Fender Stage Lead II. Help remembering basics! Lol

[Indyryder02]

Ok.. this one is surely something simple and stupid!
And if I knew solid state tech better I’d already have the answer. But I’m too far in the weeds now and it’s just not making sense.

first off. The amp was previously butchered by someone’s attempt at replacing caps it prob didn’t need. Ripped whole traces off the board.. broken solder joints. Amp inoperable.
Fixed all that stuff And got it up and running again.
it had a bit more hum than I wanted. Which I traced to failing c113 / c114. I also replaced 112,111,110,and 109 while I was at it. Upon firing up slowly with a variac I did notice immediately something wasn’t right. (Should have had my lightbulb limiter going too but was feeling cocky ♂️
I prob didn’t give it more than 20v at input before realizing it wasn’t happy . And sure enough, in my haste, managed to install c113 or c114. Backwards doh’. Reinstalled it correctly.
amp fired up ok. No dead silicon. Amp works but now has a loud 120hz buzz. Checked the rest of the caps and solder connections etc. Here’s what’s got me stumped.
when inserting a cable in the power amp in. The noise goes away, signal can be inserted here and heard cleanly from the speaker. So this has always told me. The power amp section is fine. And the noise is before the split. The noise is not before it though. It’s clean past Q102 and Q103. I haven’t pinpoint it after that so far… but my question is. Why does inserting a cable in the power amp in stop the noise, If the noise is in the power amp section. Something has to be missing a ground etc, and inserting the cable is giving it back? Just not making sense.
I’d think one of the jacks was bad. But wouldn’t imagine it happened while replacing a handful of caps.
Again surely something simple I’m missing. But I can’t spend anymore time on it going in circles getting nowhere. So needed to reach out to the hive for a better train of thought and more wisdom than I have!
Thanks in advance.

[The Dude]

Does it still hum with volumes down? The power amp doesn't use the LV supplies except for a muting FET line, so maybe one of your LV supplies is not filtered well. I'd scope those if you have a scope. If not measure ACV on those low voltage supplies.

[Indyryder02]

Does it still hum with volumes down? The power amp doesn't use the LV supplies except for a muting FET line, so maybe one of your LV supplies is not filtered well. I'd scope those if you have a scope. If not measure ACV on those low voltage supplies.

Good suggestion. I do have a scope.
so is that what’s going on there then at what looks like Q101 just below the effects loop jacks with the 15v supply? Does that mute the preamp when the power amp gets an insert? Just trying to better understand how that works.

I wish I could stay out of solid state and just work in tube amps.
but I’m glad I’ve learned as much about solid state as I’ve been forced to so far, since these days so many tube amps have solid state circuits in them too. Grrr! Haha. I don’t mind the extra knowledge. But it’s been a lot tougher to grasp how things like that FET circuit work compared Tube tech.

anyways, thanks. I’ll “scope” it out and see if come up with anything. Need a break from it. Getting lost in the weeds and going in circles. Never good.

[g1]

Q101 mute is for power-on, not for the pwr.amp.in jack. It prevents turn on thump.
It shorts power amp signal line to ground while C103 charges up during power on (Q101 is on until turned off by voltage at it's gate).

[Indyryder02]

Q101 mute is for power-on, not for the pwr.amp.in jack. It prevents turn on thump.
It shorts power amp signal line to ground while C103 charges up during power on (Q101 is on until turned off by voltage at it's gate).

Thank you. That makes more sense. The jacks / switches act as normal then.

[Indyryder02]

On a side note. What’s a good book for better understanding solid state? I’ve actually decided to try and focus only on tube stuff in the near future, as I often seem to not do well with solid state even when I’m able to diagnose it without issue. Due to obsolete parts and spending time finding new equivalents etc.
just been thinking I should focus more on tube gear…. I’m also the only tech in my area that will even work on solid state, and as I said, I often find solid state circuits for switching etc in modern tube amps… so I still think it’d be beneficial for me to have a better understanding of solid state. Like that mute circuit I was clueless about.
thank you.

[The Dude]

Without knowing the scope of your existing knowledge: Presumably (since you are already repairing tube amps), you understand how resistors, caps, and diodes work. I don't have specific resources in my brain, but I would concentrate on transistor theory. Google things like "how to check a transistor", "how a transistor works", "transistor amp theory", etc. That's a good starting place, anyway. From there you can move on to op amps, FET's, SMPS, and more complicated SS devices and circuits..

[Helmholtz]

The power-amp input jack disconnects the preamp when a plug is inserted.

Regarding preamp hum, C113/114 might be damaged from installing backwards.

[Indyryder02]

The power-amp input jack disconnects the preamp when a plug is inserted.

Regarding preamp hum, C113/114 might be damaged from installing backwards.

I did replace both of those caps already thinking the same thing could have happened. No change though.

That’s the strange part. It does disconnect the preamp. And that does make the noise go away. But I can’t detect the noise coming from the preamp. Only a clear signal. Past Q102 and Q103. (Are these considered the differential pair? I know I’ve had dc on outputs of hifi amps with these getting out of whack before) Still clear signal… only after that the signal is then mixed with the noise.. and then I can’t pinpoint where the noise starts. so I still don’t get that. If I plug a cable in the power amp input, the test signal and noise goes away. That makes sense. Without the cable inserted. I have a clean test signal past q102 and q103. No noise. Is there any way plugging the cable in could cause noise to go away after these transistors? Or am I just being tricked and led down the wrong path…
in any other case. This would be preamp noise surely. Due to the noise going away when inserting at the power amp in.
Yet I have clean signal past this point up to q102 and 103.
I’m surely missing something basic here. I’ll have to do some more tests.

Geesh. I only replaced those 6 caps. After the initial board repairs. And the amp was working ok with just a bit of hum. Which replacing the caps I don’t think even helped much.
I’ve tested the caps to be sure they’re still ok. Connections of outboard wiring is good. No loose ground etc.
only thing I could think of was a diode or transistor failing from the cap being backwards. But usually they short and are easy to find. Plus from my spectrum analyzer it appears very much to be 120hz. Which would lead us back to filtering.
And hence the circle I got lost in! Need to go back to basics.
I’ll scope those power rails as previously suggested and recheck filtering.

[Helmholtz]

So you have no hum at the collector of Q102 (are you using a scope to detect hum?) but at the same time hum/buzz at the speaker output?

[g1]

First get the hum you are hearing at the output to display on the scope (scope at spkr jack). You should not need the spectrum analyzer as it should be visible. Make sure it disappears when you insert at pwr.amp in jack, then you will know you are seeing the same hum you are hearing.
Once you get it to properly display on the scope, check again at Q102. You will need a much more sensitive scope voltage setting at Q102 compared to spkr. jack.

[Indyryder02]

So you have no hum at the collector of Q102 (are you using a scope to detect hum?) but at the same time hum/buzz at the speaker output?

Thank you.
I was not. I was using my audio probe, which I do like to use a lot but it certainly can fool me.

[Indyryder02]

Ok. I got it. Q114 was shorted. I of course thought I’d checked all the transistors. But of course must have missed that one.
thank you for all the great suggestions. It led me to the culprit.

ultimately my mistakes were using my audio probe only without the scope. And with signal applied I could hear it clean n Q102 / 103. Because I had the signal louder than the noise without realizing it. Neither my scope or audio probe could pickup the noise coming into the power amp.

once I realized this.. and went back to basics. Reassuring myself that it has to be preamp noise, and I just can’t detect it at that Low level. (This has burned me before)
also knowing it was 120hz not 60hz…. I went back The Dude’s suggestion of scoping the power rails. Now I could finally see something!
yep one side wasn’t getting filtered, which led me to Q114, sure enough shorted. Something I wasn’t even looking for as usually when I have a shorted transistor it either blows a fuse or burns a nearby resistor etc.

Thank you all again for the help. You guys are truly the masters!

side note : My scope goes down to 5mv and has a 5x magnification which just makes anything a blur at that point. It’s not a high quality scope by any means..
i’m sure it’s cost me lots of time not showing me what I need.

what parameters am I looking for when trying to buy a new good quality scope for amp work? I think I’ve read 100hz at least 2 channels for my needs.

of course I’d like the highest quality I can get for the least cost haha. I’ve seen some that look pretty good around $350-$500 Rigol, and a few others I’ve seen.

Are the new digital ones definitely better than older scopes with phosphor etc? I saw a video debunking the myths about cleaner sine waves on old scopes… and that the old ones just didn’t have the sensitivity to see the super fine noise new digital ones show. Opinions? Thanks so much!

[Indyryder02]

Is there a better way to detect noises at low levels like this before they get amplified? Otherwise it can be super hard to pinpoint the noise with a scope or audio probe. That one amp with the evil yellow glue that was making the crackling noise drove me crazy cause it was so low I couldn’t pick it up once I got close enough to the problem. But then I never would have expected the glue to be the problem either! I’ve had several issues from that glue now.

[The Dude]

I presume you meant 100MHz and not 100Hz. I'm a fan of CRT scopes, unless you spend good money for a high res digital scope.. You can find good quality used analog scopes on eBay for very good prices and I think it's a better value. There are so many great options, so what follows is just my personal experience and not the only good option. I use a Sencore SC61 and have had it for years. It's a great tool. One of the reasons I like it is because there is, aside from the CRT, a digital display that can display DCV, VPP, or FREQ per channel. It saves you having to count squares and hash marks. I'm not trying to sell you on this scope. There are many other good choices.

Edit: Post edited- incorrect link.

Edit (again): Audio frequency range is only up to 20K, so I don't think you need a 100MHz scope for amplifier repair. Also, a dual trace scope is a good thing, but not a necessity.