Fisher x100c hum problem revisited

Discussion in 'Fisher' started by mp53, Nov 6, 2017.

  1. mp53

    mp53 Member

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    Hello, This is a follow up on a privious post concerning the buzz I'm getting through the speakers out of mt rebuilt Fisher. I did conclude that the buzz was coming from the 7199 driver tubes. I bought some adaptors and replaced the 7199's with 6gh8 . These 6gh8 tubes came out of a working tuner. I still have the buzzing.
    But I have discovered that I inadvertently left out the heater reference circuit for the amp section . Just dont know the best way to fix it sense I have installed the IBAM and individul carhode resistors. I have so far no discussion here on it so any suggestions or resources is appreciated.
     
    Last edited: Nov 8, 2017
  2. mp53

    mp53 Member

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    Attached is the heater string and how I wired it
     

    Attached Files:

  3. audmod01

    audmod01 AK Subscriber Subscriber

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    MP53;

    I reviewed the schematic for the X-100-C and I only see 12AX7 and 7868 outputs. Recheck your model number as the X-100-C did not use 7199 driver/phase inverter tubes. If we have the correct model number then we may be able to help you.

    I would add that it appears insulated posts have been added near the tube sockets that are not original to Fisher manufacturing. Most of us would not do that. However, heater wiring in general has to be twisted pair wires to prevent radiating 60Hz hum into nearby high gain circuits (if any), so changing heater wiring of tube circuits has to be done with great care and rechecking of performance as changes are made. Keep in mind that changing from 7199 to 6GH8A tubes can be done with amplifier circuits that used them, you are introducing extra parts by using adapter sockets and extra parts add more points of possible failure. I have a Dynaco ST-70 power amplifier that I modified to use 6GH8A tubes by cutting circuit traces and rewiring directly under the tube socket. It works well and no hum was introduced in the process, but the changes were to circuit traces that were close to the socket. Have you tried different 6GH8A tubes? I would suggest that first, providing that it worked well without hum right after the modifications were made.

    Joe
     
  4. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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  5. audmod01

    audmod01 AK Subscriber Subscriber

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    Larry;
    Thanks for pointing me to the previous thread. Now I understand what the approach is. I took time to redraw the partial schematic to make it easier to understand. Here it is:
    7199 mod 2.jpg
    One thing to be careful of is to make sure that the filament of the 7199 has a resistor connecting its filament back to a B+ source somewhat near to the voltage that appears on the cathode of its triode section. It is there to help prevent breakdown between the insulated cathode and the filament. If it does breakdown (or short) it will indeed place a 60Hz hum into the audio. Look closely at how Fisher handled this issue with the original 12AX7 driver/phase inverter. Also, remember to use solid wire for filament strings in order for wires to keep the position you place them in against the chassis. Use about 2 twists per inch or more of the filament wires if you can manage, but don't go overboard. You don't want wires to spring back into the air above the chassis metal. Stranded wire is not the best choice when handling filament circuits for that reason. Dress the filament wires well away from signal grid leads and components. Recheck solder connections of all grounds at the tube socket or any bypass capacitors leading to the 7199 tubes.

    Another unrelated item is to add a 100 ohm resistor in the screen grid circuits of the 7868 B+ feed to the tubes. This will help protect against arc-over inside the 7868 tubes. I believe Dave recommends a 1/4 watt resistor so it can serve as a fuse in case of an internal short of the screen grid.

    Joe
     
  6. audmod01

    audmod01 AK Subscriber Subscriber

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    I went back and re-read your first comments here. You mentioned heater reference circuit - I think you were thinking about the DC voltage at the cathode/heater circuit of the triode (which is where most of the voltage difference would be). According to the RCA tube manual, the 7199 allows a 200V peak difference between the two and a normal steady maximum difference of 100V maximum. All of the tubes in the set have the same filament to cathode breakdown voltage ratings and so does a 6GH8. This heater to cathode voltage may not be a problem. It could still just be due to a poor connection or poor lead dress of heater wires versus components handling signal.

    Joe
     
  7. mp53

    mp53 Member

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    I do understand what you are saying but originally there was no heater to cathode voltage wired in. I was asking the best way to implement one. So this is how I did it. Took two 100 ohm resistors wired each to one side of the ac filament supply then the opposite ends both to the hot side of the cathode resistor. This did help the hum problem. Unfortunately this was just the beginning of more undiscovered malfunctions. Evidentially wherever I soldered any signal wiring the heat of doing so melted the insulation enough to short it to the shield. I've replaced three shielded wires so far and probably replace all of them in the end.
    Right now it still has a hum but better, right channel has a lot of spurious noise and both channels have hiss. Also the volume pot is not consistent .
     
    Last edited: Nov 20, 2017
  8. audmod01

    audmod01 AK Subscriber Subscriber

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    mp53;

    You did it the right way. What you did provides essentially a center tap of the filament AC voltage and attaching the mid-point of the two resistors to the DC present at the cathodes of the output tubes does reduce the DC voltage difference between the phase inverter stage cathode and the overall filament circuit. The cathodes of the output tubes are filtered by the electrolytic present in that circuit and also provides the center-tapped AC ground for your implementation of that design modification.

    Soldering leads near other wiring is always a risk. This is especially true for those of us who are growing older and have hands and fingers that shake. Using a small tip soldering iron with adjustable temperature helps. Also you can use hemostats or seizers to help carry away heat from the wire and help avoid melting insulation. Be careful in your choice of audio coaxial cabling materials as their capacitance from center conductor to shield varies quite a lot depending on brand and type selected. Too large a capacitance per foot can result in degraded high frequency response. Fisher used audio coax cable material that had a carbon film shield with a tracer lead in many of their products. This material had low capacitance properties and yet was small diameter, allowing less space to be consumed under the chassis.

    One way to tell if a particular stage filament circuit is causing hum is to find a way to interrupt the 6.3VAC feed to that particular tube while the rest remain connected. If the hum goes away, your efforts can be concentrated in that location. The audio will remain strong for quite a few seconds until the tube's cathode temperature drops and emission falls. It will give you enough time to judge whether the filament/cathode circuit is contributing to the problem or not.

    Joe
     
  9. mp53

    mp53 Member

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    Thanks for everyone's input, I have some more questions and observations to ask.
    1] What I
    have for the coax cable replacement is a 24gauge twisted pair silver plated copper multi stranded wire with teflon coating inside a copper braid. So i dont know what the capacitance would be?
    2] I thought about replacing the two 100 ohm resistors for the heater reference to a 50 ohm pot. That way i could adjust for lowest hum. it would be easy to do because I have a 50 ohm two watt pot and I could put it in the spot where the pot for the cathode adjustment was.
    3] Another concern I see is when I measure with my volt meter set at AC , I get a 70 volt reading at one of the legs of the power transformer AC input and chassis ground, the other leg reads much lower. Could this be a source of hum ?
    MP
     
    Last edited: Nov 20, 2017
  10. audmod01

    audmod01 AK Subscriber Subscriber

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    MP;

    Item 1) I would say go back to using conventional audio coaxial cable instead of twisted pair multi-strand wire. Multi-strand wire tends to not stay in physical position as well as solid wire in twisted wire applications as mentioned previously. Also coaxial cable is often much better shielded from hum pickup than twisted wires can be. You should be able to find decent audio coaxial cable material from a number of sources. Choose the one that has the lowest capacitance per linear foot or linear meter specification. Single shielded should be good, Double shielded is fine except that its outside diameter may begin to be a problem fitting into curves etc. of the placement in the chassis. Always follow the original lead dress when replacing wires. Consult your pictures of the underside of your chassis to help you get lead dress correct. Use the small size cable ties if needed to keep wires that were originally bundled together grouped together as you work.

    Item 2) Using an adjustable control to provide a way to minimize hum is a good idea. It was a common practice by many manufacturers years ago and is certainly worth a try. As long as a 50 ohm control does not overheat you should be OK there.

    Item 3) The 115-125VAC input leads of your house wiring has a hot feed on the black wire at the AC outlet. The other side is called neutral which is ostensibly the ground side of the AC feed from the power company. The third wire at the AC wall outlet is a green safety ground which normally traces back to a ground rod just below the AC meter on the outside of your house. It is common to see a difference between the AC input white neutral wire and the green safety ground. So seeing a difference in AC measured to chassis ground from the two AC input wires inside a set is normal and not much of an issue. These older tube type Fisher chassis did not connect to the AC receptacle safety ground. If the original AC filter caps of the chassis on the AC input leads have not already been replaced, it would be good to replace them with modern safety related parts just to be on the safe side. Use new parts with the same capacitance value.

    Joe
     
  11. mp53

    mp53 Member

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    Found another cause of one of my hum problems. The wire that runs from one side of the balance pot to the volume pot when moved cut out one of the hums. So I looked at the volume pot terminals and saw that they were kinda too close I then bent one of them just a little to widen the gap but after that some of the hum came back and now moving the wire doesnt change it. So there is a problem with the two pots. So what is it about these pots that can cause hum? Shouldn't that wire be shielded to? And isnt the volume pot a linear one because how it wires to the bass boost circuit? So I know what type of pot to replace it with.
    Another question Joe, You mentioned concerning the cathode to heater reference that the purpose was to protect the cathode insulation from breaking down. I ran the amp for an hour at least untill I wired it correctly, Do you think I may have damaged the 7199's?
     
    Last edited: Nov 30, 2017
  12. audmod01

    audmod01 AK Subscriber Subscriber

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    mp53;
    1) If moving a wire or audio cable that connects from one end of the balance control to a connection on the volume control made hum go away, it appears that you may have a poor connection on one end of that wire/cable. Check for loose connections or poor solder connection on both ends of that wire/cable and if it is a shielded cable, make sure the shield is properly grounded at whatever point it is connected to.

    2) The filament aspects of this design may be a red herring - not the real cause of the problem. Try some new 7199 tubes or 6GH8 tubes if you are using them and the adapter sockets to see if you may have damaged a tube. That is a simple test that may help you get on the right track.

    I am somewhat confused as you say you added IBAM to this set and the schematic you posted indicates that it is still operating with cathode self-bias on the four output 7868 tubes. That implies that all the tubes operate with the same bias other than what small differences are developed across the 10 ohm resistors you added at each 7868 tube's cathode. Those 10 ohm resistors allow you to measure what each 7868 tube's idle current is (and they are seldom identical even when using a "Balanced Quad set") but the overall cathode voltage is adjusted using the original R79, a 5K ohm variable control. If you somehow implemented Individual Bias Adjustment for each 7868 tube's grid you did not show it in your schematic.

    3) On the issue of the 3-wire AC cord you added to this set, it is possible you may have introduced a ground loop that is causing hum to be introduced. Temporarily disconnect the safety ground in the chassis (the green wire and insulate it from the chassis. Then try the amp once more to see if this has any impact on the hum you hear. Adding safety ground to sets not originally designed for it can introduce problems.

    All four output tubes and the driver/phase inverter tubes and the pilot lamp have their filaments on a common filament winding of the power supply according to the original schematic. That winding is not originally referenced to ground - in other words it was floating with respect to ground. That actually provided some protection against the cathode shorting to the heater in a tube. Tubes V5, V6, V7, V8, V9 and V10 are on that filament circuit. Since there is already a positive voltage at all the 7868 cathodes due to their combined conduction you could add say a 100K ohm 1/2 watt resistor from the Junction of R78 and C79 + terminal (the 7868 cathode circuit) to a terminal strip and then join that to two more 100K ohm resistors each of which would connect to each leg of the filament wiring for those tubes. The common point of the three 100K ohm resistors would need to be bypassed with a suitable electrolytic, say 100uF@50VDC or 75VDC so that any residual hum at that point would be minimized. This would place the normal 39VDC of the output cathodes on the filament string and reduce the difference between the cathode of the triode sections of the 7199 tubes and their heaters. However, to do this you would have to remove any variable resistor you may have added from both sides of the filament string to ground. The filaments would have to be floating again.

    Check the wire/cable connections first, then try the tubes and then the safety ground removal before doing anything to the filament circuitry.

    Joe
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    you can individually bias stuff with a common cathode resistor. I don't have the schematic up but basically it involves a set of trimmers to vary how much cathode voltage is fed back into the grid. Basically 4 trimmer and resistor chains vs the single one shown in the schematic above. Get the voltage drop across each 10 ohm the same and its as matched as you could ever want it to be. The trick is that every adjustment has some effect on all tubes so its a bit of a dance to get it settled out. I have the same setup on my Fisher TA-600, it works well but its not quite as easy as adjusting fixed bias amps.

    Is this 60hz or 120 hz hum? That will change what the likely issue is. 120 is power supply related, 60 is usually ground loop, heater-cathode leakage, wire routing problems, that kind of thing. If you don't have a scope, there is probably a phone app that will identify it.

    General related questions, are all the tube shields in place and does it change with the bottom plate installed? Volume-dependant or no?
     
  14. audmod01

    audmod01 AK Subscriber Subscriber

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    Gadget;
    All good points, especially about recognizing whether it is 60Hz or 120Hz.

    With the individual bias adjustments you described, probably the safest way to start the adjustment process would be to adjust for minimum current in the output tubes and gradually walk the relative negative grid bias down until each tube reads about equal current while delivering the best overall performance. The brand of tubes will make a difference, with the new Tung-Sol tubes able to handle the highest current while delivering best output power level.

    Good luck.
     
  15. mp53

    mp53 Member

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    A few things need clarifying: 1) There is no earth ground 2) There is no heater reference to chassis or ever has been 3) It does have the IBAM, I qualified this in the post with my pencil diagram.4) There is no feedback circuit as of yet, I do have a scope so I was going to dial in those last.
    Right now I have distinguished that there are three noises occurring at the same time. Two hums and a spurious one. One of hums is coming from the volume pot wire going to the balance pot, I'm replacing the pot because it doesn't track right and I have rebuilt it twice allready. The second hum is very low in pitch and volume. The volume pot does not effect it.The spurious noises ,I'm hoping will be gone when the the first two hums are fixed.
    I havent installed the bottom plate or the pot cover either so that could make a difference.
    Joe, your scheme for the heater reference using three resistors. Do you mean 100 ohm resistors or 100 k? And the capacitor is positive end at the juction of the three and negetive to ground?
     
  16. audmod01

    audmod01 AK Subscriber Subscriber

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    MP53;
    Yes 3 100K ohm resistors and the capacitor + to the junction of the three 100K ohm resistors and capacitor negative to ground..
    The hum that does not change with volume is likely due to the power supply in the chassis and/or an issue right in the push-pull output stage(s).

    I have seen volume control elements that developed an open ground at the lug that gets grounded in the circuit. Of course that usually causes multiple symptoms at the same time, possible hum and some loss of volume control, especially at the low end of the control. An ohm meter check will usually spot this. Rotate or wiggle the shaft of the control while checking to see if mechanical movement is causing erratic ground of the element.

    A possibility we have not mentioned is that sometimes the rivets used to ground the tube socket frame becomes corroded between the rivet, tube socket saddle and chassis metal. This can usually be spotted with an ohm meter on the R X 1 scale, especially with a good digital meter. Double check against other tube sockets in the chassis to get a good idea what to expect for a normal ground at a rivet. Sometimes there may be a telltale corrosion around the rivet head on top of the chassis, but not always. However, I think you said you had changed the tube socket?

    Joe
     
  17. mp53

    mp53 Member

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    Yes, all the tube sockets have been replaced with NOS types. So the sockets themselves need to be grounded to the chassis? Why is that? I mean the ones without tube sheilds.
    The symptom you describe about the volume pot is exactly Whats happening. At the low end of the dial one channel is allmost completly out untill you turn it. . So I m just going to replace it with something better, make an upgrade. Then see what happens.
    Yea and also all the electrolytic s have been replaced to. When I got this amp the output transformers measured open. So I didn't have any basis to work with.
     
    Last edited: Dec 6, 2017
  18. rufleruf

    rufleruf AK Subscriber Subscriber

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    I've been reading about what to do with a bunch of Heathkits I recently acquired and most of the threads I've come across that involve any major rewire or component swaps warn that there will be hum problems introduced if great care isn't taken with wiring routing, twisted pairs etc.

    Maybe the issue is not a component at all?

    And echoing gadget73: I was recently auditioning a stock X100B and noticed a slight hum between songs. I dropped the missing tube shields on the middle two 12AX7s and the hum disappeared.
     
  19. mp53

    mp53 Member

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    I do have some extra tube shields to try out and see but that volume pot is causing one of the hums.
     
  20. audmod01

    audmod01 AK Subscriber Subscriber

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    Gadget;
    Yes you are right. Also wholesale replacement of tube sockets is not a good idea because it is too easy to wind up introducing errors and problems that mask other issues with a unit. It is best to proceed one stage at a time and recheck performance as each stage is worked on. That way if you introduce a problem of any sort you can correct it before moving on to a successive stage. It becomes difficult to troubleshoot a unit that has multiple problems. It can be done but requires a good deal of knowledge and experience to achieve a good end result. It is very easy to create a bad solder joint when replacing a tube socket or even a terminal strip on a chassis. Good soldering technique is important to achieving good results and that is another area where experience over time is important as well as rechecking each change before proceeding to another component or stage in a unit.
    Joe
     

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