The Amp Repair that Almost Broke Me - Marshall JCM800 Combo

Comments · 771

• TH

  @TheGuitologist · 9 years ago · pinned

  Hey guys...thanks for the overwhelming response to this one. I've gotten a lot of questions regarding what was the final diagnosis because apparently I wasn't clear enough in editing. The switching transistor TR1 was faulty. Replacing that allowed the Boost Channel to come alive. The phase inverter tube, V5, was causing some sort of issue as well, as was shown in the video, so it was replaced. It tested good on my tube tester prior to troubleshooting. I edit out tube testing from most videos because it gets laboriously long - 50 minutes even without showing tube testing is getting longer already than most attention spans. In fact, you've probably already stopped reading this. The V1 tube tested on the weak side of good on my tester, but I left it in because it should have still worked just ok, which it seemed to do because it is also part of the Boost Channel which was working before I replaced that tube. The V2 tube was the one I finally replaced at the end which allowed the Normal Channel to function properly and fully. The tube I was using in that position during the troubleshooting tested good on my tester, but for whatever reason, this amp didn't like it. The fact that it tested good on my old Precision tube tester is what led me to assume it wasn't the tube and explore other options. Also, if you'll notice on the schematic, the switching is tied into the grid of V2, which is how the Normal Channel gets muted during Boost Channel operation. My theory was that switching in the IC might have also been at fault. Anyway, hope that clears thing up for those of you who had questions. Watching the video back, I can see why it was confusing.

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• OF

  @officialmetalwill · 9 years ago

  I'd rather watch an hour long video of you tinkering with an amp than 99% of the garbage they have on TV these days. :)

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• SM

  @smacman68 · 9 years ago

  These videos are like a murder mystery! Who dunnit? Well, it wasn't Professor Capacitor in the main board with a leak...let's try Colonel Transformer in the mains section with a short...lol

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• FR

  @FransvandenBergeMuziekschuur · 9 years ago

  I love the way you explained the proces. My kid was watching you like a hawk. This will help many many youngsters. As a DAD I want to thank you.

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• RI

  @richardeaton6119 · 9 years ago

  The toddler babbling in the background is fantastic.

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• RE

  @rebeldedelsur8830 · 8 days ago

  9 years later........still awesome.

• KE

  @keithmccormick1272 · 9 years ago

  EYE TROUBLE I USE CAPS :) . I REPAIRED TVS FOR 30 YEARS AND AM RETIRED NOW . I ENJOY WATCHING YOU AND THOUGHT ABOUT KEEPING MY HAND IN REPAIR DOING EXACTLY WHAT YOUR DOING I.E. WORKING ON TUBE AMPS . I AM OLD ENOUGH THAT I WORKED ON TV'S WITH TUBES IN THEM FOR A FEW YEARS AND RETIRED IN 2005 . BUT WATCHING THIS VIDEO SAYS EXACTLY WHY I DONT LOL - I AM ENJOYING MY RETIREMENT . STILL LIKE TO WATCH YOU TROUBLE SHOOT . GOOD JOB. KEITH

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• SI

  @simonlinser8286 · 3 years ago

  It's not everyday you see a nice Marshall out in the shade, in nature. Refreshing.

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• VI

  @vinceparke5740 · 6 years ago

  I don't understand a lot of what you're doing but I have to watch the whole thing. The suspense, the mystery! Awesome vid.

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• BL

  @bluestudio67 · 9 years ago

  I owned a combo just like this from 1985 to 1987. Best amp I ever owned. A beast. Wish I had it back.

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• ER

  @EricBlackmonGuitar · 8 years ago

  I salute you man! You have an unbelievable degree of patience!

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• JJ

  @jjiacobucci · 8 years ago

  Kudos to you!  I recently had my amp go bad and brought it to an amp technician who made it well again.  Thank God for folks like you who help make things well when they go bad.  Great job!

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