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troubleshooting M40 Echolette with loud hum

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  • #61
    Hi Oc , the problem is that the heater seem to be grounded and in purpose (see the attached pic) on pin 4&5 of the far right output tube, we find H2+the 2 wires that go to the light bulb + connection to the center tap of the tube socket and the 2 black wires that go to ground.. I don't understand this.. i think i saw somewhere on the web that those heaters has to be grounded but i'm not sure.. It's gettin difficult, i take a look at light bulb but it seems ok, i continue the researches

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    • #62
      Don't worry about what you heard.... the amp should match the schematic unless there is an error in the drawing.
      I cannot see on either of the 2 diagrams where H2 connects to ground !!
      Is the cathode next to it grounded ? Does it look like the previous owner soldered it ?
      I would disconnect the ground to H2. Make sure all the cathodes are grounded EL34....

      From your readings the light bulb measures .6 of an ohm. I presume it lights up ?

      With the ground disconnected from H2 re-measure both heater wires to ground. Shouldn't be any resistance at all ie should be open.
      Attached Files

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      • #63
        ok i disconnected this, actually it's the H2 wire on pin4 that seems to be longer in purpose and then soldered to the center tap of the tube socket, so pin3 and 4 are connected to ground..
        i found this pic on the web and it's hard to see but it looks the same, pin3&4 connected to ground,
        anyway you're right now that i disoldered it i measure a short on H2, now i wish i had a good PT to test it cause it still overheating and smelling bad..

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        • #64
          Ok it's looking like the transformer is about to die. Before it does and assuming you have a light bulb limiter with at least a 60 watt light bulb,in series measure the A.C. voltages just the 2 heaters and importantly the ht voltage but be careful.
          I'm a bit concerned about removing the primary side and reconnecting it on the table. Even though it's AC the direction of the current should be the same.
          With those primary windings that connect in series for 240v its important the temp wires/wiring is exactly the same on the table as it was in the amp.
          If one of them was reversed it would be opposing the others. I am still concerned there may be a fault still lurking in the amp. I assumed you would be using the LBL
          all the way through. It seems the main fault was the two resistors touching which created the hum and sent the previous owner on a wild goose chase (do you have that expression in France?) replacing capacitors until he gave up and sold it to you.
          On a lot of these forums you only have to whisper hum.. and every one will say "change the capacitors" when a lot of the time it's not that simple.
          It now seems after very stressful testing the transformer is about to die. The best replacement would be one from another broken unit ..say one with a blown output transformer. You are more likely to find one in Europe... maybe put a "wanted" advertisment in a local music magazine or on local forums near you.
          Getting another brand or generic replacement power transformer will be easy electrically but difficult to physically fit ... mounting holes etc.
          There are places that rewind transformers back to original but that could cost more than the whole M40.
          Rectifier valves/tubes can get internal shorts .. be aware that the ez81's may be faulty now after red and white glowing.

          Back to the grounded H2 ... it's normal to use various methods to connect (reference) the heaters to ground to reduce hum.
          Look at some Fender circuits , the old Fender twin had two 100 ohm resistors connected to ground from each heater wire.
          In some amps Fender introduced a hum balance control similar to your 100 ohm pot on HT 1.
          In yours after they drew the schematic and actually built one , they could have found a slight hum which disappeared when one side of H2
          was grounded.

          So ideally you need a replacement transformer with 2 x 6.3v tappings and what ever you measured for the HT and bias tap (between CT and lead that went to the 5k Resistor. How about getting 4 x 1n 4007's and 1 x 100ww 5w resistor and build a temp solid state rectifier for testing purposes and reconnecting everything for a last hoorah ? Look at a fender ss set up. The 81's may have failed some time back when you blew the house fuse and have gone on to destroy the transformer ...well started the internal insulation breakdown.

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          • #65
            Hi Oc, yes in France we say "go hunting the Dahut!" (partir à la chasse au Dahut) which is an imaginary animal we could say also "runing after the moon" (courir après la lune), i'm glad to learn new english expressions!
            ok so i did a quick test,(cause i'm going out for the w-e and madam is waiting for me..) and almost from the begining of this troubleshooting i did my tests with the light bulb limiter (115w) hope it didn't false the results, i did measurement on the primary and i can't find any 220VAC, i read a 152VAC on one terminal, and crazy readings on the others has if i didn't have a stable current, sometimes i could read 20V on one of the terminals, but it's overheating quickly, i'll try further measurements next week but so far, unless i'm doing wrong, i should have 220VAC somewhere on one of the terminal? I already tried measuring the disconnected wires of the primary and i couldn't read any voltages..

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            • #66
              Hi again, I finally took off the baking paper that was already torn on the PT, see the attached pic,(should have done this earlier..) and i found the label with the voltage on it.
              I would like to have your advices before doing stupid things, cause i want to solder directly the 2 wires of a 220VAC power cord (with the light bulb limiter in series) that i took from a light, but i don't know which terminal i've got to use. If we look at the schematic, the terminals are labelled 1-3-2-4-4-6-5-7.
              -I did a continuity test on those terminals, and we have continuity between:
              1-2 ; 3-4-5 ; 6-7
              - i was thinking of soldering the power cord like this : the - on 1 and the + on 7, like on the schematic but i don't know if i've got to hook other terminals between them also cause there is no continuity between 1&7.
              What do you reckon?
              Cheers N
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              • #67
                Well that was a bonus , to find the covered voltage listing..

                I'm feeling uncomfortable about encouraging you to experiment with a possibly lethal power transformer.
                I suggest you find someone at a university or college (or repair shop) near you who speaks your language.
                Pin 7 (at transformer not VOLTAGE selector) although it's connected at one end , the other end goes to
                the voltage selector lever (5) which is not switched to anything !
                Likewise on the VOLTAGE selector (4) (1) and (6) don't go anywhere when switched to 220 AC volts.
                It's important to understand the windings have to be in phase the magnetism from the coils in the transformer has to rise and fall
                in the same way. You loose bass for example if your speakers are reverse polarity one moves forward while the other moves backwards.
                When transformers are made because of physical problems (wires crossing tangle) they often don't come out of the guts in a nice neat order.
                Thus the funny peculiar order.. not 1 2 3 4 5 6 7 !

                Somebody ?


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                • #68
                  Hi Oc, thanks for this explanation, i didn't realize this "phase" aspect by the way,
                  well i gave it a try and did things with precautions, and now we're fixed that the PT is shorted!, i thought it wasn't but after let say 50secs the burnt smell started..
                  I'll ask around about the cost of a PT rewinding, i'm affraid has you said that is gonna cost a lot more than the all echolette, but for the moment a broken unit on ebay is quite expensive too
                  and still you're not sure about their component reliabilty, but as soon as i get it fixed, i'll do a report
                  cheers

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                  • #69
                    You might ask here:

                    RITTER electronic systems--Die Röhrenspezialisten seit 30 Jahren!
                    Welter Audio Electronic********************************************** ************************** Seit 1975

                    After quickly reading through this thread, i am afraid that the OT was ok initially but has been killed during the experiments.

                    Did You use a load resistor during Your measurements? You NEED such a resistor in order not to kill the power stage. 100 or 150 W / 15 Ohms attached to the 15 Ohm connector should be ok.

                    When i try to repair my old amps i avoid switching them on and try to measure everything possible at lower voltages first.

                    Do You understand the function of the blown resistor? At least its location in the circuit?

                    (Yes, i know, trouble shooting an M40 may be hard. I have such a project going on for over10 years).
                    Last edited by bea; 08-09-2015, 11:30 PM.

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                    • #70
                      thank you bea! for those informations, i have sent a email to them to see how much it would be,
                      i'm looking for auctions on ebay but it's expensive for a defective unit which you don't know if the PT/OT are actually in good condition..
                      When i did the measurments switched on etc.. i had the speakers plugged in, i know it's not the best way.. but i 'll build a dummy load later on, cause it's true that it's much more easier with it + you don't risk to blow out the speakers.
                      I precize that i'm a beginner and for now i don't have a lot of equipment,( it would be nice to buy a variac for exemple),
                      i hope you're not right about the OT..What do you mean by "the blown resistor" by the way?
                      Cheers

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                      • #71
                        You showed a photo with a resistor with signs of overheating.

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                        • #72
                          oh yes, actually it's still working, it's just that it was creating a short with another resistor nearby due to it's uninsulated shell,
                          the resistor is for the grid but i don't really see the purpose, maybe to feed the grid with some voltage..
                          Anyway i got an answer from Welter, they sell the PT for 95 euros which is the price, less expensive that a PT rewinding i guess, here are the details:

                          "Netztrafo mit folgenden Daten: Primär: 230V - Sekundär: 272V-245V-0V, 0V-245V-272V, 21,5V, 6,3V und 6,3V. Der Netztrafo N17E ist für Echolette® M40 mit 2xEZ81 Röhrengleichrichtung oder wahlweise mit Silizium-Brückengleichtung und für Echolette® M70 mit Silizium-Brückengleichtung. Die Bauform und Größe entspricht dem original Netztrafo. Der N17-E hat durch die Verwendung von hochwertigeren Blechen mehr Leistung wie der original NT. Dieser Trafo ist nur als Sonderanfertigung lieferbar."

                          my german is really bad, but according to them it's the same PT for M40&M70, i hope that if i order this i will figure it out which terminal is which, maybe Bea or someone already installed this specific brand in their M40?
                          it looks like there isn't the same number of wires on the picture compared to mine which is a BV 1665.
                          Cheers
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                          • #73
                            No, i did not need to install it.


                            So the damaged resistor is a screen grid resistor?
                            If yes, this might tell You a bit on the usage of the amp: it probably has been used as a guitar amp and it probably has been heavily overdriven.

                            When You overdrive a power pentode, the current on the screen grid increases rapidly. Without additional measures this will lead to a desctruction of the valve. The resistor has mainly a protective function: if the screen current rises, the voltage drop increases and the screen voltage will decrease.

                            If You want to overdrive the power stage, You need larger screen grid resistors. Please keep in mind that these amps are not designed to withstand heavy overdriven operation.

                            And if one of them already has signs of damage, it NEEDS to be changed. Not only that - all four should be changed. Overheating must have occured to all of them, and there may be damage not visible to the eyes. After closely checking the screen grid resistors i noticed varying signs of damage in 3 of the 4 resistors...


                            The resistors are not chosen to take too much load intentionally. They somehow act as fuses to protect the grids and the transformer.

                            One of the tube sockets was worn and needed to be changed, wasn't it? If yes, there is probably some damage on the other sockets as well - pins losing their elasticity and not giving safe contact any more. Effect: the amp will be very sensitive to mechanical vibration. My M40 fuzzes if operated standing on a loudspeaker...

                            You should replace the sockets and You should be aware that the connections should not be too close to each other to ensure electrical safety. I know that's a bit hard especially for the mass wires soldered to the center pins - on recent sockets these center pins are too short. (I still need to change the sockets in the power stage of my M40...).

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                            • #74
                              BTW, as a quick reference here the voltages in my M40 (voltage selector in position 240 V, rail voltage 230 V):

                              PI:

                              183 / 183
                              27.5 / 27.5

                              Endstufe:

                              Pin 9, Usg: 332
                              Pin 7, Ua: 330
                              Pin 2, Ug1: -15.5

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                              • #75
                                Thank you for those infos bea and for your measurements their gonna make me a good reference for later on, actually the resistor is'nt damaged cause i still measure 226 ohms it's just that it was touching another resistor in the circuit creating a short. First we thought it was a loose pin9 but after i changed the tube socket the short was still here.. it took me ages to receive the tube sockets cause i stupidly ordered the wrong size first, then i received the "good ones" from china and yes quite hard to install with this short center pin..
                                This is really important what you said cause i actually want to use the sweet overdriven sound that this amp can provides, using a ABCD box to switch between different overdriven stages..
                                in this way you think i should higher the value of the 220 ohms resistor on each pin 9 to let say a 470 ohms for example? Should i also higher the value of the 2k that goes to pin2?

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