Accutronics spring reverb

The Accutronics spring reverb has been around for a long time. They haven’t changed much, except that there are more options for the transducer specs. The idea of this little missive is to get to find out when they might be repairable. The answer to that is ‘Quite often’, and for no more money than a blob of solder. But let’s clear up what a ‘transducer’ is, for a start.

The yellow thing is a transducer.

A transducer can be many things, but basically, it’s any electronic wozzit that converts one form of energy into another. In this case, this wozzit converts movement into electrical voltage/current. Or the opposite way round, i.e. electrical current/voltage into movement.

This means that the speaker in your amp is a transducer, as is any form of microphone.

The Accutronics has two transducers, one at one end of the springs we can see in the top pic, and another (looks the same but isn’t) at the other. One is the input transducer, the amp puts out a voltage /current to this, which creates a magnetic field in a dooflicky called ‘the armature’ and this acts on two very small magnets connected to the springs. This effectively creates movement in the springline which is an analogue of the electrical impulses applied to the transducer coil. That’s basically what the transducer is; a coil. The coil at the other end (the output transducer) of the spring picks up the movement of the spring, converts it to electrical signals, and the amp’s circuitry amplifies these small signals and mixes it with the straight sound through the amp.

So the amp circuitry for the input side of the reverb is completely different to that of the output. Depending on how old (or expensive) the amp is, the drive to the input might be a chip (op-amp, and cheap), or a transistor (still cheap,) or a complementary pair of transistors (not so cheap) or a valve driving a transformer (definitely not cheap. Most early Fenders had this arrangement and the later ‘reissues etc.’ not.) It makes a big difference to the quality of the sound through the Accutronics device.

From the above we can see that, if you plug the unit into your amp the wrong way round, you are up for a disappointment, because it won’t work. The two transducers have completely different specs; the input being quite low impedance and the output very much higher. If you’re going to swap a unit in an amp, you should look on the Accutronics/Belton website, because there are a lot of variations of these things these days, whereas the early models were a spring line unit that worked, and nobody bothered too much about the numbers. Anyway, there is a code printed on these units , and you need to compare this to the various specs on their website. Not difficult, just irritating. Or maybe that’s me. Damn! I was going to do this blog without any niggly quips etc.

So what might you be able to repair in one of these?


The first thing to check with a multimeter set on a low ohms scale, is the readings on the phono connectors. The input should read a few tens of ohms and the output, a couple of hundred or so. If these read ok, you’re looking for a fault in the amp reverb circuitry. A check on this is easy. Turn the reverb up and put your finger on the plug of the output lead. It should buzz. If not, problems. Not easy to test whether you have a drive to the input. Grounding on the unit comes in a variety of forms. The old ones used to be grounded through the frame of the unit via the rivets that held the phono sockets in. Corrosion often messed the ground integrity up. A ground wire between the two phonos will usually sort that out. Have a good look at the wire from the phonos on the unit to the transducers. They often break because of the constant vibration of the spring unit. A break at the phono end is repairable (easy); a break at the transducer end is usually not.

Much of this can be sorted out with the unit out of the amp case.

A cup of tea. And a macaroon!


And Now…..KAM !!!! Another fully paid-up member of ARS

“So!” I hear you ejaculate (?); “What is this ‘ARS’ that you insist on my sensibilities?”

ARS is a fine example of the group of phrases known as ’acronyms’. It is also, by the by, a fair description of the companies who deal in ‘Anonymously Ridiculous Subterfuge’. These people’s productions are soooo….clever, that they are quite convinced that there is nobody else can repair them. This not all that surprising when you discover that the schematics are encoded in Sanskrit and buried under the Sphinx.

I have to admit that most current and recent gear blends into a sort of mess of colourless nonentity when I have to recall what they are. Or what they’re called. Or even what they do, sometimes. So I can’t really be expected to remember this KAM mixer because it also blended into the undergrowth of standardised, erm, undergrowth.

The last DJ-esque thing I looked at was a Neumark Dimension something or other power amp, and the thing I remember most about it is that it had been imported from planet Zarg; and the schematics, so far as I could work out after various expeditions up the Unpopo, were still residents of Zarg. I was bloody minded enough (on that occasion, and usually, anyway) to draw the thing out with my trusty pencil on the back of a fag packet, which enabled me to prevent the owner paying out a few hundred quid to Neumark. So it was worth the effort just for the opportunity to deal a hefty fiscal boot to the groin of another member of ARS.

Back to KAM. It’s a while ago now but the most memorable bit of it was that it looked like everything else. From my point of view that means ‘made of tin and sharp edges and most of B&Q’s stock of self tappers’. And it didn’t work. I often forget that bit because when I get something it nearly always doesn’t.

In this case the left hand output was down. After week or two’s trawling the internet, I had to admit that KAM was definitely a member of ARS. In that situation one is reduced either to:-

a) giving up; b) eating the workshop carpet; or c) having to think.

This one wasn’t that bad. If you look at the mixer from the front, on the right hand side are the two output faders. After removing a tinful of screws, the pcb is revealed, as are five SIL (single in line) chips. Four of them are situated in a square formation between the faders. The really interesting one is at the top of that pcb. This is the final amplifier before the output, and this was the one that had faulted. Get your favourite integrated circuit data book out and find the pinout arrangement for that chip. All you need then (and this test works for most op amp chips) is your multimeter, set on a volt range of say 0-25 volts or so. This is a dual chip so it has two outputs. At those two output pins you should see a zero volt reading, or at most a few millivolts. A faulty chip will often give out rail volts (that’s plus or minus 12 or 15 volts-ish). That was the case for this KAM mixer. And, once again, the customer did not spend a few hundred quid with KAM for a new mixer; but about £40 with me. I doubt, however, if he was going to heartily recommend KAM to anybody except for a good kicking.

All’s Well that Ends Well. Shakespeare said that. “And now for a cuppa and a macaroon.” I don’t think he said that.

A Travelogue….dedicated to Mark, the third reader of this blog

Yes; it came about after I had relieved Mark of £110 in aid of resurrecting his lovely Fender bass amp. It had blown a fair chunk of the power supplies, and fried three output valves. He admitted that he regularly read this blog. I had to believe he read it because there was no way he could hang his laptop on the nail in the bog for more useful pursuits.

So here’s one for you, Mark. Hope you and your lovely Fender are keeping well.

                       A Travelogue:-

                         How to go Somewhere Foreign.

           If you’re English, it doesn’t take you very long to catch on that there is something seriously wrong with you.

Your first problem after being born, which was finding out how to breathe, (in England, we have a Government Pamphlet for that. I know that because we have a Government Pamphlet for everything) you are then given another pamphlet that tells you who is Foreign. It’s quite a short pamphlet. It just says ‘Everybody’.

So ‘Going Somewhere Foreign’ is a really easy book to write, because if you live at ‘27, Bargery Street, Dipstick, Doobyshire,’ for instance, you know for a fact that ‘29, Bargery Street, Dipstick, Doobyshire,’ is Foreign. As are all the other numbers in Bargery Street and Everywhere Else.

So my Travelogue could be about, say; ‘32, Bargery Street’. I would then go on to discuss useful matters for the tourist of 32, Bargery Street.

The climate for instance; ‘Overbearing’ about covers it. ‘How to get around in 32, Bargery Street’; which of course contains bus timetables, when to hitch a lift on their dog, and avoiding the bedroom at certain times.

But on this occasion, just this once, you understand, I’m going for the adventurous approach. I’m going to do ‘the Solar System’.

Think big, just this once.

‘The Solar System is Very Big’. There we are, done that.

Onwards and upwards to…….A cup of tea and…..A macaroooooon!!!!!!


Fings Ain’t What They Used to be….An unreliable history lesson

This is a digression. It won’t help you to fix anything….except maybe make a start on the World….but then, that is too far gone to warrant the effort of throwing a spanner in it.

This little dissertation comes about from my various involvements with Carlsbro, both the real one of yesteryear and the imaginary one from China. Or wherever.

The photo is of the TC60 (actually the one after it but it looks about the same). This was Stuart Mercer’s first production (not unlike the AC30 from a distance but utterly different from every other point of view). That gentleman started the business and designed the first amps. The school group (‘band’ as it would be called now, but they were ‘groups’ then) that I accidentally started in 1959 bought a complete set of this gear from a stable block in Jenford Street, Kirkby-in-Ashfield, where he made them, in 1963. That was a couple of guitar combos, a bass top and cabinet, and a 60 watt PA system with a pair of 2×12 speaker cabinets. A total of 240 watts rms of valve power, and it was bloody LOUD. The whole lot was also £600+ and in 1963, for a group of school kids, that was a lot of dough. On the never-never (hire purchase) of course.

This was the Carlsbro bass stack. Amp missing but that looked a lot like all the others.

As with most gear of the time, schematics were not considered a black art and were easily available. In fact they were often supplied in the bottom of the cabs. Or in the case of Leak, Wharfedale and the like, they were actually stuck into the bottom chassis plate. Very sensible.

To find a current schematic for most things out of China, Indonesia, Korea, India, Botswanaland, Mars etc, you would need to get up an expeditionary force to the upper reaches of the Unpopo because that’s where they’ll turn out to be. (Under the box marked ‘Man eating crocodiles do not disturb’.)

Why is that? Too complicated? Too clever? Too self-interested? Narrrr…..none of that. They just don’t have ‘CLASS’. You can’t buy it, you can’t put a price on it, but you know when it’s there. Don’t ask me why, but a falling-to-bits AC30 or a Binson Echorec absolutely reeks CLASS. Whereas your state-of-the-art Mackie desk, or Blackstar whatsit just doesn’t.

I mean just look at this thing. TOOO COOOOL. And they sound….pheeewwww.




Back to the ‘history lesson’. It must be pretty obvious that I am terminally biased (?) against modernity. The price of being an old git. But I don’t care. Anybody who hates touch screens and apps and phones that take photographs and double as life rafts and do everything, but badly, can’t be all bad. You can argue with that if you like, but as I don’t care what you think, you would need to find somebody else to argue with.

Stuart Mercer started Carlsbro, as we said, in stables in Jenford Street. Presumably after the cows had been put out. He then did a lot of sweating, building prototypes. Much of this sort of grafting work is no longer done in modern design. In fact, a bloke with a degree in computer software would be better suited to designing a modern amp (even though he knew more about elephants than electronics) because this is where the ‘models’ come from. Software programs.

But Mr. Mercer had a problem. He was a qualified TV engineer, and TV engineers know little about the design of big valve power amps, but plenty about valve preamps. So he could design the tone circuits and such, but what to do about a power amp? This is where a stroke of brilliance, much representative of those days, struck him.

He bought in a Leak 50 watt rms power amp, and SCREWED IT IN THE BOTTOM OF THE CABINET!!!! From there he had to build a connecting lead from the preamp to the Leak amp and Walla !!!! A perfect solution. Doubly so, because keeping the power and preamp sections physically away from each other is a great piece of design strategy.

There might be more of this dismal rubbish; I’ve really enjoyed this tirade of insults to current thinking. I don’t really deserve a cup of tea and a macaroon, though. But what the hell….

Righty-ho clever dick….YOU design an amp……

There is the possible beginnings of a story to this one. So, as we are all ‘friends’ (in wonderful internet-speak that is. And bearing in mind you don’t know me from Adam. Or Eve for that matter) I let you in on an interesting prospect.

A gentleman I knew from a long way back called me out of the blue to ask if I would care to figure out why the amp project he had built does nothing but make nasty noises. This is regardless of what kind of nasty noises are being shoved into the front, you understand. The answer to that enquiry, although the problem I find very interesting, would usually be something like ‘GET THE HELL OUT OF HERE!!!!’ There are cans of worms and bucket loads of them, and sorting out somebody else’s design is definitely a bucket load. But Will is no idiot, a genuine bloke, and a good businessman. I don’t know about the first two but I definitely do not fall into the last category. The idea of somebody else doing business-thingies while leaving me to sort out technical issues might work out alright, thinks I.

The problem he has is with parasitic oscillation. Basically the amp is unstable (and unusable.) Not to give away too much, part of his idea was involving a class A output stage. A push-pull (class AB of some description) output stage is a naturally stable design. The reason that power amps become unstable is often that the power rail (this is a valve amp) carries some of the signal of the output back to the preamp stages and produces a positive feedback loop, not unlike acoustic feedback, which is also a positive loop. In a push -pull stage, one valve conducts through the primary winding of the centre-tapped output transformer, and the opposite valve conducts through the other half of the winding.

This means that the power rail that supplies the centre tap, has no signal content because the two halves of the transformer cancel each other out. It’s not quite like that, but close enough. Class A, for all that is trumpeted about its wonders has three serious failings. Firstly, It has to take a high dc current from the supply. It has to be biased so that the valve(s) will conduct the entire signal. Secondly, there is no cancelling effect from the dc supply rail. So the smoothing has to be very substantial, in order to kill off the inherent signal in the dc rail that is pulled by the output valves. Thirdly, the push-pull arrangement also cancels out smoothing ripple at high current drain. Again, class A relies entirely on major smoothing capacitors, iron-cored chokes etc to eliminate the 100hz ripple generated by high dc current drain.

So, although I’ve not seen the amp design yet, these are the things I’ll be looking for. Class A amp building is not an easy assignment. I’ll keep you posted on the hair loss and macaroon situation.



Marshall AS100D…and further hair loss.

This is a quick report on the weird and wonderful AS100D. You might remember that the lower of the two channels (channel 1; but don’t rely on that. In fact, don’t rely on any information you might come across on this amp. The schematics I sourced from Marshall’s database were as useful as a road map of Birmingham to somebody lost in the Gobi Desert.) was low gain.

The nearest chip to the input jack (not marked on the pcb, but referred to as IC100 on the Gobi Desert map) is a TL072, and had dry joints under the input terminals (2 and 3 ) which meant that (unlike the symptoms of dry joints on the rail pins (4 and 8) ) they didn’t affect the dc offset to the output pin (pin 1). Dc at either output pin (1 or 7) is an excellent clue to the chip’s integrity, but when everything looks ok, it’s big trap to fall into. There were also suspect joints to the push switches on that channel, so all those had a going over. After that it was on test for an hour or so, and was fine.

If you really must stick your head inside one of these things, it’s as well to remember that the AS100D from China has not much in common with the one from India. Or Korea. Or UK. Or Mars.

My next project (after a cup of tea and a macaroon) will be to superglue back, the hair I lost around the bench. I might just glue the carpet on there. It might make the next one easier.

Marshall AS100D….lunacy in progress

I don’t usually put up posts like this one, which I haven’t solved yet. It’s a bit like telling your grandma how suck eggs when you don’t know what one looks like.

“Maybe if I write something down” I thought “it might make a bit of sense?” Not so far, I’d have to admit.

I use Marshall imagebank a fair bit. Of all the companies (possibly excepting Fender and Mesa) Marshall are a million miles ahead of everybody when it comes to support. However…..

Out of the four different versions (so they say) of the AS100D I can’t find a schematic that fits it. They have been made in Korea, India, UK (?), and the one I have was built in China.

If you don’t have to take it to bits, I wouldn’t. I would prefer to take a Jaguar XJ220 gearbox to bits. Six screws on top. Easy. Two screws, one either side. Easy. Push the amp from the front and it slides out. For about two inches. It’s then that you find out about the speaker cables. There are four (two each speaker) and NO CONNECTOR. It’s hard wired into the speaker cabinet. The speaker cabinet is a totally enclosed design, so you can only get to the speaker connections by taking the speakers out. Take off the spade terminals, and then struggle to get the things through the straightjacket hole in the cabinet.

I can’t believe that an amp manufacturer of such a vast experience as Marshall has ever seen this amp. Straight from Tihibongwizzle, Taiwan to Joe Smith’s music shop Cleckheaton. Do not pass go do not collect £200. And don’t spend it in the first place if you’ve got any sense.

But we’ve not even started to fix anything. I thought I’d figured out how the schematics might fit by using this one from the Korean version, this one from the UK version, this one from the Martian version (for all I know). It seemed to work out from the outside (oh dear) but inside? The component markings were probably as relevant to a Hotpoint spin dryer as an AS100D.

The fault isn’t a desperately serious one, but having a channel low output without any control of the bass on it, is a niggle and not a great advert for Marshall either. Finding something like that fault with misleading schematics is bad news.

As a parting shot, I can tell you that in the power amp sections (there are two 50watt amps, each driving a separate speaker) are very similar in design (but not physically) to the AVT series of amps, in that they use TDA power chips. If you have a problem with blowing fuses, nasty smells, or nothing happening at all, these chips are a good place to start. They quite frequently blow holes out of themselves. That’s a dead giveaway if you’re fault finding. The AVT series have the chip(s) built onto a separate little pcb that just plugs in. Not so the AS100, but the idea is the same.

Where is the KETTLE!!!!!??????

And now, an ‘Easy One’…a Fender Super Champ

Some of the most monstrous faults can turn out to need a blob of solder or a component worth 20p to fix it. Easy !!!!!

Except it took you up to retirement in the next life to figure it out.

I’ve got one of those in the form of an led stage light. The fault has to be on a piece of pcb small enough to lose on my bench. Come to think of it, an elephant is small enough to lose on my bench… Lets not go there. But I’m totally lost with it. It defies all reasonable laws of logic, and I’ve got to stage where I’m dancing round it with a woojie stick. That didn’t work either. It’s just about at this point I get my ‘Bloody-minded’ hat on, and I’ll be prepared to run myself into bankruptcy if that’s what it takes to get bastard to work. I digress. But not much.

Here’s a nice scenic pic or two of a little Fender Super Champ. Calming.












On this restful vista, is a couple of brown cables terminated onto spade terminals. Supposedly. This was one of those simple jobs that required a bit of hunting about and a blob of solder. On the channel selector switch. It had an intermittent fault that made the little channel led inoperative. As a side issue, it also caused the amp to make no sound at all other than a fizzling noise in the background. Not a great set list for a successful show, you wouldn’t think.

The blob of solder to the channel switch worked a treat. It was on test for a half hour or so, and everything hunky dory. Then Mike paid out his hard earned cash, took the little Fender for a therapeutic ride up the road…..and it didn’t work. Well, all right, he got about five seconds of glorious sound.

So, now what? I fix it again, (for nowt, as a warranty) tug my forelock apologetically and start again. The fault actually turned out to be on the same track as the previous fix, so that hurt.

In this photo you see the two crimp terminals shown earlier. They supply a low ac voltage to a row of four black diodes (1N 4002 or something like it). These are arranged in a bridge rectifier configuration and supply a low dc voltage to an LM7805 regulator. This supplies the 5 volt dc to the digital stuff and also other things like channel switching. Which means if you haven’t got your 5volts, you ain’t got nothing.

In this case, the fault was a badly pressure crimped terminal onto the connectors of the brown wires. Not easy to find, and this fault could occur in the diode rectifier, the channel switching, or the regulator.

It’s a happy bunny now. And I richly deserve a monster cup of tea and a lot of macaroons.







Mackie 808M….it distorts.


This one I’m going to do in more than one part. You’ve got admit that was, for this blog site anyway, sensible.

In the Houses of Parliament, you will find the repository of all wisdom, sincerity, truth seeking, beneficence and self-effacing personal sacrifice. Well, you can’t be sensible all the time, eh?

The Mackie 808M is quite a lethal package for a small box. At 600 watts rms per side, that’s a lot of watts; and it’s not fan cooled. But it has a nice chunky, cast alloy heatsink on the back, that you could probably chuck a few burgers at and double as a Barbie when used in anger. There’s one born every minute.

The reason for this serialised blog, is twofold. First, if I keep going too long, it’s only goldfish that will be with me at the end. Mostly because they think it’s still the beginning. Secondly, it broaches the subject of switched rail supplies, and that could do with a piece of its own. Any conventional class AB amplifier (which is most of them) rated at over, say 300 watts rms, is almost certainly going to have switched rail supplies, and that means that, it has an extra pair of voltage rails (often at twice the voltage) in addition to the usual + and – rails.

So what? Well, from a practical pint of view, a fault in the power amp can look very serious, and in fact it’s the switching that has gone down. It might make the difference between a parts bill of say £40+ to one of say £4-ish. Not to mention a nasty shock with the labour costs. We’ll have a look at the Mackie, then. (You don’t say! I thought you were going to waffle on about goldfish into next week.)

This is the front of the Mackie 808M…..(Duh!). Unless you’ve had a serious bad time with whoever sold to you, there will be something behind it.



The ‘something behind it’ looks like this.






Unless you’re a victim of one of those dubious types that infest ebay, in which case it will look like this…..


You won’t get any problems with distortion with this latest piece of electronics from ‘Wonderfully Clever Inc.’



Let’s assume for a minute that the inside of yours isn’t a house brick. Referring to the pic above that, there are two components marked ‘High rail switching mosfet’ and ‘low rail switching mosfet’.

The basic idea behind rail switching is that the mosfets (in this case an IRFP150N and IRFP9140N) switch the higher rail voltages onto the power transistors MJL21194. So what? So that the higher rail voltages give a much higher peak to peak output level before clipping. So a big increase in power output capability.

The fault in this amp was that the output distorted, it also had a dc offset; not a lot ( about a volt or so) but enough to know that all was not well in Offset Voltage Land.

It turned out that the low side mosfet switcher had shorted. So the voltage to the high side was about +45 volts and the low side rail was -90volts. Quite enough to upset the output dc offset and produce a lot of a-symmetrical distortion. This fault was cured by replacing a fiver’s worth of mosfet. Un….fortunately, there were other problems, but at least I wasn’t tempted to take the output stage to bits.

I’ll do a bit more in detail about rail switching in another blog soon; but I have to take my goldfish for walk.

And here’s a little aside, just while you put your bedsocks on. I had a repair shop for few years in a little town on the North Wales coast. I had to close it after a visit from somebody called ‘Bailiff’ and somebody else called ‘Taxman’. They both wanted the same thing. Called ‘Money’. I digress. Just across the road from my estimable establishment was an emporium called ‘Aladdin’s Cave’ run by Tony. He came across on this particular day. He’d just bought in a TV set (for sixty quid if I remember right), which he had, unwisely, not tested before parting with his money. We sat in front of this machine on his counter, twiddling every knob and switch we could see, and nothing happened.

I took the top off and inside, completely alone and taped to the bottom with gaffa tape, was a house brick. That was an expensive brick.

And now to put the kettle on.




Studiomaster 1200D stereo power amplifier


I noticed that these blogs could be loosely categorised. There are those that are specific and useful to a very small minority. One, actually, and that’s me. There are those that are nonspecific and useless. They appeal to nobody at all…well, alright I like them.

There are those that are, really, quite loony. As most folks seem to be serious, these don’t have any general level of appeal except for people who wear tartan jackets, very long shoes and a red nose. Suffice it to say that’s what I turn up to work in.

This is a high elevation, high definition, uni-chromic, digitally encumbered, perspective enhanced, indigenously florid…. well, it’s actually a picture of a 1200D amp with the lid off. But the marketing department got there first.


The blue pcb’s at the top are the power amp modules. As the whole of this amp was designed and largely hand-built in the UK, so were these.

The module consists of two pcb’s and their heat sinks which screw together with insulating plates. The top one is the rail switching pcb and the bottom one, the power amp. You can see one of the power transistors MJ15022/ MJ15023 with its T03 case screwed to the heat sink.

This is a shot of the two halves of the pcb separated. All the connections are by spade terminals and there are three interconnecting cables that link the top to the bottom pcb.

All the cables connecting the power supply pcb to the module are colour coded and also marked as to their function and voltage.

This is design and manufacturing with repair and servicing at the forefront. The schematics were (and still are) easily sourced and most of the components are still readily available. We could really build an amp, when we wanted to. There are many of these twenty-ish year old amps still doing the business, and if they do fault, you would expect to be able to repair them.

But this is not a do-it -yourself Studiomaster fixit blog. Sorry about that. Most faults that do occur in these, involve rebuilding the power or switching sections and, if it’s really not your day, both of them. But the modules are separate, so work on channel A won’t affect channel B, so that’s a plus.

It is nice to think that something designed and built within a hundred miles of here twenty years ago can still kick the arse of last week’s offering on all counts, and not break into a sweat.

I’m going to celebrate that with a nice cup of tea… and even…a macaroon!!!!