Hi. My friend's Rumble 100 bass amp suddenly, overnight, has developed a problem and I have been asked to troubleshoot it. The sound with a bass or guitar is totally distorted and loud, and this distorted sound exists with ALL of the controls set at ZERO. I've attached an MP3 file of a few seconds of strumming a guitar plugged into the amp. I have already tried disconnecting the internal speaker and hooking it up to my guitar amp and it sounded fine. I opened the amp up and all I found that looked suspicious was a 10k resistor on the power supply that looked like it had gotten hot and was leaning against a .33uf safety cap (I've since straightened it up). Both components tested good in circuit, well within tolerance, and they also tested good, well within tolerance, after I removed them from the board. Where do you suppose that I should start looking next?

I think your meter is probably measuring AC volts, I would expect peak-to-peak to be 2.8 times that value, minimum.
Here is what someone measured (attack) for peak-to-peak with a scope.  I would expect an extra hot pickup to put out even more, and maybe allow for up to 1.5 or 2V p-p with your input stage.
http://www.muzique.com/lab/pick.htm

The cap has the 4 as a multiplier, not as a digit.  So 33 with 4 zeroes, in pF.
For the transistor sub, power and current & Hfe look ok but I don't think the Vceo of the TTC015B is sufficient.  I would look for the TTC004B.

Quote from: g1 on November 03, 2025, 11:14:12 AM

Quote from: edvard on November 03, 2025, 03:07:24 AMI can't imagine any electric guitarist saying "Hmmm... perhaps I should plug into the 'Lo' input, as I wouldn't want to overload the front end of this fine amplifier". 

Seriously?  Even if they're using active pickups and the jacks are labelled 'passive/active' instead of 'hi/lo'?  That person would have issues that can not be solved through electronics. 

Dude, we're talking possible Metal players here.  Nice bunch of folks, but when it comes to subtlety, well, you know.  "Why should I turn it down or plug into 'Lo' when there's a perfectly good 'Hi' input right there?  'Hi' means 'above', and that means better, right?  TURN IT UP TO ELEVENTY-FIVE AND KICK OUT THE JAMS BUTTER TRUCKERS!!"

QuoteHere is an example arrangement.  You can see that the only difference between the passive and active sides of the buffer is the single resistor change after it, R105 vs R106.

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Oh, I'm fully aware of the arrangement and method.  I'm just thinking of a "no-brainer" option. 
**Dubious math and assumptions about how meters work follows, please let me know where I go wrong**
I made some possibly inaccurate measurements of my hottest output guitar on an op-amp no-gain buffer with my trusty DMM, and I captured peaks of 150 to 275 millivolts. That's probably at best an average, so lets call it ~300mv max, and assume my cheap DMM is showing me peak-to-peak RMS, so ±212mv.  With that running into an op-amp powered by ±10.7v (which is a ±12v DC supply with noise filtering that drops it a bit) and wired for a gain of 10, I can still hear clipping on some transients from hard strums.  At a gain of 10, my now-±2.12v should come within ~8 volts of either rail.  So, unless my pickups are producing transients 10 times as high as I measure, or my math is WAY off (a very HIGH probability), there shouldn't be any rail clipping at all, right?

Meanwhile here is what I think for the capacitor

Found what seem to be some suitable replacements but not sure what voltage rating I need

Might be some info in the schematics kindly provided so I'll take a look

Btw wasn't sure if 334K means 334,000 picos or not

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And with that I'm gonna try to get out of y'all's hair I appreciate the help I reckon y'all have much more important stuff going on 

Thanks guys I appreciate I have to put forward that since I so greatly appreciate your help I have to let you know this is for all essential purposes a cadaver that is being worked on and not something I desperately need like working for my son on Christmas day or something 

With that said let's carry on

I may have found a replacement for TR-201 but I'm in a little over my head with the ratings

Idk if it ( Toshiba TT015B) will be able to endure the rigors of the environment

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For what it's worth, here is a partial schematic I made the last time I saw this model.

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Quote from: Loudthud on November 04, 2025, 02:10:50 AMI think speakers use an odd ball size, something like this:

I'd forgot about that in-between size (.205), which seems more common in offshore speakers.  Not a good fit if you try to jam a 3/16 on there.  I guess in fractional it would be about 13/64.

Most Japanese transistors like that omit the '2S' prefix.  If you see a transistor with just A,B,C, or D in front of a number, that is usually the case.
So what you have there is a 2SD569.

Thanks. Will try that. I spose I could do the same with the gain.