A few questions about the Yamaha b100 power amp and pre amp boards

[Gioanpj]

I recently purchased a Yamaha b100 bass amp off ebay with the intention to recap the amp. I have the list of caps ready to order (Only 12 total!) When I got the amp, I checked the fuses. As it turns out the 5a secondary fuse had tin foil wrapped around it. According to the ebay listing, the "amp works great". So, I changed the foil covered blown fuse for a non-blown fuse, and then I plugged the amp into the lightbulb limiter before turning it on. The amp played fine for about 5 minutes (and damn, does this amp sound rad!). It had the usual scratchy pots and then the limiter went off when I cranked the volume. Then the amp started cutting in and out with its volume and the red light on the front started going crazy. Now the output is very low.

So, I shut off the amp and opened it up. I can smell that some component started cooking , but I'm not sure which one. So, I intend to replace all the electrolytic caps, clean the caps, and I'm assuming that that should solve the issue-- unless one of the transistors is starting to go, or the bridge rectifier in the power section. However, I noticed that the preamp (lc 80123) and power supply board (lc15965) have several spots for capacitors on the board that are blank. I'm assuming that on the preamp board, the circuit was a generic model that would get used in many different amp models, and the section that is left out is probably for a tremolo or reverb effect or something like that. Wouldn't it be pretty easy to add in the missing components to make a reverb/tremolo circuit? The section with lots of missing components is to the right of the distortion pot. As for the power amp board, it is missing two capacitors next to the two 470uf 50v caps (right behind an ms161 that is on the board) and perhaps another ms161. What do these extra 2 caps do?

Does anyone know what kind of circuit these missing components comprised? Also, should I add extra caps and an ms161 to the power supply board where it appears the two are missing?

If you look at this video, the power amp and preamp board appear to be the same as in the b100. However, they have the extra components added. The power board has two extra caps, a fuse, and... I'm not sure what the ms-161 component actually is. The packaging is unfamiliar. It looks like a bridge rectifier. https://www.youtube.com/watch?v=0QvOepREO4k



--Phill

[The Dude]

With risk of sounding terse, we have no idea without a schematic and/or pictures. Do you have a schematic you can upload? If the amp has been fiddled with in the past, sure, it's possible somebody removed parts and didn't put them back in. It's more likely those parts aren't used in this model and you don't need to put them in. A schematic would tell you that. Look at the solder side. Does it look like something was soldered there at one time?

By the way, that was quite the nest!

Edit: Did you play through the amp on the LBL? The limiter is just to test initial function and for problems. When the amp is proved working correctly, you should remove the limiter before actually using the amp. Also, if there wasn't any hum and the amp is functioning normally, I don't see any reason to wholesale replace caps.

[Gioanpj]

Here is a link to the service manual: https://www.google.com/url?sa=t&rct=...7WOMuaz2TS4avZ

I'm fairly certain that no one removed components, I think it's more likely that these boards were just used in other amps.

Also, a caveat, that video is not my particular amp, just a model that probably uses the same circuit boards with a few component changes.

[The Dude]

I would start by verifying the power supply. Verify 36V on the cathode of D6 or D7 (same point). Verify 137V on the cathode of D2 or D3 (same point).
Also, point "5" from power supply to amp board should be roughly 1/2 of the 137V supply. If it's not (and power supply is good), there is an amp problem.

[Enzo]

Your amp is starting to burn up parts. First, replacing all the caps. Even if that is a good idea, we have no reason to assume it would cure burning parts. Second, you want to fill in the empty parts. Make the amp work first, then worry about modifying it. We need to systematically find the cause of your overheating parts.

The service manual for the G100B shows the board in guitar amp use, including the reverb circuit.

The drive IC might be hard to come by, AN374. A more modern replacement might be cobbled into its place

http://pdf.datasheet.live/a8e51bb5/p...o.jp/AN374.pdf