Hi & Need Help with a RD112-50. Thanks!

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'm not finding anything loose. This has a nice clean waveform, but with a obvious DC ripple(probably 60Hz) running thru the middle. BTW, probably not related, but the polarity switch doesn't do anything. However, I do float the ground on my Variac so I don't get a loop with my signal gen. & scope and it on. Using a 1k signal at 200Ma gain. Never been an issue with anything else over the last 18 years.

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

Hi! Sorry for not getting back sooner. I've been buried by a couple of Vox AC30's.. I will check these suggestions. If I remember, the hum sort of starts at the phase splitter. I'll get my probe back into it in a few minutes.
Thanks!!!

Is there a board layout for this amp that I could get from someone? Just wondering..
Thanks!

O.k. Coming out of IC#3 it's clean. Coming out of the reverb pot it's clean. Both channels are clean coming out. From the input of IC#4 it's ugly! I just wish I had a board layout to see where everything is. I have the schematic and this is a Version A board.
I normally do not work on Solid State circuitry except for overdrive pedals, fuzztone's etc. I stick pretty much to tube circuits
So any help would be greatly appreciated.
Thanks!

Great! Thanks! I will try this in the morning. I'm pretty sure I swapped out C24. It's an electrolytic if I recall.

Well everything starts between R42 & C24 but coming out of the Reverb pot into R34 the signal's clean. The only electrolytics that have not been replaced are C38 & C43. Kinda hard to find this value as I don't do much Solid State stuff. I know that CE Distribution doesn't carry anything near this and I buy the majority of my caps thru them.
Anyway, I'm stumped. This is a weird one to say the least..
Thanks!!

I would really appreciate any comments or suggestions as I'd like to get this done before Xmas.. It belongs to Blues guitarist extrodinare Joey Delgato of The Delgato Brothers..winners of the Memphis Blues Society band of the year and Joey also won Best Guitarist this year..
Thanks!!

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.

I'll check this out this afternoon. Thanks Guys! Trying to fix a newer Twin Reverb that someone decided to pull the pcb and installed a PTP type board with big cloth covered wire. Their solder skills leave alot to be desired.. move a wire and the signal goes away.

Stay warm Lars! I know how cold it can get up there in "The Great White North" I've played in Canada many, many times.

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

O.K. I went backwards from IC4 to IC3. The signal is great on pin#1.. Wish I had a board layout for this..

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

The signal is clean. Nice on the input side of R42. The output side of R42 is pretty nasty. If I lift the feedback loop coming from the output jack is ugly and lifting the feedback loop on IC4 (R44) it just goes into clipping and is a square wave(distortion) and the ripple is very pronounced.
Thanks for the layout! Comes in quiet handy!

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.

I've replaced all the electrolytics except the two 400uf @ 25v ones. Trying to find them as they're not typical values that I use in the tube amps I normally work on. I'm trying to see what's on Mouser's site.. I did plug in the ft.sw. and no difference. I will try replacing R42. I did swap IC1 & IC2..& no difference their either.
Thanks!
** Update: Mouser Electronics has these. I just ordered 4 of them along with some other caps I need. I ended up ordering some 1000uf @ 100v caps for the bias supply in my 1964 Fisher 800 C Tube Stereo! Love these old Fishers!! I have 3 of the (101x, 500 & the 800 C).

Both Mouser and Newark Electronics carry all the necessary caps.

So can't seem to access the other posts here. I finally got the 400uf@ 25v caps and installed those.. No difference. I'm just feeling really stupid here as all the obvious parts have been replaced(all electrolytics), swapped out the TL072's IC's, changed out R42, etc... I even installed a new Mercury Magnetics Mains tranny that is the exact one they make just for this model, swapped out(well, tack soldered in) an output tranny, nada..

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

O.K. Have the right voltages at all these spots. I'll check with my probe/scope on the ripple. Been a tough day having dental work done and just hit the aspirin & a low dosage THC gummie bear for the pain. Don't normally do the gummies, but can't smoke due to a 34 year run of Bronchitis that's been under control pretty well for the last 3 years. I use to get it about very two months..
Thanks!

Good amount of ripple on both sides of C25 & 26 and R62 & D9...

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

I pulled the 6L6's and there was no ripple on C24 or C26. I replaced R42 & IC4 before that. But with the 6L6's in I get the ripple.. Should I just start shot gunning parts in the phase inverter and output section? This is one weird amp..but I'm not going to quit until it works right. It's a challenge!!!!
Any observations, suggestions or comments are very welcome!
Thanks!

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

I do have ripple on both sides of D9... I've resoldered all of the filter caps(all of them are brand new..) checked the voltages and all are good. However, the 4016 B chip is bad. Had to order some from Mouser and I also ordered a quantity of TL072's as well. Cheap enough @ .75 cents (I think..). So I'll see what happens when they get here Monday.
Diodes are all working correctly. I have a diode checker on my Fluke meter(an older 112 model).
Thought kind of interesting that the IC for the OD channel went out.. Might have shorted across a couple of pins without knowing it with my scope probe..
All suggestions, observations and thoughts are welcome!
Thanks!!
Don

...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.

So if I catch what you're saying, you think it's a diode issue? I did do a continuity check on all the diodes. But maybe I should just replace them(?)..
Thanks!

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...

Having a hard time trying to find any zener's and diodes at Mouser or CE Distribution.. Weird.. I know Mouser has dropped a lot of items over the last 4-5 years..
I do have enough IN914's & IN4003's, but where do you find MR510's, IN4751A & IN5353's?