Marshall Amp Repair Update

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Dickie Fredericks, Mar 16, 2006.

  1. Dickie Fredericks

    Dickie Fredericks Member

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    Guys, Just got the JCM 900 4102 combo back from the shop. Total charge of $56.15 to "fix the power supply". Heres what he did/parts:

    1 - T4 Fuse Hex
    4 - Diode 2.5a Hex

    Those are the total parts. They totalled $2.00. He said the T4 fuse was bad/dead and needed to be replaced. He said 2 diodes were bad/dead and they come in a set of 4 (not sure if he meant 4 to a pack or 4 in the amp) so he replaced all 4.

    FWIW, the amp works again and I didnt think the price was bad. What do you think? Anyone care to shed some info on these parts and what they do?

    Dickie aka A happy guy right now...
     
  2. Dave C

    Dave C Gold Supporting Member

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    Price seems pretty reasonable to me. Not sure which fuse is T4 or what hex means, but is probably the HT fuse (high tension....high voltage). The diodes are part of the power supply, your amp uses 4 of them for the rectifier circuit.
    Dave C
     
  3. Dickie Fredericks

    Dickie Fredericks Member

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    Good to know. Thanks for the reply Dave. Yeah I thought it was very reasonable and was expecting there to be a 1 before the 56 LOL.

    Hopefully Ive found a good tech who is close enough for my comfort level. When I lived in Orlando I was very close to Peter Wells who is a great tech. I always had a good relationship with him. Hopefully Ive found that here as well.

    Now Im wondering if running the thing with 2 Yellow Jackets and the MV up on 7-8 caused this. I sent the YJ's back for a replacement as a precaution as there was a noticable buldge in one of them.

    Dickie
     
  4. Dave C

    Dave C Gold Supporting Member

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    Sounds like you're running those yellow jackets at or maybe even a little above their max ratings . That bulge would concern me too . I'm not saying don't use them , they probably sound great, but if they're being run as hard as I suspect they are.......well I'd expect some failures along the way. Just my .02.
    Dave C
     
  5. John Phillips

    John Phillips Member

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    If the rectifier diodes shorted (which is most likely, diodes tend to fail that way rather than open-circuit) it could possibly have damaged the Yellowjackets - they would have been fed high-voltage AC for a few seconds until the fuse blew. Ask THD.

    The repair cost sounds reasonable too - the parts are cheap enough, but most of it is the labor to take the amp apart and back together - you have to get one of the boards out. I'd estimate up to an hour to do the job properly including finding the fault and a bench test afterwards, so around $50 plus parts is in the right range. I would have changed all four diodes too.
     
  6. Dickie Fredericks

    Dickie Fredericks Member

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    Thanks guys for the replies. John, I guess its your vast knowledge on these things that causes it but I wait at every thread for your reply. LOL

    You are the man no doubt !!!

    The YJ's are being replaced and they actually had that buldge when I got em. Its ever so slight. I wonder if the YJ's didnt cause it. Either way, I have a new set of YJ's coming. Im going to use 4 now I guess so I wont have to turn the amp up to 8 as I have to with 2 YJ's. Maybe the 3 db volume increase will help. Ill let yas know...

    Dickie
     
  7. Dickie Fredericks

    Dickie Fredericks Member

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    John/Guys
    Remember way back a month and a half ago when I was telling you about the amp doing this treble thing and losing power? Ive a few questions as since the amp is back from the shop this is happening again.

    Brief history of today. I got a new Strat today and plugged it in. Im getting that nasty trebly thing again. I decide to check something and I bypass the preamp section by plugging the guitar into the BD-2 and then into the return of the loop. Now I have that Marshall tube tone again. I am wondering now if I dont simply have one or more bad preamp tubes.

    Can you guys tell me which tubes are V1/V2/V3 and how each affects the tone or what each does?

    OK maybe this is crazy but... When I was a kid I had a Fender Super Reverb. One time it sounded not so good and a friend of my Dads (who I was in a band with at the time) told me to check the tube by tapping them with the amp on and I would hear the bad tube. So I did it back then and I cant remember if I changed the tube and it fixed it or not.

    Flash forward to today again. I decide to remove the covers off the pre amp tubes and tap them. I hear glass one the first one. I tap the second and I can hear it through the speaker (which is how I was told to see if they were bad). I tap the 3rd one and hear glass again.

    Is it possible that the 2nd tube is bad? I have another set of tubes that unfortunately I wont be able to get to till tomorrow.

    Is any of this possible? Your thoughts of course are appreciated.

    FWIW I have had the amp for 8 years and have changed the preamp tubes only once... (runs and hides)

    Dickie
     
  8. Dickie Fredericks

    Dickie Fredericks Member

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    Just for fun I switched the tubes already in the amp. Put V3 (closest to the power tubes? ) into the V2 socket. Same thing. Also you can kinda wiggle the tubes in the V1/V3 positions with no issues but moving V2 just a little causes a loud hum and no sound at all comes from the amp.

    Maybe a bad tube socket? Solder joint?

    Dickie
     
  9. jazzygeoff

    jazzygeoff Member

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    LOL! That's exactly what just went south on my Demeter exact same parts and price. Don't tell me your amp started to sound like a real deep old Fender Trem was switched on right?
     
  10. Dickie Fredericks

    Dickie Fredericks Member

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    No mine simply stopped working. Wouldnt even power on. I got it back and turned it on to see if it fired up and it did so I was happy. Now Im back to having distortion that sounds like a buzz saw instead of smooth. Its real trebly and sucks.

    I really gotta get this thing going. I have no other option. I guess techs just fix what you tell em to and dont look for any other problems. However, maybe the techs dont know what a dang Marshall is supposed to sound like?

    Im about to give up and buy a DSL head. geez. I love this amp. I cant believe its this difficult for me to figure out...
     
  11. Dave C

    Dave C Gold Supporting Member

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    Dickie , I just had this happen on a friends Marshall , it turned out to be a cold solder joint on the socket , you could wiggle the tube and it would make lots of noise and occasionally would make contact and act normal , but would eventually happen again. While looking at connections on the tube socket while wiggling the tube I saw one wire not moving with the wiggling of the socket. It can be deceptive....the joint "looked" good but in fact wasn't , and the problem was intermittant which are the toughest to find. I also know if I was your tech I'd want to know about it if you were still having a problem. He could have easily missed it if it wasn't acting up when he had it. I know it's a pain , but whatever it is CAN be fixed , I'd give your tech another shot at it before getting frustrated and buying another amp. Check the socket , clean it , and the pins on the tube , try another tube , check all solder joints ,reflow/resolder anything that looks questionable if you're handy with an iron . I'll bet it's a simple ,easily repaired problem. Finding it is the tough part sometimes. I wish you well.Be safe and drain the caps first of course.
    Dave C
     
  12. John Phillips

    John Phillips Member

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    What was the cause last time? I can't remember what you said. (FX loop jacks maybe?)

    I still think you may have an issue with something in the power section. It also sounds like you have a bad socket or joint on the V2 socket, yes.

    Some techs are like that, yes. Even with the best intentions, it's often difficult to fix faults which don't show up under testing - and there are some like that, although if the owner tells me it's intermittent I will generally go through the whole thing really thoroughly and poke every single component, bench test it at full volume, etc (which unfortunately costs time and money). Even then sometimes they don't show up until the next time the owner gigs it, which is highly frustrating...

    Don't. If you think you have reliability problems now, wait until you're a few years down the road (or if you're unlucky, sooner) with a DSL. Buy a spare 900 if you have to, they're cheap enough :). For all my criticism of the 900's build quality I think they're still better-engineered than the DSLs, and if you can find the fault and have it fixed properly it won't give you any more trouble.
     
  13. Dickie Fredericks

    Dickie Fredericks Member

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    Yeah John I could have sworn it was the FX loop jack as circumstances pointed it that way. I went ahead and put some deoxit on a 1/4" jack and pushed it in and out of the jacks to clean it but this time to no avail.

    There are other things to consider I suppose. The 5881's I have in it until I get the YJ's back and the pre tubes which have been in there a long time.

    After hearing the tapping of V2 through the speakers, I took the chassis back out of the cab and looked at the tube socket and such. Tpped on the tube with the chassis out and no noise through the speaker. So I put the chassis back in the cab.

    Its not doing that noise through the speakers when tapping and doesnt do that hum thing.

    I did a little research last night and found out that um Im not sure of his first name or what have you ... but the gentleman who makes the Clark Gainster (I think thats it) also owns Clark Amplification in Cayce SC which is only a bit further than Columbia. In fact Cayce seems to be just a burb of Columbia.

    Im simply gonna take it to him one day. Tell him my sob story and tell him to go through it with a fine toothed comb. It cant cost as much as a used 900 combo which are going for 650-800 on Ebay right now (I paid 750 for this one used in 98 when it was 4 years old).

    I may after getting this one fixed purchase a spare down the road and before that may buy one of the solid state heads and use it for temp when the 900 fails me.

    Dickie
     
  14. Rich M

    Rich M Member

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    FWIW, I have a 4100 head that had a bad (dirty) pot on the clean channel. Made the amp sound aweful. Hitting it with spray clean/lube didnt help. Pulling the can off it and spraying with resudue free cleaner did.
     
  15. Dickie Fredericks

    Dickie Fredericks Member

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    Pulling the can off? Huh? You mean pulling the knob off?

    Im gonn ahead out to get a new iron and some solder. Also some cleaner....

    BBIAB

    Dickie.
     
  16. Rich M

    Rich M Member

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    The can is the metal shield around the back of the pot. There is usually 2-4 metal tangs that hold it on. When I had the problem, I was able to deduce that it HAD to be a problem with the pot. I pulled it and sure enough the wiper was not making any contact with the carbon trace. Opening it up allowed me to wet the whole trace and run the wiper over it. Make sure the can is properly seated when you recrimp it.
     
  17. Dickie Fredericks

    Dickie Fredericks Member

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    OK One more clue to the puzzle for your consideration and analysis before it goes back to the tech. FWIW I used another can of deoxit on everything short of taking the "cans" off the pots. Still sucky. Took it to practice and had to switch over to a borrowed Fender Deluxe which just sounded great.

    Heres another clue to the puzzle. When I turn the amps treble and presence to 0 and leave the mid at halfway and the bass on 10 the amp sounds like it should although it doesnt have 100W. I know its low on power because the 40W Fender Deluxe was louder even with all the controls maxed out.

    Maybe the amp just sounds like crap normally and the treble is dropping out which makes me think its all good? LOL nah

    Maybe a cap or diode or something has a short and is causing the power drop and hence the treble buzz saw from hell?

    Just wanted to see what you think before I take it back in. I understand alot of folks find the 900 series quite bright but this is silly treble. Like no bass treble although turning the bass pot yields results like its working.

    Whatcha think?

    Dickie
     
  18. John Phillips

    John Phillips Member

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    I think that it has a drop-out of one side of the power section.

    (Still :).)

    That would give a thin, harsh, buzzy distortion (in fact it would be difficult or impossible to get a clean sound out of it) with about 1/4 of the normal power.

    The most likely (but not only) cause of this with a JCM900 is the high/low power switch. It could also be a fault in the output transformer, or something in the phase inverter circuit (tube or component). If anything like this fails you will get half the waveform only, which will give you exactly the results you're experiencing.

    I'll bet you a beer I'm right ;).
     
  19. Dickie Fredericks

    Dickie Fredericks Member

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    Im gonna copy the page John and take it to the tech. Tell me kind sir, just what type of beer would you like? Should you be correct I will send you a six. Heck Ill send it to ya anyway for all the time you've spent typing on my behalf. PM me the addy and its yours. :) Your help is always appreciated my friend.

    There must be a reason why folks post in this section of the gear page with your name in the title of the threads and I know what that reason is.

    Thanks again John and everyone else who threw their 2 cents in. Ill let ya know what I find.

    Dickie
     
  20. John Phillips

    John Phillips Member

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    It might be expensive to send a six-pack to Scotland :). Just hold onto it until we're ever at least on the same continent ;).

    Make sure I'm right first anyway, I might not be, even though I think it's by far the best explanation for what's going on. Those amps can have some strange problems, like Rich said.
     

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