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.