A Very Quiet Fender Twin Reverb

I hadn’t seen John for some years, but he turned up out of the blue with the Fender described above.

Even if I looked at the amp from a distance of a lot of miles I would have known it was a ‘Reissue’. I’ve just looked up ‘Reissue’ in ‘Ballooning for Aquatics’ and it said ”Consult ’Skiing for Quadrupeds’.” Which said ‘Consult “Brain Surgery with Lump Hammers”.’ So I finished up guessing. It must mean, I thought, something old brought out again, much later. Wrong.

What it really means is ‘Nick a label off a really nice amp and make a new one that’s nothing like it.’

The overriding problem with current reissues of ‘classic’ amps, is that it’s impossible to do unless you have a raft of prospective customers with bottomless back accounts. I’ve got one of those, but only because it has a big hole in the bottom. A reissue amp has to use pcb’s. If it used solder wells, tagstrips, turret board; as the original Vox, Hiwatt, Marshalls(old ones), Fender (old ones), Selmer, Supro, Carlsbro (old ones) etc….etc…. it would be monumentally expensive (there are still a few of this breed around) because people would have to do much of the building, as opposed to computers.

This gets me (eventually) to John’s Fender Twin reissue. The almost insuperable problem of valves and pcb’s not being mutually on speaking terms rears its head. In this case the output valve bases (4x 6L6) are soldered directly through the pcb, and…… as the valves hang from the bases the heat rises and…..they get hot!!!! And if we heat up a solder joint a lot of times (it doesn’t even need to be all that hot; the melting point of tin is 232degrees c and lead is 327 c so anything over 232c burns off the tin and leaves the lead.) So it drives off the tin content of the joint and you finish up with lead. And usually cracked lead. The alloy of solder is usually 65%tin to 35% lead, and the resistivity of tin is 11,5 ohm.m whereas lead’s resistivity is 21,3 ohm.m. This means that the solder joint has twice the resistance when the tin has gone.

Another property of lead is that its flexibility is poor, and the expansion and contraction of the metal with heating produces cracks and poor contact with the joint. At its most extreme the joint can become an insulator, and your nice guitar sound has become nothing at all at the other side of this joint (which is known as a ‘dry joint’ or a ‘cracked joint’).

It’s about here when I wish I had a bit more interest in photos and things, because it would be useful. However….the fault with John’s amp was minimal, but as with a lot of pcb-based amps, not that cheap. To correctly replace a component in a pcb, you have to be able to get to the to the UNDERSIDE of the board. Pheeew! That can mean taking off wiring, marking where it came from, taking the pcb out, all to get to the track side of the pcb. If you don’t do that (it’s possible to cheat by cutting off the component and soldering to the resultant wire sticking out) you run the risk of dry joints, shorts to chassis and missing any burned tracks that might be under there. In the original version, the solder wells are visible, and the pcb dismantling zero, because there aren’t any. So the job takes five minutes, and the rest of the time can be usefully spent cleaning up and servicing.

Also, on this amp there are two wirewound dropper resistors (270 ohm if memory serves) that run hot. That’s ok, they drop the voltage from around +- 50 volts down to +- 17 volts to supply the chips and relays that the amp uses for channel switching. These two resistors are flat to the board (not ok), and so heat up the tracks underneath, and are also close to two 1000uF 35 volt capacitors which don’t like the heat. For an amp of this age (1994-ish and on) these components are ready for replacement and you need to have a good look and resolder all the joints under there.

Just as an aside, it’s interesting to note that any mortgage you might have taken out to buy a Mega-Hugely-Marvellous-Platinum Plated- Radar Controlled…..erm….guitar lead….would be a totally daft investment if you had a dry joint on the input socket.

To put this problem right would cost you 0.00000001% of your investment with Wonderful Guitar Leads Inc.

Which is but a small increase on my macaroon outlay for the decade. Tea.


Universal Audio S-610 preamp

This is brief set of impressions regarding a nice American made valve preamp. This one blew fuses, so not quite as nice as it should have been.

On the back of the unit there is a lot of blurb about which fuse to use for which mains voltage. What it is less than clear about is that you don’t just swap the fuse and everything is ok for the different voltage. What it says is that a 250mA fuse is for 110 volt operation and a 120mA fuse is for 240 volt operation. What is not obvious is that there has to be changes made inside the unit to effect the voltage change. You might notice that if you trawl through the user manual, but not necessarily.

There two four pin connectors mounted on the internal pcb near to the IEC input, and the free plug that fits these (looking from the back of the unit) needs to be on the left hand one for 240 volt operation or the right hand one for 110 volt operation.

Unfortunately, if you’ve bought one of these units, and it was internally set for 110volt operation, and have plugged into 240 volt mains, it will have blown the fuse in the IEC socket. You might be lucky and after changing the voltage setting to 240 it might be fine. But the electrolytics in there will have been subjected to a high over-voltage and it would be sensible to check these.



Ampeg (i.e. Loud Technologies) BA300 115

If you happened to be one of those few misguided folks who followed this blog, you may have noticed that….there hasn’t been any for quite a while. My excuse is that I have to get enough steam up to vent my spite on unsuspecting pieces of electronic equipment, and, more often than not, their manufacturers.

The Ampeg BA300, whether in the 1x 15″ or the 2x 10″ versions, seems, from the various forums I’ve read, to be leaving a trail of very disgruntled musicians in its wake.  Many of them having had these amps switch themselves off, mid-phrase. The amp then switches itself back on again after a leisurely 10 seconds or thereabouts, and then, if the volume settings remain the same, does it all again. Although the amp itself seems not to suffer any embarrassment, the same cannot be said of the bass player.

I’d like to be able to say I’d successfully repaired one of these, but the honest statement is that I’ve done no such thing. But I have spent many hours inside one, and can let you in on a few conclusions.

The first mystery is why any builder of amps can think that it’s a great idea for the amp to switch itself off if too much power is demanded of it. The traditional answer to that problem is that the amp goes into clipping distortion, the player hears that, (not pleasant) and turns it down. If the music demands a heavy distortion, the amp just doesn’t go any louder if turned beyond it’s rated power output.

This amp is a ‘clever’ amp. This means that it’s complicated, not necessarily for any good reason except for, maybe, profit boosting purposes. It is a ‘class D’ amp. The ‘D’ stands for ‘dismal’ in my book. But it actually means that it uses a modulated carrier signal (of about 450 kHz if memory serves). There are different versions of the modulated carrier technique; this one uses ‘pulse width modulation’.

They also use a switchmode power supply. It’s a lighter alternative to a traditional isolating transformer, but it has its own problems.

The power amp section is unusual, and not a bad idea. It uses a bridgemode technique, which means that the speaker is driven from both sides, instead of the usual hot drive to positive and ground to negative. The power rail voltages can be much lower than in the traditional configuration, and this results in better efficiency in  the power amp section.

There were two faults showed up on this amp. The first was easy and fairly obvious. It didn’t work. With certain faults in mosfet devices, (these are !RF640 used in a push-pull configuration) an output signal can be produced which disappears when there is no load (speaker) connected. This amp has four !RF640 mosfets. Each pair produces a push-pull output, and the two pairs are out of phase; and applied to the speaker at either side. This was the bridgemode arrangement described earlier.

These devices were replaced and…It worked!

Un…fortunately I found out that a transient (a ‘pop’ or a slap on bass) switched it off. It returned after a delay and would do the same thing again. Etc…etc…

The ‘protection’ (if that’s what it is; one could argue that it’s musical assassination) switches off the carrier signal mentioned earlier.

Clever? Yes, but not very sensible.

Time for tea.

The Unfortunate Case of the Roving Vox AC30TB

There is a short but entertaining story to this, which deals with a range of matters from lack of respect for quality to wanting to kill people from a distance to destroying irreplaceable artefacts to dealing with insurance companies to fixing the unfixable to boldly going where……you get the idea.

This little story, although sort of entertaining, was definitely not so for Marc; who shall otherwise remain anonymous. He sold his lovely old (mid sixties) Vox AC30TB to a bloke in Spain who desperately wanted to buy it. My own involvement up to that time was that I had rebuilt the output stage for him a couple of years earlier. Although you could have bought one of these new in 1965-ish for a hundred and twenty quid or so, you certainly can’t now. So, the deal was done and the lovely Vox sailed away to Spain having been packed and cased by Marc, who is the soul of conscientiousness. So it would have been very securely shipped.

After a week or so, the Spanish bloke decided that he wasn’t actually as keen on buying as he had been, and took this up with Paypal. He had a full refund, and sent the amp back. All this, although a sad indictment on honourable dealings seems (to the modern mind, at least) alright. But it didn’t stop there. The lovely Vox AC30TB came back in the in same box without any packing. I did mention ‘lack of respect for quality’ did I? It was, unsurprisingly, very much the worse for its journey, with the front baffle and casework badly damaged. It also appeared to have had less than expert hands inside it, than it deserved. It’s about here where I get involved again. Marc needed an estimate for repair for Paypal as he was making a claim for damage in transit.

The big problem with an estimate in this situation (i.e. major accidental damage) is that you need to do all the repairs to find out what it will cost. There is so much in an amp that might look (and test) alright, that actually isn’t. Not reliably so, anyway. The valves would all need to go, for a start, whether or not they tested ok.

The result of this was that the amp was a write-off, mainly because the speakers  (original Vox Blues) were damaged, and therefore it would not have been possible to bring the amp up to original spec.

What a shame. These amps should be treated with kid gloves, not hobnail boots.

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.

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.