General Sound Repair Tips.
The sound on WPC games is very robust; it just doesn't fail too often. But here are some things that do fail related to sound:
* No sound or intermittent sound? Check the speaker in the bottom panel of the cabinet. If one of the leads is off the speaker, or the speaker is broken, sound won't get to the other speakers (hence silence)! Sometimes the bottom speaker wire connectors are intermittent too. So when a solenoid fires, the sound can cut off and on.
* Re-seat all the sound board ribbon cables. Surprisingly, this fixes a large number of WPC sound problems!
* Check the EPROM chips for bent pins and incorrect insertion! This is very common. The EPROMs are about the only socketed chips on the sound board, and often people will remove/replace/update the chips. And sometimes in their haste, when the chips are plugged back into the sockets, a pin or two may bend over (underneath the chip), or bend outside of the socket. If this happens, just unplug the chip, straighten the bent leg, and reinsert carefully. A worse problem is if the EPROM chips is plugged into the socket "backwards" (notch on the EPROM not matching the notch on the socket). This will ruin the EPROM chip. In either case, bent pin(s) or backwards EPROM chip(s) can cause the sound board to not work at all (no sound), or to work intermittently.
* Speakers blown: yes this happens more often than you might think. If the game was in a noisey arcade, the volume could be up so loud it blows the speakers. You can test the speakers (with the game off) using a 9 volt battery. Momentarily hook the battery up to the leads of the speaker. You will hear the speaker cone pull in if the speaker is good, when you attach the battery to the speaker. Make sure you check the speaker in the bottom of the cabinet too. Often if one speaker is blown, the others will not work.
* Main amplifier is bad: On pre WPC-DCS games, the sound board uses a LM1875 as the main amplifier. This device has a large heat sink attached to it. Often, this component has heat failure. The sound works fine until the game warms up for five minutes or so. Then the sound starts cutting in and out. You can use a logic probe on the leads of the LM1875. If the probe's beeps correspond to the cut in sound on one of the leads, the LM1875 is probably bad. The LM1875 is at U1 on the WPC audio board (not used on WPC-DCS or WPC-95).
* Main amplifiers are bad: On WPC-DCS and WPC-95 games, the TDA2030A amps are pretty fragile too. On WPC-DCS this is at U27 & U28, on WPC-95 at U5 & U6 (not used on pre WPC-DCS games).
* Check both of the TL084 op-amps too. Depending on the revision of the sound board, these audio amps can effect a certain type of sound they amplify. On WPC these are at U7 & U8, on WPC-DCS at U21 & U29, and on WPC-95 at U1 & U2.
* Bad rectifier diodes on the sound board. Often these become leaky and can cause intermittent problems before they total short.
Volume up FULL and Can't turn it Down.
The volume control on all WPC games is electronic. On pre WPC-DCS games, this is controlled by an electronic prom pot. This E-pot is a X9503, at location U5 on the sound board. If turning the volume up or down has no effect, and the volume is stuck on full blast, this is the first component that should be checked. Also the capacitor C18 (47 mfd, 25 volts) that connects to the E-pot can fail too, and should be checked. As described above, the electronic volume control can be disabled by removing A-12738 sound board jumper W9 to disconnect the software controlled volume circuit. Then connect a potentiometer (any value 5k to 200k ohms should work) to connector J507:
* J507 pin 2: To center pot leg
* J507 pin 4: To outside pot leg (Analog ground)
Static Noise and Loud Whistle.
Problem sound boards can produce a large amount of static. The TL084 quad Op-Amp (U7 & U8 on WPC, U21 & U29 on WPC-DCS, U1 & U2 on WPC-95) can be the cause of this. Also the TDA2030A (WPC DCS and U5 & U6 on WPC-95) amp can also cause this. Finally the large filtering 4700 mfd or (or 10,000 mfd on WPC-95/WPC-DCS) 35 volt capacitors can also be the problem. Aldo check for cracked solder joints on these large filter caps (solder jumper wires, as done to the bridge rectifiers explained earlier).
Another problem I saw on a WPC DCS sound board was a really high pitch whistle as soon as the game was powered on (in this case Jackbot). The volume control did not the whitle volume, and the game play sound could be heard behind the whistle. The whistle was so loud and obnoxious it was difficult to have the game powered on for more than a few seconds.
First thing done was to isolate the CPU from the amplifier section. This was done by removing the ribbon cable from the sound board, and by removing the sound EPROM. This way the sound board could not execute any code, and the CPU was basically detached from the amplifier. The whistle contined, indicating the problem was not in the processing of the sound, but in the sound amplification.
Looking at the schematics showed that the only things really not involved in computer processing of sound is the pair of TL084 Op-Amp chips and the TDA2030A ampifiers. In this case it was a bad TL084 causing the problem.
Static/Minor Hum and the Sound Board Filter Caps.
These are often the cause of minor sounds problems such as hum and static. Cracked solder joints at these capacitors is common. Soldering jumper wires from the PCB traces directly to these capacitors' legs often solves many problems (as described previously on the power driver board's bridges and capacitors).
* WPC-95: C36 and C37, which are 10,000 mfd at 35 volts.
* WPC-DCS: C20 and C21, which are 10,000 mfd at 35 volts.
* WPC: C24 and C25, which are 4700 mfd at 35 volts.
Static & Scratchy/Tinny Sound on Early WPC-95 Games.
Early WPC-95 games (Sacred Stiff for example) have two capacitors installed at locations C47 and C51 on the A/V board. These two capacitors are located between chips U5/U6, and near connectors J505/J504. With later WPC-95 games, these two capacitors were *removed*. If an early WPC-95 game has some static noise or just thin tinny scratchy sound, a good first step is to completely removed these two capacitors. It doesn't cost anything to remove them, and often provides a solution to the static.
Intermittent Sound Cuts and Shrieks.
After playing a WPC game for a while (5 minutes or longer), the game sound starts to cut out or in some cases it will emit an extremely loud tone that can get louder and louder until the speaker (or your eardrum) blows. If you turn off the game and re-start, the problem will appear again very shortly. Sometimes you can play a while and it never happens. Often the heat sink attachment to the TDA amps can be very poor and cause the amp(s) to overheat. You can feel the amp(s) get red hot. Solution is to simply un-bolt the sinks, re-grease them and re-attach, making sure to use a locking washer or kep nut to stop it from coming loose.
TDA2030A Amp Chip.
This is a fragile chip used on WPC-DCS and WPC-95 games. It comes in two flavors; the TDA2030 and the TDA2030A. You want the TDA2030A version, as the TDA2030 does not have a high enough power rating, and can distort under higher volumes.
Loud Hum from the Speakers.
Problem: a loud hum from the speaker which does not change in loudness as you change increase the game's volume. This is often caused by the large filter caps on the Audio board (as discussed above). For early WPC games, this is capacitors C24 & C25. On WPC-DCS games, this is capacitors C20 & C21. And on WPC-95, this is capacitors C36 & C37. To fix this problem, check for cracked solder joints on the leads to these capacitors. It is a good idea to solder jumper wires to the two capacitor's leads to ensure good continuity (like you did on the Driver board's large capacitors).
"Popping" Sound, Hot LM1875, and Speakers Shorting.
Problem: pre-DCS sound board works, but eventually shorts the speakers. First the speakers start to "pop" (not very loud), every second or so. Eventually the speakers short and are ruined. Also the sound board's LM1875 heat sink gets very hot. DC voltage was measured at the speakers, and found to be 40mV (there should be no DC voltage).
Solution: At first the LM1875 was thought to be bad. But the real problem was the capacitors feeding the LM1875. Caps C46-C47 (1 mfd Tant), C20 (10 mfd), C22 (22 mfd), C23 (.22 mfd) were replaced, and the problem was solved. Also the LM1875's heat sink now ran cool. The giveaway here was the DC voltage at the speakers, pointing to the capacitors. There should be zero DC volts at the speakers. As little as 5mV DC at the speakers can cause the LM1875's heat sink to run hot.
Replacement Speakers.
All speakers in a WPC game are 4 ohms. No other speaker value should be used in these games.
The most common speaker to die on a WPC game is the backbox tweeter (right speaker, as playing the game). This is a small 3.5" speaker with a capacitor attached to the negative speaker terminal (the capacitor is the "cross-over", which filters out all but high frequency sounds). A quick and dirty replacement tweeter is available from Radio Shack, part #40-1233, $9.95. Though this is a 3.75" tweeter, the holes can be enlongated slightly to fit the 3.5" bolt pattern.
The 6" speaker in the bottom of the cabinet can be replaced with a PinballPro subwoofer. See
www.pinballpro.com for details. They also sell replacement speakers for the backbox.
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Topic: Machine Bride of PinBot sound problem - help me start troubleshooting? (Read 10381 times)
