I don’t really work on HiFi, why you ask? It’s a fair question and the reason is that there are just so many different makes, manufacturers that it is difficult to gather any experience on them all. So there are a few makes that I like or have success with and I will accept them for repair projects.
I really love the sound of the old Marantz HiFi amps from the 1970s. Walnut veneer chip board surrounds, glowing lights what a joy. I don’t usually work on HiFi but here I make an exception.
These amps are made to the highest standards and feature great transformers and parts top to bottom.
A couple of issues I have come across and are worth sharing to help others.
Crackling no matter volume level
Seen on Marantz 4240 but applies across the range. In this scenario the amp works as normal but produces a fairly constant rustling / sizzling bacon style crackle. The fault may increase gradually folowing switch on or be permanent. Even with no input connected the fault is present and not effected by the volume control.
The cause is leaky tone recovery transistors on the PCB driving the amplifier blocks.
Everywhere on the internet people talk about replacing capacitors. It’s a bit over stated. Often their are visual signs like rupturing. leaks and yes these should be replaced in pairs but shotgunning the whole amp is wasteful of time and money and unlikely to be worth it.
So concentrate on where it is needed.
visibly worn decoupling caps
interstage electrolytic coupling caps
If you have one of these amps that you would like to sell either for restoration or parts donor I would be interested to hear from you.
Too many controls on a Boogie’s front panel for me to be a fan but people seem to love them.
This one below was cursed with a strange fault of bias instability which would lead to humming as the bias drifted off and ultimately to red plating after 30 mins or so.
The cause of the fault was some sort of PCB contamination. I initially thought that this was some sort of chemical residue on the PCB but after much cleaning it didn’t really improve.
In some conditions the amp would actually spark as voltage jumps between worn out circuit pcb paths as in the case of the bias supply or worse when adjacent parts of the pcb are a little too close. Then unrelated HT would jump to say a channel switching circuit. Take a look at the videos for examples of this.
The solution in this case was to remove affected parts from the PCB and mount them off the board following point to point construction methods. The challenge here is the space. Boogies are famous for cramming a lot in so it is a very cramped working environment.
Seen a few of these recently and worth sharing some problems and interesting solutions that I have found.
Fender no sound or fading sound
If your amp produces no output and the lights on the back are flashing about then you might have a faulty autobias unit.
The auto bias unit fitted to these amps, as well as some Fender Bassmans I believe dynamically control each valve’s grid bias thus ensuring that your amp’s valves stay perfectly adjusted no matter what the wear level or matching level is with the tubes. The board does this by measuring the bias current and comparing it with a reference signal and adjusting the grid bias accordingly in fact it does this thousands of time a second thanks to an innovative little chip. You can adjust the control a little as well between cold, medium and hot levels which is nice.
Unfortunately this innovation seems to be a little unreliable. The two issues I have observed are either: 1), unreliable cables connecting the auto bias unit on the amp’s rear panel to the main PCB, or 2) a faulty autobias unit which seems to either permanently fail or work for a moment at start on and then fail with flashing lights and no sound. Fortunately the board fails safe and pushes the power valves bias way into cut-off silencing the amp but protecting those precious tubes. So, what is the solution to this annoying issue?
Well you could just replace the auto-bias unit and hope that it works out ok. However the amps that I have seen with this fault are just a few years old and so I wonder if replacing the autobias unit could be a just a temporary short term solution.
What about another approach such as down grading the amp to a simple bias pot and accepting that you need to always use well matched valves. Well that is an option and just what one of clients recently asked for as he was fed up having an unreliable amp.
In theory this should be quite a simple mod but the complication lies in the fact that this amp has two operating levels: club and arena and this means two different bias levels rather like the Famous Fender The Twin (or evil twin) from the late 80s.
With my mod you preserve the club and arena feature by automatically switching between two bias levels as you move between the two modes.
The mod consists of removing the auto-bias PCB completely and replacing it with a tag strip of parts, a bias pot away from the hot power valves. The bias pot is located on the underside of the amp near the effects loop well out of the way of the power valves and does not add significant noise levels. The result is an old school amp and one that is much more reliable.
This seems like a downgrade but really it is copying the design of the Fender twin which has been successfully powering amps that belong to some of the world’s most successful musicians. So why not?
Fender Supersonic Crackling Noise
Another problem with these amps is excessive cracking noise. This sounds a bit like crackling of old plate resistors or more descriptively the same as sizzling bacon in a frying pan; just a little louder and really annoying. It certainly does not have any vintage amp charm. None at all. In the case of the most recent amp it was impacted by the reverb control which can be helpful to know when trying to fault find such an issue.
What I have observed is issues with internal layers in the PCB multi layer board. The layers have broken down and make am unreliable connection and that gives you a crackle through your speaker.
If you have been affected by these issues leave a note on this page or get in touch to book yourself in for repair.
Restoration project of a 1962 Vox AC15. Clearly seen some earlier work clue being the orange drop capacitors fitted around twenty years ago. Most of the carbon composition resistors were high asnd showing signs of surface cracks so all replaced. The Mullard valves appear to be original and in fine working order. What a great find.
All the way back from 1941 this is got to be one of the oldest amps that I have worked on. Pair of 6L6 output power valves red chassis, lots of rust – sounds good as gold.
Bit of history with this amp as it was stationed in Pearl Harbour back in the day and presumably saw some action. Later on it found its way over to the UK at some point where it has been waiting for restoration for some time. Virtually all of the old resistors were drifted so much in value that the amp would barely output anything. That restored and a few worn out valves replaced and the amp is back to full power. Not surprisingly it sounds great and distincively to a different era.
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.