Marshall Valvestate 8100

[Hawk]

 Probably a simple fix but when I turn the effects knob (eff)  to the right the volume cuts out, starting at about 12 o'clock and then goes completey dead when turned all the way to the right. I used Deoxit and got rid of all the scratchiness but same problem. I removed the pot and measured 22K as it should be. As I turn the pot to the right it's as if the signal is being shorted to ground and then at the far right: dead, volume completely cuts out.

[g1]

What are you using in the loop?  Try a plain patch cord.

[Enzo]

It kills the sound because you have it turned all the way over to the loop.   That is your FX loop balance control.  All the way to that end means the only thing going on to the power amp is whatever is in the FX return jack.  Unless there is something in the loop, this will always happen.  If you do not use the loop, then leave the control at the other end.

[Hawk]

Thanks. So obvious now. Amp is working as it should. I've never used outboard effects so I've learned something new.

[Hawk]

This amp is dirty! What do people recommend for tolex/faceplate?

[J M Fahey]

Aggressive cleaners can attack plastic if they have solvents or scratch labels off if abrasive, so classic cleaner (at least for starters)  is some cloth which you wet under the tap and then squeeze so it's humid but not dripping (you don't want water seeping inside) where then you add a drop of dishwasing type detergent.

Wipe everything in many ways, pull knobs if necessary, use a similarly treated Q Tip to clean hard to reach places, on large very dirt encrusted Tolex surfaces an old toothbrush and lots of patience will help.

Then wipe everything off, the small amount of detergent will leave behind a slight foam.

Then wash cloth under the tap, squeeeze again and sweep old water+detergent off.

Repeat a couple times.

Wipe it dry with a new dry clean cloth, let it dry naturally, if you touch it you will notice it later.

It will be clean but dull, then you can apply some spray furniture polish or whatever you use on car seats.

Most people skip the cleaning process and straight apply spray polish, net result is a still very dirty, gunky amp.

Can't suggest brands because every Country has its favorites but you know what I'm talking about.

Remember cleaning knobs, strap handles, etc. with  water with a drop detergent (don't overdo it)  and an old toothbrush.

[Roly]

Remember cleaning knobs, strap handles, etc. with  water with a drop detergent (don't overdo it)  and an old toothbrush.

Dish washing liquid, warm water, several brushes and lots of toilet paper/tissue.

:dbtu: :dbtu:


Also, metho is pretty safe for cleaning off other gunk like coffee or wine, but don't use it on finger grime (front panel) because it turns it into a nasty sticky sludge.

[Hawk]

Great help and answers. Thanks guys!

[Hawk]

With this amp and others is there a way to have the circuit board out of the amp with the appropriate wires still connected so that I can test components with the amp in it's "on" state? Is it a matter of making sure the pots that need to be grounded are grounded?

[Roly]

is there a way to have the circuit board out of the amp with the appropriate wires still connected so that I can test components with the amp in it's "on" state?

Every one is different.

I've worked on a huge range of equipment, and how easy it is to faultfind and repair covers a vast range from bloody wonderful to bloody appalling.

There is a special case of Murphy's Law which states that "an intermittent fault will only occur when the last of 24 retaining bolts are tightened down".  Well I once had exactly that with an industrial controller in a big flameproof box.


The very best:

A Philips car cassette player in for service.  Open lid and find a stack of three large PCB's.  Single multipole connector on one corner - unplug.  Now the whole lot swings up out of the mechanism and flat over the back, then the rest unfolds to the right, and the final layer folds forward and the connector can again be plugged in and the whole thing run while spread out flat on the bench.  Cunning design.


One of the very worst (and there are very many more contestants for this award, the world if full of "cost-effective" stinkers*):

Early Yamaha 8-channel digital mixing desk with flying faders.  Now generally speaking Yamaha stuff is pretty good, I've got a fair bit and it all works and is built well.  But there are exceptions and this mixer was an extreme one.

The flying faders where now slipping a lot, but investigation suggested that it was a design problem that couldn't be fixed.

One of the inputs was dead which turned out to be a dead A/D converter module which was unobtainable.

The proximate reason for it coming in was that it had started to forget its presets, and I suspected a dying backup battery.

When I opened the mixer up I found several large PCB's in layers about an inch apart, and the lot laced through with many un-insulated tinned copper wire links scattered all over the boards and running from top to bottom.  I did locate the backup battery but it was soldered in to the centre of one of the middle layer boards - simply impossible to access without seriously demolishing the PCB assembly.

{* saw a very impressive looking iPod dock/amp/speakers junked after less than a year.  Inside it was boards and modules just indifferently glued to the inside of the fiberboard case, as crude and rough and sloppy as one could imagine.)


Every time I open up some item of gear I can tell what sort of people designed it and put it together, and what sort of attitudes they have to the planet and their fellow humans.

With guitar amps, Fenders the component boards are so wired down you have no hope of swing one up, but other amps, I'm thinking Overreem, all the connections to the boards come off one end - a few screws and presto, you can work on the board hot, standing on end.

We had two rack amps in the PA, a New Zealand Perreaux, and an American CueTek.  The most significant difference between them was that the Perreaux had a removable lid and bottom, leaving the amp totally accessible, while the CueTek was poured into a U-channel case and you couldn't get at anything.

A lot of gear is designed to be manufactured.  It is not designed to be disassembled.  At all.  Ever.  Much less repaired.

A large part of my life has been figuring out how to get assemblies open, particularly stuff that wasn't intended to be opened again.

[Hawk]

With guitar amps, Fenders the component boards are so wired down you have no hope of swing one up, but other amps, I'm thinking Overreem, all the connections to the boards come off one end - a few screws and presto, you can work on the board hot, standing on end.

We had two rack amps in the PA, a New Zealand Perreaux, and an American CueTek.  The most significant difference between them was that the Perreaux had a removable lid and bottom, leaving the amp totally accessible, while the CueTek was poured into a U-channel case and you couldn't get at anything.

A lot of gear is designed to be manufactured.  It is not designed to be disassembled.  At all.  Ever.  Much less repaired.

A large part of my life has been figuring out how to get assemblies open, particularly stuff that wasn't intended to be opened again.

Thanks Roly, your years of experience pretty much says it all. Especially "designed to be manufactured not repaired". I'll soon discover what I can and can't do with amps, and hopefully I'll have some stories to pass down. :tu:

[Hawk]

So this amp is working and using a voltage divider equation there should be 1.92 volts across R118 (should there?). But when I measure with my voltmeter I get .007vdc. I put a 1khz signal through the amp then I played my guitar and measured the voltage across the resistor and measured it referenced to ground but nowhere near this 1.92v. What am I missing here? I think I'm missing the obvious but what?

R117=820ohms, R116=470ohms

820+470/470=2.74X.7=1.92V

[Enzo]

R118 is a 0,33 ohm resistor in the emitter lead of TR8.  How did you decide it ought to have 1.92v across it?  That would be about 6A flowing through it.

What do you consider to be a voltage divider here, and what is it across?

R117,116 do form a voltage divider, but they do not put a voltage on R118, rather they divide the voltage that drops across R118 for use by the base of TR12.

Whatever voltage drops across R118 is also across the parallel R117,116 pair.  820+470 is 1290 total, so at the base of TR12, we expect 470/1290 x (V of R118).

This is a classic limiter circuit.  The voltage across R118 varies with the current through it, the more current, the higher the voltage.  If that current, hence voltage, gets high enough, the divided voltage at TR12 base is enough to turn the transistor on, and that then causes the collector to drag down the signal at the base of TR8.

[Hawk]

Enzo thanks so much for the response. Below I quote several times from a post on another forum that didn't involve myself. This person gave a fairly thorough analysis of this amp to someone trying to figure it out. When I was referring to a voltage divider I thought I knew what I was talking about as in this same post he referred to R74 and R73 as a voltage divider and set about making calculations 1800 + 680/680 X .7=2.55 volts. He did this to calculate Collector Emitter voltage across TR9


"For TR8 the resistors R116, 117, 118 and transistor TR12 and D5 make a current limiter.
For TR10 the resistors R96, 97, 115 and transistor TR11 and D4 make a current limitter."

So, below, is where my intial confusion came because I couldn't measure the voltage he refers to: 1.92V
"The calculations for the current are similar to the calculations at TR9.
It will result in 1.92 volts accross R118 , this will give a limit of 1.92 / 0.33 = 5.82 A
For R96 it will result in 2.14 Volts , this will give a limit of 2.14 / 0.33 = 6.48 A."

I somewhat understand what you're saying about  pulling down the signal on TR8 if the voltage gets to high. By signal are you referring to current, therefore this config. acts as a current limiter? 

One day the penny will drop, but it sure is taking awhile!

[Enzo]

The limiter action serves to limit the current through R118 and thus TR8.  If the current gets past a certain point, then the base or TR12 is turned on so the transistor conducts.  That prevents any additional current going into the base of TR8.

Off hand I don't know why there would be a difference between the voltages at R118 and R96.  I don't know what entire discussion took place with your 1.92 volts, but I suspect it was confused.  I would not expect 1.92v there, but I might suspect that 1.92v there could be the threshold for turning on TR12.  let me try that.

If I start with 1.92v across R118, that also means 1.92v across R117,116.  That is 820 over 470.  The voltage at their junction would be 470/1290 times 1.92.  I get 0.7v, which is about what ought to turn on Tr12.  I would expect the base voltage of that transistor to be closer to 0.5v, but 0.7v is the classic figure.  Thus I have to imagine they were discussing the voltage across r118 required to initiate the limiting function.  It is not a voltage you would expect to find there unless the amp was maxed out.  It would not be a steady DC voltage though.


R73,74 is a completely different circuit.