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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 20th Aug 2015, 12:45 am   #1
beery
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Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
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Default MJH Electronics Standards Converter

Hi All,

I've recently resurrected the first standards converter that I bought.

Back in the 1990s when the wonderful Dinosour converter was available, but expensive, with the slightly cheaper Pinapple converter just around the corner, the MJH Electronics converter became available.
It was advertised through 'Television' magazine and was I think only £150.

I remember asking to have it modified with 405 line video in and out. I eventually fitted the PCB into an early Saisho VCR (why was there space for it?) and wired it into the E-E circuit to enable 405 line recording. This was partly because the converter does not work well with 625 line VCR signals and also its 5V regulator tended to overheat.

I've now returned the converter to more or less as built condition, though I've given the regulator a better heatsink and have kept the caps strapped across the ICs that I added years ago to remove some of the switching noise from the picture.

The converter is a simple line dropping type that drops every third line. The ADC is a Brooktree BT218 and the DAC is a TDA8702. The heart of the converter is the Mosel MS6130 dual port ram. The master clock is from a specially made crystal of 15.55336MHz (maybe left over from another project?) which is divided by 2 to give a write clock of 7.77668MHz and divided by 3 to give a 5.18446MHz read clock.
The converter uses 74HC4040 chips for address counting, the reset pulses for both read and write address counters are provided by a programable logic chip (PEEL22). A PIC16C54 inserts broad pulses somewhat randomly which results in the picture slowly shifting up and down.

A modulator is included. A charge pump tripples the 5V rail and this is fed via a tuning pot to a varicap diode. This enables the vision oscillator to cover all of band 1. Sound is intercarrier modulated by using a 3.58MHz resonator pulled to about 3.51MHz.

The performance is ok on a 60s set, but the intercarrier sound really does not work for TRF sets. There are 3 very fine white vertical digital noise lines in the picture. The 5V regulator in its original location melted itself into the top of the case! The modulator is not great. I originally ended up attaching a David Looser style modulator to it.

However, for the price at that time, I was very happy with it. It was also very compact for its age and as far as I know the first with a tunable modulator.

Does anybody else have one or have any more info on it?

Cheers
Andy Beer
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