Mesa Boogie Dual Rectifier Problems

If you are having trouble with any of the following boogie symptoms then I may be able to help you out as I have spent more than my fair share of time on these metallic monsters.

Mesa Boogie amps are reveered by players for their unique sound. But for us Amp techs they can be a pain.  Not only do they use unique to Mesa Boogie components but they also choose to hide  circuit board markings so that diagnosis can be tricky they are helpful with schematics but they schematics are not easy to connect with the PCB in the amp as there are no component references.

Special Boogie Symptoms I have come across include:

Intermittent output, or rather occasional fading output.  This is a common problem in amps usually caused by an unreliable connection somewhere in the signal chain.  Usually such a fault will be either permanent or intermittent and respond to thump on the cabinet.  With Boogie it is extra annoying as the intermittent nature is temperature / time related and of course it will let you down when you really need it to be reliable.

Bias LED flicker

Bias light flicker – check the little red or green light on the rear when the output fades.  Sometimes the output fading and the light flicker coincide together and sometimes not.

Channel Switching Problems

Channel switching strangeness including flicker,  dim lights, no lights etc.  Seems more noticable on footswitch but that actually it is usually the same with the rear switch.

 

Marshall DSL401 

These medium sized combos from Marshall should be very cool.   They are packed with features (like multiple voicings, low noise pre-amp, fab speaker) all in a small box.

Unfortunately they are not so cool because  they run so hot and they suffer from it.  Look at the picture left.  What you are looking at is a power valve that has had its label “fried” off it.

The cause  of this problem is that the amps were originally biassed to run a little too hot and also the ventillation is limited with heat rising up but with almost nowhere to escape to.

One of the main problems can be the bridge rectifer shown below.  This is black square on the right hand side of the picture.  These components supply the pre amp heaters with the super quiet DC.  Orignally Marshall fixed these directly to the boards and they get up to around 70deg celsius.  The problem with this is that the heat so close to the PCB is enough to start melting the solder joints.

Later Marshall realised this and raised them off the board by a few centimetres, the impact was tremendous and reduced the temperature by about 30%.  I go a step further and add a little heatsink to them too.

The symptoms of the overheating is faults like cutting out, channel switching problems and general weirdness.  Once modified to remove these issues you have one of the most well thought out practical gigging combos out there.

Audio Research (ART) VP1

After a good ten years use this valve pre amp started developing a little crackling and rustling noise on some of the inputs. The cause is oxidisation on the rotary selectors.  As you can see from the dismantled switch in the picture the contacts on the left are dull with oxidisation whereas the ones on the right are shiny and would provide a good contact. Unfortunately cleaning switches is not possible they must be replaced. 

Once repaired the owner was pleased to report that the preamp sounded better than ever. Why?   Well this is because of how important a good contact is in delivering the whole signal.  Degradation of contacts over time is difficult to notice as our ears get used to it.  

WEM Dominator 45 Bass

So great to hear a 15″ speaker again. This is driven by a 30 watt class A four valve EL84 power section in WEM’s seemingly dated combo. But oh boy does it pack a punch.  Rather than 30 watts you would be forgiven for thinking 100 to 150 watts. Such is the trickery that we have become accustomed to. 

It’s called a 45 Dominator but nobody seems to know why. Perhaps it was meant to put out  45 watts but that is going to be tricky with EL84.  Maybe it was aimed at middle aged home musicians in their mid 40s. 

Couple of things to point out about this amp that make it rather special. Features that would be welcomed on many of today’s amps. 
First of all is the amp chassis cushioning system. The brackets holding the amp chassis have for rubber supports that cushion and absorb all vibrations from the mighty 15 inch Italian speaker. I would so like to see this feature added on to some of the Princeton clone amps and other small Fender combos.

The other thing that I really like is the preamp screening cans. Most amps have aluminium screening cans to minimise noise in a preamp stage. The problems with these things spring loaded cans is that they tend to resonate and rattle. here  in WEM they have chosen to wrap the preamp tube with a spring loaded piece of  steel. This simple innovation is truly a sign of the innovation that WEM was capable of. 

Headstrong Li’l King

As is typical with boutique amps every conceivable part has an up market feel to it. Even the screws are stainless steel, you can’t even see some of them.  It’s a nice touch of luxury. 

Sounds wise this Princeton clone, like the original is super loud for a small amp. Both suffer from cabinet ratttle and power tubes do seem to suffer from vibration damage more than larger amps. 

But that said this amp oozes that appeal that many guitarists seem to long for and it is a delight to play and listen to. 

HH Amps

Just finished working on a very good condition HH VS Musician from 1977.   Because it had its original vinyl cover it is in fantastic shape.   Sounds wise this amp was suffering from noise, multiple time.  I guess as things get older the problems just get on top of each other; a bit like life for humans too.

So there are noisy pots, that scratch and make the sound of the wind as they are turned.  Connectors that cut in and out, usually when you look away.

Then you have those annoying connectors on the rear that we almost never use – the effects loop.  Your signal goes through those unused connectors and they suffer terribly from dirt and oxisdisation.  Many techs will clean them and this might ease the symptoms for a while but I prefer to replace.

Most annoying is that scratchy sound that just won’t go away no matter which knobs you turn, which buttons you press or which connectors you fiddle with.  Now often probing with a chop stick can identify some bad joint on the PCB or a connector, but often it is something else and something so subtle to find that there is no way to identify it without isolation and tools like an audio probe and oscilloscope; I favour  the former as the scope requires eyes but the audio probe relies on ears – much better if you are not a multi-tasker. The fault that I am talking about is the  humble op-amp.  The little 8 pin chips dominated the HH  range ofamps and many others and to this day they are many designers first choice.  There seem to be three types of failure that I have observed:

  • total – no signal despite a clear input
  • severe distortion – where the signal turns into a robot like version of  the input.  Usually quite easy to identify.
  • subtle scratchy noise – hard to spot.  The amp works perfectly but you get a little scratchy signal where you expect silence.  It’s really annoying.  To identify which op amp you need to isolate sections of  the pre-amp and completely separate stages.

On the HH amps you need to be careful because turning the volume of say channel one does not isolate channel one, just the input coming in to it.  So if channel one op amps are playing up then there signal goes straight into the power amp and there you have it coming out of your speaker too.

Once the amp is put back together it sounds great again although the valve-sound never got me too excited.

Dansette restoration

Well the  occasional bit of Hi Fi  comes my way and today it was an old dansette Bermuda from 1965. This one  is in fantastic condition considering that it had been in the loft for the past twenty two years. Just a little surface rust on the chrome trim.

Valve driven from high output crystal pick up and stylus these items are simplicity in a portable package.  Commonly found in car boot sales and junk shops most will require a new cartridge as the old ones are damaged by damp as the crystals stick together.

Once working the tone takes you back in time what a joy.

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