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  • Electronics help?

    Since I am not an electronics sort of guy, I could use a little troubleshooting help. I bought a Marshall 3210 for $80 and it sounds great except for one thing. Hum in the reverb. The hum actually sounds more a swishing sort of noise but LOUD.If reverb is turned off or down, no hum. If I disconnect the tank, still have hum. If I turn reverb off and shake head, I get beautiful reverb spring slosh. Regardless of tank presence or not, when I turn o the reverb switch, as I turn up the reverb knob the hum increases, and conversely decreases. I opened the head and have look at all the ground connections I can find, and they appear solid. I took a non-conductive probe and tried to rock every part while amp was humming and found nothing loose. No parts LOOK damaged. I cleaned the jacks and plugs with Caig Deoxit. While I was working at NASA I took a soldering course and feel very comfortable working on the board, but have no idea what I am looking for, or what parts might be most likely. I can attach the schematic and if anyone has any suggestions, I am open to them. (Including take it to a repair shop because it is an advanced issue, not a simple find and solder.) HELP!
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    Last edited by Axisplayer; 07-27-2017, 09:39 AM.

  • #2
    Is the character or the amount of noise you get from turning up the Reverb affected by the amplifier's Master Volume control?

    If so, can you describe the effect?
    As a human being, you come with the whole range of inner possibilities
    from the deepest hell to the highest states.

    It is up to you which one you choose to explore
    .

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    • #3
      Just got in from practice. I will hook it all up again tomorrow and check that. Thanks for trying to help me. This is not my forte. THANKS!!!!!!!!

      Comment


      • #4
        Its likely the tank itself. Marshall uses some cheap assed tanks. The one in my Valvestate hums when turned up. It has gotten beat up from just moving the amp around and its not like I move it around that much.

        I'd open the tank and re-solder the wires between the jacks and the elements that straddle the springs. if that doesn't work buy a new tank for it. Tanks are very cheap to buy. If you can get an ohm reading off the reverb tank jacks that would be helpful finding a new tank for it. Any serial number on the tank and the number of springs will help too.

        Comment


        • #5
          Originally posted by WRGKMC View Post
          Its likely the tank itself. Marshall uses some cheap assed tanks. The one in my Valvestate hums when turned up...

          From the OP...

          Originally posted by Axisplayer View Post
          ... The hum actually sounds more a swishing sort of noise but LOUD.If reverb is turned off or down, no hum. If I disconnect the tank, still have hum...

          The 'swishing' sound referred to in the OP seems more likely to originate in an active device - I suspect the op-amp that amplifies the reverb return or the switching transistor that shunts the signal to ground when the reverb is switched off.
          As a human being, you come with the whole range of inner possibilities
          from the deepest hell to the highest states.

          It is up to you which one you choose to explore
          .

          Comment


          • #6
            I haven't hooked it up this morning (so far,) but I CAN hear clear, hum free reverb springs when I bump the head even if the reverb is off, it just doesn't send the guitar signal, so I took that to mean the return circuit was OK and quiet and that it was probably in the send circuit. It isn't until I engage it that it hums, and increases with the reverb knob rotation. Since I hear it clear before I engage the reverb pedal, would it most likely be the switching transistor? It does sound like a ground problem, except it swishes like your ear in a seashell instead of pure hum. If it is that, which part number on the schematic am I talking about?

            EDIT:
            I just it hooked it up. Here is the situation. If there is no guitar cable plugged in, I don't get noise regardless of whether I engage reverb or not, and regardless of amount of reverb or volume dialed in. When I do engage it with cable inserted, I get no hum when disengaged, regardless of volume. When I engage the reverb, I get hum (mild swishing) proportionate to reverb level, and unaffected by the master volume. The sound of the reverb hum remains constant in character.

            Thanks and hope this helps pinpoint it.
            Last edited by Axisplayer; 08-01-2017, 08:16 AM.

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            • #7
              The input jack on the amplifier is known as a switching jack. When there is nothing plugged into it, the switch shunts the signal to ground at the effects send jack. The effects send is after the Master Volume and Reverb controls in the circuit and any sound or amplification that occurs before the jack will not get through to the output section.

              You mentioned that you still get the noise with the Reverb springs disconnected. To me, that points to a very small part of the circuit that can be the source of the noise.

              If you look at the digram and identify the reverb springs and look for the Reverb control you will see a few componants in between them.


              Something you could try is going from the effects send to a second amplifier and see if the noise shows up there. If you also plug into the effects return, you will stop the sound of your amp getting through to the speaker so you will pnly hear the sound from the second amplifier.
              As a human being, you come with the whole range of inner possibilities
              from the deepest hell to the highest states.

              It is up to you which one you choose to explore
              .

              Comment


              • #8
                Originally posted by Axisplayer View Post

                EDIT:
                I just it hooked it up. Here is the situation. If there is no guitar cable plugged in, I don't get noise regardless of whether I engage reverb or not, and regardless of amount of reverb or volume dialed in.

                When I do engage it with cable inserted, I get no hum when disengaged, regardless of volume. When I engage the reverb, I get hum (mild swishing) proportionate to reverb level, and unaffected by the master volume. The sound of the reverb hum remains constant in character.

                Thanks and hope this helps pinpoint it.
                You get no hum or swishing noise with the reverb turned off and instrument turned on. You do get a swishing hum with the reverb engaged.

                I still say your problem is with the tank. Have you pulled the tank out, inspected the wires going to the electromagnets. I suspect the input element is at fault. I have this same issue with my 15W Marshall valvestate. The only reason I haven't replaced it is because the reverb isn't a separate tank. Its part of the chassis.

                Your master volume is before the tank and the reverb level is after the tank. The reverb level knob is the only knob which will attenuate the tanks output and its the only thing besides your send and return jacks and power amp.

                The master volume will attenuate what's going into the reverb unit. It has no affect on what's coming out. With the reverb on and reverb level up you have a constant output from the tanks (as with most amps)

                R37 which connects between the master volume and power amp is the mixing resistor which is in parallel with the tanks input and output so you have dry sound mixed with the wet. Many amps use a pot in place of that resistor to act as a reverb depth knob.

                The switch connects directly to the base of TR3. When the switch is on it provides a voltage to that transistor turning it on and allowing the return of the reverb tank output back to the power amp connection where it mixes with the dry signal.

                That transistor is obviously working BUT, I have a question for you here. Are you using a Marshall switch, a clone or some generic foot switch?

                The Marshall switch is a two way latching switch to a stereo plug. The cable ground is shielded. If you use an unshielded cable, it may inject hum into the base of TR3 at a different phase from whatever hum is feeding into the input of the reverb spring.

                In other words your problem may be two fold. A bad ground on the tanks input element and an unshielded switch pedal cable.

                If you unplug the foot switch and simply turn up the reverb level, do you have that same out of phase hum?

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                • #9
                  I unplugged the tank input and output cables, and still get the hum/swishy sound with the tank removed electronically. The tank makes no contact with the unit physically or electronically at that point. I have a Marshall pedal that came with the amp. It functions, and I have checked the cables. It works perfectly for the channel switching and induces no hum. I metered the ground from the reverb switch inside the pedal to the cable plug and that is also fine. The hum continues with or without the pedal connected. I also can operate it fine with a pedal from my Valve amp, but the hum persist and I get identical results. Additionally the Marshall pedal works perfectly with my Valve amp, and induces no issues. I THINK that eliminates the pedal and tank from the issue.

                  I also tried moving and pulling on each wired connect from circuitboard the anything else in the Marshall and nothing made noise like an intermittent connection, and nothing seemed to be loose. I also visually inspected solder points and can't see anything that appears dull like a cold connection, or showing bulges or burn marks. Parts APPEAR to be fine visually.

                  I just tried a test with two amps. I took the line output from my Univalve and put it into the FX return on the Marshall. I get no hum or noise. When I take the FX send from the Marshall and insert it into the input of the Univalve I get the hum/swish just as I do using the Marshall alone, so the Marshall is sending the hum through the FX output to the Uni.

                  Sure seems like it has to be on the circuitboard of the Marshall. It hums without the tank or pedal, when reverb is turned up and it is not humming using the FX input through the power section of the Marshall. Anything else I can try?
                  Last edited by Axisplayer; 08-02-2017, 07:43 AM.

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                  • #10
                    You've done all an inexperienced person can do at this point. The amp needs to be signal traced by someone who knows how to troubleshoot them. This involves injecting a signal into the input and using a scope or an amplifier probe at each stage of amplification till you get to the component that has a clan input and a dirty output. At that point you've found the culprit that needs to be replaced. you also have to measure the supporting components to be sure they are good so when you solder in the new part it doesn't blow out like the old.

                    The only other suggestion I have is to replace the cable going to the tank. If the tank uses those cheap RCA jacks with the plastic ends they are notorious for developing bad ground. Unplugging the cables from the tank isn't going to matter nor change the level of hum if the shielded ground isn't there. I'd focus on the return cable especially.

                    There's only one op amp on the return from the reverb and it does appear to be an issue with the return side. That op amp looks to be a dual op amp given the fact its marked IC 4A on the input side and IC 4B on the output side. If you can locate that IC and look at it carefully. Any signs of the print printed part number looking gray or any buckling of the surface may indicate the IC overheated and it would be the cause of the problem. Transistors too can discolor or crack when they short. Maybe you have one bad transistor that work as a push pull pair and you're only getting a half wave through the good one mixed with hum from the shorted/open transistor.

                    Visually identifying bad components isn't a given. If there's a catastrophic short and the voltages are enough to cook them you might see something. They could easily be bad with no visible signs however.

                    If you knew your way around you could test the 15 and 34 volts that feed the components and see if there's AC contamination to the DC supply. The caps they use in many of these new SS amps are total garbage. I'm thinking about changing the power caps in my Marshall Valve state soon. I did a gig a few weeks ago and when I got the amp home and plugged it in I got a noticeable hum which wasn't there before. I still need to do some troubleshooting. It may be the new preamp tube I put in it just prior to that gig. I had been using a 5075 low gain tube but switched to a high gain 12AX7.

                    As far as paying to have the reverb fixed you'd expect to spend at least $150. I'm not sure that amp is worth it when you consider the prices on new ones. In my case, I don't even use the internal reverb on that little 15W. It sucked for tone when it was working and I use pedals with great sounding verbs so its not worth my time messing with it.
                    Last edited by WRGKMC; 08-02-2017, 03:04 PM.

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                    • #11
                      I just hate having it with a known problem. It is a tool, not an investment, so I might pony up to fix it. On several Marshall boards they said that these MOSFETs were the best solid state Marshall ever made. They liked them better than the newer ones. This is around '88 or '89 head. It sounds great, so it is usable either way, but I don't paying to fix it. Current prices on these are going up and I now see numbers of them on eBay getting $300-375 when I look at completed listings. Ads now are asking even more. Having gotten for $80 I can afford to put some into it before I have overspent, and as said, it isn't investment, just a tool. I hate dragging a $2500 combo to practice when I can drag this. Guess I will drop it at the shop nearby and let them take a look.

                      Thanks for all the advice guys. I thought it was over my head. I have played for almost 60 years, and do my own adjusting and intonation on guitars, as well as the wiring. I also don't mind biasing my amps, but at the component level, I get lost real quick. LOL. I should probably learn about it now that I am retired and have time to tinker with things I like to do.

                      I will let you know the outcome. THANKS!!!!

                      Comment


                      • #12
                        You could give signal injecting a try. You'll need to do some research, maybe find some videos.

                        The tools you need can be whipped up from scratch. You need a sound source to feed the amp. It can be a CD player feeding music, a computer outputting sound from its sound cards line out to the amp, even a cell phone connected to the amp playing music.

                        If you connect a computer you can download a free test tone generator. You just have to play sound through the amp so you hear the symptom occurring. Just remember, an amps input has higher gain levels so a line level input needs to be turned down so it doesn't overdrive the input and you want to get the level about the same as a guitar would be.

                        Next you need a small amp with a speaker. Those little battery operated amps are ideal for this purpose.

                        Next you need to convert a guitar cord into a probe. You can connect a long wire to the ground and connect it to the amps chassis ground wherever its convenient. On the tip of the cord you need to connect a .1uf capacitor. The other end of the cap is your probe. We used to use an old meter probe for this so we had a wand to make contacts on the board, but you can be as creative as you need here. The trick is to keep the unshielded wire to a minimum length so it doesn't induce addition hum into the small amp. I'd heat shrink or tape the cap and wire off so you only have a small electrical contact at the end of the probe.

                        You can test it by touching the end of the probe and it should hum like an unplugged guitar does,

                        From there you play the music through the amp. You can plug a jack into the effects send or return jack so the power amp is disconnected. We know the power amp is fine and one of the jacks is switched so it will disconnect the power amp and quiet the speaker. You could also just unplug the speaker. SS amps will run safely without a load.

                        The master volume and reverb levels need to be set where you hear the symptom as I said.

                        Next you'd use the probe in conjunction with the schematic. The cap prevents the small amp from loading down the circuit and prevents the probe from shorting out any DC voltages.

                        You want to find the points on the board or components where the signal passes through the components. The schematic is your road map.
                        The first point I'd check is pin #3 on IC 4. It should be clean because we know the signal comes from the master volume and the clean signal through the amp had no hum. You adjust the probe amp so the tone or music is clearly heard.

                        Next you move to Pin #1 on the same chip and test it. By the way the chip usually has the #1 pin marked on the board or uses a dimple or dot on the chip to mark it. You then count in a clockwise order to count the pins. 2, 3, 4 etc.

                        The output on pin two will be louder because the device amplified. if you hear a clean signal going in and that hum problem coming out you know the chip is bad. if its clean you continue along the road map to the next components are two diodes then two transistors, TR4 & TR5. We know the output of IC4 is clean. We want to localize the problem to see if any of the components between the chip and tank are causing the problem. we leapfrog to the transistor outputs. You can make it easy and go right to the hot wire feeding the reverb tank.

                        If its got the noise, then you backtrack to check the transistor bases or the individual transistor outputs. If the basses have a noise then we have a bad diode, if the bases are clean and the outputs are noise we have a bad transistor. If all are clean at the reverb input then the send circuit to the tank is good.

                        I walked you through step by step here. You can easily test this entire circuit by unplugging the tanks input, and connecting it to the test amp. If its clean, you know the send circuit is working, if its got the hum, you know the problem is between the tank and master volume and can test it step by step as I just mentioned.

                        If all is good up to that point, you can jump to the tanks output. Unplug the tank and connect the tanks output to an amp. If you have a hum, you have a bad tank. If the signal is clean the tank is good and can be plugged back into the circuit. (The tank output level will be very low before its amplified so this should be expected. You're looking for the hum)

                        Next stop can be the input to IC4 at pin #5 but we've already tested the output of the tank. I'd leapfrog to pin #7 In my gestimation, you should have the hum here. If Pin #5 is clean and Pin #7 is dirty, you know its the chop or the voltage supply it is carrying AC noise.

                        If the signal is clean at pin #7 go to the center leg of the Reverb pot. If the center leg has noise and the chip doesn't, your problem is likely TR3.
                        You can safely remove TR3 and see if the noise goes away. If it doesn't then I'd look at caps C50and C26

                        I skipped over a bunch of other caps that can be culprits but the key here is to isolate where the symptom begins. This will narrow down the possible components and we simply replace the most likely culprits.

                        I hang on this site pretty regularly during the week. This circuit is very simple to troubleshoot and both Onelife and myself are seasoned amp techs who can help walk you through this problem. Once we figure out the components causing the problem we can focus on getting exact replacements. This is usually the toughest part of the process because many of these components are made in japan and china and they may not have good crossover numbers, but gives the simplicity of the circuit I'm not seeing a big issue here so lets remain hopeful this is a simple problem. Just keep in mind, the trick here is to compare the input vs output. If sound goes in one ear and it doesn't come out the other properly it obviously met with an obstruction.

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                        • #13
                          Originally posted by WRGKMC View Post
                          ...
                          The switch connects directly to the base of TR3. When the switch is on it provides a voltage to that transistor turning it on and allowing the return of the reverb tank output back to the power amp connection where it mixes with the dry signal...
                          First of all, bi-polar transistors are current operated devices. When a current is applied to the base of the transistor, it turns the transistor on.

                          Secondly, if you read the schematic correctly then you will notice that TR3 is positioned across the Reverb Level control. When TR3 is switched on it shunts the Reverb signal to ground effectively turning the Reverb off.


                          That being said, TR3 is in a position where it can introduce noise into the circuit.



                          Last edited by onelife; 08-03-2017, 11:07 AM.
                          As a human being, you come with the whole range of inner possibilities
                          from the deepest hell to the highest states.

                          It is up to you which one you choose to explore
                          .

                          Comment


                          • WRGKMC
                            WRGKMC commented
                            Editing a comment
                            Fair enough I stand corrected. To an end user however, when the switch is depressed to turn the reverb on, its actually turned off/open and the transistor is non conducting.

                        • #14
                          Ok, I have homework to do. I might actually learn something...

                          I will make the probe up and try to follow the schematic. Thanks! You guys are amazing. I will report back once I muddle through it all.

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                          • #15
                            A friend who runs a music store told me he would walk me through this. Took about an hour of work to open it up, find the problem, fix it, close it up, and pay for it. It was an op amp after all. Parts and labor at $53. I am happy and learned a little. Thanks guys. I am still going to get a book and do some reading. I really SHOULD know these things. Appreciate all the help.

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