Having completed the project to perform a Hantarex MTC-90 to MTC-900 Conversion I now needed to put together more MTC-900 ITT FB chassis to replace two more MTC-90 chassis in cabinets. Having run out of spare ITT FB chassis, attention turned to the S4 chassis to investigate if it was possible to convert an S4 chassis into an ITT FB chassis for use with an FB neck board.
Some Zaccaria cabinets came with a Hantarex CRT compatibility chart stapled to the inside that described the chassis configuration
variations between various CRT and neck board configurations. The chart showed various FB, Philips HFB-PH and Video Color HFB-S4
configurations but no ITT HFB-S4 configuration that I suspected would be the closest match for the ITT FB.
A friend in Europe kindly sent me a spare FB neck board to try out for the conversion. At least for now, this negated the need to have to
reproduce an FB neck board PCB for this project and prototype the chassis conversion first.
The first step was to identify the various S4 chassis to determine which one was already the closest match to the ITT FB chassis.
From the few chassis I had it was clear there were at least two S4 configurations. The first configuration matched the one
described in the Hantarex CRT configuration chart in the 20" V.C. A51-421 Hi-Foc. row for the Video Color CRT. The second S4
configuration wasn't listed in the chart. The ITT FB configuration and two S4 configurations are listed below for comparison:
| . | GIOGO (yoke) | ZOCCOLO (socket) | TELAIO (chassis) | TRASF. EAT |
MODULO E.W. | R 78 | C 70 | P 18 da EAT | B 5 BOBINA PONTE (bridge coil) |
C 69 | C 68 | R 110 | D 23 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 20" ITT A51-211X | 89-253100 | FB | Standard | 551001003 | S1 | 1.2R | 4.7nF | da pin 16 | 523802807 | 0.68uF | 22nF | 4.7R | BY448 |
| 20" V.C. A51-421 Hi-Foc. | YS-51 877-1 | HFB S4 | Cambia comp. | 551001003 | NO | 1.2R | 4.7uF +1.5uF | da pin 16 | 523920903 | 0.33uF | elim. | elim. | elim. |
| 20" ITT A51-231X | ? | HFB S4 | ? | 551001003 | S1 | 1.2R | 4.7nF | da pin 16 | 523802807 | 0.33uF | 22nF | 4.7R | BY448 |
The capacitor in question is not a standard capacitor however, it's officially described in the Hantarex MTC-900 manual as a
"double metalized polypropylene capacitor".
This is a special type of "safety" capacitor intended for use in the line yoke drive circuit of TV's & monitors.
I wasn't able to find anywhere to buy such a capacitor new, however I guessed that maybe the MTC-90 chassis had similar tuning
components since it drove the same CRT & yoke. I wasn't disappointed, the MTC-90 schematic showed that the chassis actually used
two of this type of capacitor.
Comparing with an original factory MTC-900 ITT FB chassis showed that the capacitor was the same in both MTC-90 & MTC-900.
Removing one from the MTC-90 chassis confirmed the full part code is ".68J250 PLESSEY 1.76 MKP L 7".
The two capacitors are different sizes but the MTC-900 chassis is pinned to accept either size.
The MTC-90 0.68uF capacitor installed with no issues into the MTC-900 chassis, completing what turned out to be a very simple conversion.
Finally it was time to put the chassis back together and connect it up to the MTC-90 ITT A51-211X CRT to see if the conversion actually
worked.
First power on yielded a reasonable picture. Pretty much everything needed to be adjusted as expected but the base picture
was pretty good. The slight bleeding & uneven colour I suspected was due to old capacitors on the chassis.
After a cap kit followed by geometry and colour adjustments the picture quality was looking good (that center wide band in the
photo is camera aliasing). The chassis ran for a few hours without issue and was ready to go into a game (Super Galaxian in this case).
