Hantarex MTC-900 S4 to FB Conversion

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.

Chassis CRT Compatibility

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.

Hantarex MTC-900 CRT Compatibility Chart

FB Neck Board

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.

Hantarex MTC-900 FB Neck Board

ITT FB, Video Color S4 & ITT S4 Chassis Comparison

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 last row was derived from looking at the S4 chassis itself. Comparing all the components across the table showed that just one component, C69, was different between the ITT FB chassis (0.68uF) and the ITT S4 chassis (0.33uF).

The Capacitor

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.

MTC-900 S4 C69 .33J250

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.

Hantarex MTC-90 0.68uF capacitors

Comparing with an original factory MTC-900 ITT FB chassis showed that the capacitor was the same in both MTC-90 & MTC-900.

Hantarex MTC-900 C69 0.68uF

Capacitor Replacement

Removing one from the MTC-90 chassis confirmed the full part code is ".68J250 PLESSEY 1.76 MKP L 7".

.68J250 PLESSEY 1.76 MKP L 7

The two capacitors are different sizes but the MTC-900 chassis is pinned to accept either size.

C69, .33J250 and .68J250 1.76 MKP

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.

MTC-900 FB C69 .68J250

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.

Hantarex MTC-900 FB on A51-211X

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.

Converted MTC-900 S4 picture

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).

Converted MTC-900 S4 picture, adjusted with kit