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!!!!




Musical Fidelity Nu-Vista preamplifier

These things are/were a lot of money. Not in the astronomical terms of the Kuiper Belt audiophiles who have light frequency eardrums, but in normal-ish terms of something that might cost the equivalent of a newish (but still second-hand) car.

This little missive is a toe in the sulphuric acid towards getting going again on the blog. I’ve noticed that the whole of the internet becomes bereft when this little blog disappears. EVERYTHING is SOooooo Serious……..AND…….IMPORTANT!!!!!!!! Unless it’s pseudo light-speed-hearted. Which is even more serious because it’s usually written by folks who think that ‘U’ is the way you spell ‘you’. God knows what happens when they get to a hard one like, say Wednesday. That must be ‘Wdndy’. But that could be short for ‘fart’. I suppose.

Anyway, this Nu-Vista valve preamplifier had me rushing to my valve detecting machine cupboard (I’ll spell that cpbd) because there wasn’t a valve of any description in it.  What we did have was a reservoir capacitor (1000uf @ 100volt if memory serves) that had pretended to be a Mills grenade, and there was quite a lot of it plastered around the inside. This capacitor had shorted and taken out a dropper resistor and had also spiked a voltage regulator further along.

The whole job was made conveniently intractable (that’s cvnty itcbl) by the internet (or at least Musical Fidelity) being as devoid of schematics as goldfish in my sulphuric acid footbath. At least George Orwell had ‘Newspeak’ which told you nothing at all but sounded important. What we seem to have now is ‘No-speak’ which tells you nothing at all because it doesn’t exist.

But my kettle does. Exist. I hope. (flrnsclh) (No it doesn’t mean anything I made it up.)


QSC CX 404….Don’t try this at home

I had a worrying thought. It might have been yesterday. Having not very many of them (thoughts, that is) I ought to be able to remember one. So that was a bit worrying as well. It was regarding the prime minister. I realised that I didn’t know who it was/is; then I was even more worried by the fact that I didn’t care who it was/is.

He has paid me a hundred quid towards my heating bill, and I thought it would be nice to send him/her a note. Obviously to 10, Downing Street.  But you can’t address it to ‘Somebody, 10, Downing Street. The cat might get it. Anyway, lead onwards and outwards, Macbeth!

Ah….yes! The CX 404. The amp with more things to go wrong in it than a politician. It has a complex switch mode power supply, with a load of things to go wrong in that. It has four separate amps in it to go wrong. It has a bagful of protection circuits to protect it from going wrong, all of which can go wrong. The only way you should take on a job like this is with a bazooka levelled at your head (or somewhere) and Mills grenade taped to both hands. That’s the most useful information I might come up with. Suffice it to say, I recently repaired one of these. This I put down to stupidity of a high order.

The power supplies had blown. These are IGBT (insulated gate bipolar transistors) that do the power switching, and they’re not cheap. There are two chips that generate the switching pulses and provide the drive to the IGBT’s. If the IGBTs are cleaned out it’s pretty likely that so are these two chips. But if there is a fault in any of the power amps, the amp will cycle on\off etc. and the power supplies won’t operate even if they’re ok, because of the dc sense circuits. Or, of course, it could be because the sensing circuits are sensing something that isn’t there at all.

The power transistors you need to take out to check if they are the ones in the schematics. The one I had, had two pairs of completely different ones in it, to the ones in the schematic. The drivers were the same MJE15032 and 15031 I think.

There is another issue with these. The heatsinks clamp down onto the top of the power transistors (there are four separate ones, one for each amp. That’s fine when they’re first built, but I discovered that different manufacturer’s devices can vary a bit in the physical thickness (back to politics) which means that your heat sink might not make such a good contact with the device. Not good news.

I got fair number of useful lessons out of this amp. The main one certainly to be very wary in taking them on.

Being deserving of at least several cups of tea and a pallet load of macaroons, I shall attend to those matters.


Peavey VK 100……no, seriously.

The VK 100 head is the same amp as the VK 212 combo. I’ve had two of these in the last couple of weeks; the first was fairly mundane….but the second! Wo-ho! There have been a few reports on various forums etc. about anode resistors blowing in these things; the reason for this is interesting. But, to start at beginning…..

Anode resistors do blow. Older amps are more susceptible than newer ones. No they’re not, necessarily. Resistors tend to go high as they age, and that can make them heat up more and that can make them go higher, and that can make them……you get the idea. The anode resistors in an old amp are often half watt rated, sometimes more. On a new-ish amp they are often quarter watt and sometimes less.

There is a term in engineering design known as ‘over-engineering’. This means that the design in some way uses higher rated components than necessary. This makes for better reliability and (sometimes) performance. Peavey’s designs and constructions when built in the States, were plentifully over-engineered. Which is presumably why there are still plenty around from way back. Same could be said of Fender, Ampeg, Matchless, Gibson and a load of others. Open a new-ish offering of those things, and it becomes pretty obvious that it’s the profit margins that are over engineered (in China?) and they cut their fingers on everything else (personal opinion). This is where we are with the Valve King 100.

The two red marked sections are heater diagrams, and they reveal something very  peculiar about this amp.

V4, V5, V6 and V7 are the heaters of the output valves. They are in series, so that’s a bit strange; the heater supply is dc; so that’s also a bit strange; the three preamp valve heaters are in parallel with each other, and the whole lot are in series with the output valve heaters. So what?

In the first place, there is about 32 volts to the heaters. Each valve has a 6.3 volt heater and the voltage drops through the chain of heaters so that each heater gets it’s 6.3 volts. So what’s wrong with that? Nothing, until a fault develops. Here we need to remember that a hot heater has a radically different resistance to a cold one. When first switched on, there is a surge, because all the heaters are a fraction of the resistive value that they are a few seconds later when warming up.

In this amp, if you take out a power valve, none of the heaters will work, because of the series connection. However, if you take out a preamp valve, this will upset the heater balance and put, say 9volts to the other two. Take another out and that becomes, say 12 volts. Heaters don’t last long in that situation. But there’s worse to come.

If, having blown the hell out of a couple of preamp valves, you put in a cold valve, this will upset the heater supply to all the valves, because of the low resistance of the cold valve. But there’s worse to come.

A fault that can happen (and had on the VK100 I had) is that an over voltage on the heater can distort it so badly, that it shorts its length against the cathode tube it is housed in. In this situation the dc heater voltage can be applied to the cathode. This upsets the bias of the valve no end and it draws a lot of current which can blow the anode resistor. Maybe then you might do a bit of swapping around with this dud valve and take out more anode resistors. R103,R104 and R101 were open on this one, but R144 and R145 on the phase splitter could easily have gone the same way.


Yes, alright, the capitals are a bit over the top. But so is your overdraft if the matched quad of mesa valves you got a second mortgage on just made a lot of smoke.

A nice cup of tea……..and maybe…….a macaroon!




Elm Tree Soup

You might have guessed from the title that this is not going to be helpful. You may even be able to buy Elm Tree Soup off the shelf (so to speak) at your local garden centre. Any shop that has the gall to sell recycled plastic reindeer droppings with little bells in June might just do the same for Elm Tree Soup. At that last resort, I would have to say that anything home made will taste like soot. If I made it.

There is a serious philosophical dissertation coming here, so dust off your MA in soup-making. This is how it goes.

If I could find something so utterly gormless to build my thesis around, I wouldn’t have to agree with anybody, and still get a ‘First’. Or in the realms of Elm Tree Soup-making, a ‘Thirst’. The problem with getting all these ‘levels’ (you know ‘A’ or ‘O’ or ‘Spirit’ or ‘Mezzanine’) is that you have to AGREE WITH PEOPLE. It’s no good on the exam paper saying, “I don’t like the way you fart, so my answer is much better than yours.”

That’s a non-starter. What you have to do is memorize by heart every last hiccup of the examining board, disengage any suspiciously subversive brain cell activity, and write like hell. Anything. Unfortunately, I have a lot of trouble agreeing with anybody, which is why I repair electronics. If I disagree with it sufficiently, I can stamp my climbing boot on it.

This option, I hasten to add, is not usually one you might come across in an exam room. Neither do they ply you with tea and macaroons. Which is something else I can’t agree with.