It Pays to Have a Little Fun—Fixing a Marshall Master Lead Combo 5010

Have you ever checked to see if an amp or pedal was working by sticking a cable in and tapping the other end? Or noodled on the guitar for a moment to make sure that whatever you just fixed works? My latest repair highlighted the importance of taking the time to ensure that the problem is indeed fixed. And as a mere dabbler in repair, this is something I need to remind myself from time to time.

I recently was given an Marshall Master Lead Combo 5010 to fix (Fig. 1 below). When I tested it, it had output, but it was intermittent and there were occasional dropouts and static pops. Turning the Preamp control caused a lot of crackle and garbling of the signal. There was crackle with all the other controls as well, but not as bad.

Fig. 1. Marshall Master Lead Combo 5010 Solid State

I removed the chassis and thoroughly cleaned all the pots. I plugged in a guitar and it sounded fine, even when I turned all of the controls. No static. So I played for a few moments more and thought I heard a pop. I stopped and waited, but didn't hear it again. I played again, and after five or ten seconds, heard another pop.

I did the chopstick test, poking all of the components (of which there are not many—see Fig. 2). The corner of the board near the two big caps was pretty loose, and when I poked it, I heard loud crackling. I poked at the caps and one of them was markedly looser than the other. I looked at the solder joints with an inspection mirror while I wobbled it, and sure enough, I could see the wire and solder moving freely back and forth as I did so (Fig. 3)

Fig. 2. The simple layout of the board was refreshing when troubleshooting.

Figure 3. A very loose and sloppy solder joint

Apparently, when I played, the vibration was causing that bad joint to wiggle enough to cause the pops.

When I removed the board, it was immediately apparent that previous repairs and other sundry misfortunes had been visited upon this poor amp (Fig. 4). It all seemed to work except for the joint in question, so I ignored the other stuff, resoldered the bad joint, plugged in the guitar and played for a bit. None of the problems were evident, so that did the trick.

Fig 4. Some of the copper traces had previously been scraped and exposed for reasons unknown.

After cleaning the pots, if I had just done the cable-only test or noodled on the guitar for a moment, I would have assumed that the problem was fixed. The bad solder joint might not have had the opportunity to cause a problem, and I would have called it good and returned it as supposedly working. Playing it normally for a little while saved me that embarrassment.

When I fix an amp, I always play it just for fun for five to ten minutes. Often, it's likely the only opportunity I will have to try a particular amp. It's usually an amp I haven't played before and it's almost certainly one I will never own, so it's a nice chance to try a new amp in the comfort of my own home. But more importantly, this indulgence also serves the purpose of making sure I'm not too hasty in calling it fixed.

Marshall JCM 900 Dual Reverb Repair (No Presence Control)

My latest repair was a Marshall JCM 900 100 watt combo, which sounded great except that the Presence control did not have any apparent effect. A bad pot didn't make any sense. Usually, if a pot goes bad, there is some sort of funkiness, like scratchiness, odd dropouts, etc.

A visual inspection revealed nothing particularly noteworthy. I did notice that the two reverb pots had the back removed, pictured here:

Fig. 1. The backs had been removed on both reverb pots. Note the board version
on the left. You might need that when tracking down the correct schematic. 

Both were double gang pots, so removing the back only disabled the half that wasn't connected to the board anyway. That is worth noting only because the presence control (immediately next to the left reverb pot) was also a double gang, and one row of pins were snipped off so that they looked like they were connected to the board but were not.

I removed the board from the chassis by removing the knobs, the plastic nuts on the input and footswitch jacks, and finally the nuts on the Preamp (Ch. B) control and the Ch. B Reverb controls using a 10mm socket (pictured below).

Fig. 2. You'll need a 10mm socket for the nuts on the Preamp (Ch. B) control and the Ch. B Reverb pots

With the board out, the problem was quickly apparent. Someone had already worked on the Presence control and installed a jumper incorrectly, so that all pins went to ground regardless of the position of the control. I think the jumper was installed because it looks like the solder pad had lifted.

Fig. 3. I did not do this.

I would like to add that I did not put all of those scratch marks there. This work looks like it was done in the back of an offroad vehicle. Anyway, I removed that jumper and installed a longer jumper to the correct spot (C28), after which the Presence control worked fine.

Fig. 4. The new jumper. 

And here it is. Lovely.