2017, 09:14:34 am
content: Hi,I have been contemplating that idea as well, no need to bypass the modulator, the C-64 already has a composite/s-video output connector.It would be nice if I could fix the tuner though, just to learn how to. Plus, I might find more old video devices (got an atari 2600 but that stays at home)

author: Bendba
author_rank: Regular Contributor
timestamp: May 16, 2017, 11:14:26 am
content: Well, I eventually worked it out, call me dumb on this one. It appears that the Commodore 64 has a UHF output, not VHF like the atari. I was trying to tune in on the wrong band. What I don't get though is that I still had a faint image on the low VHF, how comes?

author: Theobald
author_rank: Contributor
timestamp: May 17, 2017, 07:06:06 am
content: Hi,in this kind of products, UHF is made by using harmonics of a low cost crystal. The VHF harmonics are weak, but your TV set tuner is very sensitive.Theo

author: Bendba
author_rank: Regular Contributor
timestamp: May 17, 2017, 11:09:55 am
content: Is it? I always thought it was based on a LC oscillator.What was the interest in modulating in UHF instead of the low frequency UHF?I am probably still going to add a connection on the portable tv set to bypass the RF, hopefully that'll make it usable in text mode.I have another issue but I'm unsure weather I should start a new subject about that. I can get anything out of the tape deck of that CX-500. The read head doesn't seem to read anything (very faint signal out of the head) and there is no amplification (i think that's only a corrosion issue, needs cleaning up some switches and reflow some solder joins)And I would really like to get it working and add the same circuitry as the Commodore tape deck inside the casing to save me having to transport the original C2N

author: Theobald
author_rank: Contributor
timestamp: May 18, 2017, 08:51:15 am
content: You're right, it's a LC oscillator. See thishttps://gareth.halfacree.co.uk/2012/09/the-astec-um1233-rf-modulator1) Regarding the stability of the crystal vs LC, we don't care, because your TVset has an AFC circuit.2) The LC oscillator can be tuned over several TV channelsVHF or UHF depending on the country, sounds good also.In your case, same answer: your modulator may be designed by filtering the 3rd or 5th harmonic from a VHF LC oscillator and the TV was tuned on the fundamental.By the way, you can make a VHF or a UHF RF modulator by changing values in the output filter.Theo

author: Bendba
author_rank: Regular Contributor
timestamp: May 20, 2017, 04:22:14 am
content: Hi Theo,Thanks for your input.Would there be a frequency in the range the modulator can handle where the image would be of better quality? Or is the quality of the image independent of the modulator's frequency?Also, if someone has any tips on testing the cassette deck read/write head, that would be welcome. I have continuity on both windings (stereo deck) but I can seem to detect any signal when playing a tape. Could both ferrite rings be broken? What is the most common failure on those heads?Thanks

author: mzacharias
author_rank: Frequent Contributor
timestamp: May 20, 2017, 12:22:13 pm
content: Quote from: Bendba on May 16, 2017, 08:52:42 amHere is a link to a service manual (should be anyway, haven't tried it.)http://elektrotanya.com/?q=showresult&what=jvc%20cx-500&kategoria=&kat2=schematics

author: Bendba
author_rank: Regular Contributor
timestamp: May 20, 2017, 01:37:35 pm
content: Hi,Thanks heaps for that.According to the schematic, it's a mono head. So I'm guessing what I tough I was measuring as continuity is a short.Well, I took the cassette unit apart. It is indeed a mono head.

author: Theobald
author_rank: Contributor
timestamp: May 24, 2017, 07:29:41 am
content: Hi Bendba,No, the quality is usually the same on the few TV channels covered by the modulator.The composite video input and the RGB inputs are always better than UHF/VHF. You may improve the picture by fine tuning the TV set.In my opinion, the RF input gives the 'old fashion' touch to your C64 !Theo.