Jcm800 2210 C38 Cap Marked On Pcb But Missing Etc...

Discussion in 'The Workbench' started by John Boner, Aug 8, 2018.

  1. John Boner

    John Boner New Member

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    Hi all!

    I'm not a compelete newbie to amps&electronics, but my understanding is quite limited. Though now I'm studying to be an electrician (my third vocation...) and I've started to fiddle more with electronics in general, and since I have a degree in music&technology, a better understanding of these things probably helps me along the way.

    Anyway, to the subject at hand: I'm working on my '88 2210. The amp has a few issues which I've pretty much ignored for the 15+ years I've had it. The amp has had some work done to it, I followed the schemes today with the amp open. So, the reverb has been acting up every now and then. The amp sat around for quite a few years, I fired it up pretty recently and noticed that the issue has gotten way worse, the reverb sounds weak and when turned up over about 2 o'clock a pretty nasty brumm manifests, it gets a little quieter when dimed. Though I believe I found the reason for it, the pot measured little over 40k ohms on the worst spot and on full dropped some. It's a 100k pot and turns way too easy. I'll find out tomorrow. The 'verb not working properly hasn't bothered me since I really don't need it at all. With my previous heavy-rock band there was a few guitar parts where I would have used it at max but rarely even bothered to hook the footswitch.

    Then there's the channel bleed issue which to my knowledge should have been fixed in a model this late. The normal ch bleeds to dirty, most noticeably on bass frequencies. I've checked a lot of posts on the issue, there are quite a few,can anyone point to the most obvious fix? Though I assume it has something to do with the fact that c38 is marked on the PCB, but the capacitor itself has been removed or maybe not even installed. Though I didn't yet remove the PCB and take a look underneath since the amp is in my school and I didn't want to leave it laying around with all the knobs etc out. I've checked the scheme and can't find it printed anywhere. Still I searched around and found pics with the cap in it's place. Also I don't know what kind of a capacitor it is. In the pics it looked exactly the same as many others nearby, I'll go with the same type if somebody can confirm me on that. The location is near the normal ch section. Would this fix the bleed issue? If I remember correctly, one of the suggested fixes was to check c40 and maybe replace with a bit bigger one. Other was about the IC operating voltage, swapping the two feeding diodes with schottky's. Any ideas on this issue would be extremely welcome. Someone suggested they might have accidentally marked c38 on the board but on the scheme it would be c40. This issue hasn't bothered me much either, I use the dirt ch for what it's meant, usually with a slight volume boost with a pedal, and the normal channel cranked for AC/DC etc tones. I've got another amp for clean(er) tones, and almost never have had any reason to switch the channels on gigs, I just kick the booster off and roll the guitar volume down. Cleans up enough for me, the gain on the amp isn't all the way cranked but the volume quite high (with a dummy load, of course, the amp is pretty damn loud. As a Marshall should be).

    Then the last one, the amp sounds a bit muffled for a Marshall. I haven't had the chance to compare it with another 2210. I've taken care of that by using a Sonic Stomp on the fx loop with the "Process" dial turned up just a tad. The pedal apparently slightly delays bass/low mid frequencies which in a way reaches your ear a bit quicker due to longer wavelength. Or something like that. I'm not really looking for simply more treble/presence. The caps on the amp are most likely original, the filters are LCR brand. Yet still, the amp doesn't make a whole lot of noise, the normal channel keeps pretty quiet and the dirty (of course) hisses pretty much but nothing that I'd assume would have to with the filter caps. Now I really don't like to fix something that works, but the caps being as old as they are and when I take the PCB out and work on it anyway, would that "remove the blanket in front of the cab"? I like the idea of keeping it as original as possible but those electrolytes probably are about 30 years old...

    This post came out pretty damn long with a quite a few issues, so if you made it this far, congratulations are in order!

    All ideas welcome & thanks in advance!

    P.S. I'm new here and I'm from Finland, I hope my "little novel" doesn't contain too many grammatical errors...
     
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  2. Chris-in-LA

    Chris-in-LA Member

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    JCM800 amps are notoriously bright. I have to run my 1986 models with the midrange, treble and presence way down to compensate, around 3. Turning up the gain to 10 will also darken the sound. The midrange knob is what I use for treble. Even if my presence and treble are at 0, turning the mids up to 5 can make the amp bright and fizzy sounding. If your amp sounds like it has a blanket over it I would start there.

    If you can find a schematic that shows you what the voltages should be at certain points I would start checking inside to see if things are working as they should. If the voltages are way out I would also test as many of the resistors and caps as I could. Let us know what you find.
     
  3. John Boner

    John Boner New Member

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    The only schematic I've found is probably the same everyone else finds on the internet, I don't believe there is anything out there with voltages. Resistors are easy to check, but the caps... I do have access to a capacitance meter, but to get the correct readings I'd have to disconnect the caps from the board and measure then. Though I should be able to get the readings at least in the same ballpark. I just might replace all the caps, doesn't cost much but takes a lot of time. The common conception seems to be that 2210:s are fizzy&bright, mine isn't, the frequency response is actually quite good. Often when I've recorded 2203:s they tend to be a bit bass heavy. It's usually not a problem, sounds good on it's own but can mess a busy mix. (That's why we have EQ:s)
     
  4. Chris-in-LA

    Chris-in-LA Member

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    I’ve seen schematics for the 2204’s with voltages marked on them. Maybe there’s one for your amp, too. It would be good to at least check that part of your amp. Let’s see how f someone else has a link for you.

    This is the one I found, they might have other schematics as well.

    https://mhuss.com/MyJCM/JCM800_2204.pdf
     
  5. John Boner

    John Boner New Member

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    Thanks, will check that out. The power amp is almost the same at least on 2203&2210
     
  6. RickyLee

    RickyLee Well-Known Member VIP Member

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    Wasn't C38 only on the 2210 100W amps?

    I used to know where C38 went but can not remember as it has been too long since I have had my Split Channels out.

    You should be able to look at the PCB and see where C38 is going. See if it does relate to C40.

    Then I am remembering some of these amps having two caps hidden on the bottom/solder pad side of the PCB as well. And then some had only one cap on the bottom.
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    On the wrong side of the tracks.
    I looked at the schematic and found no C38 in this amp.

    You need to specify if the C38 is in the preamp or power amp...

    You can't measure pots in the circuit. You have to disconnect them to measure them.
    That's why 100K pot reads 40K.

    If there is an issue w/ the reverb, the first thing to check is the reverb tank, the RCA plugs can oxidize and you will lose the signal...the parts of the reverb tank go bad over time. You may need a new reverb tank.
     
  8. NewReligion

    NewReligion Well-Known Member VIP Member

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    Mad Amp nailed my first point, disconnect pots to get accurate measurements. The later models dedicated V2 to the clean I believe. Pull V2 see if it affects the Drive channel. It well may help you understand what the Drive channel should sound and act like.

    Next, blanket is likely because your large Blue Dual 50uf caps dried up on you due to non plying for an extended period of time. Replace them. In addition install a new quad of EL34’s and bias to 65%. Let the new caps set on play for 8-10 hours before judging as they will need to flow and settle.

    Preamp Tubes rarely go bad but if you have Rusdian pre’s toss them for Chinese. V1 & V 5 (PI) will have the greatest impact. You want the best balanced 12AX7 in the PI as only 1?out of 5 are close to equal as for out as a dual tritode tube.

    Disconnect the reverb until you get the tone sorted out. It could be a tube but most likely the tank.

    Once you have great tone then work on the simple stuff as you have too many target issues. Prioritize!

    I played post 1986 2210’s for nearly 25 years before building my own amps based off of Cameron (Mentor), Friedman, Bogner and arriving at my own Pandora design. The 2210 is my FAVORITE stack Marshall period.

    CLEAN all pots, Jacks & Sockets with non oil based cleaner. You may be amazed.

    Just my .$02 after 25 years experience playing them.

    I created this video 10 years ago just for guys like you who love their 2210’s. Dig...

    Good luck, David

     
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  9. NewReligion

    NewReligion Well-Known Member VIP Member

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    That is what a 2210 should sound like. John Norum & Michael Schenker etc...

    These are what I build now. Just plug straight into the front. All tube analog gain. Cleans up with guitar volume.

     
  10. John Boner

    John Boner New Member

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    Thanks! Just came back home from fiddling around with the amp, the reverb problem got even worse when I resoldered the old one back on (clipped the legs when taking it off). Definitely a ground/contact issue, the noise varied a lot when I wiggled the other wire going to the tank. I can't really test the amp with enough volume right now, but after playing a while with a dummy load between the amp&the cab I did get a more trebley sound than I remembered. I'll have to move to a practice space to confirm. I don't think there's anything wrong with the tubes/valves, they have been replaced a few times and always properly biased. First thing I did was trying different preamp tubes on almost every slot. It has sovtek's on the fx loop and reverb stages, tung-sol 12ax7:s on preamp and JJ EL34:s on post. The filter caps look fine and I don't notice any 100 cycle hum (50hz freq in Finland), the amp is really quiet on the normal channel.

    I did find the so-called C38, it's wired like the C40 on this preamp schematic:

    I added a 1uF cap in parallel with the .022uF as suggested on some forum, still bleeds. I'll have to try one more, or if any of you have other suggestions they'd be more than welcome.

    Tomorrow I'll resolder/replace the 'verb pot and check the tank. Can anyone tell me what kind of resistance or other readings I should be getting from a good tank? Or the correct voltage from the amp when maxed, or anything that'd help me diagnose?
     
  11. John Boner

    John Boner New Member

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    Couldn't post the link... but it's the one from DrTube dot kom
     
  12. John Boner

    John Boner New Member

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    Oh, if I didn't mention, I've had the amp for close to 20 years. I can't post a direct link but if you search on youtube "jackhammer demo 2007" (channel Flesh Nugget), there's a 2203 one side with an SD-1 on front and my 2210 with just a boost on the other, I'll let you guys/gals guess which is on the left and which on the right...
     
  13. Yugedrums

    Yugedrums Active Member

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    I'm with you on that! My first Marshall was a 2210 that I bought brand-new from a local music store for $600'ish, because it had a dent in the rear grill panel. I don't know what year it was exactly, but it couldn't have been any earlier than a 1989, because I'm pretty sure I bought it in 1990. It was pure magic and perfectly recreated all the sounds I heard and wanted at the time. It would do any metal you desired and would especially nail Metallica type tones all day long with tons of gain, no pedals required whatsoever... which is why I was so disappointed in later years with the low-gain of a JCM900 DR I had... even a 100watt SLX I had didn't seem to have the "beef" that the 2210 had. I foolishly sold it at some point. I was probably hard up for money and I still miss it to this day. I will have another one some day. I won't get it for $600 though! :noplease:
     
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  14. Yugedrums

    Yugedrums Active Member

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    A 2203 with an SD-1 is very, very close to the tone of a late-80's 2210, in my experience. :hbang:
     
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  15. John Boner

    John Boner New Member

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    Got the reverb issue solved, just a bad ground. I let the amp "idle" for quite a while and tested the bleed issue again having added the 1uF cap to C40, seems it somewhat alleviated the problem, now it happens only when the master is turned up enough. I'll add another 1uF and see what that does.

    Checked the plate voltage too, 475VDC. I'm a bit baffled about how to set/check the bias on this, can't seem to get a reading/access the correct halves from the OT transformer. I did bias my Peavey classic 30 without any problems using just a dmm and doing some calculations. This subject has probably been discussed&solved many many times but I can't seem to find a simple answer, except adding a 1 or 10ohm resistor between the cathodes and ground.

    Like I said before, I'd like to keep the amp as original as possible. Anyone have a link for clear instructions on how to set the bias without adding resistors or using a "bias tool"? Though I don't think I'll adjust it, the tubes have been good, sound (at least with a dummy load on almost full dB reduction, can't crank it enough at home) is good and the last time it was biased/retubed I told to the tech to bias it as hot as possible without going overboard.

    The input transformer makes a slight buzz, but I haven't heard an older marshall which wouldn't buzz more or less. I think I'll eventually replace the electrolytes, but right now I'm broke. Though I will test the amp and compare to other similar ones when I have the chance.
     
  16. NewReligion

    NewReligion Well-Known Member VIP Member

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    I assume you meant output Transformer?

    Not being an ass here but Caps are different than resistors when adding. They must be paralleled not is series. Complete opposite of transistors. Be careful messing with PT primaries...It is a good way to wake up smelling chicken, but its you. Seriously, be careful...♫
     
  17. John Boner

    John Boner New Member

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    Yes, I meant the output transformer. And of course I wired the caps in parallel. Also I'm very careful with electric stuff. Safety first, of course. Thanks for the comment, any input is very welcome. My current studies are mostly on basic electric installations on households, also on automation, 3-phase systems etc. It will expand to grid voltage works (here grid means the high voltage power supply to transformers on houses etc). This kind of amp stuff is just something I've always been interested in, I'm incorporating it in with my studies. Yet still, best to consider me as an amateur when it comes to amp tech.
     
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  18. RickyLee

    RickyLee Well-Known Member VIP Member

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    Measure across a pin 3 to pin 3 on each side/pairing of the power tubes. Then to find the OT readings that will add up to that, try from that each side pin 3 to the HT fuse. You should get your each OT leg side that adds up to the entire winding.

    Now did you mean your meter was acting up not giving you the OT halves/sides readings in ohms? That is something my Fluke III does on certain amps. It could be that there is some residual B+ in the filter caps that is messing up your meter reading. I have a cheap Craftsman meter that I use for this when the Fluke does this.
     
  19. John Boner

    John Boner New Member

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    Back again with the amp. Installed the 1 ohm resistors to check bias, measured the dc across. On each socket they read around 18mV, about half of what it should be by my understanding. Could there be something wrong with the power supply or is the amp just wired so that I'll just have to double the value? Then it would set in the right ballpark. Tried another set of EL34:s, no change.

    Also, can increasing the C40 cap affect the reverb? It started acting up again.

    The sound seemed ok, but I can't really turn it up loud enough to say for sure. The plate voltage was still around 475VDC and varied when turning the bias trimmer, as did the mV:s on the 1+8 sockets to ground.

    Also, the amp has only one fuse, the one next to the mains input. It's the "push the red button to reset"-kind.
     
  20. RickyLee

    RickyLee Well-Known Member VIP Member

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    So now that you have the 1 ohm bias resistors in place, you will need to adjust the bias trim pot to bring the bias idle up to where it should be. Or you have it turned all he way up and 18mV/18mA is the highest it will go?

    C40 should have no effect on the reverb circuit.

    I want to clarify, the bleed issue is: While on Boost channel, the Normal channel controls are bleeding into the Boost channel?
     
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