Music Man Amps

I'm not as familiar with the RD's as I should be but have you started at the front and worked your back to the power and established WHERE it starts? That's usually my route. Also, are y'all sure it isn't just a bad ground situation? Maybe even an input jack with the shorting bar bent? I'm guessing here but without knowing WHAT you've checked, it's hard to start unless it's at the beginning. How loud is the hum? Does a signal pass with it humming? Just askin'. Mike.

OK- so you've checked the finals and there's hum there. Maybe get the audio probe and start at the front and work your way back. this SOUNDS like a electro-mechanical issue. Solder joint, bad ground, crappy lead dress-hell, I don't know! Get the audio probe out and see where it starts and we'll go from there. Mike.

I will ASSUME that you checked to make sure the tubes weren't DRASTICALLY out of balance. Signal before PI? Signal out of IC's? Badly out of balance power tubes can cause a nasty hum in the finals. Mike.

Is there any AC ripple on the low voltage DC supply lines? ie. +/- 16V and +/- 8V?. They gotta be clean and stable. -mgriffin

Is there any evidence of where the hum is originating? If you ground the input to the phase splitter IC section, does the hum go away? (Junction of 15k/22k/0.47uF). If the hum does not change when this point is grounded, remove IC-4 and see if that changes anything. If this still does not change the hum, I'd look at the +30V supply to the 6L6 input grids.

If, on the other hand, there is no hum when you ground the input of the phase splitter section, the bad stuff originates upstream. Start simply and pull the limiter 12AX7 tube. That will silence the dirty channel but would also let you know if the tube was causing a buzz as both the clean and dirty intermediate amp stages and associated tone stacks are fed signal at all times. The switching circuit selects which of the two volume pots is active.

Cheers,
Lars Verholt

OK- I've been out of the loop here but to catch up, it's DIRTY going INTO IC4 but clean atR42? That really only leaves the source from the reverb send. It's CLEAN before R42 but dirty AFTER? And dirty before going into C24? Like Lars says, if you disconnect the feedback circuit, does it go away? What happens if you disconnect the reverb send from the junction BETWEEN R42 and C24? I SUPPOSE it could be feeding some noise in if the rails were dirty. Just spitballing here. Mike.

Also, have you verified that the ground at 3 and 5 on IC4 ARE actually grounded? Mike.

Wait- are you SURE you're getting a good signal at teh start of R42 but it's dirty AFTERWARDS? Are you sure you're checking BOTH sides if R42? Did you disconnect the run from reverb send? By the way, By the way, I DO have layout. Don't have a clue where it came frmo though! See attachy. Hope it helps. Mike

I know this is an INSANE question but have you thought about replacing R42 and getting THAT the hell out of the way? this is starting to get weird! Aliens? Bigfoot? My mother in law? JUST KIDDING about the mother in law. Mike.

Thanks for the compliment but Lars is a guy with knowledge. I'm just a mechanic! And YOU have a great deal more knowledge tha I do. Like I said, I'm a nuts and bolts guy who just figures chit out! Mike. (Oh, and I'm an old b=tard!)

Have you tried swapping IC-1 and IC-3? If IC-4 is bad you should get some funky signal on the wiper of either of the two treble pots. What you saw lifting R44 is expected behaviour. What happens if you apply signal from your tone generator at the wiper of the reverb pot? (You'd have to open the pot a bit so that you're not at ground) - do you get a dirty signal at pin 2 of IC-4 still?

Also - double-check that C25 and C26 are not passing any DC.

Cheers,
Lars

No expert, but the RD50 is probably my favorite MM amp, and I've been inside quite a few of them. Anything I know I learned from the great minds and generous souls on this board (and in this thread), and of course the legendary Terry Loose.

Have you replaced ALL the electrolytics? Please see this thread for details what to replace with what:

http://pacair.com/mmamps3/node/2141

Double check they are all installed with correct polarity.

Checked the zener diode (D9)? All the others? One bad one grounding out can ruin your day... Also check R62, it fries when D9 goes bad.

These are the usual suspects I look at when an ailing RD50 crosses my path.

Good luck getting this one going.

Both Mouser and Newark Electronics carry all the necessary caps.

OK-in your previous posts, you state that the noise starts AFTER R42 . I never noticed the different voltage supplies for IC3. Have you checkedthe +8 and -8 voltage supplies from the rails? C40 and C41? The diode bridge? I'm thinking some nasty chit in the supply going to IC3. Or just a bad IC3? That's my guess and I'm sticking with it! Mike.

Hi TM1, Sorry the shotgun approach hasn't yielded the results you desire. Several folks, including me have suggested you get out your DMM and check the low voltage power supply rails. Perhaps you have and just not reported to us. If you haven't, the easy way is to check the voltages at the zener diodes. D10 regulates the +16V line. D11 regulates the -16V line. The voltages should be close to the expected values but may not be exact. While you're there, check the +/- 8V lines at C40 and C41. Finally, check D9 for +30V. It supplies the fixed bias to the control grids on the tubes. These diodes are a common failure mode as they live a horrible life and are under heat stress. If D9 turns out to be shorted, most likely R62 will also be fried from current overdose. If they need replacement, mount them a little higher off the motherboard for better air circulation/cooling. Please tell us the voltages before replacing anything else. Lastly, look at those 3 power supply voltages with your OScope to see if there is the AC ripple you've described. There should be none. :) Good Luck. -mgriffin

Hi TM1, For grins, try removing the tubes (powered off of course) and then check for ripple with the amp on. What do you see at both sides of C25, C26, D9 and R62. Just lookin' for clues. Is there any ripple on C37, C42 150uf 50V caps in the power supply? Like mm210 mentioned, it might be a bad diode in the bridge rectifier (D12,13,14,15). -mgriffin

Hi TM1, I thought that might break the ripple loop in the amp section. Cool. That also kinda points to the power supply. Did you check to see if ripple is still on D9 zener? If yes, you need to trace it back through the power supply components that feed D9. Check to see if one or more of the 4 bridge diodes (D12 - 15) is open. Also check for loose solder joints at C37 and 42. -mgriffin

...you might want to pull the chip and see if a good cleaning of its legs and its socket mounting slots helps any. With such small signals/voltages, any dirt or corrosion could cause all kinds of strange problems. Just ran into a similar problem recently, so may be worth a few minutes with some contact cleaner and small pointy tools.

Did you check the OUTPUT of the diode bridge to see how much Annie Green Springs was there? (Oh wait-you said Ripple). Anyway, same question. I still think it's in the PS source from the diodes. A diode checker may NOT show what we want. SCOPE! Mike.

I just think you should LOOK at the output of the rectifier. If you have ripple, it's getting there somehow. I never like the idea of throwing parts at it until it's fixed. I want to understand WHY! Mike

On a different model MM amp, a zener diode had shorted, grounding out itself and some other connected components. This allowed an AC buzz and blocked signal flow. Replaced the offending zener, and the amp is working well.

In other words, I'm betting you have one or more diodes in distress...