update tommy chong jcm 800 2210 howling

Discussion in 'The Workbench' started by Exotic, Dec 5, 2019.

  1. Exotic

    Exotic Active Member

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    caps arrived today tech is going to work on it this afternoon ,wish us luck ? i asked him to save my old parts ,is that a good idea? so if the tubes are ok ,how about the tube sockets themselves? maybe they need de oxit? or re tensioning? can that cause a howl? cause the reverb knob to affect the volume ?
    cause the amp to sound bad?
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    We can't troubleshoot an amplifier unless you are looking at it. Everything is guessing not testing.
     
  3. Exotic

    Exotic Active Member

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    what city r u in amp mad scientist? you should be working on this amp : ) if the filter caps don't do it ,i will ask him again to come here and talk to you .
     
  4. NSV-Andy

    NSV-Andy New Member

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    I am in Northeastern Wisconsin.

    The easiest way to check if the amp is an early run is to look at the fuse in either of the fuseholders on the rear panel . If it is the larger fuse, a 3 AG, then it is an earlier run. If it is the smaller 5X20 mm fuse, it is a later run and unlikely to be the one with the grounding problems.

    Your problem sounds like an integrated circuit used for channel and reverb switching, a CA3046. I have had to replace one of these in the last couple of years that had problems similar to what you are describing. There is a certain amount of funkiness in the way that the channel gain and reverb controls work even with a known good IC, but what you are describing goes far beyond that. Tell your guy to look at the IC and the circuitry surrounding it (there's a reverb kill transistor also associated with it).

    An open filter capacitor will often look perfectly fine. What it means is that the capacitor is doing nothing, just like it was taken out of the circuit. A shorted capacitor will behave as though it were replaced with a piece of wire. Capacitors can also change value, e.g. a 20 Mfd cap can only measure 5 Mfd on a capacitance meter, which can cause a variety of performance problems.

    While hum is an obvious symptom of a bad filter capacitor, an open filter capacitor in the "right" spot can cause oscillation. This is because it is not isolating one part of the amplifier from another, in effect, causing feedback. The full explanation can get more involved than that, so I'll leave it there.

    Hope that info is helpful. Good luck!
     
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  5. Exotic

    Exotic Active Member

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    wow yeah that is very interesting ,i want to tell the tech about that but i haven't heard back from him yet ,so i'm still waiting ,but thanks .
     
  6. Exotic

    Exotic Active Member

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    i got an email today that read in part the following :

    Exotic, your amp is done and will be ready for pickup Monday. I replaced
    the 3 capacitor cans as we discussed, and I cleaned up a couple of small
    issues I believe were created by a previous tech repair. One was related
    to the replaced master volume pot...when the original pot was replaced,
    the PCB traces were damaged, and not correctly repaired. I think that
    was the cause of howling and increase in volume as the reverb level was
    raised. Everything is working normally now. I removed an extra added
    resistor to allow for more range in the bias adjustment, and I did
    slightly lower the bias current, from 60% of max down to 53%. A moderate
    change, but I think it also helped with the howl issue.

    what do u think of the biasing ?
    what do u think of the resistor being removed?
    what u guys think? one thing i can say is ,the howling started before the pot was changed . so must have something else? well the only thing different was the caps ,because all the stuff was there when it was working fine .
     
  7. Gunner64

    Gunner64 Well-Known Member Premium Member

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    Sounds like it was related to the pot to pcb connections..the connections could have been compromised, which lead the last guy to replace the pot...
     
  8. Exotic

    Exotic Active Member

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    let me clarify please , the amp was running fine for like 4 months after a retube ,from one day to the next it started up with the howling and the reverb knob acting weird .
    so i took it to my usual tech ,he couldn't find what was wrong , but i asked him to replace 2 pots ,the reverb because the amp was acting up with the reverb pot and the master volume because it wouldn't change the volume gradually ,it would just jump from nothing to screaming . so i took it to the second tech, he changed the caps and made some tweeks and now he says it's back to normal .
    but until i get it back monday and try it out ,i have some concerns .
    1.the resistor he removed ,was that a good thing or a bad thing?
    2.the lower biasing ,will it have less crunch now?
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    and I did
    slightly lower the bias current, from 60% of max down to 53%. A moderate
    change, but I think it also helped with the howl issue.

    Huh?
    Why wouldn't you bias this amp at 70%?
    It still creates more questions than it answers.
     
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  10. Gunner64

    Gunner64 Well-Known Member Premium Member

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    My 2205 acted up one day out of nowhere, for seemingly no reason at all. The pcb in these are partially suspended by the pots themselves, which is stressful on the solder joints. My point here is the pots may have been fine, and were just losing contact with the traces.
     
  11. Exotic

    Exotic Active Member

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    ah ,i see what ur saying ,they went bad causing the problem ,and the caps being changed was ok because they were old .

    as for the biasing, 53 to 70 % seems like a big jump to me ,making me think the amp won't have much crunch and sound weaker now ,sure the tubes will last longer but it won't have as much gain ,do i have that right? and at 70 percent do the tubes still last a reasonable amount of time? and is the sound more distorted ?
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    If the amp howls at 70% there is still something wrong.
    You should be able to adjust it for 70% without incident.
    Colder bias 53% means that there is crossover distortion making the amp sound fizzy.
    The circuit traces solder might be broken cracked true and this is common where jacks and pots are soldered onto PC boards.

    But one thing that really comes to mind is that the primary of the output transformer may be reversed causing the oscillation.
    And you really should take this to a knowledgeable tech who can test it to make sure it's really working correctly.
     
  13. Exotic

    Exotic Active Member

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    i'm trying to find out .
    but what about the removing of the extra added resistor? do u agree with that or might it have been left in?
     
  14. Exotic

    Exotic Active Member

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    If the OT primaries were reversed, it would howl at any setting. Your amp OT primaries are not reversed.
    Regarding the output tube bias...I can set the bias current anywhere you want it. I like to set non master volume amps at 65-70% of max, and master volume amps that produce preamp distortion (like yours with channel volume and channel gain controls) at 50-55% of max. Because I think they sound better that way. But like I said, I will set it wherever you want it....but you should play it like this and see what you think.


    that's what i got from my latest communication ,i'm picking it up tomorrow God willing ! : ) so i hope it's ok now wish me luck .
     
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    If the OT primaries were reversed, it would howl at any setting.

    Not necessarily.
     
  16. Exotic

    Exotic Active Member

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    ok tommy chong rides again it's back and it's fixed ,the howling is gone and the reverb works fine ,it's a screamin monster now ,but with the 6 band eq it has a bit of hiss ,noise ,i don't know if it's same or more or less than before but i think that's just pedal noise . IMG_2037 (Phone).JPG IMG_2039 (Phone).JPG IMG_2040 (Phone).JPG IMG_2041 (Phone).JPG
     
  17. Exotic

    Exotic Active Member

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    if you download the bottom pic u might be able to read what it says if u zoom it a bit ,if not let me know and i'll type what it says .
     
  18. Gunner64

    Gunner64 Well-Known Member Premium Member

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    Cool. Great amp. The loops are noisy on these however, if you run anything in the loop it hisses quite a bit...at least mine did.
     
  19. Exotic

    Exotic Active Member

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    ok is that why there's noise gates? time for one of those huh?
    ok did anyone say the caps? i don't think anyone did .
    i know someone said 2 times the solders on the pcb board by the pot ,bingo for u !
    so the caps and solderings and a couple of resistors ,oh man i forgot a photo .
     
  20. Exotic

    Exotic Active Member

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    IMG_2043 (Phone).JPG IMG_2044 (Phone).JPG

    these are the parts he took out /replaced .
    notice the date on the caps ,justified?
    anyone recognize the resistors?
    anyone got any photos of the solderings on the pcb board by the pots?
     

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