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Base theme by DesignModo & ported to Powered by Vanilla by Chris Ireland, modified by the "theFB" team.
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The burned-out component was a resistor not a diode, and it was in the valve-monitoring part of the circuit rather than the 'Vari-Class' - although it had the same result (no output) since the circuit shuts down the amp if the bias is outside the correct range - or if it thinks it is because one of the resistors has burned out…
It's sort-of a good idea, but it still strikes me as unnecessarily complicated and likely to cause more problems than it solves - particularly as with a major power valve short, it fed the full HT into a 15V part of the circuit with rather predictable results! The dead valve also took out its pair, and the phase inverter valve… I'm not sure how but I find it hard to believe it's a coincidence. But after cleaning the board, replacing the missing resistor with a conventional 1/4W one (which should show the small scale of all this) and fitting some new valves, it all - somewhat to my surprise - seems to work fine.
I still wouldn't buy one though.
Edit - bugger, that didn't last long! Ran for about ten minutes then redplated and shut itself off again. At least the new resistor didn't burn out so it's not exactly the same fault again. The next question is what else is wrong with it...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I have to change the Presence pot on one - it's had a bash and the shaft was bent and wouldn't turn properly, and of course when I tried to straighten it I got to about 90% of the way there and then it snapped off.
Replacing the pot is a nightmare - to do it properly you have to remove the front PCB which holds all the pots… so far so normal, but this one is joined to the main PCB with about 20 short solid wire jumper links, so to get the board out, either I'm going to have to cut every one of these and solder them back together afterwards (I don't really like doing this) which is still quite a big job, or to avoid that the only other way is to remove the main PCB as well, which is a *huge* job.
So I've decided to bodge it - by destroying the old pot above the board, soldering some flying leads to the solder pads and fitting a chassis-mount pot through the hole. I'm not proud of this, but the owner is just trying to sell the amp and I don't really want to cost him the thick end of a hundred quid's worth of work when I can do it for thirty. It's lucky it *is* something I can do this way rather than something where there would be no choice...
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
*An Official Foo-Approved guitarist since Sept 2023.
I finally got around to checking out the Trimmer pot shown in the picture and it is definitely A Bias trimmer pot. I biased the Amp yesterday using the trimmer pot.
Cheers Ian
That amp is indeed a total sod.
I've seen several that motor boated when the fx loop return level was turned up due to poor filtering.
The design hangs 8 (yes eight!) triodes of one power supply node, which is asking for trouble.
Removing the main PCB is a major undertaking.
well that was easy, jack socket switched contact is bent and doesn't ground when lead is pulled out. The other fault is a bit of a bugger, when the second channel is switched to line where's no signal. Now the switching seems to be logic gate or flip flop triggering many relays all over the board, never seen so many relays in an amp and no idea why it needs to be this complicated bearing in mind the 3 different amounts of gain needed to reproduce mic \ line and Piezo
Disassembly isn't too bad, the IDE cables are glued but only require a some gentle heat to release. If you bend the front panel slightly you can remove the pre amp PCB without taking out the rear PCB first. It's a mixture of thru hole large component and a lot of SM but nothing tiny
Unless anyone's see and done this fault before and can help @ICBM it looks like it's gonna be a long afternoon hunting down the fault
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I've worked on a few AERs, but as they don't supply schematics, and are often very complex, I usually swerve them.
It does help we work opposite Westside Distribution, so can send people over to them.
Roland KC-880 keyboard amp.
It has top-mounted controls, which as you probably know by now I dislike - one of the reasons why is that it's far too easy to damage them, if something gets put/dropped on top of the amp. This one has had something heavy dropped on it by the look of the panel, and it has smashed the master volume pot.
No problem you would think - except that it's a PCB-mounted *5*-gang pot!! So not only extremely difficult to unsolder without damaging the board - although my usual method of carefully destroying the pot should work, it will be very time-consuming - but good luck finding that... I'm sure I've never even seen one before. I'll try Roland but past experience suggests they won't have it in the UK, if at all.
WHY?! Surely even in a stereo amp - which this is - a dual-gang should be all it needs? This just seems like needless complication for the sake of it.
Worse, the owner needs it for an important gig next week... so the simplest solution is just to jam the remains of it in the full-up position, solder across the top two pins on each gang to make sure, and call it a field repair, since it still has individual channel volumes .
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
https://uk.farnell.com/chip-quik/smd1/removal-kit-smd/dp/1850214
It's a very low temp solder and flux that is really aimed at SMD work, but I also use it sometimes for through hole components, especially getting the DPDT switches off MXR pedals as the traces are verrry thin.
It's not cheap, but it's great at getting components off the PCB without damaging the traces.
My default method before this was similar to yours, cut the component down so that the pins can be removed individually, apply some flux and leaded solder and remove the pins one by one.
Very - the side handles are horrendous, there's too short a gap between the crossbar and the top so they try to break the back of your hand. Very poorly designed amp in so many ways.
I'm actually hoping that the 'field repair' is sufficient to make it unnecessary ever to do the proper job... or see it again. But I have a feeling that it will be back like a bad smell.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
A chap I know had the 1x15” one, that sounded nice too - he used it for a long time before he finally got fed up with it breaking down, after its second OT failure.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
An object lesson in how to build an amp badly. And the vinyl comes off the flimsy MDF cabinets easily and snags the chassis when you're trying to work on them.
Totally different amp from the JTM30 and JTM60.
All the reissues are extremely well-built and reliable.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein