I have an MTH Proto-Sounds One PRR F-3 ABA. The engine runs fine buy makes a loud hum that you can hear over and above the engine sounds. The horn sounds distorted as well. My question is: Is the problem the speaker or the QSI Sound Board. Any help or advice would be welcome.
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My answer is neither. The hum is often the charging circuit trying to charge a failed or failing old 9V battery that is either very low in charge, or possibly just so old it no longer is capable of charging.
It could actual board damage in extreme cases, but that would be identified after replacing the battery with a known charged new battery or possibly a BCR.
Again, it could be the board, but the root cause in most cases I have seen is the old original white battery- now near 20 years old was not replaced by the user. That in turn loads down the charging circuit- sometimes to the point of damage.
Again, in this order typically is troubleshooting path or things to swap with known good components in my experience when dealing with PS1 "hum" problems. MTH parts still carries PS1 boards and even a few electronic components (example LM337 regulator). https://www.mthpartsandsales.c...lists/199?type=lists
#1 the battery
#2 the charging circuit (example trying to replace the LM337 and/or other components)
#3 replacing the entire lower PS1 board.
| AD4500001 | PROTOSOUND 1 BOTTOM BOARD / UNIVERSAL | $60.00/ea |
As Vernon Barry stated it is usually the LM337 voltage regulator but if you run to much with that issue you can damage the audio amp also!
Alan
The unit has a BCR. Would a bad BCR cause such a problem? Could it be the speaker?
@triplex posted:The unit has a BCR. Would a bad BCR cause such a problem? Could it be the speaker?
In my experience, those 8 Ohm classic speakers are not subject to a high failure rate. Further, to make the hum, it (the speaker) would have to be shorted to frame AC. So again, no, more likely this is a bad BCR (they too can fail- rarer but possible) or the charging circuit is damaged. Something else- it's imperative the LM337 is electrically isolated from it's heatsink. It's part of voltage regulation and charging circuit. That insulation is both the plastic shouldered washer and the mica between the backside of the tab and the metal heatsink. Any failure of either one can damage the circuit.
Picture grabbed from here https://ogrforum.com/...9#169655768976849149
First thing I do is replace the 337 regulator pictured above. Most of the time, that fixes the hum.
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https://www.mthpartsandsales.com/shop/lists/199?type=lists