I had the fortune to repair one of these valve compressors recently. Two 12AU7 valves, optical coupling, built to the standard of a battlefield weapons system. I was expecting it to sound good. Some compressors just seem to be so subtle that you sometimes don’t even notice them. Not so with the squeezebox in extreme settings the squish gets so much that bass takes on a percussive tone.
If you need a repair of one of these, get in touch as I do mail order repairs on small units like this that can be sent in.
Mackie Studio Monitors HR624 MK1 – no power or wont stay on.
These great little monitors suffer from a regular power failure problem where they will not turn on. Not always, but quite often, this is actually a fault with the standby circuit which shuts the amp off as it thinks the input signal is too low.
The solution is remarkably simple and just involves the replacement of a few capacitors. If you think you might be affected by this issue get in touch as amp repair is less than you think. Individual amps cost 1.5 hours time and a pair can be done for 2.5 hours + postage.
Best thing is to remove the amp from the cabinet by undoing the 8 torx screws on the back and packing the amp up and sending it in to me.
This amp, unusual with its master volume control had become unreliable. Occasional red plating, bit of noise.
The cause is heat damaged circuit board. Just look at the pictures below showing the damaged sections – the dark areas indicating heat damage.
The problem is that heat damage eventually changes the circuit board material from a good insulator to a partial conductor. The burns do have to be severe mind you. You will usually see inner layers of fibreglass exposed, carbon particles. The only real cure is removal of the effected area. For small areas of damage the effected area can be drilled out but for larger areas a section may need to be cut out.
In this boogies case I took inspiration from a post on the boogie board where the owner replaced the section of pcb with hard wired chassis mounting sockets.
Add metal plate to chassis
Cut holes for B9A sockets and fit them
Remove section of PCB
Wire up new valve sockets for connections to output transformer, screen supply, grid drive, cathodes and heaters.
Wire up PCB and compensate for any circuit breaks following removal of damaged section.
Post 1990 Fender amps seem to have a variety of input jacks fitted. The great American amp manufacturer seemed to have tried lots of different styles of Jack but they all have in common flakey behaviour and robustness compared with old amps from 50s to the 80s.
In older amps you commonly see open jacks manufactured by the company switchcraft. This is the same company that makes the jack found on your Fender and Gibson guitars.
The parts cost a little more and that is because they are heavy duty nickel plated and designed for years of use in telephone switchboard.
A great improvement to any twin, supersonic, deluxe reverb etc is to upgrade the jacks.
As illustrated below the jacks are hard wired in and this prevents the inevitable strain and eventual pcb damage on the factory part.
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 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 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.
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.
Wow what a treat. This 1966 AC100 needs a service.
Well after all these years yes I agree it’s about time.
One owner from new, original cover and used twice weekly in the band. The sort of thing vintage amp enthusiasts dream about.
This rarity is one from the erase when pop music was really developing. The Beatles had stopped touring and concentrating on recording Revolver, Jimmy Page was coming out of the studio. Pop was progressing and diversifying.
100 watts doesnt sound like much by todays standards. But don’t think this is a slouch though. This amp drives speakers very deeply indeed and even at low volumes is absolutely room filling.
Few gut shots below and keep your eyes open for one of these in your attic.
Some of the early DT models hum intrusively. This can be reduced by lead dress improvements and altering the positions of the interstage and output transformer. The improvement is significant with 500 percent reduction in hummmmmmm.
If you would like me to modify your harp gear amp just let me know. Sending the amp in is a good option if you are not in the Bristol area.