January, 2018 - Unbroken String

Marshall AVT50X Combo Amp Has Issues

January 29, 2018 by latchaw

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

Peavey 1810 Bass Enclosure Crossover Doesn’t (Crossover)

January 22, 2018 by latchaw

Most of what you see in this post works well. However, a crossover network inside the speaker cabinet doesn’t work at all. Could the Unbrokenstring Crew sort out a solution that would put this cabinet back to work?
Here is some file old hardware from the 1980s. This cabinet has an eighteen inch subwoofer underneath two ten inch loudspeakers.

Behind this panel is found a crossover network that routes the input signal to the proper destinations. The crossover automatically routes the really low frequency stuff to the 18 inch loudspeaker and the rest is routed to the ten inch loudspeakers. Or, you can specify which signal goes to which driver using the BI-AMP jacks.

The sheet metal screws hold this circuit board in place. An inductor plugs into the pins in the center of the picture.

The input jacks are seen in this view. The capacitor is part of the crossover network, and has been replaced. As this is in a high powered audio network, the electrolytic capacitor is a non-polarized variety.

We can examine the circuit side of the printed circuit board to figure out the schematic for this assembly.

This inductor is cooked. Unfortunately, this part is no longer available.

An examination of the inductor may give us some clues that we could possibly use to fix it and use it again.

This component can take the place of three different inductors, thus the three wires. Any two wires yield a different inductance. Unfortunately, the insulation is thoroughly cooked. No salvaging this guy.

The functions of each ‘net’ on this circuit board is labelled with a felt tip marker.

For this particular model of crossover, the inductor wires we need to use are indicated by this inductor symbol.

This is an inexpensive crossover kit, with similar specifications. We can harvest this inductor for use in the crossover.

Here are a few details, listed on the end of the box.

This inductor is has a laminated bar core.

We can remove this inductor from the circuit board and use it in the Peavey circuit.

The back side of the donor network is covered with some self-adhesive foam rubber.

The foam sticks really well! But now we have access to the solder joints that need to be unsoldered.

The mechanical mounting scheme is VERY robust for this heavy part. We can use all of this in the Peavey circuit.

These white plastic caps are handy to hold the mounting nut and to insulate the exposed iron core.

We will use the original circuit board as a template for finding and marking places to drill new mounting holes in the Peavey circuit board. We can search around for a practical mounting location.

The mounting holes are marked. Away We Go!

One end of the inductor is electrically wired here.

The other end of the inductor is wired here.

We will make lock the threads of the mounting screws with this stuff.

The thread locking compound is thin enough to seep into the threaded fasteners and lock them.

Now, we can reassemble the crossover network assembly.

The original inductor was mounted in the foreground. This doesn’t look to tacky, does it?

One last look before it disappears into the enclosure.

The panel is ready for reassembly into the enclosure.

No. Wait. We need a new gasket between the I/O panel and the cabinet enclosure. This self-adhesive foam strip material is just the ticket for this application. Note the mitered corners. Because I’m OCD like that.

Holes for the mounting screws are cleared with the Exacto knife.

OK, NOW is one last look at this assembly. The wire pairs go to the loudspeakers in the cabinet.

This system is pretty awesome. Everything tests out at full power. Life Is Good!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Mackie 808S Powered Mixer Blows A Fuse

January 15, 2018 by latchaw

This powered mixer lives two lives; During the week, it rides quietly in the back of the sound van; When the weekend comes, it works hard as the sound system on the main stage for outdoor festivals. After years of incredibly reliable service, the unit just stopped. And it already had a date on the main stage at The Lonestar Rally in Galveston for the following weekend. Could the Unbrokenstring Crew see what happened and bring this unit back to life?

This unit was cold and lifeless on the bench. But where is the main AC line fuse? We need to get inside.

Mackie is top of the line name brand, nearly ubiquitous is the sound reinforcement industry and in recording studios.

Name, rank, and serial number, please.

These screws hold the front panel in place. The lock washers are captive.

We note that some of the machine screws are a different length. This is noted in the notebook.

The screws in the back of the enclosure have large fender washers around them. We note this, too.

The front panel is free. Ah, but we are just getting started.

A large ground cable and a smaller white signal cable is seen in this view. These are taped to the side to get them out of the way.

At the other edge of the front panel is this relatively fragile flat cable. This requires special care as these are easy to damage and pinch.

Next, the power section of the unit is removed from the case.

This is what it takes to handle 600 watts per channel of audio power.

And here at last, at the very back corner, is the AC line fuse. This fuse has fatigued over the years and finally opened.

The actual fusing element inside most cartridge fuses is a soft metal strip, suspended at each end. They can fatigue and fail open, even under normal use. I believe that this is what happened here. The green tool is a real fuse installer. Because I’m OCD like that.

There appear to be no other problems with this unit. All the screws going back into the right places for testing.

So we tested the unit with a little Lennie Kravitz.

This unit has operated at full power for four hours, as do all of the finished repairs at The Unbrokenstring Shop. I am deaf.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey Artist Combo Amp Refurb

January 9, 2018January 8, 2018 by latchaw

This Peavey Artist combo amp was WAY too distorted to suit even the most extreme metal head. Could the Unbrokenstring Crew look into this and put this unit back into service?
First, a tour. You can have two channels, or a mix of the two ‘Automix’ inputs. This was a ‘thing’ back in the day.

The other controls are straight-forward.

The standby switch is in the front, whereas the AC power on is in the back. Actually, I like this because if both power switches are in the back, half the time I switch the wrong one.

The AC line duties are all squared away on this side. The City Of Los Angeles has their own version of UL. That’s the yellow and red sticker.

The right hand side of the rear chassis has the ins and outs for this amp.

These are all Peavey-branded tubes. They are all in good shape and will stay in this amp for now.

So we put a clean sine wave in, and this is what we get out. The positive power supply is weak.

An overall gut shot shows power on the left, preamp on the right, and power amp on the bottom.

These capacitors have begun to swell and push the seals outwards.

We have signs of overheating. These resistors handle power distribution and are somehow related to our problems.

The other power supply has a cooked resistor as well.

Here I am just documenting all the plugs and wires so I can get them back in the same place.

These capacitors are also bulging and will be replaced.

Time to remove the power supply board and work it over.

This circuit board holds the tube sockets. We have an intermittent short to ground under this assembly.

At first I thought that the short was under the tip terminals of these jacks, but that was not the case.

I am going to pull this assembly out and look it over as well. The blue, red, and brown wires are high voltage.

The blue capacitor in the upper right is the ‘death cap.’ If it shorts, 115vac is connected to the chassis. Not good if you ever touch the amplifier. Fatal if you touch the amplifier with one hand and grab a microphone with the other hand.

This circuit board is supported by the tube sockets. All four sockets will be unsoldered.

Out this guy comes.

Here is our short circuit. These are component leads from parts installed on top of the circuit board and soldered from the top. I guess if the excess length is out of sight, then it is out of mind.

Here are some of the parts on the top side. I don’t think these were replaced in the field, but rather it came from the factory with the untrimmed leads. Sloppy.

However, wires that are too long are easier to deal with than wires that are too short.

I am cleaning up the bits of crap in the bottom of the chassis, using some sticky tape as a way to capture the crap.

I have installed new bleeder resistors and new capacitors on this assembly. That big blue resistor is a high voltage dropping resistor. This part is fine and will not be replaced. However, those are not made anymore, and I have some of the last remaining stock of the OEM resistor. You’re welcome.

Everything gets trimmed and cleaned up before reassembly.

The power supply board has new caps everywhere.

The Ty-Wrap was my idea. These big parts need some mechanical support, but I’m not big on lots of hot glue.

The original power resistors were way out of spec, so these new parts are higher wattage to take the abuse.

These caps on the preamp board were replaced.

As was this guy.

These were the overheated resistors that we saw earlier.

These new resistors are actually more robust than the parts they replaced. And they are flame-proof.

We are back on the air!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Ibanez Compressor Pedal CP10 Repair

January 2, 2018 by latchaw

Mysteriously, this pedal just went mute while playing. Can the Unbrokenstring Crew get it going again?

I find this to be a unique design. But I cannot help but think that the assumption was made that everyone uses their right foot to run the pedal. Using your left foot is a little awkward.

Let’s make a quick tour of the unit. The pointers on these molded knobs are actually easy to see when the unit is on the floor. The LED indicates that the unit is in circuit and active.