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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    thanks ,but finding a 3rd tech around here sounds like a real drag ,plus the caps r really old aren't they? isn't it about due? what happens when u change caps? what r the symptoms that tell u it's time to change them ?does it ruin a vintage amp like a jcm 800? does it sound better ,worse, the same?

    since my amp is going under the knife ,is there some simple mod he can do to crunch it up a bit more? i'm going for a van halen 1 album type sound mixed with a zakk wylde sound on ozzy no more tears song ,is a mod a bad idea at this point due to the price or the fact that the amp is still acting bad ?

    i plan on running my amp with a cab with 2 jbl d120f's ,a reissue greenback and a evh speaker .
     
  2. scott-colorado2016

    scott-colorado2016 Active Member

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    Couple of things...
    While I don't completely disagree with @pedecamp I am a service technician and I do know that somethimes there is more than one fault. That being said, there are some "consumable" parts in a tube amp, included in these are 30 year old filter caps. If his fee of $100 to replace them is inline with the customary pricing for the job, do that cause it needs or will soon need to be done. He seems fair on what he has done, is doing and what you will be charged. Now, it would be nice if he had much better troubleshooting tools beyond just the guess work of "if this, then this" and a multimeter. The oscillation is coming from somewhere in the amp, without an oscilloscope, you will be hard pressed to find it.
     
  3. Exotic

    Exotic Active Member

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    should i ask him if he has one and is going to use one?
     
  4. ricksdisconnected

    ricksdisconnected Well-Known Member

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    end of story.
     
  5. Gunner64

    Gunner64 Well-Known Member Premium Member

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    Good advice on the scope.any tech worth his salt will have one..I will mention this once more. The pot to pcb solder joints. They break and do some strange stuff. Reflow them all
     
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  6. pedecamp

    pedecamp Well-Known Member

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    I guess you just answered your own question, go ahead and have this guy replace parts as an alternative to actually troubleshooting which he's demonstrated he doesnt know how to do. :shrug:

    And again, I'm no tech this is just my gut talking to me based on your information.
     
  7. Exotic

    Exotic Active Member

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    ok i will ask him to reflow .and see if he is gong to use a scope? thanks for all ur ideas and i imagine there won't be much new until the parts arrive and he gets to it ,bummer fingers crossed .
     
  8. scott-colorado2016

    scott-colorado2016 Active Member

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    @Exotic aren't you in or near the Los Angeles area? I would think you would be tripping over them, or if you threw a down feather, you'd knock 6 of them out! A great amp tech should be easy to find there!
     
  9. Exotic

    Exotic Active Member

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    well there might a few around but the trouble is i don't want to drive an hour on the freeway each way . both the techs i found around here are like 10-15 minutes away .
     
  10. South Park

    South Park Well-Known Member

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    If you live in the LA area I’m here in Ventura county. I can take a look at it
     
  11. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

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    I get that dude, but wasting $100 on tubes or caps here n there and not fixing your amp may be worth an hour drive. Or this just continues. IMO. The 15 min drive has done nothing to help the problem.
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    "If you live in the LA area I’m here in Ventura county. I can take a look at it..."

    Don't ever let this guy work on your amplifiers.
    He is probably one of the least qualified persons imaginable.
     
  13. Exotic

    Exotic Active Member

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    yeah it's pretty grim ,it's been a month and a half ,the tubes i needed, that was like 7 months ago ,but the potentiometers were my fault . and this caps recommendation seems ok to me since they are 30 years old .ima have to do it soon anyway ,best to avoid a bad timing incident or more damage ,so lets see what happens after he changes the caps ,anyone out there think it might be what the amp needs to make it work right again? really don't want to drive an hour and a half one way, 6 hours total for an amp repair ,it would give me nightmares .
     
  14. Sustainium

    Sustainium Well-Known Member

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    I’d package it up and mail it with insurance to the expert tech of my choice is driving was to far.
     
  15. Exotic

    Exotic Active Member

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    i don't have a expert of my choice and shipping an amp is an ordeal in itself ,my jcm 900 came inside a box surrounded by packing paper ,inside of a box the size of a vw bug ,and the reverb still got damaged . i don't even want to think about what it would cost both ways .
    i want to think positive and root this guy on that he will solve it ,and bring my legendary jcm 800 signed by tommy chong back to it's former glory ,now who's with me ! ? : ) IMG_1952 (Phone).JPG
     
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  16. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Randomly changing parts trying to fix an amplifier by accident is a horrible way to go.
    I will bet you $1 that this will not fix the problems.
    You are just throwing good money away after bad.
    "Changing the capacitors" is something that novices do when they have no clue how to troubleshoot a problem. Shot in the dark.
    (at least 90% of the time, it's not capacitors causing any problem)

    You should have brought this "tech" into the forum, and he should have followed directions from the real techs here in the forum.
    After you get done banging your head against the wall :wallbash: come back and we will show you how it's done.
     
    Last edited: Dec 12, 2019
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
  18. Exotic

    Exotic Active Member

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    not really banging my head against the wall(anymore) because my jcm 900 has solved that ,i'm getting great tone ,and my dog doesn't even have to leave the room like with my jcm 800 .
    do you think that leaving caps in that are 30 years old is a good idea? i'm a rookie all over again but i'm thinking why didn't the first tech inspect the caps. you're probably right about the caps not fixing the issue and u will most likely get ur opportunity to say i told u so ,but if it's as u say ,what is there to look for after the tubes and the caps ?
    and i can't drag the guy here by force ,i have asked him nicely to come and talk to u guys more than a couple times ,but he seems adamant at putting himself through the paces his own way . and it's not over yet ,he hasn't given up . this whole caps things is probably going to take a couple weeks?i wish he would talk to u that would be great .maybe after he gives up?
     
  19. NSV-Andy

    NSV-Andy New Member

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    This is an interesting problem. I have seen similar problems before and this reminds me of an experience I had with a pair of JCM800s, both appearing identical.

    First, an open filter cap in the B+ divider chain can be causing an oscillation in the preamp. It is not unreasonable to change all the filter caps due to their age, but scatter-shot component replacement - shotgun style - is to be avoided. In this case, it's a good thing to replace the caps, though. As a side note, JJ ECC83 tubes generally have less mutual conductance than others (such as Russian or Chinese versions) and have slightly less gain. This may make them more stable in a circuit on the edge of oscillation.

    I had a JCM 800 (forget which model variant it was) brought to me with awful hum and oscillation. I could see it was a long-standing problem as I could see evidence of at least 1 other person trying to repair it. He had replaced a wire going from the input jack to the grid of the first preamp with a shielded cable. He also changed a couple of resistor values in the preamp stage (it had one stage before the volume control).

    After thinking about it and looking at the grounding system of the amp, I came to the conclusion that the grounding system was poorly designed. Oh, it might have worked fine when the amplifier was new and there were no oxidized connections, but would be unstable once the thing got old.

    I completely rewired the ground connections to a single-point ground and eliminated all ground loops (they were aplenty) and the amplifier became stable and the hum problems disappeared.

    Now here's what convinced me that my troubleshooting was valid. While I was working on this amp, the customer brought me another amp of the same model. I had to replace broken output tubes which failed due to it being put in a very cold, windy outdoors after a show. I opened it up to take a look at the grounding system and lo and behold, it was completely different. This was a different version of the same amplifier, but completely reworked. It even had 5X20 mm fuses rather than the 3AG fuses in the other amplifier.

    I mention this for anyone who may work on this amplifier or another like it. Oscillation and excessive hum? It may have been an early run of the amp before the problems were worked out and changing the grounding system could be the answer.

    This was not the worst I have seen. The worst was a Splawn Nitro that was a design mess. It had multiple ground loops and a ridiculous placement of the filament rectifier setup. It was bordering on gross. I had never seen anything so poorly designed and haven't since. Took me a good long time to straighten that puppy out.
     
  20. Exotic

    Exotic Active Member

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    where do u live? j/k ,no really ,where ?
    i think i had found out using one of the numbers on the back that this jcm 800 2210 is from the year 1990 ,so in this case is that a good thing?
    i didn't notice any hum ,but from one day to the next, all of a sudden the reverb knob wasn't doing it's job ,it was trying to be a volume knob ,and the amp didn't sound right no matter how i would try to set the knobs . i'm pretty sure i should be able to put both the volume and the gain on 10 and control the loudness with the master volume ,and that's where the amp has that howl ,oscillation noise that should not be there .
    i agree after the caps i don't want him replacing every part on there hoping to find the solution ,if he runs out of ideas i will show him all this if he will listen .
    btw .what does open filter cap mean? like oozing ,gone bad ?
     
    Last edited: Dec 13, 2019

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