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I just joined and need help. I'm trying to fix the amplifier in my Chicago coin speedway arcade machine. It basically works but it has a bad hum in it. I did replace a couple capacitors that were in bad shape but the hum is still there. I finally checked the two germanium transistors in the preamp stage and they both seem to be shorted or out all together. I did order a couple new ones but my problem is I think the schematic is not correct. I'm going to replace the 2N2429 transistor and the A104 transistor. What's bothering me is the schematic calls for an A104 transistor which is a PNP transistor. The symbol shows a NPN transistor. Can someone tell me what is really suppose go in that spot?

schematic diagram

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  • \$\begingroup\$ It's really unlikely that bad transistors would allow the amplifier to work but create a hum. It has to be something else. \$\endgroup\$ – Dave Tweed Apr 21 '18 at 1:49
  • \$\begingroup\$ Welcome to EE.SE. It is not unusual for a schematic to have boo-boos in it. Transistors and diodes in reverse of actually what is on the board is more common than you think, especially from the old days of hand drafting. That A104 is a 2SA104 PNP. Replace both with a silicon version. \$\endgroup\$ – Sparky256 Apr 21 '18 at 1:49
  • \$\begingroup\$ I already ordered the transistors but thanks for telling me about the substitute. Just in case, what would be the substitute for the 2N2429? \$\endgroup\$ – Rod Apr 21 '18 at 3:04
  • \$\begingroup\$ I did check the rest of the capacitors and they all check fine. Does anyone know how I could download the schematic? \$\endgroup\$ – Rod Apr 21 '18 at 3:05
  • \$\begingroup\$ There is an icon that shows up when you are editing your question. It is for adding pictures. Sometimes, it will reject the picture for size or other reasons. If so, then either use PAINT and save it in a different format or different resolution or else load it up to some other site and give us a link to it. We can get it inserted for you if we have a link we can access. \$\endgroup\$ – jonk Apr 21 '18 at 7:40
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you should recap all the capacitors considering its that old first. any leakage current at c1, c2, and c7 will really make it hum. I doubt that the transistors have go bad. Because you would have to physically remove them (or the resistor around them) to really test the transistors.

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  • \$\begingroup\$ I did remove the transistors. I'm just not getting good readings from them. I'll check them again tomorrow. It's 2:30am and I just got back form a gig. I did replace C1. when I took out that capacitor to check it, it fell apart in my hand. It was one of those "clown" epoxy capacitors. The only capacitor that I could find that was 16uf was a 400 volt electrolytic one. It was really big for the space so you guys know where I could get a smaller one, let me know. \$\endgroup\$ – Rod Apr 22 '18 at 6:37
  • \$\begingroup\$ mouser.com/ProductDetail/Nichicon/… 16uf@16V is an oddball size. There is quite a range you can use with coupling caps as long as you stay at or above the voltage limit. but a 22uf@16V is the next standard size up from it \$\endgroup\$ – drtechno Apr 22 '18 at 9:50
  • \$\begingroup\$ the thing I kind of wonder about is the quality of the 400V cap, because their leakage currents are typically greater than the lower voltage rating caps. and in this part of the circuit, you would want to have very little leakage there. I also check that volume control as the wiper will deteriate causing dead spots that the wiper is not contacting the resistive element very well and will cause low sound with hum in circuits like this. \$\endgroup\$ – drtechno Apr 23 '18 at 14:18
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    \$\begingroup\$ Ok guys, here is the up date. I replaced all the capacitors known to mankind in the game and it still hums. I'll describe the situation better. when there is sound going on (like the motor sound and the crash sound), there is no hum. The hum only is there when the game is not making those other sounds. I did disconnect both sound effects to the amplifier and that's when there is a constant hum. All that is left in the amplifier to change are the resistors and the transistors. So, where should I look now? \$\endgroup\$ – Rod Apr 25 '18 at 19:05
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    \$\begingroup\$ Hey guys, I think I finally found out the problem with the amplifier. I de-soldered the wires running from the RCA chassis mounted jacks to the circuit board and the humming stopped. So, I have a couple RCA jacks with a bad ground. I wanted to stop by the local Radio Shack to pick up a couple but it seems that they don't exist any more. I order a couple gold plated ones off of Amazon and they'll be here next week. Hopefully, this solves the problem. \$\endgroup\$ – Rod Apr 26 '18 at 0:00

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