Marshall JCM900 Tune Up

This wonderful old Marshall JCM900 lives in a recording studio. It was due for a set of tubes and a million-mile checkup. Could the Unbrokenstring Crew refresh this head and resolve the tiny issues that had arisen over the years?

 

In simple terms, this head has two channels that share a common tone stack, effects loop, and reverb tank. The amount of reverb, as well as the gain and volume, are independently adjustable.

 

Name, rank, and serial number, please.

 

The effects loop is accessible from the back. This unit is recording-friendly, with outputs for ‘wet’ and ‘dry’ signals.

 

The Business End. This amp can be switched to 50 or 100 watt output power.

 

Two fuses are used in the high voltage plate supply for this amp, which is a nice touch and will add something to the story later. IEC mains power socket and a line fuse rounds out the rear panel.

 

These power tubes have pushed billions and billions of electrons around, and some of those electrons have interacted with the inert gas inside the glass envelope. Do you see the frowning face in the upper insulator? The brown scorch mark is his beard.

 

These great tubes have delivered a long service life and are now just about worn out.

 

Interestingly, Marshall delivered these heads with 5881 tubes, a military 6L6. Later 6L6GCs dissipate more power and take higher voltages. You can read Internet posts regarding the battles between Marshall in England and American importers; the latter changed the tubes on new amps to 6L6GCs because they believed the 5881s would not last through the warranty period.

 

And here we have the reverb tank.

 

A walk through the bottom of the unit shows us the output transformer. The red and black leads to to the reverb tank.

 

On the left is the preamp circuit board containing the input jack, tone controls, and signal switching. The tube sockets are discretely wired, and on the right is another circuit board handling the effects loop jacks.

 

More views of the preamp board on the left and the output jacks on the right. Tube sockets are in the middle.

 

At the lower right side of the output circuit board is the power supply power resistors, rectifiers, and fuses

 

The large blue items are the filter capacitors. These are in excellent condition and will not be replaced today.

 

The power transformer and power switches are mounted directly to the chassis.

 

This blue control sets the idling current (bias) for all four tubes. The current splits thru R28 and R29 to manage a pair of tubes each, part of the 50W/100W power control circuit.

 

The Unbrokenstring Crew are big fans of DeoxIt products. Here, we have sprayed a little D100 into the cap, and then soaked a pipe cleaner in the solution.

 

The pipe cleaner works well to clean and recondition each individual octal tube socket contact.

 

We will also wipe off the pins on the bottom of each tube.

 

So with the tubes installed and operating into an 8 ohm resistive load, we set the idle current for one pair of tubes. But the two sides don’t match.

 

Here, I’m using my good Fluke bench meter to confirm that one pair of tubes is idling at 50 milliamps, while the other pair is idling at about 41 milliamps or so. Both meters are in good agreement with the values measured, but I’ll stay with my good Fluke to investigate the situation.

 

Plate current causes heat to be dissipated in each tube. The V1 and V4 tubes are about 114 degrees C. while idling at about 41 milliamps.

 

The V2 and V3 pair are a little warmer. These tubes are idling at 50 milliamps. The temperature difference confirms the validity of the different idling currents… but why are they different? They share one transformer winding. We paid big money for matched tubes (which, when swapped around, make no difference…) More work!

 

Remember seeing separate fuses for plate current on the back of the amplifier? Checking voltage drops in the entire plate circuit, we see that this fuse drops about 0.2 volts across it more than the other fuse. Does that tiny voltage drop make any difference?

 

The fuse for the V1/V4 pair of tubes measures over half an ohm (meter zeroed for test lead resistance.)

 

This is the other fuse, for the V2/V3 pair plate circuit.

 

This fuse measures a tiny bit smaller resistance from end to end. Does this actually account for the higher current?

 

Sure enough, those voltage drops and differences in resistance accounts for about 10mA difference in plate current. New Fuses, Please!

 

While we’re at it, we will clean the fuse caps with DeoxIt, just as we did with the tube pins.

 

And the fuse holders will be similarly cleaned. (Hint – these pipe cleaners are perfect for cleaning other hardware besides your tobacco pipe.)

 

This line filter capacitor is scorched by a power resistor that was pushed up against it, perhaps a result of rough handling during shipping.

 

Components that are used on AC power require all sorts of safety certifications, which this part has.

 

I could probably leave this part in the amplifier, but film capacitors are cheap and if this were my amplifier, I would want it taken care of in a proper manner.

 

So here is the new line capacitor. The power resistor will be moved away from this guy when it is installed.

 

The filter capacitors in the bias circuit were also replaced, while troubleshooting the plate current imbalance.

 

Of course, replacing those parts requires access to the bottom of the circuit board.

 

While we have the circuit board up and out of the way, we can catch a glimpse of the discrete-wired tube sockets. This is a much better way to wire vacuum tube sockets, rather than solder them to a printed circuit board IMHO, because the tube sockets expand and contract much more than the circuit board material, whereas the discrete wire can just flex with the expansion and contraction.

 

This little bit of trimmed wire was stuck on the bottom of the circuit board. This will be no issue unless it comes loose, which it might do just as you are ready to go on stage and start the set.

 

Now this amp is running like a clock. The waveform represents the voltage across eight ohms driven with 110 watts, with a 440Hz sine wave injected into the input jack.

 

The chassis goes back into the case. I removed the power tubes for this step because I didn’t want to risk breaking anything in case I got stupid. The red and black cables to to the reverb tank.

 

Everything is checking out!

 

The sheet metal rear panel is much easier to align when the unit is face-down on the bench.

 

Zenith televisions were advertised with the slogan “The quality goes in before the name goes on!” After a four hour burn-in, the sticker is affixed on the output transformer side of the rear panel.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE

281-636-8626

Marshall AVT50X Combo Amp Has Issues

The customer was headed to the recording studio with his favorite amp.  However, our patient has two issues. When the unit operates, the fan is noisy and the output signal comes and goes when you tap around the headphone jack. When you don’t hear the fan anymore, the whole thing has quit. Can the Unbrokenstring Crew make this work AND make the fan quiet at the same time?

This hybrid combo amp has a lot going for it. Besides being Marshall loud, the unit is light-weight and has some moderately cool features such as a lead and rhythm channel, solid state reverb, a 12AX7 preamp tube for that tube sound, a headphone output, and a CD input so that the aspiring bedroom rock star can play along with their favorite real rock star.

 

An unexpected bit of excitement is, this is actually a Bletchley-built unit, factory-built for export to the United States.

 

Removing the chassis from the cabinet is straightforward.  Here is a view from behind the front panel.  The power switch is to the left.  The headphone jack and the CD input jack are in the center.

 

The front panel controls are those green vertically-mount potentiometers.  The input jack is on the right.

 

Looking inside the rear panel, we see the IEC power jack.  The power transformer and heat sink dominate the center of the chassis.

The rear panel jacks on the rear panel have their own circuit board.

 

We need to remove the main circuit board, as The Unbrokenstring Crew has quickly identified many intermittent solder joints that have caused this unit to quit.  We are going to be giving the soldering iron a workout today.

 

These pictures that follow document where the cables plug in.  All of the connectors are polarized.

 

The blue wires go to the final amplifier, which is a high power integrated circuit.  Power for the amp is carried by the white wires.

 

These cables carry the output signal to the rear jacks.

 

More cables.  The white wires are the AC mains cables.

 

These cables go to and from the power transformer.

 

The knobs and now pulled and the retaining nuts are removed from everything.

 

The main circuit board is free!

 

Now that all those knobs and jacks are out of the way, we can give the front panel a good cleaning.

 

The Gibson Guitar Pump Polish does a good job of removing grime and fingerprints.  And it smells nice!

 

The headphone jack has a wide circuit board footprint.  Here is the replacement jack compared against the output jacks.

 

The original shield from the bad headphone jack is moved onto the new part.  We’re changing the headphone jack because the preamp output signal goes here and either passes thru to the output amplifier, or goes to headphones.  When NOT using headphones, the signal to the output amplifier was intermittent, because of the dirty switched contacts on the original jack.  These cost less than a dollar so replacement is faster and better than cleaning the original.

 

The headphone jack is 100% now!  Because it’s brand new.

 

Some of the controls needed to be changed out.  The new parts arrived today!

 

Have you ever seen a four-terminal potentiometer?

 

These are all changed out.

 

Next, we need to look at the noisy fan.  This is the power amp / heatsink / fan assembly.

This is an inexpensive 12vdc fan, similar to the ones used in personal computers.

 

Perhaps we can oil this guy and shut him up.

 

Well, that didn’t work.  It’s worn out.  Off it comes.

 

We can salvage the electrical connector and install it on a new fan.

 

The new ‘silent fans’ came as a pair, so this heat sink is going to get two fans.

 

This fan is oriented to move some air across a power resistor on the main board.  Because I’m an Engineer, that’s why.

 

The other fan is oriented the same way as the original fan.  IMHO this makes a little more sense than the original setup.

 

This is the top view of the final setup.

 

When removing cables, a lot more than the cable came loose from the circuit board assembly.  See the hole?

 

This pin used to be soldered to the board through that hole.

 

These pins are tin plate over steel.  They have a larger thermal mass than the other components on the circuit board, and therefore MAY have not been at a high-enough temperature long enough to make a good solder joint.

 

These pins will be re-tinned with tin/lead solder.  The entire circuit board will be reworked to make it NON-lead-free.

 

This stuff is rosin-activated solder flux.  This enables good ‘wetting’ of the tin/lead solder joint.

 

This is a beautiful NON-lead-free solder plate over steel.  The whole amp gets this treatment.

 

Lots of those pins came loose when the cables were removed.

 

Those bad solder joints go a long way in explaining why this amp became intermittent.

 

This amp was manufactured while the world was converting to lead-free solder.  So, the process guys were still learning what worked and what didn’t work.  These didn’t work.

 

All of the solder joints in this amplifier were reworked by removing the tin solder and reflowing with tin/lead solder.  The Unbrokenstring offers this service for those who wish to have the MOST reliable gigging and recording equipment.

 

Lead-free solder is NOT reliable.  Exemptions from the lead-free directive (RoHS) have been issued to automotive, avionic, energy/down-hole, and medical electronics precisely because it has proven to be unreliable.

 

Here is our new headphone jack again.

 

We are ready to assemble the unit.  Now where did I put all those knobs?

 

Four hours of continuous testing proves that this combo amp is in top shape!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626