C.W. I wish I could help. I had this same problem with the loco we run around our Nativity display and posted about it here on the Forum. Someone put me in touch with a "guru" who repairs these boards. This is what I wrote:
"I have a loco vintage 1971-72 with Electronic Mighty Sound of Steam. The chuff volume has become very faint. Last year it would suddenly get quiet after about 10 minutes of hard running, but if I turned it off and let it sit for a while, full volume would be restored. This season, I can hear the static, but no matter what I do the volume is low all the time. I removed the tender shell; there are no integrated circuits on this early version of the board. There are three large electrolytic capacitors; my guess would be that one of them has dried out or gone bad.
Please note, this is the variant without electronic whistle. The speaker works, the foam looks good and the components aren't shorting to the metal tender chassis. It's also not the loco-- I tested the circuit by touching the tether to the center rail. There is a "chuff" sound (static) but it's very faint. I'm familiar with component level repair and I think I can fix it with components I have on hand if you can tell me what you think needs to be replaced. "
Here was his response:
"Although the capacitors do go bad, I more often usually end up changing 2 or 3 of the transistors, but in these cases you usually hear nothing.
Since you can hear the faint chuffing, I would guess its one of the two output transistors, the output (speaker) coupling capacitor, or perhaps the speaker itself or the connections to the speaker. Also look for any cold solder joints.
I also usually replace the main power filtering capacitor. These do get tired and a new one (even larger value) usually reduces the hum a bit. Can't get rid of it all."
Since you can hear the faint chuffing, I would guess its one of the two output transistors, the output (speaker) coupling capacitor, or perhaps the speaker itself or the connections to the speaker. Also look for any cold solder joints.
I also usually replace the main power filtering capacitor. These do get tired and a new one (even larger value) usually reduces the hum a bit. Can't get rid of it all."
Unfortunately my vacation was almost over so I didn't get a chance to try the repair. To hedge my bets for next Christmas I bought a new old stock Sound of Steam board and a speaker from Harry Henning in Lansdale. So I'm pretty confident that one way or another my loco will be chuffing in December. If you get yours working, please update us with what you replaced. -Ted
