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.


Can I test a valve with a Multimeter??????

The slick (but rather obvious) dude reply to this would be, ”Why am I going to take a second mortgage out on a valve tester, if I can do it with my three quid Maplin Multimeter?” You’d have to admit that even an accountant could have worked that one out.

So the answer is, clearly, “No you can’t.” But just a minute; this is a thinking blog! I made the choice between a Weetabix and a piece of toast this morning, so it has to be. The useful answer is “Up to a point”. So here we go, onwards and A&E wards.

At the top of this is a layout of a B9a valve base. It’s also the layout of the pins of the valve looking from the bottom, and what they connect to in the valve. This one is the good old ECC83, (or 82 or 81). The characteristics are different but the pin layout is the same. These are the things you can test with your multimeter on the ohms scale.

The heater integrity. The ohms reading across pins 4&5 will be very low (just an ohm or so) and the reading across either of those to pin 9 will be half that reading. If it’s open on any of those readings, the heater is broken and the valve is dud. It’s hard to tell if it’s shorted or not, but the usual reason for a shorted heater is that it’s shorted to the cathode, and the next test will tell you that.

Heater to cathode resistance. This should read an open circuit, pin 3 and pin 8 to pins 4,5 or 9 (or at least a lot of megohms if it’s a damp day). If not it has a heater- cathode short and is one for the bin. It will usually put out a very loud hum in that condition. Although on an ECC83 this is just a nuisance, on a power valve it can be a disaster, because it can destroy the bias and do a lot of damage.

Anode to anything resistance. You can test pin 1 and pin 6 to any other pin and it should read open circuit.

Same for the control grids (pins 2 and 7); open circuit to everything else.

So why bother with a valve tester? A valve tester tests a valve under dynamic conditions, which means it has voltages applied to it and various currents are measured to determine the characterists of the valve. A multimeter can only put a few millivolts across the various elements.

What it can do is tell you when a valve is definitely dud. A broken heater or shorted cathode to grid, grid to heater etc., and your valve will not do anything . But there are other faults that can look ok on a multi meter but the valve will not work.

So there we have it. Whatever it is.

Time for tea. Hmmm….maybe a macaroon??????





Mesa Boogie DC5… with distortion fit to drill teeth

Without doubt, Mesa Boogie are a class act. But you will be very daft to try to repair one. I’ve had a better time pulling out my own toe nails.




Here’s the inside of one of these legendary DC5 machines.

You really have to take a lot of things to bits to get anywhere near a repair. Anyway, not to carp on. (For once.) This one had a more distorted view of the world than Hitler. There was nothing you could do to get a clean sound. Except maybe drop it out of a plane. As I don’t know anybody with a plane, I had to fix it.

This is me looking intelligent. The thing that I am asleep on, is a schematic of the preamps of the two channels

Each round looking thing (not me) is one half of an ECC83 (12AX7 across the water). Looking from left to right the three gain stages of this clean channel lead one to another (it’s called ‘cascade’) finally leading to the switch on the far right. The lower channel does the same thing except through four gain stages. The tone filters are a different arrangement but they’re essentially doing the same thing. That also leads to the switch, and this is the channel switch footswitch socket.

This is the next schematic in the series, and the output from the switch of the previous sheet arrives at the arrow at the far left of this drawing. There are then four more round things. These are different, they’re transistors. What happened here, was that although everything looked fine and dandy up to the input of this stage (which drives the fx send signal) at the send output it looked like something Salvador Dali might have cooked up. This is obviously referring to oscilloscope displays.

All the transistors in this section need replacing. This is where they are in the amp.

The four little black things in the centre of the picture are the culprits. MPSA 70 (two of) MPSA63, MPSA20. Cost about ten quid all told.

This is a fault that you could spend a lot of money on (revalve etc.) after which you find its exactly the same. Depressing.

Anyway, it’s tea time. Might have to consider a macaroon.





Korg SDD3000 (The old one).

I like rack mounted effects units. I know the hot fashion for little boxes that sit on the floor and you stamp on them. But they really don’t do your stage persona much good when you’re grubbing around to find a three millimetre diameter knob amongst a wash of Carling Black Label and fag ends. Still, each to his own, live and let live, and all that.


This is the inside of the rack mount Korg SDD3000. Unfortunately I was too late to do a ‘before and after’ thing on it. I’d done the job before I caught up with the brain cell that deals with ‘before and after’.

This is the battery that was in there before the thing I replaced it with. Sorry ‘with which it was replaced’. I could do it in Italian, but I don’t know any. This original one was a NiCad, rated at 250 mAh @ 3.6 volts. The mAh (miliampere-hour) rating tells you that it will put out 250mA for an hour. Or 500mA for half an hour. You get the idea; it’s the capacity of the battery to store charge. It’s not that important electrically, especially in this context, where the current draw is minimal. In other words they last a long time. This particular battery had drained to about 2.6 volts. With an old unit like this, checking the backup battery is first job.

But the voltage is important. And in this case, if the battery voltage drops too far, not only do you lose your custom settings, it also doesn’t work. At all. Dave’s unit came in to be checked over and serviced. Servicing is generally straight forward. Separate all internal connectors and spray them out with something like Servisol 10 (NOT WD40). Same with pots and switching and jacks.

Here’s also a timely warning. There are various static sensitive (Cmos) devices in here. They don’t like fingers much. Strictly speaking, these blogs are not a sort of instant-do-it-yourself-manual. Sometimes they might save you a few quid, but often they are just interested in informing.

The only battery I could find that was 3.6 volts and would fit in (with a shoe horn) was an AA size lithium battery. It did fit in with the correct clip, the only other mod being moving the 100 ohm current limit resistor. The custom presets had to disappear because the original battery voltage was too low to hang in a temporary d.c. supply, but Dave was ok with that.

There was another problem waiting to happen on this one.There are two separate bridge rectifier networks in this. The four diodes in the pic have replaced the originals. There was nothing wrong with the originals, but they had obviously been running hot for a long time, and joints underneath them had dried/cracked and the pcb was discoloured. I replaced these because with the longer leads they can be spaced off the surface of the pcb and dissipate heat, so that future dry joints won’t happen.

Time for tea. And maybe a macaroon, to celebrate something.





Let’s Hear it for Mesa Boogie and Traynor!!!!!

Yes, it’s true! You haven’t inadvertently flipped over to an American Evangelical site selling you a place on the Bus to Heaven!

I sat down in front of this hated machine this morning, ready to do battle with the Lords of Rock Surveillance and their Secret Minions. (You know, Roland and Soundcraft and Laney and just about everything Chinese and….well you’ve got the picture long before this.) Yes, the shock was nearly as dramatic as finding out that Marc’s strange AEI amp had 415 volts on the ground of a line in. I measured it out of curiosity after it had killed me. A tricky proposition but clearly not impossible. I had paid my subs to the Heavenly Bus Co. however.

To return to the topic. That’s a fresh approach, at least. I typed into my WEB BROWSER (I had to put that in capitals because I only just found out what one is) YCV20, and within half a minute I had a complete set of schematics for this Traynor amp. A SERVICE MANUAL, no less.

And then (shock horror!!!) I did the same with a Mesa Boogie DC5 and the same thing happened. I had clearly caught Heavenly Bus. Ecstatic!!!!!!!!!!!!

Please take note. I did say ‘half a minute’ and not ‘half a month’. If I’d been looking for a new Wharfdale or a new Carlsbro, I wouldn’t have been able to see the screen without having had two haircuts.

So the flag outside the workshop now says ‘Mesa Boogie and Traynor are Brilliant’.

I’m now off to catch the Heavenly Bus to a cup of tea.

Dedicated to Dave and Marc, the two readers of this blog

I heard, in person, face to face, unequivocally (didn’t think I knew that one, did you?) first hand, that there are two gentlemen who actually read this junk. I mean ‘these literary expositions’. So that they do not lose their respective jobs for their rank bad taste in reading these literary triumphs, they will be forthwith referred to in code as ‘Dave and Marc’. Ooops.

So here is blog of your very own, kind sirs. It’s actually a short story about Dick Big and Barney the Stoat, private dicks. When I get tired of writing this blog bollox, I turn my attentions to……writing some more bollox! What else?


                    The Dick Big Detective Agency. 

For best effect, recite through a mouthful of gravel, preferably in a down-market bar. If the venue is misjudged, the recitation may be improvised through a mouthful of loose teeth.


            Dick Big was suspicious. That was o.k. It was Dick’s job be suspicious. And to talk in short sentences.

Sometimes. Very short. Sentences. (Breathe here.)

            Barney was his sidekick; but he was starting to show the bruises. Dick would have to kick him with the plimsoll.

            It all started on a usual, short sentence. Day. But Dick knew that something big was up. Not enough roughage. Barney scratched a match along the hatch, and catched; sorry, ‘caught’; a cheroot with the side of his mouth. That was side that had a lot of blisters. He needed the practise.

            “O.k. Barney, this our big break. We get the lowdown on this trip and we’re fixed-up floosied for good.” Dick didn’t know what any of this meant, but it sounded helluva good. He was going to have to look up ‘helluva good’ in Roget’s Tyrannosaurus.

            The dry, dead, sylvan carpet sounded loud as a erm, stoat, in the still night mist of the forest, leaves of the twisted birch rustling their rhythmic salsa underfoot, and the owls owling their percussive pizza. Otherwise all was quiet. Except for the M63 in the background; delicately humming its low harmony; its irritating drone; its intolerable blood-curdling roar, driving the innocent to maim and kill and murder and COMMIT HORRIBLE CRIMES AGAINST HUMANITY………

Other than that it was quiet. Too quiet. It must have been because Dick Francis said so somewhere. So did everybody else for that matter.

            Dick Big had a clue. It was the first one. He was, generally, clueless. As they walked together towards the ancient chapel, the flicker of a bloodied candle stabbed the forest through a cracked finger-nail. They dived into the ditch at either side of the forest trail as the hooded figure in flowing shawl slunk from the ancient chapel of devil-worshipping hamsters, and writers who put far too many words in a sentence and should be dismally ashamed of themselves; casting hunted glances, the ceremonial axe at its back dripping ceremonial blood, that whipped up in a petulant wind like a erm, stoat. The sleek furry creature followed closely and silently; and moving swift and deadly leapt at the back of the fleeing figure in black, ripping at the cowl like a, erm, stoat. (“Have I done that one already?” “Yes. To death, actually”)   There was the sudden wail of a screaming banshee through the catacombs of death. Or Waitrose car park at 2.00 am. The difference is a subtle one. 

            “I’ve broke my fuckin’ ankle”. It was Barney. He had. It was bad. They were back to short sentences again. Dick was supportive;

            “With all those bruises I’m surprised you noticed a broken ankle.” Fortunately for Barney, Dick was no good at first aid. He sympathetically levelled things up by breaking his other ankle.

            “That should hold it, till we can get help.”  He looked up. Quickly. Very quickly. In fact. The stoat was expanding in a very unstoat-like fashion, standing over the prone figure of the skeleton monk and hacking indiscriminately with the bloodied ceremonial axe.

            This was Dick Big’s big chance. He threw his plimsoll in the air.

            “O.k. Barney. If the shoe hits the ground, you go take the monster out. If it doesn’t, I go. Fair’s fair.” The shoe floated around at head height. Barney turned around. Dick had gone. Back to short sentences.

            “You bastard.” Barney slipped in this short sentence of his own. But Barney was made of stern stuff. He had a well-stuffed stern. Drawing himself up to his full height, he dusted off the dead leaves and stared into the alien’s kneecap.

            “Growl!” it said using the first exclamation mark in the whole piece. The axe sang through the night and hacked Barney’s head off.

            “Barney!” shouted Dick, extravagantly wasting another exclamation mark. “It wasn’t ‘heads you lose’.

            “Growl” said the blood-sucking vampire alien from planet zgrvtrblblet.

            “Growl” said the M63.

            “There’s only room for one blood-sucking alien in this piece and I’m it, buster” said the blood-sucking alien from planet zgrvtrblblet (which we will, for convenience sake, call Brian), running onto the M63 in a rage and no safety first procedure and was squashed flat by a Virgin; er…Virgin Express, having taken a wrong turn at the Maidenhead junction. And now back to short sentences.

            Dick Big was benighted. Barney had a head transplant from a stoat.  Everybody lived happy ever after. Except Brian who was dead.


                                   Nearly The End.


Well, there we are. The literary firmament has hit a new low. But only because it hasn’t yet read the continuing saga of Dick Big and Barney the Stoat.

Tea time. And I hope Dave and Marc imbibe also. It might improve the after taste.