Marshall JCM 900 fried! Need help :)

[g1]

Yes it's so much easier to be the leader of the opposition than to be the government.

It's easy to jump in and say tsk tsk about what is being done wrong, it's a little different when you are the one trying to help the drowning man, with very little help from anyone else as has mostly been the case here.
With all due respect Stan, I have seen a lot of posts where you say what is being done wrong, or the right way that a "pro" would do it, but you are also opposed to novices doing their own repairs, as is the case here. Perhaps this is a good example of why they shouldn't, but some of us here refuse to refuse to help.

[sibo]

Seriously? What's this elitist bullshit about novices shouldn't be doing repairs?
How would anyone be a pro if they didn't start out a novice?
For the help I am getting I am truly thankful and you all seem like nice guys but I just can't let that sentence pass as it's the opposite of everything I believe in.

Now that that's out of the way - how do I proceed?

[g1]

Hold on a moment please. Stan did not say that in this thread and I did not mean to put words in his mouth. If I have gotten that impression elsewhere, that is just my opinion. His post here seemed to be constructive criticism and I hope it was not aimed at anyone personally.

[Audiotexan]

Seriously? What's this elitist bullshit about novices shouldn't be doing repairs?
How would anyone be a pro if they didn't start out a novice?
For the help I am getting I am truly thankful and you all seem like nice guys but I just can't let that sentence pass as it's the opposite of everything I believe in.

I appreciate the kind words before, and I'm all too happy to help.
But I've also got to say this. Please try not to judge Stan by this alone Sibo, nor someone else's comments. Hang out and read awhile before coming to your own conclusions about anyone here. I have secondary opinions (as I'm sure does g-one and others), but those are a result of my (our) own interactions, and watching other's interactions as well. IMO, the measure of a man is only partly in what he says, but largely in what he does. So I'm already "partially-screwed" on the first part to some extent, as I struggle with words all too often. LOL
But I hope that my actions/intent outweighs/overcomes my shortcomings with 'how it comes out' at the keyboard.

Stan's a VERY intelligent man, with a ton of BIG (large volume) business experience under his belt, much more than I -especially in the arena of tubes. Which I why I hold a great deal of respect for him. He's mainly trying to say that this could be a whole lot easier...for a tech. I get that. I just took issue that he jumped in half-assed, and started criticising before he'd even (read) taken the whole thing in to put it in context. If he had, I believe he could easily see where I was going, and what I was trying to do, even if it was a slightly longer way around. If you only listened to half of a customer complaint, that'd be a critical error in troubleshooting as well. Hell, even when folks have stated things flat out in a thread, I've missed them now and again. We're all human, we all eff up sometimes. No biggie. We learn, we try not to repeat the same mistakes, we move on.

On a seperate note, the more eyes/expertise on this the better. Everyone has a slightly different approach, and that just gives more opportunities to provide insight/tips to getting to a repair faster. I'm all for it!

Now that that's out of the way - how do I proceed?

Well personally, given that you have a bad power supply cap, I would recommend leaving it off until you can get the parts that are on the way replaced. As those parts in particular (anytime you have power supply problems) can cause issues further down the line in secondary functions/aspects if left running unchecked. (Leaking AC into DC areas, or allowing higher voltage surges than what's supposed to be there.)

Again though, don't forget to drain all the HV caps *just before* you go to install the new replacements! They can rebuild a charge just sitting unplugged.

[oc disorder]

I don't want to spoil the party by butting in - but here's my 2cents worth
Given the situation and particularly as Sibo noticed a different speaker lead upon it's return why haven't we checked the output transformer first?
For a newbie with a Marshall (the temporary owner) it's not uncommon to forget to plug the speaker in.
That would be my first port of call. From the photos the primary wires are coloured orange ,brown and white.
Find the centretap the one that goes to the power supply. and measure between that and the two outer windings which go to the output tubes/valves anodes.
You should get similar approximate readings and if you do ignore this rude post!
If you don't......... it wouldn't be the first time somebody neglected to mention they initially forgot to connect the speaker !

EDIT: Thought I had better add - do this with the power off and meter set to low resistance reading

[J M Fahey]

before anything else
1 measure wall voltage
2 measure filament vooltage
3 measure HV secondary voltage

post here

the DC voltages posted earlier are CRAZY high ... and all derive from some AC source so it;s useless to measure parts or internal voltages until we sort that FIRST

I very well understand Stan.s frustration when reading this thread

[sibo]

Hold on a moment please. Stan did not say that in this thread and I did not mean to put words in his mouth. If I have gotten that impression elsewhere, that is just my opinion. His post here seemed to be constructive criticism and I hope it was not aimed at anyone personally.

Got it!

don't forget to drain all the HV caps *just before* you go to install the new replacements! They can rebuild a charge just sitting unplugged.

Will do, once they get here.

I don't want to spoil the party by butting in - but here's my 2cents worth
Given the situation and particularly as Sibo noticed a different speaker lead upon it's return why haven't we checked the output transformer first?
For a newbie with a Marshall (the temporary owner) it's not uncommon to forget to plug the speaker in.
That would be my first port of call. From the photos the primary wires are coloured orange ,brown and white.
Find the centretap the one that goes to the power supply. and measure between that and the two outer windings which go to the output tubes/valves anodes.
You should get similar approximate readings and if you do ignore this rude post!
If you don't......... it wouldn't be the first time somebody neglected to mention they initially forgot to connect the speaker !

Of course I’ll take any aspect of this into consideration and try that aswell. I’ll get back with readings when I get back home.

before anything else
1 measure wall voltage
2 measure filament vooltage
3 measure HV secondary voltage

1. 233VAC
2. 7.2-7.3VAC (measured between pin #2 and pin #7 on output tube socket)
3. I’ll get back with this asap.

Tyvm!

[km6xz]

There are two likely problems that could cause those readings to be high, the common one to all the readings is simply that the meter is not calibrated.
Unfortunately, contrary to the speculation in prior threads, I did read the posts and imagined along the way what mishmash of theories and tests suggested would seem like to a beginner in troubleshooting. Still, any parts proven to be bad except for the one resistor?

Think of the symptoms, plays ok for a period of time, meaning no significant component such as transformers or the circuit in general is in a failed state.
After a period of playing the gain increases and then distortion and a burning smell. The power supply capacitors suggested would not give that sequence of symptoms, they might be bad but they are not causing this symptom.
For the sound to increase in gain, then distortion the gain structure is changing, either in the gain of the preamp section or in the power amp section. Of those two which could cause the gain increase and then distortion, only one would fit the full set of symptoms, the output section. A change in gain, onset of distortion and heat would be expected if the bias voltage on the girds varied. That is why the very first test I suggested was to check and monitor the bias at the grids of the power tubes, not at the bias power supply.
Just to eliminate the pre-amplifier as the cause, insert a signal into the effects return jack of about 200mv RMS when the problem is present, if the distortion disappears, it is the pre-amp but I would bet against it.
Before going any further, measure the bias as the power tube grids without a signal. If it increases in voltage, look for a leaking coupling capacitor at the plate of the phase inverter. If it drops, look for bias power supply filter cap. The smell of heat could easily be the power tubes getting hot as the bias dropped, thereby increasing plate current.

Thinking of the symptoms instead of parts. Some diagnosis conclusion will satisfy all the observed symptoms, the theories that don't satisfy all the observations is wrong so don't start changing parts hoping that it is right.
There are two possible causes of this method not working, one is that not enough symptoms are observed, and two, that the circuit is not understood well enough to know what to expect. In this latter case, the owner is often better off by stopping and taking it to a tech who is familiar with good troubleshooting technique and knows the circuit concepts. It will be cheaper and faster than making a wrong assumption and following the wrong path.

[Audiotexan]

Perhaps this is a good example of why they shouldn't, but some of us here refuse to refuse to help.

There are two possible causes of this method not working, one is that not enough symptoms are observed, and two, that the circuit is not understood well enough to know what to expect. In this latter case, the owner is often better off by stopping and taking it to a tech who is familiar with good troubleshooting technique and knows the circuit concepts. It will be cheaper and faster than making a wrong assumption and following the wrong path.

All the other aside, bottom line...you suggest he takes it to a tech.
Sadly, the only thing he learns there is how to open his wallet. Except that I imagine he already knows how to do that.

The first post you reference as a starting point was from a third party. The buyer. Who could have said anything to get out of the deal knowing it was now blown. But I haven't seen Sibo say a word about the gain increasing or distortion.

Could've been from as simple as mentioned previously, and turning it on and not having a cab plugged in. (Another good call/possibility).

At any rate, I prefer to start with the person whose hands are on the amp in front of them (first party) to assess current condition:

There's currently potentially two different issues:
-first ticking
-then buzz/hum

Though they could also possibly be the same component further deteriorating, and changing the symptoms. They could also be two separate issues.

Post 33: When I switched bypass to ON there was some ticking coming from the tubes. I've never played brand new tubes before so I guess that's normal.. But then, after like 5 seconds it started buzzing so I turned it off instantly.

Post 63: (repeat p33) After like 30 seconds there's a vague but increasing sound like the one when you tap a tele jack that is connected to your amp with your finger. The overall hum starts to increase and something smells burnt so I turned it off again.

However, my normal first reaction to hearing a complaint about hum/buzz is to check the power supply. Then I move to other areas.

Regardless, I'm happy to see more eyes on this.
But for now, I'm going to remove myself for the time being.
I need a breather.

[J M Fahey]

Quote Originally Posted by J M Fahey View Post
before anything else
1 measure wall voltage
2 measure filament vooltage
3 measure HV secondary voltage

1. 233VAC
2. 7.2-7.3VAC (measured between pin #2 and pin #7 on output tube socket)
3. I’ll get back with this asap.

What I ask in 3) is the AC voltage across Red to (Red?) wires which feed main HV rectifier bridge D2/3/4/5 .
Be careful, it's over 300VAC there and is not referred to ground but floating.

And then read DC voltage across main filter cap (50+50x500V)

And then reading bias voltage at the junction R26/27 .
Not contradicting Stan at all, but trying to know first what do we have available.

He suspects not enough bias voltage reaches actual grids, which is entirely possible, I would first want to know whether said bias voltage actually exists to begin with

Either a leaky bias capacitor (C13/14?) or a wrong bias voltage dropper (C15?) may conspire to bad bias.
Note: "?" at the end of a name or value means it's not very readable on the schematic drawing.

The schematic states 220V primary ... sibo measures 233V, that would make the 6.3V winding show 6.7V .... the 7,2 measured might indicate an unloaded transformer, would love to have readings later, when amp works fully.

As of the ticking sound by itself, might indicate arcing somewhere ... or plain "new tube warming and stretching for the first time" .
Unspecified "burnt smell" may also mean "new tube warming for the first time" .
In Argentina for sure: dumb Customs guys affix import stamps anywhere, often to the glass bulb.
Nobody removes them of course, for fear of being accused of using bootleg stuff, so 80% of the time a new tube announces itself by the burning paper and glue smell .... not kidding.
And the "hum/buzz after 5 seconds) may mean just that the tube is starting to work.
Problem is, maybe nothing wrong is happening and any Tech won't even notice that ... but a worried noob (no disrespect meant) may get very nervous about that.
Just remember the tons of posts about "the mysterious blue glow inside tubes".

I use the graphic schematic instead of the PDF so it's already here without opening new windows.



Note: measure bias voltage at the resistor junction without power tubes, just bto play it safe.
Also measure bias voltage at each of the tubes grid pin, what Stan suggests.
It will show lower than normal, because the meter input current drops voltage across the bias resistors, but it's a fair indication, specially about path integrity.

[J M Fahey]

All the other aside, bottom line...you suggest he takes it to a tech.
Sadly, the only thing he learns there is how to open his wallet. Except that I imagine he already knows how to do that.

With due respect, I guess any new damage to the amp will make sibo's wallet to open even more

And as of learning, there are steps and steps to be followed.

As in: sitting somebody who wants to learn driving straight at the wheel in a Formula1 car ... even in NASCAR .... won't help him much to get his driver licence.

No doubt those will be some real exciting (last?) 60 seconds in his life, that's for sure .

[Audiotexan]

With due respect, I guess any new damage to the amp will make sibo's wallet to open even more

And as of learning, there are steps and steps to be followed.

No offense taken, and I completely agree!
I'm glad you're helping

[km6xz]

Ticking sound? Sure, stress tubes and get the plates really hot and you will heat ticking as the metal contracts and buckles as it cools. Another piece of evidence that supports the bias fault theory.

There are many ways of learning, but destroying a $1500 desired asset is not a very effective way to learn. Unless the fundamentals are learned, as a foundation upon which to build a scaffold of understanding interconnected electronics principles, all that will be learned this was is when this exact set of symptoms are found, replace C2xx. Without context, the OP just becomes another of 1,000,000,000 other internet experts who repeat comments to others without understanding what it means. 90% people on this forum are doing that and this forum has one of the best signal to noise ratios around in music and electronics.

How about learning on something more instructive like a old tube table radio(really old so it has a power transformer) where the circuit is less complex and expendable? How about referring to books on the fundamentals? Even the Radio Shack kids experimenter kits where simple circuits can be assembled or rearranged while calculating expected results and then measuring them? A battery and a handful of $0.01 resistors and a volt/ammeter will teach more than diving into a dangerous valuable tube guitar amp as first step.
You have to consider motivation also. If someone is not really interested in the basics, only the results, they will be far ahead getting to know an honest skill tech and letting them deal with the learning aspect(by having done all that messy book learning decades before but still studies). A beginner motivated by a desire to know can be spotted in an instant, just look at their library at home. When aspiring recording engineers or techs came to ask to be trained, it was very easy to spot the few who were motivated and would actually make it. A person claiming a passion has already devoured every book on the subject they could get their hands on. The rest really just wanted to learn the end product of that study, which knob to turn when to be a star. Talk to any good tech or music related engineer, when they started and whether they learned it in school. The answers for almost all I ever met were "about 12" and "no, the classes were too slow".
Fast instant answers does not teach anything, except over confidence that will ruin more gear and possibly be dangerous to health.

[sibo]

What I ask in 3) is the AC voltage across Red to (Red?) wires which feed main HV rectifier bridge D2/3/4/5 .
Be careful, it's over 300VAC there and is not referred to ground but floating.

When you tell me to be careful, exactly what do you mean? I get that those points aren't directly conncted to ground but are you warning me not to touch it or should I know something else before measuring it?

There are many ways of learning, but destroying a $1500 desired asset is not a very effective way to learn. Unless the fundamentals are learned, as a foundation upon which to build a scaffold of understanding interconnected electronics principles, all that will be learned this was is when this exact set of symptoms are found, replace C2xx. Without context, the OP just becomes another of 1,000,000,000 other internet experts who repeat comments to others without understanding what it means. 90% people on this forum are doing that and this forum has one of the best signal to noise ratios around in music and electronics.

How about learning on something more instructive like a old tube table radio(really old so it has a power transformer) where the circuit is less complex and expendable? How about referring to books on the fundamentals? Even the Radio Shack kids experimenter kits where simple circuits can be assembled or rearranged while calculating expected results and then measuring them? A battery and a handful of $0.01 resistors and a volt/ammeter will teach more than diving into a dangerous valuable tube guitar amp as first step.
You have to consider motivation also. If someone is not really interested in the basics, only the results, they will be far ahead getting to know an honest skill tech and letting them deal with the learning aspect(by having done all that messy book learning decades before but still studies). A beginner motivated by a desire to know can be spotted in an instant, just look at their library at home. When aspiring recording engineers or techs came to ask to be trained, it was very easy to spot the few who were motivated and would actually make it. A person claiming a passion has already devoured every book on the subject they could get their hands on. The rest really just wanted to learn the end product of that study, which knob to turn when to be a star. Talk to any good tech or music related engineer, when they started and whether they learned it in school. The answers for almost all I ever met were "about 12" and "no, the classes were too slow".
Fast instant answers does not teach anything, except over confidence that will ruin more gear and possibly be dangerous to health.

Noted. The way I see it I already learned a lot and my aim is set (and always was) to be a player, not a tech. This is just a hobby for me and clearly we see things from a different perspective. This amp hardly sold for 400€ in Sweden but I guess prices are different in Russia. I don't like this phrase very much but let's agree to disagree and move onwards. Also, thanks for getting involved in this thread.

[km6xz]

If there is a problem with bias, one single test point can determine if that is the source of the problem, testing noting else, measure the control grids, if it is right, move on, forget all the rest of the bias supply. If it not there, fine, you have narrowed the problem to a very simple circuit that can be tested with any VOM DMM or VTVM. He can check that without any power tubes plugged just in case lack of bias is the cause, it could save a set of from being overstressed. Testing various parts of the bias supply individually still does not tell you whether the all important bias is actually on the control grids. An open resistor or broken connection between the test point and the control grid can easily cause the symptoms but not be detected by metering earlier part of the bias supply.