Zaccaria/Seevend/NSM-Lowen Zaxxon Maintenance 2022

16/07/2022 - Replacement bleeder investigation part 2

The company had four types of bleeders still available as new old stock that looked visually compatible with the Hantarex MTC-900 LOPT. These were measured using the same two methods (voltage and current) as the original with the following parameters:

• Supply voltage = 237.0 VAC
• Meter resistance = 10.25 MOhm

HR-1687

• Bare focus wire
• Focus voltage = 15.6 VAC, resistance = 145 MOhm
• Focus current = 1.7 uA, resistance = 129 MOhm

HR-1690

• Spade focus wire
• Focus voltage = 14.5 VAC, resistance = 157 MOhm
• Focus current = 1.7 uA, resistance = 129 MOhm

HR-1689

• Spade focus wire
• Focus voltage = 14.6 VAC, resistance = 156 MOhm
• Focus current = 1.6 uA, resistance = 138 MOhm

HR-1688

• Spade focus wire
• Focus voltage = 14.6 VAC, resistance = 163 MOhm
• Focus current = 1.5 uA, resistance = 148 MOhm

All the bleeders had approximately similar measurements. Given the inaccuracy of the measurement method and a theory that the only significant difference between them all was the focus wire termination and HV lead length, I selected HR-1687 to try first since it already had a bare ended focus wire matching the Hantarex. All the bleeders had a stripped area on the HV lead to allow for ground attachment that I modified by shrink wrapping a fly lead to match the Hantarex fly lead ground connection. I also coiled the fly lead wire around the braid instead of using solder to prevent damage to the HV insulation.

18/07/2022 - Game PCB #PA20180615 repair

Over the years the audio hum had become worse, and I removed the game PCB to test on the bench to try to determine if it was a cabinet issue or game PCB issue. The Zaccaria cabinet 1B1126 regulator PCB had already had a cap kit fitted that had made little difference. There was a large amount of noise even on the bench, and that noise was controlled by the volume control so it was coming from the pre-amp mix of several game sounds. The schematics identified net "SJ" as the combined audio net and each individual sound was mixed in via a 51K Ohm resistor. Shorting "SJ" to ground cut the noise aside from a faint background "wah-wah". After the short the noise took ~1s to return. Shorting any of the mix signals to ground had the same effect. Using a scope to try to isolate a specific individual sound as the source didn't conclude anything - the noise seemed to show on all. I began to suspect bad smoothing capacitors. The 6 x 470uF capacitors on the +12V and +6V supplies were replaced and the noise appeared to be significantly reduced. The PCB was set aside to test back in the cabinet.

02/08/2022 - Game PCB #PA20180802 repair

Pre-show testing discovered that this spare game PCB had a +12V short to ground. The +12V supply was used in several places throughout the sound section. I thought about trying unlimited +5V to blow out the short but on this PCB there was a lot of tracking that could go up in flames. Instead, the safer plan was to setup a current limited load using the +5V supply through a 3R6 10W power resistor to give 1.3 Amps maximum and then measure the tiny voltage drop across the PCB tracking to find the low point at the location of the short. The voltage measurements on the +12V supply net were:

Location	Voltage
Capacitor C1	88.3mV
Amplifier U1	84.2mV
Capacitor C7	78.8mV
IC U7 pin 4	73.4mV
IC U8 pin 4	73.2mV
IC U20 pin 4	73.6mV
IC U19 pin 4	73.6mV
IC U5 pin 4	74.4mV
IC U4 pin 4	74.4mV
IC U2 pin 4	58.8mV
IC U3 pin 4	57.9mV
Capacitor C64	56.7mV

There was a definite significant drop on the IC U2 & U3 subnet and the tantalum capacitor C64 recorded the lowest voltage point. Lifting one pin of C64 removed the short and replacing C64 (22uF/16V) fixed the game.