Experimenting with Casio Portable Analog TVs from eBay

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Recently I won an eBay Auction for 2 portable TVs, the EV-570P and the TV-880D. Although both are analog TVs with a 3″ LCD screen, the EV-570P supports both VHF and UHF while the TV-880D supports UHF only, as indicated in the channel ruler below the LCD:

Both sets have an automatic tuner and there is no option for manual tuning. Pressing the tuning buttons (next, previous) will scan and switch to the next available channel. A vertical scrolling bar (red for VHF, green for UHF) will be displayed showing the current channel (which is what the channel ruler is for).

I was disappointed to find out that although both TV sets are able to tune into Singapore channels and display the picture properly, only the EV-570P is able to play audio while the TV-880D simply plays white noises. I remembered the auction description clearly stated that both sets support PAL, which is the analog TV standard in Singapore. So what is the problem? I soon learned my lesson by looking at the back of both sets and by reading this Wikipedia article about different PAL standards:

The EV-570P supports PAL B/G/H/I while the TV-880D supports PAL I only. And Singapore is using PAL B/G standards. The following table from the above Wikipedia article explains why the TV-880D could not play the audio properly:

While PAL B/G and PAL I are color-compatible, the audio is not as it is transmitted on a different subcarrier frequency (5.5 MHz for PAL B/G and 6MHz for PAL I). The set was attempting to play the audio from a non-existent frequency, resulting in white noise. Notice that while this difference prevents the TV-880D from receiving Singapore channels in PAL B/G, the composite input is unaffected and can be used with other portable devices that have composite output such as VCD or DVD players

With some free time, I decided to disassemble the TV-880D to see if there is any hope of modifying it to support PAL B/G:

I quickly identified a through-hole crystal (highlighted in RED). However, with my oscilloscope, the frequency measured is 4.43MHz, the PAL color subcarrier frequency, not the sound subcarrier frequency.

My next attempt is to search for the service manual of the unit, which may offer some insights. I found manuals of other similar models, the TV-770B (for NTSC only), TV-770C/D/G (for PAL B/G/H, PAL I, PAL M respectively). The block diagrams of these models are quite similar and I can almost match it to my TV-880D:


Further studying the part list, I learned that the only major difference between the PAL B/G/H/I/M or NTSC set is the tuner (highlighted in green):

For TV-770B:

For TV-770C/D/G:

 

In other words, all that I was hoping to modify, the sound subcarrier frequency and the tuning range (VHF or UHF), is inside that “black box”, the tuner. One of the replies to my post on this forum suggests that it may be possible to disassemble the tuner, change the crystal filters used for the audio subcarrier frequency and re-assemble it. However, as I found no elegant way to do that without break-opening the tuner metal can, I reassembled the unit unmodified and simply used it with my home circuit transmitting in PAL I.

The service manuals of the TV-770 and other Casio portable TVs can be downloaded here. I hope this will help others with similar problems.

See also:
Casio Pocket TV

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ToughDev

ToughDev

A tough developer who likes to work on just about anything, from software development to electronics, and share his knowledge with the rest of the world.

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