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REPAIR: Marshall 50w Plexi 1967 - DOA

Discussion in 'Amp Input - Normal or Bright' started by vid1900, Nov 17, 2019.

  1. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    Guy: Buddy, I need an emergency amp repair, like today!!!
    V: Ahhhh, we don't really do "emergency work".
    G: I'm *****************, you have heard of me?
    V: Yeah of course [sighing]....what kind of amp?
    G: Marshall, old one. C'mon man, I'll give you $300 over and above your rate!
    V: 80s?
    G: No old, like 1960 man. Old, old, old. Completely dead.
    V: Did you check the fuses?
    G: Yeah man, they look fine.
    V: Alright, bring it....

    The moment I hung up, I had that bad feeling of accepting the work. But I kinda wanted to see what this king of the blues guy was touring around with.

    I knew it was going to be an 80s Marshall, but my morbid curiosity made me play along.
















    The joke was on me, because this sure was not an 80s Marshall, lol


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  2. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    Removing the back screen panel we find:

    1. Original iron - good sign that nobody ever over-fused the amp and killed the most expensive parts.

    2. General Electric 6550 Tubes - yikes, those are expensive. I'm praying those mothers are not dead.

    3. Newer Caps - Somebody has worked on this amp, because the originals are bare aluminum cans.

    4. Footswitch Jack - I've never seen an actual Tremolo Footswitch for a Marshall, they are always lost to the ages.

    5. Missing Speaker Jack - Not a good sign.

    6. This is a USA export model in the transition year. That means that you have to manually change the voltage jumper inside if you bring it back to the UK. Usually the chassis is already punched under the plastic, so if you are often touring overseas, you can add the proper Marshall selector.

    7. IEC Jack. These would have had a round metal plate with a captive, ungrounded power cord originally. This is a very good sign that a competent tech converted this amp to using a Safety Ground. There are only 2 IEC Jack brands that fit into the original, round hole; so the tech knew what he was doing.

    8. Tremolo Preamp Tube Socket - clearly this guy is not using the Tremolo circuit. Metal heads will often use that 12AX7 as yet one more gain stage (I'm not kidding). There is already so much friggin gain available in these amps....

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  3. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    The amp is indeed, stone cold dead.

    No pilot light, no output, no hum.

    Q: Is the cord bringing power to the chassis?

    A: Yes sir, it is. 122 VAC


    3.jpg

    Sometimes the problem is that simple. But not this time.
     
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  4. ChrisD

    ChrisD Senior Stratmaster

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    I love this stuff! Looking forward to seeing how this works out:thumb:
     
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  5. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    No matter what you have been told, you CAN'T look at a fuse and **see** if it's good.

    You MUST check it with a meter in Continuity mode.

    ^ read the above again, until it sticks in your head ^

    Although you once in a while see a blackened, blown fuse; a fuse can blow and look brand new.

    -

    I expected the 3amp Mains fuse to be blown, but nope, it's fine!


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  6. dante1963

    dante1963 Strat-O-Master

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    Keep it coming! This is fascinating!
     
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  7. CB91710

    CB91710 This is a Custom Title Gold Member Strat-Talk Supporter

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    Would have been nice to make $300+ just to change a fuse.
    Switch?
    I'm sure this isn't one we can blame on RoHS solder.

    No pilot is not a good sign unless it is just a bad connection. I hope the PT is good.
     
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  8. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    The HT Fuse is not our problem, but I check it because it is often overfused by goofy guitarists - voiding the protection.

    If it was blown, we'd suspect the 6550s.

    Fuse was good, and the correct value:

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  9. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    I'd still have to figure out why it blew.

    Maybe 1 in 50 times a fuse blows for no reason. The other 49 times the meter says it's drawing higher than normal amperage, and just waiting for a reason to blow again.
     
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  10. clydethecat

    clydethecat Strat-O-Master

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    What's the extra control on the front panel?
     
  11. CB91710

    CB91710 This is a Custom Title Gold Member Strat-Talk Supporter

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    Ya... Sometimes it's just age. The cap adhesive has failed and vibration has broken the connection at one end.
    I used to run into that a lot with traffic signal equipment when they still used glass fuses. High vibration environment, honestly not unlike being on stage, or transported from venue to venue. Amps probably have a harder life actually, but glass fuses worked just fine in cars for decades.
     
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  12. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    Some past tech took out the dangerous Polarity Switch and Deathcap.

    Then they put in a Master Volume in the hole.

    If there is one thing every Plexi needs, it's a Master Volume, lol

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  13. CB91710

    CB91710 This is a Custom Title Gold Member Strat-Talk Supporter

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    I'm just trying to figure out who would fit this description who would not have an amp tech on the tour with him that wouldn't be able to handle it.

    Certainly Bonamassa could just have a new one FedEx'd from his man cave :D
     
  14. dante1963

    dante1963 Strat-O-Master

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    Just as a wild guess...I’m wondering if it rhymes with “Malter Smout.”

    ‘Cause that guy pinned my ears back with an old Marshall not that long ago.

    :)
     
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  15. Tone Deaf

    Tone Deaf Senior Stratmaster

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    This is very interesting.
    Great subject.
     
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  16. clydethecat

    clydethecat Strat-O-Master

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    Isn't this a 1987T not a 1967? :):):)

    But seriously, I need closure.
     
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  17. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    Alright, were goin' in.

    The best part about any old Marshall is that you can fix it backstage or on the bus.

    You don't need any schematics, any amp tech can just look inside and know how it works.

    No circuit boards to burn up, no traces to melt, no relays to carbon up, no class D chips or gobs of epoxy hiding the circuit design.

    These things were built for service.

    -


    The first thing I do is just "take it all in".

    Smell. I stick my giant snoz in there and smell if anything (like a transformer) smells cooked.

    Is the wiring clean? Odd reroutings? Of course after 55 years, these amps have seen service, but does it look like a pro did the work?

    Any carbon on the tube sockets from arcing?

    Any strange resistors that look underrated for the job?

    Any 50 year old caps still in service?

    Has the Deathcap been removed and the Polarity Switch disabled? (very important).

    Is the amp properly grounded? (very important)

    -

    Lets see what we found in here:

    It looks like a time machine with mostly stock parts!


    1. Bass amp tone cap - the only difference between the Marshall Bass and Guitar amps is this cap. The Marshall factory was famous for just putting any cap in back in those days, and this one has been a **bass amp** for 55 years. lol. Pretty common mistake, and it probably gives this amp some serious balls.

    2. Tremolo Footswitch - only runs to the Tremolo board. No channel switching or fancyness here.

    3. Missing 2nd speaker jack - I'll add this back in before it leaves.

    4. Missing power resistor - Most techs add a 1K 5W power resistor on the two 6550 tubes. Those old GE tubes can live without them, but any crappy new tubes won't last long. I'll add them before it leaves the shop.

    5. Voltage Selector Punch-out - on these early USA export models, Marshall skipped the switch. If you tour overseas, you can add it back here in the punch out.

    6. Unused Tremolo tube socket

    7. Replacement Dual Cap - this thing needs a zip tie or hot glue to keep it's heavy weight from breaking the solder joints. I'll add zip tie before it leaves.

    8. Bias Pot - this has been added. Good mod, rather than swapping resistors every time you get new tubes. Work looks solid and clean.

    9. Tremolo Board - not in use here, but it's fun to see a little solid state component inside.


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  18. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    10. Fuse Holders - seem solid, and uncracked (common problem). Wiggle them around and see if any cracks open up, if so replace.

    11. IEC Power Cord Jack - It's super important that the HOT lead, goes to the 3amp Fuse Holder. I don't know why, but 75% of the time, someone has wired the Neutral lead to the fuse (very wrong!). No matter what the internets say, FUSE THE HOT for safety. This amp is wired correctly.

    12. Big Dual Caps - these are high voltage, and will zap you. Test that they are drained before servicing the amp!!!

    13. Power Ground - Of course this amp was un-grounded when it was new, but if you don't want to get electrocuted on stage like Ace Frehley, this connection must be clean and tight with a toothed washer. This one is tight, but I'll add a toothed washer that will never come loose.

    14. Voltage Selector - This wire decides what voltage winding on the transformer is chosen. It is correct for 120v mains.



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  19. clydethecat

    clydethecat Strat-O-Master

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    Perf board in a Marshall gets me all fizzy inside.
     
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  20. vid1900

    vid1900 Most Honored Senior Member Strat-Talk Supporter

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    Moving to the front panel:

    15. Master Volume - You can see it has been installed in the previous Polarity Switch hole. A shielded cable runs back to the circuit.

    16. Power Switch

    17. Standby Switch - Lets the amp heat up to full power, without the amp being on.

    18. Pilot Lamp - Tells you the amp is powered, regardless of the Standby Switch position.

    9.jpg
     
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