MarkBassChat

Switching from 9V to 18V does not provide any extra boost. It provides only some more headroom which is not the same as extra boost. So switching from 9V to 18V does not make much difference/sense. Mark

Nice tutorial.It seems that previously you mentioned that you could not remove the knobs. What was the problem? Mark

Check this video: [url="http://www.youtube.com/v/AJ1B4vVXZTI"]http://www.youtube.com/v/AJ1B4vVXZTI[/url] to see/hear the difference between piezo and magnetic pickups sound. Mark

This usually happens when you replace output transistors without replacing driver transistors, or bias circuit. But a tech should know it Mark

I think that I mentioned that I did it once. It's not easy but once you start doing it, you will do it. It takes less than 1 hour and you waited already 4 days :-(. This is the amp I was fixing - all modules were removed from the enclosure, including front panel. [attachment=134879:RH450.jpg] Mark

Information from: [url="http://www.voxamps.com/valvetronix/vt-series/"]http://www.voxamps.com/valvetronix/vt-series/[/url] : "By using the optional VFS5 foot switch you can switch programs, turn reverb or other effects on/off, or make speed/tap settings using your foot. This is a great advantage when you're creating sounds or performing live." Why do you think it's mono? Mark

Which exactly model you have (there were at least 4, or 5 different models of the amp)? The one that I know uses VFS5 footswitch, which requires a stereo cable. The footswitch contain electronic circuit and it cannot be replaced with a footswitch of other manufacturer. How do you know that the one that you have uses mono jack? Mark

Being frankly I don't remember (I was fixing the amp 2 years ago). You have to know that this is very advanced amp - switching power supply, class-D power amp, digital pramp, figital front panel. And opening it is also not that simple. Try to remove the torx ones and see if this is enough. Regarding the knobs - it depends which ones are already removed. Why can't you remove all the knobs? Are they glued? If you post a photo of the amp, I may try to remember which screws I removed. But you can just feel it. Mark

[quote name='big rob' timestamp='1368304032' post='2075581'] Maybe your right but tc are sending me the replacement buttons and say its just a case of opening it up and popping off the old and replacing with the new ones. Someone must have opened one up, would really appreciate the help it anyone has. [/quote]Yes, you just need to open the amp and replace the buttons. The most tricky part is opening the amp. As far as I remember I had to buy a hex key or even a torx key to do it. Mark

I hope that you are joking about dropping the amp from from a great height. If the amp works correctly (sometimes), then this is just a minor problem, maybe just a dry joint, as mentioned. Or maybe a problem with the volume pot. I wonder how it is possible that Marshall couldn't fix it. Maybe they just couldn't reproduce the problem in their lab. Mark

Be warned that the amp contains dangerous voltages and there is no room for mistakes when you deal with such voltages. Previously you said that the amp blew the power fuse when you disconnected the power amp module from the pre-amp. In the last post you said exactly the oposite: when you disconnected the power amp, the rest of the amp is working. Which way is it? I think that the second post is more accurate. First you have to identify modules in the amp: the preamp, the power amp, the power supply and the transformer (part of the power supply). I susspect that what you are doing is disconnecting the power amp from the power supply (correct me if I'm wrong). In this case the preamp works (you can check that the valve is glowing and you have signal both of the Effect Send output as well as on the DI output). The Power LED os on and the fan is working. This proves that the preamp and the power supply (including the transformer) is OK. This means that the power amp is shorted (output transistors). And in this case I'm sorry but it is too difficult for a novice to desolder the transistors. Appart from the output transistors quite often driver transistors fail (along with the quiescent current regulator). So my advise is to confirm that you disconnect the power amp from the power supply (and not from the preamp), verify that the preamp is working, check the output transistors for shorts (with a digital multimeter) and that's it. If the transistors are shorted, the only thing you can do is to take it to a tech. If you are not sure what you are disconnecing, make a photo and post it here. Also make sure that the fuse that you had previously is of the correct type: it should be 4A - slow blow. Sometimes people use fast fuses and this is the whole source of the problem. Mark

Failed power supply, or the power amp. The most frequentlt the problem is caused by shorted output transistors. Mark

If they are active (and it looks like they are), you are not measuring the resistance of the pickup but just a resistor on the output of the preamp. Mark

I don't know why you have no sound with the power amp but the power amp clip light suggest that one low voltage rails (either +15, or -15V) is missing. Can you measure the voltage on Zener diodes (after large, white resistors) in the power supply? I'm only guessing but I would start with this measurement. Mark

This is clearly related to the preamp. Either there is something wrong with the preamp itself, or with wires soldered,or plugged into the preamp. In any case you can take a look at the preamp with a magnifying glass. If there are plugs (from pots to the preamp), check that they are OK. If you cannot see the problem, ask a tech to service it. Mark