wolf1419

I have a problem with an amp blowing fuses as soon as it starts-instantly!Where can I look first and proceed with this process. I have a multimeter, soldering skills, but very limited diagnostics of electronic circuits, schematics other than continuity testing. Please point me in the right direction without going out for service. This is a project and learning experience for me. We can walk through this together if there is a tech willing to hold my hand and teach.

Adwex

The Valvestates have 1 preamp tube? 12AX7 / ECC83 ?
Try it with the tube removed, see if the fuse blows again.
If it doesn't, get a new tube.

__________________
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wolf1419

Step #1 Removed tube, fuse still blows, so tube is now eliminated as source
Step#2 Visually checked all wires, output transformer, solder joints, etc, ....on top side of board, visually no burns, loose connections.

Where do you check next, and how is it accomplished, etc.....???
I'm ready for lesson#2 of my diagnosis, on my project learning curve.

Adwex

Darnit! I was hoping it was the tube, that's the easiest thing to try.

I'm not at all familiar with the Valvestate amps, sorry. Here's a link to the schematics if you want to poke around yourself:
http://www.drtube.com/marshall.htm#VS2

I took a quick look at the schematic for the power section...There's an "autotransformer" in there that could've failed.

__________________
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wolf1419

Thanks for your help. Any real techy's out there that can stear me through the diagnosis process.....?

Pete240885

Thanks adwex for the schematics, a simple way you could tell if its the transformer is disconect it from the amp, If the fuse still blows its definitely the transformer if it doesnt The first thing i would check is the Bridge rectifier, see if one of the diodes inside it has blown and is cause therfore causing a short circuit.

wolf1419

Thanks for the support guys! I'm wondering if I only need to unsolder one, a few, or all wires at the transformer. I want to get this right before I apply any power to this thing.
I am a novice, but very practical and cautious so please bear with me. (my boss always said I was over cautious - "your mom must have had 2 diapers on you to cover your butt". LOL Yeah maybe, but I've always found it safer and cleaner in the longrun to try to get it right the first time.

Pete240885

Assuming the schematic is correct, remove both white wires, these ends will be live when you switch it on so make sure you insulate the ends before you do.

wolf1419

Thanks for working with me Pete. I've been studying these schematics, trying to read them, researching, etc......as part of my learning experience, how do you read in the schematic, where and what color wires you're dealing with, or if I'm on the primary or secondary side of the transformer. I'm trying to get a true handle on signal flow, AC, or DC current, etc....this is the "if you can teach a man to fish(or read schematics/test circuits), he can eat for the rest of his life scenario, vs. just handing him a fish, even though that's much easier."
If the two white wires removed is taking the transformer out of the circuit, and the fuse does blow, then it would not be the transformer.....is that correct?

Pete240885

your on the secondary side of the transformer, if you were on the primary side nothing would be connected to the mains at all, I know the wire are white because it says Wh1 and wh2 on the schematic - this could be different on your transformer but hopefully its standard

A rectangular symbol With two wires joining to it indicates a resistor (e.g R1) - on some schematics this shown as a zigzag line

A rectangle with two wires and a third wire with an arrow at the end of it it av ariable resistor/potentiometer (e.g VR1)

A triangle represents an amplifier (e.g IC1a),

Two short lines represents a capacitor( eg C2),

A rectangle and line represents an electrolytic capacitor (eg C1) - the rectangle is the positive connection

A triangle with a line at one of its tips represents a diode, Look at the component BR this component actually consists of 4 diodes connected together, wough can see that what appear as white wires from the transformer on the schematic connect to either side of it.

A thought has just occoured are the fuses your using marked with a T?

Pete240885

Ssorry missed your question out, i've been at work all day and its getting late here, If the you take the two white wires out the transformer is the ONLY component in the circuit and so if the fuse does go its a problem with the transformer

wolf1419

The closest fuses I could find were AGC(fast acting)2A 125V. Where can I get the T2A-120V fuses, which I think could be part of the issue if they should be "Time delayed, or slow blo".
I'm looking at these schematics, but still can't find wh1, wh2, for wiring colors. I'm looking right at the mains(L,E,N)socket........but nothing but lines(wires), FS1(fuse?), earth(ground),SW2(mains switch?), etc....very incomplete as far as I can see.

Pete240885

yeah the T does mean time delay, you could get them from maplin, or farnell most electronic compent suppliers should have them.

I've just noticed that i have the wrong schematic, that is to say i've saved them with wrong names on my pc like an idiot, can you try the fuse thing while i take a closer look at the schematics and then re-think if necessary?

wolf1419

I found a 2A slo-blo 250V, tried one and it blew also, so I surely have an issue somewhere.
I learned a lot about elements of electricity today(electrons, atoms, current, ohm's law, DC vs. AC current, circuit symbols, simple flow of series and parallel wiring), so I'm trying to get a handle on this stuff. I really appreciate your help and I'm looking at Marshall 8080/8100 schematics(preamp and output stage-2 sheets)PC0689.DGM

Pete240885

Thats a start but you really need the correct fuses before we can go any further, T fuses are slow to allow for larger start up currents than the current drawn in general use, fast acting blow quicker than standard fuses so regaurdless of anything else these do need to be changed! you can get them from CPC CPC | CPC - Over 100, 000 products from one of the worlds leading distributors of electronic and related products. thats the UK page though and i've checked they definitely sell them.

While your waiting for these fuses however take your multimeter and look at the diode bridge DB1. Bellow is the Pin out for it



Now how much do you know about Diodes? I'm hoping you can figure out how to test it yourself with little a reading? Looking at the schematic you should be able to test this while its on the PCB

yup me too now that i've renamed them on my PC

And now i can see the auto transformer that adwex mentioned too which means his thoughts make more sense.

Hmmm you should already know this if your inside your amp but i'll just check C53 and C59 can and will hold HIGH voltages for a VERY long time after the amp has been disconnected from the mains - if you wish to remove this voltage connect one end of a wire to Ground and then touch the other end to R109 (the end that connects to the Tube) for a seccond or more

wolf1419

The good news is that I actually am relating the parts you mention on the schematic and then finding them on the actual board, and have researched that diodes will read continuity when read with proper polarity, and should have no continuity when opposite polarity is observed. Yeah!!!!!
The bad news is I think I might have to completely remove the board to measure diode bridge DB1 from the opposite side of the board to access the contact points(the side opposite the tubes, resistors, etc......??? Is that true?
I also see the wh1, wh2, blk1 right in that area of DB1 on the actual board itself, not showing in the schematic, but that probably relates correctly to your read of the other schematic where you wanted me to disconnect wh1, wh2 in our initial test that you wanted me to try(after I get the proper fuses).
Are you in the U.K. or U.S.A., as "T" from U.K. in U.S.A. appears to be called "slo blo"(time delayed) fuses, in which case the ones I have might be alright.....as I am in the U.S.A........does that sound o.k.?
I know this must be like trying to teach a baby to fly a jet plane, but i'm slowly getting a sense of this, so please stay with me teacher...!!! I'll make you proud, I hope..!!!

Pete240885

I'm in the UK and yes those are just different names for the same fuses, however you mentioned that most of yours are quick blow

Yes you probably will have to remove the board from the chassis in order to access them can't see from here but in most cases this is the case

wolf1419

Hi Pete,
Excellent. I have a mate in the U.K., that's very cool! Yes, I had the fast fuses at first, then got the slo-blo(time delayed), and will stick with them. O.K., I'll be digging in and taking the board from the chassis, which will give me a better view of the whole situation.
This might take some time for me to get the courage and know how to get in there, while keeping all in order, so you'll get a nice break. I have some other guitar projects going at the same time. I'll let you know how I make out.

Pete240885

Alright cool - just watch those voltages...they hurt

wolf1419

Would you recommend going back to your first thought about just disconnecting the secondary side of the transformer before I try to delve any deeper? There are 2 white, and 1 black center tap. Can I just desolder right at the transformer(all 3, or just 2 white outside legs) to avoid getting into, or near the board just yet to test that transformer?
Somewhere in my research I found something about working around the fuse issue. Is there some workaround with using a 100 watt bulb attached to the 2 ends of the fuse link, which was said to save you fuses while working out your problems.....and how does that affect your diagnaosis, if that is even adviseable, or workable?

Pete240885

Working round fuses is not advisable, bypassing them can lead to faliure of more expensive parts nad/or fires and as such is not something i would recomend.

You might be able to unsolder at the transformer however i would reccomend unsoldering at the Board and tbh i think the diode bridge (DB1) is a far more likely suspect than the transformer - I'm trying to look at this from the point of view of whats the most likely fault and start from there - but you could start with the transforer if you wish. Disconnecting both white wires would be sufficent you can leave the black one in place or disconnect it if you feel more comfortable doing that.

wolf1419

Hi Pete,
I finally got one part of this out of the way. I couldn't gain enough access to the board side of the secondary wires, so I unsoldered them at the transformer, replaced the fuse, hooked it up and the fuse held. I tested the fuse just to be sure and it was fine, so the transormer is not shorted, just as you figured.
Should I test output voltages now while the transformer and everything is still basically intact to be sure it's all operating as it should to that point? Any tips on that.....? Should it be +15V/-15V white to black, other white to black at the output of the transformer...?

Now the fun part seems to be getting the board out. I removed knobs, nuts, and released the plastic anchor pins, it's all free, but it just won't slide, or lift, etc. without desoldering a lot of wires first, removing the transformer, etc......Is there some trick that I'm missing...? You can't get access to the underside of the board for most of this testing, etc.....

Thanks, Ron

wolf1419

I think(using analog meter-ACV) I have measured black center tap to each white(2) as 30ACV @, and 60ACV white to white on the secondary side of my transformer, which seems to be at least consistent and balanced, so it should be a good thing.
Please also read my prior post about difficulties getting the board out. I'm slooo....ooowly getting through this tutorial, and thanks so much for your support.

Enzo

k

wolf1419

If your amp has insert FX, just try a little contact cleaner on a 1/4" jack and plug in/out a whole bunch of times(like sex...) to clean your FX loop, which is usually "in line of the circuit", which has caused many an amp of mine to seem like it had problems and it was just losing continuity at this point, and very erratic. One day it's fine, then drops off til you clean them, maybe returns, etc, until you get in the habit of doing it just before playing, plug in/out of your FX loop(even without cleaner), and it will remain clean and well connected. Hope this helps.

Anybody have any tricks on removing the boards from the chassis on a VS 8100 head without desoldering a bunch of wires/transformer, etc.(see prior posts)?

Pete240885

Is it not possible to remove the transformer aswell?

Pete240885

yeah those voltages from your transformer sound about right, thinking about the 8040 that i have upstairs when i repaired that i had to disconnect the main power socket, as it happens in order to test DB1 you don't need power to the amp

wolf1419

Thats what I figured about removing and gaining access to the board, but I wanted to be sure I wasn't taking apart more than I needed, and was "hoping for a magic trick". Oh well, it's back to the workbench to do some dismantling.
I guess the good news is the transformer checking out o.k., as I imagine that's the most costly part to purchase and replace.

Pete240885

depends on your suppliers...after shipping and such i'm having difficulty getting valves at an affordable price, but your right they're far from the cheapest part!

wolf1419

I desoldered the transformer wires and removed it, was able to gain acces to the solder side of the board, and think that DB1 is shorted. I have continuity from +(red) to each adjacent leg, -(black) to each adjacent leg, but a short "0" reading from +(red) to -, or -(red) to +(reversed polarity) That means it's bad-correct?
I've got to find a supplier, might have one in Florida, USA, that does not have a minimum order and fair shipping, so they say, but I'll check a few places.
Would you guess that just replacing this one part should fix my problem, or are there other checks that should be done also before putting it back together?