Marshall Valvestate 2000 AVT150

[terada1957]

I have a Valvestate 2000 AVT150 amp. I can only get a acoustic sound now. the buttons and foot switch to the clean and overdrives 1&2 light up but will not stay on. Any ideas or advice thanks

[sumgai]

terada,


Welcome to The NutzHouse!


Can you provide a link to a schematic for this beast? I think I already have the answer, but I want to be sure.






sumgai

[JohnH]

One thing I remember reading about that amp is that acoustic-sim mode is the only setting that doesnt use the tube. So maybe something wrong around the tube or its switching/circuitry?

[terada1957]

i found this one if it helps.   www.schematicsunlimited.com/m/marshall
I am not good with forums etc and a bit of a Dummy. not sure if this how you post but you help is really appreciated.

Cheers
Tom

[sumgai]

terada,


I'm sorry to have to say, that set of schematics is incomplete - I see nothing about a power stage (tube or otherwise), only the input and preamp sections.

Equally sadly, my Bingle-fu is alarmingly weak, no search engine seems to want to find anything further than the link you already provided.  It's as if no one was interested enough in this amp to take the time and effort to save these diagrams on the web for posterity... including Marshall themselves.

From what I saw though, my initial surmisal, that of bad opto-couplers, was wrong - there are no such devices insofar as I can tell. Perhaps they exist in the non-published diagrams, but not here.

The problem likely exists as John suggests, in a switching setup between the pre-amp outputs and the power amp's inputs. But there is one more thing you can try before giving up.


Like all other British amp makes, Marshall was no stranger to keeping things cheap. I'm speaking in particular about their 1/4" jacks, which were (and still are) the lowest of low quality. In short, they were designed to fail, and require replacement within a few years after leaving the factory, not several decades like you get with American amplifiers. (Who almost universally use Switchcraft parts.)

Thinking that a jack is kaput, try this: Open up the map, and using a short length of wire, short directly across the terminals of one of the footswitch jacks, and see what happens. If this means having to remove the circuit board in order to gain access to those points, and if it further means leaving things like connectors unplugged from their circuit board sockets while testing, then perhaps you'd be better off taking the amp to a qualified (and trustworthy!) amp repairperson.

That's about all I can come up with, sans any further documentation. My apologies for being so late with this report.

HTH




sumgai

[JohnH]

I did find the schematic, from some foreign place:

elektrotanya.com/marshall_avt150_avt275.pdf/download.html

Just click on 'get manual' below the preview image. It looks to be the right one, ir the Valvestate 2000 line rather than the later generation. My pc is still fine after downloading it. The diagrams also cover AVT275 which was a stereo version

But there's heaps going on in this circuit, mainly solid-state. But if I had to guess, given the acoustic part is OK, just focus around the valve part. Does the valve glow? how about the connector to its board?

[sumgai]

John,


Nice find!

---

terada,


Let me look this over and see what I can deduce just from your described symptoms. Later today, or early tomorrow, promise.




sumgai

[pj]

Terada, just to clarify, is this fault present with the footswitch and cable disconnected?

If I'm not mistaken by my cardiac addled memory this is the multi button footswitch with a serial type cable. I had two students with issues with these one was a mashed cable and the other was an abused connector

[sumgai]

t,



pj (above) led me to look more closely at that footswitch arrangement... My Gawd, that's about the worst thing Marshall (or any amp maker) could've done, use a D-submini connector like that. Talk about asking for trouble!! (This is even worse that Roland's 13-pin cable and plug/jack for their hex pickups.)

The control lines aren't in serial, the number of pins needed/used is indicative of that. Not to mention that they run in tandem with the front panel switching almost exactly, so serial is out. However, you need to tell us that the problem exists even without the footswitch plugged in, pj is correct about that.

And you might want to check that all the pins are straight, nothing is contacting any other pin or the outer shell.

Awaiting your answers....







sumgai

[pj]

Ah it's that one. Poor idea as you point out, they're not rugged enough for their original intention let alone the rigours of MI amplification. I thought it was perhaps the one that was terminated with the rs232 serial printer type. That may have been a laney model thinking about it.